# International Workshop on Next Generation Gamma-Ray Source

C. R. Howell  Department of Physics, Duke University, Durham, NC 27708
Triangle Universities Nuclear Laboratory, Durham, NC 27708  M. W. Ahmed
Department of Mathematics and Physics, North Carolina Central University,
Durham, NC 27707  Triangle Universities Nuclear Laboratory, Durham, NC 27708
A. Afanasev  Department of Physics, The George Washington University,
Washington, D.C. 20052  D. Alesini  LNF-INFN, Via E. Fermi 40, 00044 Frascati
(Rome), Italy  J. R. M. Annand  SUPA School of Physics and Astronomy,
University of Glasgow, Glasgow, G12 8QQ, UK  A. Aprahamian  Department of
Physics, University of Notre Dame, Notre Dame, IN 46556, USA  D. L. Balabanski
Extreme Light Infrastructure Nuclear Physics (ELI-NP), Horia Hulubei National
Institute for R&D in Physics and Nuclear Engineering (IFIN-HH), Magurele,
Romania  S. V. Benson  Thomas Jefferson National Accelerator Facility, Newport
News, VA 23606  A. Bernstein  Physics Department and Laboratory for Nuclear
Science Massachusetts Institute of Technology, Cambridge, MA 02139, USA  C. R.
Brune  Department of Physics and Astronomy, Ohio University, Athens, OH 45701
J. Byrd  Argonne National Laboratory, Argonne, Illinois 60439  B. E. Carlsten
Los Alamos National Laboratory, Los Alamos, NM 87545  A. E. Champagne
Department of Physics and Astronomy, University of North Carolina, Chapel
Hill, NC  Triangle Universities Nuclear Laboratory, Durham, NC 27708  S.
Chattopadhyay  Fermi National Accelerator Laboratory, Batavia, IL 60510  D.
Davis  Department of Physics, North Carolina State University, Raleigh, NC
27695  Triangle Universities Nuclear Laboratory, Durham, NC 27708  E. J.
Downie  Department of Physics, The George Washington University, Washington,
D.C. 20052  M. J. Durham  Los Alamos National Laboratory, Los Alamos, NM 87545
G. Feldman  Department of Physics, The George Washington University,
Washington, D.C. 20052  H. Gao  Department of Physics, Duke University,
Durham, NC 27708  Triangle Universities Nuclear Laboratory, Durham, NC 27708
C. G. R. Geddes  Lawrence Berkeley National Laboratory, Berkeley, CA 94720  H.
W. Grießhammer  Department of Physics, The George Washington University,
Washington, D.C. 20052  R. Hajima  National Institutes for Quantum and
Radiological Science and Technology, Tokai, Naka, Ibaraki 3191106 Japan  H.
Hao  Department of Physics, Duke University, Durham, NC 27708  Triangle
Universities Nuclear Laboratory, Durham, NC 27708  D. Hornidge  Mount Allison
University, Sackville, New Brunswick E4L1E6, Canada  J. Isaak  Institut für
Kernphysik, Technische Universität Darmstadt, 64289 Darmstadt, Germany  R. V.
F. Janssens  Department of Physics and Astronomy, University of North
Carolina, Chapel Hill, NC  Triangle Universities Nuclear Laboratory, Durham,
NC 27708  D. P. Kendellen  Department of Physics, Duke University, Durham, NC
27708  Triangle Universities Nuclear Laboratory, Durham, NC 27708  M. A.
Kovash  Department of Physics and Astronomy, University of Kentucky,
Lexington, KY 40506  P. P. Martel  Institut für Kernphysik, University of
Mainz, D-55099 Mainz, Germany  Ulf-G. Meißner  Helmholtz-Institut für
Strahlen-und Kernphysik and Bethe Center for Theoretical Physics, Universität
Bonn, D-53115 Bonn, Germany  R. Miskimen  University of Massachusetts,
Amherst, MA 01003  B. Pasquini  University of Pavia, Pavia, Italy  D. R.
Phillips  Department of Physics and Astronomy, Ohio University, Athens, OH
45701  N. Pietralla  Institut für Kernphysik, Technische Universität
Darmstadt, 64289 Darmstadt, Germany  D. Savran  GSI Helmholtzzentrum für
Schwerionenforschung GmbH, 64291 Darmstadt, Germany  M. R. Schindler
Department of Physics and Astronomy, University of South Carolina, Columbia,
SC  M. H. Sikora  Department of Physics, The George Washington University,
Washington, D.C. 20052  Triangle Universities Nuclear Laboratory, Durham, NC
27708  W. M. Snow  Department of Physics, Indiana University, Bloomington, IN
47408  R. P. Springer  Department of Physics, Duke University, Durham, NC
27708  C. Sun  Lawrence Berkeley National Laboratory, Berkeley, CA, 94720  C.
Tang  Department of Engineering Physics, Tsinghua University, Beijing 100084,
China  B. Tiburzi  City College of New York, New York, NY  A. P. Tonchev
Nuclear and Chemical Sciences Division, Lawrence Livermore National
Laboratory, Livermore, CA 94550  W. Tornow  National Institutes for Quantum
and Radiological Science and Technology, Tokai, Naka, Ibaraki 3191106 Japan
Department of Mathematics and Physics, North Carolina Central University,
Durham, NC 27707  C. A. Ur  Extreme Light Infrastructure Nuclear Physics (ELI-
NP), Horia Hulubei National Institute for R&D in Physics and Nuclear
Engineering (IFIN-HH), Magurele, Romania  D. Wang  Shanghai Institute of
Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China  H. R.
Weller  Department of Physics, Duke University, Durham, NC 27708  Triangle
Universities Nuclear Laboratory, Durham, NC 27708  V. Werner  Institut für
Kernphysik, Technische Universität Darmstadt, 64289 Darmstadt, Germany  Y. K.
Wu  Department of Physics, Duke University, Durham, NC 27708  Triangle
Universities Nuclear Laboratory, Durham, NC 27708  J. Yan  Department of
Physics, Duke University, Durham, NC 27708  Triangle Universities Nuclear
Laboratory, Durham, NC 27708  Z. Zhao  University of Virginia,
Charlottesville, VA  A. Zilges  Institut für Kernphysik, Universität zu Köln,
Zülpicher Straße 77, D-50937 Köln, Germany  F. Zomer

Disclaimer:  This report was prepared as an account of work sponsored by an
agency of the United States Government. Neither the United States Government
nor any agency thereof, nor any of their employees, makes any warranty,
express or implied, or assumes any legal liability or responsibility for the
accuracy, completeness, or usefulness of any information, apparatus, product,
or process disclosed, or represents that its use would not infringe privately
owned rights. Reference herein to any specific commercial product, process, or
service by trade name, trademark, manufacturer, or otherwise, does not
necessarily constitute or imply its endorsement, recommendation, or favoring
by the United States Government or any agency thereof. The views and opinions
of authors expressed herein do not necessarily state or reflect those of the
United States Government or any agency thereof. This workshop and the
resulting whitepaper are supported by the US. Department of Energy Office of
Nuclear Physics under grant number DE-SC0014616 and by the Triangle
Universities Nuclear Laboratory (TUNL).

###### Contents

  1. 1 Executive Summary
  2. 2 Introduction
  3. References
  4. 3 Nuclear Physics Research and Applications with Gamma-ray Beams Below 20 MeV
    1. 3.1 Nuclear Structure
      1. 3.1.1 Nuclear Resonance Fluorescence
      2. 3.1.2 Photon-Induced Nuclear Fission
    2. 3.2 Nuclear Astrophysics
      1. 3.2.1 The Origin of the Elements: $p$-Process and $s$-Process Nucleosynthesis
      2. 3.2.2 The Life of Stars: The 12C($\alpha$,$\gamma$) Reaction Cross Section
      3. 3.2.3 The Death of Stars: $r$-Process Nucleosynthesis
      4. 3.2.4 Neutron Stars: the EOS of Neutron-Rich Matter
    3. 3.3 Hadronic Parity Violation
      1. 3.3.1 Hadronic Parity Violation in Few-Nucleon Systems
      2. 3.3.2 Hadronic Parity Violation in Light Nuclei
    4. 3.4 Nuclear and Homeland Security
  5. References
  6. 4 Nucleon Structure and Low-Energy QCD
    1. 4.1 Compton Scattering
      1. 4.1.1 Insights from Lattice QCD
      2. 4.1.2 Extracting Polarizabilities from Data
      3. 4.1.3 Review of Recent Accomplishments
      4. 4.1.4 Overall Goals of a Compton Program
      5. 4.1.5 Experiments
      6. 4.1.6 Target Technology
      7. 4.1.7 Detectors
    2. 4.2 QCD Origins of Charge Symmetry Breaking
  7. References
  8. 5 Accelerator Concepts of Next Generation Laser Compton Gamma-ray Beam Facilities
    1. 5.1 Introduction to Compton Gamma-ray Sources
    2. 5.2 Technology for Next-Generation Compton Gamma-ray Sources
      1. 5.2.1 Related Technology Development
      2. 5.2.2 Electron Accelerators
      3. 5.2.3 Photon Beam Systems
    3. 5.3 Next Generation Compton Gamma-ray Sources: Examples and Capabilities
      1. 5.3.1 Storage Ring Based Medium Energy CGS
      2. 5.3.2 Storage Ring Based Low Energy CGS
      3. 5.3.3 Energy Recovery Linac Based Low-Energy CGS
  9. 6 Appendix
    1. 6.1 Workshop Agenda
    2. 6.2 Working Group Conveners
    3. 6.3 Workshop Participants

##  Chapter 1 Executive Summary

The photon is a theoretically well-understood probe for investigating the
structure of matter over a wide range of distance and energy scales. The
angular-momentum selectivity and unconstrained transfer of isospin in most
gamma-ray ($\gamma$-ray) induced reactions enable highly precise strategic
investigations of nuclear and nucleon structure and of collective motion
responses of the internal degrees of freedom associated with electric charge
and current distributions. In addition, high-energy photons are well suited
for non-intrusive material analysis applications in areas such as homeland
security, nuclear security, structural integrity assessments, and medical
diagnostics.

Bremsstrahlung radiation produced by an electron beam incident on a high-$Z$
target has been the workhorse photon-beam source used in nuclear physics
research and applications for over a century. The energy spectrum of a
bremsstrahlung $\gamma$-ray beam enables measurements of nuclear structure and
reaction dynamics over a continuous energy range in a single experiment.
Tagged bremsstrahlung sources provide the capability of associating the energy
of the photon inducing a reaction with the photons detected in the final
state. Also, coherent bremsstrahlung beams produced using a crystal target
offer researchers a partially polarized $\gamma$-ray beam at energies near the
end point of the bremsstrahlung spectrum. The scientific insight provided by
research conducted using these sources is impressive, and facilities with
advanced bremsstrahlung photon sources continue to be used in highly
productive research programs. However, the continuous energy nature of
bremsstrahlung $\gamma$-ray beams, which facilitates survey measurements over
a broad energy range, also has the adverse effect of creating backgrounds that
limit the sensitivity of the experiments. These backgrounds make it difficult
to investigate nuclear phenomena with extremely low cross sections or to
perform measurements on targets with small sample sizes, as is the case for
isotopes with low natural abundances.

The nearly monoenergetic $\gamma$-ray beams produced by laser Compton
scattering offer an alternative to bremsstrahlung beams, providing an enhanced
signal-to-background ratio in basic and applied research, and reducing the
radiation exposure in material analysis applications. In addition, laser
Compton $\gamma$-ray beams can be produced with a beam polarization greater
than 95% for both linear and circular polarization. Linearly polarized laser
Compton beams enable unambiguous determinations of the spin and parity of
excited nuclear states, and beams with both linear and circular polarization
are used in studies of the spin structure of nucleons.

Over the last three decades, laser Compton $\gamma$-ray beam facilities have
provided intense polarized and nearly mono-energetic $\gamma$-ray beams for
research programs in basic and applied nuclear physics. These facilities
include GRAAL at the European Synchrotron Radiation Facility in Grenoble, LEGS
at Brookhaven National Laboratory, LEPS at the SPring-8 facility, NewSUBARU at
the University of Hyogo, and HI$\gamma$S at the Triangle Universities Nuclear
Laboratory. However, HI$\gamma$S and LEPS are the only facilities worldwide
that are operated with nuclear physics as the primary research focus. These
facilities will soon be joined by two more that are currently under
construction, ELI-NP in Romania and the $\gamma$-ray beam line at the Shanghai
Synchrotron Radiation Facility (SSRF) in China. These facilities will produce
$\gamma$-ray beams with energies below 12 MeV (ELI-NP), and below 20 MeV and
above 300 MeV (SSRF).

Given that the time span between initial planning and the end of construction
of an accelerator-driven light source is about a decade, consideration for
next-generation laser Compton $\gamma$-ray sources should start now.
Technological advances in electron accelerators, lasers, and optics made
during the last decade create new options for producing intense polarized
$\gamma$-ray beams with narrow energy widths at beam energies from around 1
MeV to several GeV, the energies relevant to nuclear-physics research. A
workshop on The Next Generation Gamma-Ray Sources, sponsored by the Office of
Nuclear Physics at the Department of Energy, was held November 17–19, 2016 in
Bethesda, Maryland. The goals of the workshop were to identify basic and
applied research opportunities at the frontiers of nuclear physics that would
be made possible by the beam capabilities of an advanced laser Compton beam
facility. To anchor the scientific vision to realistically achievable beam
specifications using proven technologies, the workshop brought together
experts in the fields of electron accelerators, lasers, and optics to examine
the technical options for achieving the beam specifications required by the
most compelling parts of the proposed research programs. An international
assembly of participants included current and prospective $\gamma$-ray beam
users, accelerator and light-source physicists, and federal agency program
managers. Sessions were organized to foster interactions between the beam
users and facility developers, allowing for information sharing and mutual
feedback between the two groups. The workshop findings and recommendations are
summarized below.

Findings of Topical Working Groups:
The topical sessions at the workshop focused on five research areas: low-
energy quantum chromodynamics (QCD), nuclear structure, nuclear astrophysics,
fundamental symmetries, and applications. The advanced accelerator and light
source technologies working group recommended laser Compton scattering inside
an optical cavity as the primary technology for the next generation
$\gamma$-ray sources. The working group concluded that two $\gamma$-ray beam
facilities are needed to meet the beam requirements of the research presented
in the topical sessions at the workshop (1) a medium-energy source with
$\gamma$-ray beams in the range of 25 and 400 MeV, and (2) a low-energy source
capable of delivering $\gamma$-ray beams from 1.5 to to about 30 MeV. The
topical sessions at the workshop focused on five main subjects: nucleon
structure and low-energy quantum chromodynamics (QCD), nuclear structure,
nuclear astrophysics, fundamental symmetries, and applications. Probing
nucleon structure requires medium-energy photons, i.e., $\gamma$-ray beams
with energies from about 60 to 350 MeV, while the four other research topics
would use $\gamma$-ray beams at energies below about 20 MeV. Descriptions of
the research opportunities for all five are in the science sections of this
document. Summaries of the findings of the topical working groups are given
below.

Medium-Energy NGLCGS Facility
Understanding the emergence of hadron structure and the nuclear force in terms
of QCD are key questions at the frontier of nuclear physics. These phenomena
are a consequence of quarks and gluons interacting at confinement-scale
distances, where color forces are strong. The beams at a medium-energy NGLCGS
facility will enable measurements that uniquely probe hadron structure and
hadronic interactions in this non-perturbative regime of QCD. The experimental
program will investigate LEQCD phenomena with unprecedented precision in the
photon energy range from about 60 MeV to the nucleon-to-Delta(1232)
transition. The initial research program at such a facility will two main
components: (1) Compton scattering to investigate nucleon structure and (2)
photopion production to investigate the QCD origins of isospin symmetry
breaking in the strong nuclear force. Both program components will require
high-density polarized targets.

  * •

Low-Energy QCD and Nucleon Structure
The beam capabilities at a medium-energy NGLCGS will enable high-precision
measurements of the scalar and spin nucleon polarizabilities by Compton
scattering from unpolarized and polarized targets. Such measurements, together
with advances in calculations using lattice QCD and QCD-based effective field
theories, will explore the QCD origin of nucleon structure associated withe
the collective response of nucleons to electromagnetic impulses with
unprecedented sensitivity. For example, the experimental programs at the
NGLCGS facility are proposed to improve the statistical accuracy of the
nucleon spin polarizability measurements by more than a factor of 10 from
current values and to impact the dynamical scaler polarizabilities of the
neutron and proton with precision from energies below to above the pion
production threshold.

  * •

Low-Energy QCD: Photopion Production
The high intensity and high energy resolution of beams at a medium-energy
NGLCGS facility will enable determination of electromagnetic s-wave and p-wave
amplitudes to the $\pi^{0}$ production with sufficiently high precision to
observe charge-symmetry violation. Photoproduction of pions at energies near
the reaction threshold provides mechanisms for investigation of the QCD origin
of breaking of isospin symmetry in strong nuclear interactions.

Low-Energy NGLCGS Facility The beams at low-energy NGLCGS facilities will
enable high-accuracy measurements in nuclear structure, nuclear astrophysics,
fundamental symmetries, and $\gamma$-ray beam applications in nuclear and
homeland security. The research opportunities in each area are summarized
below:

  * •

Nuclear Structure:
The beams at an advanced $\gamma$-ray source will enable systematic studies of
weak collective dipole and quadrupole nuclear excitations with unprecedented
precision. Such studies will provide nuclear structure details and information
about the symmetry energy of the nuclear equation of state that are difficult
to obtain by other means. The high $\gamma$-ray beam intensities will enable
mapping of states accessed through M1 transitions from the ground state in
nuclei with a level of detail and breadth that will contribute to modeling of
coherent neutrino-nucleus scattering and to calculating nuclear matrix
elements for neutrinoless double-beta decay. Also, a next-generation
$\gamma$-ray beam facility will enable new exclusive measurements of
photodisintegration of few-nucleon systems with a precision that provides
sensitivity to three-nucleon interactions.

  * •

Nuclear Astrophysics:
New $\gamma$-ray beam capabilities will enable measurements that contribute
broadly to open questions in nuclear astrophysics, questions such as big-bang
nucleosynthesis, helium burning in massive stars, and synthesis of heavy
nuclei. One of the most important reactions in stellar modeling is
12C($\alpha$,$\gamma$)16O. The rate of this reaction relative to the carbon
forming reaction 3$\alpha\rightarrow^{12}$C determines the fate of massive
stars. The measurement of the rate of the 16O($\gamma$,$\alpha$)12C reaction
at energies approaching the temperatures at the core of stars is a grand
challenge in nuclear astrophysics.

  * •

Fundamental Symmetries:
The beams at an advanced low-energy $\gamma$-ray beam facility will enable
measurements of parity violating photodisintegration of few-nucleon systems.
In particular, a measurement of parity violation (PV) in deuteron
photodisintegration near threshold is sensitive to a nucleon-nucleon (NN) PV
amplitude that is not accessible using other systems. Such measurements sample
the short-range part of the NN interaction, providing unique quantities for
comparison with calculations using lattice gauge theory.

  * •

Applications:
The intense mono-energetic $\gamma$-ray beams at low-energy NGLCGS facilities
will enable photonuclear reaction measurements important for technologies and
techniques used in homeland security and nuclear safeguards. Programs to
develop field-deployable system will benefit by having a target areas equipped
for evaluating concepts for $\gamma$-ray beam interrogation of cargo, nuclear
fuel, and special assemblies at these facilities. The new beam capabilities at
advanced $\gamma$-ray sources will also create opportunities for applications
in medicine.

Findings of Advanced Accelerator and Light-Source Technologies Working Group:
Details of the various technology options considered are presented in the
chapter on accelerator concepts for NGLCGS facilities. A summary of the
findings of the working group is below.

It is unlikely that a single $\gamma$-ray beam source can meet the
requirements of both the low-energy (Eγ < 20 MeV) and medium-energy (Eγ > 60
MeV) parts of the field as described in the working group summaries above and
in the science sections of this document. In the options discussed, the
$\gamma$ rays are produced by Compton scattering of electrons from photons in
an optical cavity that is pumped with an external laser. Two options for the
electron beam accelerators for the low-energy $\gamma$-ray source were
considered: a storage ring and an energy-recovery linac with superconducting
RF cavities. For the medium-energy $\gamma$-ray source, a storage ring was the
primary option. There is confidence that a high-quality electron beam with low
emittance and low energy spread can be maintained in modern storage-ring
lattices, thereby enabling production of $\gamma$-ray beams with low energy
spread. The new facility construction cost of the storage-ring option for
either a low-energy or medium-energy next-generation Compton $\gamma$-ray
source will be about $150M. The working group cautions that this estimate is
extremely uncertain; it is intended only to set the scale within about a
factor of two. For the low-energy sources, less expensive options, such as
upgrades to existing facilities, were also discussed.

Recommendations:
The working groups through consensus make the following recommendations. The
order of this listing is not prioritized.

  * •

High intensity $\gamma$-ray beams with circular and linear polarizations will
be produced at next-generation $\gamma$-ray sources by Compton scattering of
photons from relativistic electrons inside a high finesse optical cavity. The
optical cavity will be pumped by a laser system with high precision control of
beam polarization. The electron beam accelerator will use proven technologies,
either a storage ring or an energy-recovery linac. The main technological
challenge is the production of reliable optical cavities with the technical
specifications required for the next-generation $\gamma$-ray sources.

The highest priority R&D work for the next-generation $\gamma$-ray source
should be the development of high finesse optical cavities and the associated
laser and optical systems. It is important for this work to include testing
and optimization of the cavity under $\gamma$-ray production conditions.

  * •

For photopion production experiments, small angle electron scattering (virtual
photon tagging) with intense electron beams was discussed as an alternative to
measurements using tagged bremsstrahlung sources or $\gamma$-ray beams
produced by Compton scattering. This alternative technique would be
implemented with high-current electron beams, possibly in a storage ring, and
with thin targets that allow detection of the low-energy charged particles
produced in the reaction. This method promises to be much more effective than
conventional photon tagging techniques.

R&D should be supported to develop an alternative to Compton scattering for
producing $\gamma$ rays with energies above the pion-production threshold. A
system for virtual photon tagging in small-angle electron scattering is a
promising candidate for such studies.

  * •

Low-energy QCD phenomena will be explored at NGLCGS facilities with
unprecedented precision at energies from below the pion-production threshold
through the Delta (1232) resonance region. The core experimental programs will
involve Compton scattering to study the spin-dependent electromagnetic
response of nucleons and near-threshold photopion production. Both programs
require polarized beams and targets. The measurements enabled by NGLCGS
facilities, together with advances in calculations using lattice QCD and QCD-
based effective field theories, will explore the QCD origin of nucleon
structure and charge-symmetry breaking.

Investments in polarized targets are needed to prepare for experiments at the
NGLCGS facilities.

Investments in nuclear theory are needed to support the planning and analysis
of low-energy QCD experiments at the NGLCGS facilities.

  * •

To ensure full realization of the scientific potential of a NGLCGS it is
crucial that a strong theory effort in this area be maintained as the machine
concept is developed and implemented. This will facilitate planning for
experiments that optimally realize the scientific goals articulated in this
document and continue the strong tradition of synergy between theory and
experiment in low- and medium-energy photonuclear physics. Mechanisms that
will ensure there is a strong international theory community working on this
physics that is fully engaged include workshops with small lead times and
durations of up to a month and partial support for postdoctoral researchers
and/or graduate students working on theory projects related to the NGLCGS. In
addition, computing resources can help address the challenge of solving QCD in
this regime and contribute to the 2015 Long-Range Plan’s recommendation of
“new investments in computational nuclear theory that exploit US leadership in
high-performance computing.

  * •

The 12C($\alpha$,$\gamma$)16O reaction helps regulate the efficiency of helium
burning in massive stars and ultimately determines the mass of the iron core
in the incipient supernova. The uncertainty in the measured cross section for
this reaction substantially limits our understanding of the late stages of the
life of massive stars and the details of the nucleosynthesis under the
explosive conditions of supernovae. The beams at NGLCGS facilities will enable
measurements of the 16O($\gamma$,$\alpha$)12C reaction that determine the
reaction rate of $\alpha$-particle capture on 12C at center-of-mass energies
lower than have been achieved with other techniques. Measurements of angular
distributions require thin targets to allow detection of the $\alpha$
particles along with charged-particle detectors with wide angle coverage.
Options include time projection chambers with optical and charge readout and
silicon strip detectors with thin solid or gas targets.

Investments in active targets, such as time projection chambers, are needed to
carry out the highest impact nuclear astrophysics measurements at NGLCGS
facilities.

##  Chapter 2 Introduction

111References are included at the end of each chapter

Over the last decade, substantial progress has been made in developing
formalisms and computational methods that contribute to theoretically coherent
descriptions of nuclear phenomena with origins in QCD. An ultimate goal would
be to describe nuclear matter, over wide distance and energy scales, with a
QCD Lagrangian. Achieving this aim will likely require the effort of
generations of scientists, as is often the case for grand challenges in
science and technology. Figure 2.1 shows a schematic diagram of a plausible
hierarchy for organizing the theoretical treatments of nuclear systems
spanning a variety of phenomena. The scheme starts at high energies, where the
most fundamental degrees of freedom are quarks and gluons, and progressively
evolves in complexity. This diagram is intended only to represent gross
features that should be included in a coherent picture of strongly interacting
matter. At the top of the diagram, mean-field potentials that describe nuclear
structure properties, the collective motion of nuclei, and nuclear reactions
should be derived from residual strong interactions between nucleons. Ab-
initio calculations of the structure of light nuclei [1, 2, 3, 4] and few-
nucleon reaction dynamics [5] enable refinement of two-nucleon and multi-
nucleon interactions using effective degrees of freedom. Current theoretical
tools for describing the strong nuclear force (two- and three-nucleon
interactions) include semi-empirical potential models (see, e.g., Refs. [6, 7,
8, 9, 10]), effective field theory formulations of two-nucleon (2N) and three-
nucleon (3N) interactions [11, 12, 13, 14], and lattice-QCD (LQCD)
calculations of few-nucleon systems (see, e.g., Refs. [24, 16]). Descriptions
of the collective properties of nucleons in terms of effective field theories
(see, e.g., Ref. [11, 12]) and LQCD (see, e.g., Ref. [24, 18]) are steps
toward bridging gaps between QCD and theoretical treatments of few-nucleon
systems.

Photon beams are a highly-selective probe of the electric-charge and magnetism
distributions of nuclei and nucleons. Measurements of photon scattering and
photon-induced reactions provide information on the collective response of the
internal degrees of freedom of composite nuclear objects, as depicted in Fig.
2.1. For almost a century, bremsstrahlung $\gamma$-ray beams have been the
workhorse for investigating nuclear and nucleon structure and for measuring
photon-induced nuclear reactions. These beams have the advantage of allowing
measurements to be performed over a broad energy range in a single experiment,
thereby providing information about the energy dependence of phenomena.
However, the continuous energy nature of bremsstrahlung $\gamma$-ray beams has
the disadvantage of limiting measurement sensitivity due to backgrounds
created by the photons with energies outside the region of interest. A
complementary tool is monoenergetic $\gamma$-ray beams, such as those created
by laser Compton photon sources. The narrow energy bandwidth of laser Compton
$\gamma$-ray beams enhances the signal-to-background ratio in comparison to
what is achievable using bremsstrahlung beams. In addition, the high beam
polarization available for both linear and circular polarization in the
$\gamma$-ray beams produced by laser Compton photon sources enables
unambiguous determination of the spin and parity of excited nuclear states and
facilitates studies of the spin structure of nucleons. Over the last several
decades, laser Compton $\gamma$-ray beam facilities have provided intense
polarized and nearly mono-energetic $\gamma$-ray beams for research programs
in basic and applied nuclear physics. The facilities include GRAAL at the
European Synchrotron Radiation Facility in Grenoble, LEGS at Brookhaven
National Laboratory, LEPS at the SPring-8 facility, NewSUBARU at the
University of Hyogo, and HI$\gamma$S at the Triangle Universities Nuclear
Laboratory. New laser Compton $\gamma$-ray beam facilities under construction
include ELI-NP in Romania and the $\gamma$-ray beam line at the Shanghai
Synchrotron Radiation Facility (SSRF) in China. The current generation of
laser Compton $\gamma$-ray beam sources are based mostly on either a single-
pass light pulse from an external laser scattered off a single-pass electron
beam bunch or a single-pass laser pulse scattered off electron beam bunches
circulating in a storage ring. The technique of intracavity Compton scattering
of electrons circulating in a storage ring that is employed at HI$\gamma$S is
indicative of the technologies to be used in next-generation laser Compton
$\gamma$-ray sources. Combining the high beam current of storage rings with
the high photon density inside optical cavities can potentially produce
monoenergetic $\gamma$-ray beams with intensities more than three orders of
magnitude higher than HI$\gamma$S, which is currently the most intense laser
Compton source in the world.

Figure 2.1: Schematic diagram for a coherent theoretical treatment of nuclear
systems starting from high energies (at the bottom), where perturbative QCD
can be applied, and ending with low-energy nuclear phenomena, where mean-field
potential models are most efficient. This figure is adapted from ref. [19].

This whitepaper describes research opportunities that would be opened up by
the $\gamma$-ray beam capabilities at next-generation laser Compton
$\gamma$-ray beam facilities. Its content is based mainly on discussions and
findings of The International Workshop on the Next Generation Laser Compton
Gamma-Ray Beam Facility that was held November 17–19, 2016 in Bethesda, MD.
The workshop brought together both researchers from the international low-
energy and medium-energy nuclear physics communities, along with accelerator
physicists and experts in optics. The accelerator and optics experts anchored
this exercise into the constraints of beam performance parameters achievable
with technologies that can be implemented in the coming decade. For both
practical reasons and scientific considerations, the upper energy reach of the
$\gamma$-ray source was limited to 500 MeV. Within these boundaries, the broad
areas of nuclear physics considered correspond to the upper three panels of
Fig. 2.1. It is probably fair to state that most participants began the
workshop with the belief that one laser Compton source would be capable of
serving both low-energy experiments, with $\gamma$-ray beam energies (Eγ)
below about 20 MeV, and medium-energy experiments, with E${}_{\gamma}>60$ MeV.
The questions addressed in low-energy and medium-energy $\gamma$-ray
experiments have considerable intellectual overlap and the intense,
monoenergertic $\gamma$-ray beams of a NGLCGS would facilitate progress in
both regimes. However, the $\gamma$-ray source considerations mean that
research at low and medium energies likely must be carried out at two
different facilities. For this reason, the discussion of the research
opportunities created by laser Compton $\gamma$-ray sources is organized by
the two energy ranges of the facilities presented at the workshop, Eγ $\leq$
20 MeV and 60 MeV $\leq$ Eγ $\leq$ 350 MeV. The areas explored in each energy
region are illustrated in Fig. 2.2. While low-energy QCD is mentioned
explicitly only in the upper half of the circle it is also strongly connected
to the proposed research on parity violation in hadronic systems and the
studies of few-nucleon systems and light nuclei that compare ab-initio
calculations to data. Indeed, since all nuclear dynamics is underpinned by QCD
low-energy QCD in fact pervades all the opportunities that would be created by
an NGLCGS. These include parity violation in hadronic systems and studies of
few-nucleon systems and light nuclei using ab-initio calculations. Details of
the opportunities are presented in the topical chapters that follow. A
synopsis of each topical discussion is given below.

Figure 2.2: Illustration of the research areas impacted by advanced
$\gamma$-ray source technologies. The upper half of the graphics represent the
main research at $\gamma$-ray beam energies from about 60 MeV to 350 MeV. The
lower half represents the research areas covered at energies below 20 MeV.

The main opportunities at energies below 20 MeV are presented in chapter 3 and
are in the areas of nuclear structure (including photon-induced fission),
nuclear astrophysics, and parity violation in few-nucleon systems and light
nuclei. Studies of collective modes of excitation can provide information
about short-range correlations between nucleons in nuclei and can reveal
features of the nuclear equation of state. The giant dipole resonance (GDR)
dominates the nuclear collective response and is well understood. The
rotational (rigid rotor) and vibrational (scissor) modes at energies below
about 4 MeV have been extensively studied and are described by well-
established models. The nuclear excitations on the low-energy tail of the GDR,
above about 5 MeV, and below the particle separation energy are not well
characterized. Much of the difficulty in understanding the nature of the
excitations in this energy region is associated with distinguishing between
effects due to the GDR and those due to other mechanisms. The dipole
excitation strength in excess of the GDR tail is referred to as the pygmy
dipole resonance (PDR). The generally accepted mechanism for the PDR is the
vibration of a neutron skin off an isoscalar core. If this picture is correct,
a study of its strength as a function of isospin and $A$ should provide
information about the density dependence of the symmetry energy term in the
nuclear equation of state. The beam intensities available at the next-
generation laser Compton $\gamma$-ray sources will enable nuclear resonance
florescence (NRF) measurements on nuclei at extremes of isospin where the
natural abundance is low and consequently target material is sparse. In
addition, the beam at next-generation laser Compton $\gamma$-ray sources will
enable thorough measurements of the M1 $\gamma$-ray strength in nuclei.
Special attention will be given to nuclei in the mass vicinity of isotopes
used in double-beta decay experiments. The beams at the next-generation laser
Compton $\gamma$-ray source will also enable photon-induced fission
measurements at energies near threshold, where the cross section is low, and
the study of low-yield fragments. Such experiments will provide new
information about the potential energy surface that governs the evolution of
the fission process. Examples of the initial nuclear structure research made
possible by the NGLCGS facilities are described in section 3.1.

The beams at the next-generation laser Compton $\gamma$-ray sources will
enable strategic measurements of a variety of NRF and photon-induced nuclear
reactions that contribute to modeling $p$-process, $s$-process and $r$-process
nucleosynthesis. However, the highest impact contribution to nuclear
astrophysics will be in determining the reaction rate for alpha-particle
capture by carbon via the 12C($\alpha$,$\gamma$)16O reaction in massive stars
during the alpha-burning stage. The goal for all measurements of this reaction
is to determine the cross section at a center-of-mass energy of 300 keV. In
$\gamma$-ray beam facilities, the time reversed reaction
16O($\gamma$,$\alpha$)12C will be measured, and detailed balance will be
applied. This measurement will be the flagship nuclear astrophysics experiment
at advanced $\gamma$-ray sources, and, as such, should be measured using a
variety of techniques. Nuclear reaction rate measurements enabled by the
NGLCGS facilities that are at key points in stellar nucleosynthesis networks
are discussed in section 3.2.

In the area of fundamental symmetries, the primary focus will be on studying
parity violation in hadronic systems. Such parity violation is a measure of
the weak interaction inside systems of strongly interacting particles. The
ultimate goal is to measure parity violation in photodisintegration reactions
of few-nucleon systems, such as 2H and 3He. Because these measurements must be
performed near the reaction threshold energy, and because the parity violating
asymmetry is extremely small ($\sim$ 10-7), these experiments make the most
stringent demands on the source-performance parameters. That is, a
$\gamma$-ray source that delivers beam intensities an order of magnitude below
what is required for these experiments will enable all the nuclear structure
and nuclear astrophysics discussed in this whitepaper. Measurements of parity
violating photoabsorption asymmetries on parity doublets in light nuclei will
be used to assess beam and instrumentation asymmetries in the early stage of
developing the capabilities for performing 10-7 asymmetry measurements. For
this purpose, nuclei with photoabsorption asymmetries of 10-4 to 10-3 will be
selected. Hadronic Parity Violatoin studies using photon-induced reactions are
described in section 3.3.

In addition to the basic science research enabled by the NGLCGS facilities,
these $\gamma$-ray sources will provide capabilities that support technology
and techniques development work for nuclear security. How such facilities
might be used in efforts to advance $\gamma$-ray beam interrogation systems is
outlined in Section 3.4.

Quantum chromodynamics is the fundamental theory of the strong nuclear force.
When written in terms of quark and gluon degrees of freedom, it is deceptively
simple. Indeed, QCD’s asymptotic freedom guarantees that this form of the
theory describes strong interactions at sufficiently high energy. However, in
that regime, QCD does not bind quarks and gluons into neutrons and protons.
The task of fully and rigorously deriving the presence and properties of
neutrons, protons, and nuclei from the Standard Model remains a grand
challenge for physics, requiring us to understand the emergence of new degrees
of freedom as the theory becomes strongly coupled. Without such an
understanding, treatments of the nuclear force based on the fundamental theory
will remain elusive, and so too will the ability to model nuclei in a reliable
manner that is grounded in QCD.

High-intensity beams of polarized $\gamma$ rays in the energy range of 60–300
MeV provide a unique opportunity to test our understanding of the emergence of
neutron and proton structure from the standard model. In this regime, a
description of experimentally observed phenomena in terms of neutrons and
protons and their low-lying excitations is efficient, thanks to the chiral
symmetry of QCD.

The initial research opportunities in nucleon structure and low-energy QCD at
NGLCGS facilities are described in Chapter 4. One of the main goals at these
energies will be to map out the spin-independent and spin-dependent
polarizabilities starting at energies below the pion production threshold,
extending over the pion production threshold energy, and continuing through
the Delta(1232) region. These measurements will be carried out using Compton
scattering from unpolarized and polarized targets. The other major aim in the
medium-energy domain is precision near-threshold pion photoproduction data.
Such data would provide the opportunity to extract the s-wave $\pi^{0}$p
scattering length and so gain a new window on the QCD origin of charge-
symmetry breaking..

In the limit of vanishing up-, down-, and strange-quark masses, the QCD
Lagrangian admits a global chiral symmetry: SU(3)${}_{L}\times$ SU(3)R. This
symmetry is broken spontaneously, which implies the existence of eight
pseudoscalar massless Goldstone bosons. Furthermore, since the quark masses
are finite (but small), these Goldstone bosons acquire a small mass and are
identified with the pions, kaons, and etas. The interaction of these Goldstone
bosons with themselves or with matter fields such as nucleons is weak. This
allows for a systematic low-energy expansion in terms of small momenta and
quark masses: Chiral Perturbation Theory. Chiral perturbation theory
($\chi$PT) describes meson-meson interactions in this energy domain with high
precision. It also describes the interactions of mesons with nucleons at very
low energies. Incorporating the $\Delta(1232)$ as an explicit degree of
freedom in the theory yields a chiral effective field theory ($\chi$EFT) that
describes processes at energies below the chiral symmetry breaking scale
($\Lambda\approx 1$ GeV) via an expansion in a small, dimensionless parameter.
This allows reliable quantification of residual theory uncertainties. Thus
$\chi$EFT exploits the chiral symmetry of QCD in order to rigorously connect
QCD with the phenomenology of nuclear and particle physics. Because $\chi$EFT
encodes the consequences of the standard model for low-energy processes
involving photons, pions, and nucleons, it can be used to test whether
observed strong-interaction phenomena are consistent with the standard model.
In particular, $\chi$EFT allows us to elucidate the experimental consequences
of the pattern of chiral-symmetry breaking in QCD.

Accelerator concepts for the NGLCGS are described in Chapter 5. Two options
for the accelerator configuration were discussed for energies below 20 MeV: an
energy-recovery linac with superconducting RF cavities or a storage ring. Only
the storage-ring option was presented for energies above 60 MeV. In all source
configurations, the Compton scattering occurs inside an optical cavity that is
pumped by an external laser. The electron source, accelerator structures, and
storage-ring lattices that meet the required technical specifications for
source performance are robust and can be implemented to order. The situation
is not as well established for the optical cavity. R&D is needed to develop an
optical cavity design that can meet the power, stability, and reliability
requirements of the next-generation laser Compton $\gamma$-ray source.

## References

  * [1] S. C. Pieper and V. R. Pandharipande, Ann. Rev. Nucl. Part. Sci. 51, 53 (2001).
  * [2] P. Navratil _et al._ , Phys. Rev. Lett. 99, 042501 (2007).
  * [3] P. Navratil _et al._ , Few-Body Syst. 43, 129 (2008).
  * [4] A. Nogga, P. Navratil, B.R. Barrett, and J.P. Vary, Phys. Rev. C 73, 064002 (2006).
  * [5] W. Glöckle _et al._ , Phys. Rep. 274, 107 (1996).
  * [6] R. Machleidt, Phys. Rev. C 63, 024001 (2001).
  * [7] R. B. Wiringa, V. G. J. Stoks, and R. Schiavilla, Phys. Rev. C 51, 38 (1995).
  * [8] V. G. J. Stoks, R. A. M. Klomp, C. P. F. Terheggen, and J. J. de Swart, Phys. Rev. C 49, 2950 (1994).
  * [9] S. A. Coon, M. D. Scadron, and B. R. Barrett, Nucl. Phys. A 242, 467 (1975).
  * [10] S. C. Pieper, V. R. Pandharipande, R. B. Wiringa, and J. Carlson, Phys. Rev. C 64, 014001 (2001).
  * [11] H.-W. Hammer, S. König and U. van Kolck, arXiv:1906.12122 [nucl-th].
  * [12] E. Epelbaum, H. Krebs and P. Reinert, arXiv:1911.11875 [nucl-th].
  * [13] R. Machleidt and D.R. Entem, Phys. Rep. 503, 1 (2011).
  * [14] E. Epelbaum, H.-W. Hammer, and Ulf-G. Meissner, Rev. Mod. Phys. 81, 1773 (2009).
  * [15] W. Detmold et al. [USQCD Collaboration], Eur. Phys. J. A 55, no. 11, 193 (2019), [arXiv:1904.09512 [hep-lat]].
  * [16] S. R. Beane et al., Phys. Rev. Lett. 115, 132001 (2015).
  * [17] H. W. Griesshammer, J. A. McGovern, D. R. Phillips, and G. Feldman, Prog. Part. Nucl. Phys. 67, 841 (2012).
  * [18] S. Beane _et al._ , Phys. Rev. Lett. 113, 252001 (2014).
  * [19] C. R. Howell, W. Tornow, and H. Witała, EPJ Web of Conferences 113, 04008 (2016).

##  Chapter 3 Nuclear Physics Research and Applications with Gamma-ray Beams
Below 20 MeV

###  3.1 Nuclear Structure

The landscape for the response of nuclei to electromagnetic radiation as a
function of excitation energy is shown schematically in Fig. 3.1. The electric
giant dipole resonance (GDR) is the dominant collective response of nuclei.
This mode can be modeled as the oscillation of the proton matter distribution
against the neutron distribution. In the classical interpretation, the width
of the cross-section enhancement associated with the resonance provides
information about the damping forces, i.e., the viscosity of the nuclear
matter. The nature of the nuclear response at excitation energies below the
GDR around the particle in thresholds is still rather unclear and is the topic
of considerable theoretical and experimental work [1]. The excitation in this
region has been observed to be largely electric dipole (E1). This additional
structure below the GDR has been shown to be a common feature in the E1
response of atomic nuclei, and is generally referred to as the Pygmy Dipole
Resonance (PDR). The E1 response offers the possibility of studying the
equation of state of neutron matter [2, 3, 4, 5, 6, 7]. Also the $\gamma$-ray
absorption cross section in the PDR energy region influences nuclear reaction
rates in stars [8, 9, 10, 11]. In deformed nuclei, the collective nuclear
dipole response at excitation energies below the PDR region is mainly due to
magnetic dipole (M1) transitions, which can be described as a scissor motion.
The ($\gamma$,$\gamma$’) reaction, which is referred to as nuclear resonance
fluorescence (NRF), is the most effective approach for probing these features
of nuclear structure.

Nuclear fission is a complex process in which the collective motion of
nucleons results in a strong change in the shape of the nucleus, ultimately
leading to a breakup into fragments. Photon-induced fission can provide unique
insight into the evolution of the potential energy surface during the fission
process as it progresses from photon absorption through to the scission point.
Measurements of fission product yields of isotopes with different half lives
provide information on the evolution of the nuclear deformation and breakup of
the nucleus during the fission process.

The high intensity, nearly monoenergetic, and linearly polarized photon beams
at a next-generation laser Compton $\gamma$-ray beam facility will enable
high-precision photonuclear reaction measurements. Research opportunities for
NRF and photofission measurements are described in this section.

Figure 3.1: Diagram of the landscape of the response of nuclei to photon
absorption.

####  3.1.1 Nuclear Resonance Fluorescence

The nuclear resonance fluorescence (NRF) reaction is one of the work horses in
the investigation of the dipole response of atomic nuclei. The method enables
the extraction of intrinsic properties of excited states such as spin, parity
and transition widths in a model independent way. Furthermore, due to the low
momentum transfer of photons, primary dipole transitions are induced, i.e., in
even-even nuclei states with $J=1^{\pm}$ are populated. This makes NRF an
excellent reaction to systematically study the phenomena in the dipole
response of nuclei, such as the PDR and the scissors mode over a large range
of nuclei.

Thus far, mostly the decay branch back to the ground state as been
investigated with the NRF reaction. However, the detailed decay pattern of
excited states is often connected to important details in its nuclear
structure. The decay widths to the individual lower-lying excited states or
the ground state are directly linked to the corresponding transition matrix
elements. Thus, the individual decay channels are sensitive to different
components of the nuclear-state wave function. For example, in the case of
transitions to lower-lying excited states, the de-excitation takes place via a
different component in the wave function than the excitation from the ground
state. Therefore, the observation of these transitions and the determination
of the branching ratios reveal important experimental information that tests
modern nuclear-structure models.

The method of $\gamma\gamma$ coincidences in the spectroscopy of the decay of
excited states has been proven to be a powerful tool for determining even
small branching ratios to excited low-lying states [12, 13]. The principle is
illustrated in the left part of Fig. 3.2. After excitation by photoabsorption,
the high-lying state at excitation energy $E_{x}$ may de-excite either
directly to the ground state with width $\Gamma_{0}$ or via an intermediate
state with width $\Gamma_{i}$. By detecting the two emitted $\gamma$ rays in
coincidence, even small branching ratios $\Gamma_{i}$/$\Gamma_{0}$ can be
determined with good precision, since the background is strongly suppressed.
This technique allows the branching of dipole-excited states to individual
lower-lying levels to be determined, thus greatly aiding in building up
complex level and decay schemes. An example is shown in Fig. 3.2. The right-
hand side of the figure provide, the level structure for a measurement on
76Ge, where the decay of the 1+ scissors mode has been investigated [14]. The
1+ state at 3.763 MeV is populated by photo-excitation. Gating on the decays
of the 2${}^{+}_{1}$ and 2${}^{+}_{2}$ states, as shown on the left side of
the figure, allows the order of the $\gamma$-ray transitions in the decay to
be determined.

Figure 3.2: Example of $\gamma\gamma$ coincidence spectroscopy. The spectra
show projections of LaBr3 $\times$ LaBr3 matrices after gating on transitions
from the first and second 2+ states in 76Ge. The corresponding level scheme is
given on the right-hand side. The scheme is an example for any dipole excited
state with arbitrary intermediate levels between the $J=1$ excited state and
the ground state.

The combination of large-volume LaBr3 and high-resolution HPGe detectors opens
the way for $\gamma\gamma$-coincidence measurements with sufficient
statistical accuracy to track $\gamma$-ray decay cascades. High-resolution
HPGe detectors allow for a separation of individual $\gamma$-ray transitions
from dipole excited states in regions where the level density is not yet too
high. Even in regions with a high level density, individual states can
sometimes have substantial excitation strength and are, thus, separable from
the otherwise rather continuous spectrum. This provides a way to normalize
spectra to complementary data sets obtained using bremsstrahlung beams.
Nevertheless, most dipole-excitation strength in regions of high level density
is so strongly fragmented that individual states cannot be resolved. In this
case, the large-volume LaBr3 detectors have a significant advantage over HPGe
detectors because of their higher detection efficiency. This high efficiency
allows the fragmented strength in the beam region to be separated from the
background and enables integrated branching strengths to be determined. In
deformed nuclei, where the first-excited-state energies are very low, decays
to the ground state and the first excited state are close in energy, posing a
challenge to the accelerator team to deliver beams with high intensity and low
energy spread to the target. Deconvolution methods have recently been
developed and applied to aid in extracting decays out of the detector response
(Fig. 3.3), see e.g. [15, 16].

Figure 3.3: HPGe (top) and LaBr3 (bottom) spectra from singles $\gamma$-ray
spectroscopy of 128Te at a beam energy of 6.4 MeV. The black histograms show
the actual spectra, and the red ones are the result of the deconvolution,
which includes correcting for the detector response. The beam profile is
indicated by a dashed curve in the upper panel. This figure is taken from
[15].

With the combination of HPGe and LaBr3 detectors, the high efficiency of the
latter provides sufficient statistics to observe branching decays from higher-
lying states, either in low-resolution LaBr3 or even in high-resolution HPGe
spectroscopy, as demonstrated in [12, 13]. Especially at low energies,
electric quadrupole excitation probabilities can be sufficiently large for
observation, and can be separated from the dipole strength making use of
angular distribution and correlation measurements using arrays of HPGe and
LaBr3 detectors. In certain cases even the mixing ratio of M1/E2 transitions
can be measured. This recently facilitated a first measurement of the
isovector quadrupole strengths in 156Gd [17], for example.

The capabilities of the next-generation laser Compton $\gamma$-ray source
combined with modern $\gamma$-ray detectors will enable measurements that
address a broad range of physics through photo-excitation studies. The topics
include multi-phonon structures such as quadrupole-octupole coupled states and
the search for rotational isovector scissors excitations; the investigation of
the pygmy dipole resonance (PDR) along with its structural evolution and
fingerprints in going from spherical to deformed regions; and detailed
spectroscopic studies of the M1 spin-flip resonance. Therefore, the physics
topics to be addressed embrace shape coexistence, octupole correlations,
isovector excitations, neutron skins, and photon strength functions, often
with strong relevance for the fields of nuclear astrophysics and weak-
interaction physics such as neutrinoless double-beta decay and neutrino
scattering.

#####  3.1.1.1 Shape Coexistence and Link with Physics with Radioactive beams

In the context of nuclear structure studies, a direct link between research
carried out with $\gamma$-ray beams and with radioactive beams should be
briefly discussed. Shell structure is a cornerstone in the description of
atomic nuclei as many-body quantum systems. The first data on neutron-rich
nuclei far from stability have provided evidence that shell structure evolves
with neutron excess. For example, some well-known magic numbers disappear
while others appear. Progress in the description of these exotic nuclei is due
in no small part to the development of new experimental techniques that have
gone hand in hand with the introduction of new theoretical concepts. It has
been shown that effective single-particle energies are significantly modified
in neutron-rich systems through the action of the monopole component of the
proton-neutron tensor force [18]. Furthermore, multi-particle multi-hole
excitations trigger shell evolution as a function of spin and excitation
energy [19]. In this picture, the occupation of specific deformation-driving
orbitals leads to changes in nuclear shapes and to the possibility of shape
coexistence. The region between Z=20, N=20 40Ca and Z=28, N=50 78Ni provides a
good illustration of the synergy between experiments at the national user
facilities and measurements with $\gamma$-ray beams. For example, triple shape
coexistence has recently been discovered in neutron-rich 66,68,70Ni [20].
These 0+ states are the result of multi-particle multi-hole excitations. Some
of these levels, in particular those associated with prolate deformation, are
understood as proton excitations that are predicted to be present in the
stable 60,62,64Ni as well [20], and the evolution of these states’ excitation
energy with N is a matter of much theoretical debate. The issue can be
addressed with intense $\gamma$-ray beams: in a first phase, all the low-spin
excitations can be mapped out. Following this discovery phase, information on
the wave function of the states will be obtained from state lifetime
measurements, and from the determination of decay branching ratios.
Furthermore, excitations built on the excited 0+ levels will be delineated and
their properties characterized. Other regions of the nuclear chart lend
themselves to similar studies. Specifically, shape changes and shape
coexistence phenomena, driven by multi-particle multi-hole excitations are
predicted to occur in the Sr-Mo-Zr region as well as in the vicinity of
doubly-magic 208Pb.

#####  3.1.1.2 Multi-Phonon and Rotational Low-Lying States

At low energies, in spherical or near-spherical nuclei, excited states are
often formed by phonon excitations. For example, the first 2+ state is
identified with a quadrupole phonon, and the first 3- state with an octupole
phonon. Such phonons can be coupled, leading to multiplets of multi-phonon
states built from like phonons, or from mixtures of, e.g., quadrupole and
octupole phonons. As such, the quadrupole-octupole coupled quintuplet of
states, 1- $\cdot\cdot\cdot$ 5- contains an E1-excited 1- state which
typically is the first excited 1- state, located at roughly the sum energy of
its constituents and carrying most of the E1 strength at low energies. (See
the schematic diagram of $\gamma$-ray transitions associated with multiple-
phonon de-excitation is given in Fig. 3.4.) In addition to excitations where
protons and neutrons move in phase, there are so-called mixed-symmetry states
where at least one pair of protons/neutrons has a different phase [21]. This
leads to additional states, such as a quadrupole-excited one-phonon mixed-
symmetric 2${}^{+}_{ms}$ state. Again, the phonon coupling between the latter
and the symmetric 2${}^{+}_{1}$ phonon state leads to a multiplet of mixed-
symmetry states, one of which is an M1-excited 1+ state, which is typically
the strongest M1 excitation at low energies of roughly 3 to 4 MeV. In
rotational nuclei, this state evolves into the well-known scissors mode, where
valence protons and neutrons are counter-oscillating in a scissors-like
fashion.

Figure 3.4: Schematic diagram of nuclear excitations involving multiple
phonons and illustrating mixed symmetry and octupole excited states.

Different mechanisms forming E1 and M1 excited states at such low energies
can, however, compete with one another. For example, spin-flip excitations,
where a nucleon is moved from an $\ell\pm 1/2$ orbital to its $\ell\mp 1/2$
partner, can generate M1 excitation strengths similar to those of the scissors
mode. In the E1 sector, the generation of E1 strength in valence spaces
containing either no or a very limited number of opposite-parity orbitals is
under much discussion, and there are other possibilities, such as the
formation of alpha clusters (see below). Therefore, it is not only important
to locate those dipole excited states and to determine their parity and decay
strengths, it is also crucial to measure their decay pattern, which contains
important information on their underlying structure. These tasks are uniquely
facilitated by the capabilities at a next-generation laser Compton
$\gamma$-ray source for measuring decay branches, parities, and relative
excitation strengths. Furthermore, in rotational nuclei, band structures are
expected on $J=1$ band-heads such as the scissors-mode state. Indeed, in
recent work [17], a first candidate for the long-sought rotational
2${}^{+}_{sc}$ member of the scissors-mode band has been discovered. In
addition, measurements of angular correlations made possible by arrays of
$\gamma$-ray detectors with large angle coverage, high detection efficiency,
and high energy resolution used in combination with linearly polarized
monoenergetic $\gamma$-ray beams will enable measurements of multipole-mixing
ratios. These, in turn, can reveal absolute isovector E2 strengths. High
quality data of this type are crucial in constraining effective charges in
nuclear dynamics calculations.

Another aspect of dipole excited states has recently been found and is
connected to weak-interaction physics. While the strong interaction dominates
in nuclei, nuclei are also laboratories for studying the weak interaction
through the possibility of $\beta$ or $\beta\beta$ decay. Of special interest
for many experiments, and for the characterization of neutrinos in general, is
the search for neutrinoless double-beta (0$\nu\beta\beta$) decay. The
existence of this decay mode would identify the neutrino as a Majorana
particle, one which is its own anti-particle. It would provide information
about the absolute mass of the neutrino involved in the decay through the
ratio of the measured decay rate and the calculated nuclear matrix element for
the decay. Since many candidate isotopes (mother and/or daughter) for
0$\nu\beta\beta$ decay occur in mass regions near transitions from spherical
to deformed shapes, their model description is challenging. Furthermore,
isovector parameters are seldom known. NRF measurements can be used to extract
information on phenomena such as shape coexistence and mixed-symmetry states
from the observation of the scissors mode and its decays, especially its
decays to the potentially mixed-configuration 0${}^{+}_{1,2}$ states (see Ref.
[22]).

#####  3.1.1.3 The E1 Pygmy Dipole Resonance

The strongest and best-known E1 mode in atomic nuclei is the giant dipole
resonance (GDR). It is a broad resonance structure peaking well above the
particle-separation threshold energy. Therefore, in NRF experiments, only the
low-energy tail of the GDR below the particle threshold energy is directly
observable. Often, this tail is parameterized by a Lorentzian, eventually with
modifications such as temperature dependence, e.g., parameterizations that
directly use the so-called photon strength functions (PSFs). PSFs have a
direct relation to nuclear astrophysics, since they give a handle on the
balance between particle-capture and photodisintegration processes in stellar
nuclear synthesis. Over the last decade, observations have given rise to
additional strength, on top of the low-energy tail of the GDR. This added
strength is the so-called pygmy dipole resonance (PDR) [1]. This strength,
typically a few percent of the GDR strength, appears around the neutron-
separation threshold. It would directly influence PSFs and would, therefore,
impact nucleosynthesis, which ultimately shapes the abundance patterns of
elements [23].

Various studies of the PDR have involved magic or near-magic spherical nuclei.
Usually, the origin of the PDR is thought to be an oscillation of a neutron
skin against the proton-neutron saturated core. This picture is supported by
several microscopic calculations, showing an excess of neutron transition
densities near the nuclear surface. However, other possibilities exist. For
example, the occurrence of alpha clusters on the surface, which would yield
enhanced E1 strength, has been suggested [24]. Additional problems lie in the
differentiation of PDR strength from the low-energy tail of the GDR. Alpha
scattering seems to be a possible method for accessing this problem [25, 26,
27], because, compared to NRF data, low-energy E1 strength appears to split
into isoscalar and isovector parts.

Other challenges lie in the determination of the total strength. For example,
proton-scattering data partly point to significantly more observed E1 strength
than photon-scattering data [28]. This may, at least in part, be due to
branching transitions from dipole-excited states and many weak unresolved
transitions, which would have been missed in earlier NRF experiments using
bremsstrahlung beams. At the next-generation $\gamma$-ray source, the focus
will be on identifying the strength distribution (state-by-state or average),
measuring the parities as well as branching behavior of the excited states.
From these data, more stringent conclusions can be drawn on the nature of the
PDR, its overall strength, and therefore also on the determination of PSFs. In
addition, the Brink hypothesis (the excitations that are built on the ground
state should also be built on any excited state), which is part of the basis
of the statistical model, can be tested in a model independent way using
$\gamma$-$\gamma$ decay spectroscopy, as has been recently demonstrated [29].

A further challenge lies in the deformation degree of freedom. In the end,
most nuclei on the chart of the nuclides are deformed, but our information on
the evolution of the PDR toward deformed nuclei is sparse at best. Therefore,
the experimental program should extend to address this problem. Indications of
a potential splitting of the PDR into two parts with different $K$ quantum
number have recently been observed in proton-scattering on 154Sm [30]. This
would be similar to the splitting of the GDR with respect to the two (or even
three) axes of the nuclear body. However, the high degree of fragmentation in
deformed nuclei renders a state-by-state analysis meaningless. Therefore,
measurements that track the cascade of $\gamma$-ray decays are most useful. In
addition, the very low energy of the first excited state in deformed nuclei
requires a reduced energy spread of the photon beam compared to existing
facilities in order to enable a clean separation of the decay intensity to the
first excited state and the ground state. Such measurements over sizable
isotope chains will be made possible by the beam capabilities at the next-
generation laser Compton $\gamma$-ray beam facilities.

#####  3.1.1.4 Alpha-Cluster Excitations

Below the particle-separation threshold, two underlying structures of E1
excitations have been intensively studied in the last decade, the octupole
modes [31, 32] and the PDR [1]. For both modes, a non-uniform distribution of
protons and neutron generates E1 transitions at lower energies. As pointed out
in earlier sections, these two modes could have substantial impact on nuclear
structure models and nuclear synthesis calculations. Another mode giving rise
to enhanced E1 strength at lower energies is the oscillation of an alpha
cluster relative to the remaining bulk nuclear matter [24]. For light nuclei,
alpha clustering is well-established [33], and its implications for the E1
strength have been discussed. Also, in heavy isotopes such as 212Po, strong
indications of a 208Pb + $\alpha$ system have been observed [34]. Recently,
the existence of alpha clusters in heavy nuclei was supported by adding four-
particle correlations to shell-model calculations [35]. Because the formation
of alpha clusters provides interesting insights into the formation of bosonic
clusters in strongly coupled fermionic systems, identifying new signatures of
alpha clustering in heavier nuclei is of general scientific interest.

Several enhanced E1 transitions were observed in the neodymium chain by
($\gamma$,$\gamma$’) experiments up to 4 MeV [36, 37, 38]. To shed light on
the origin of the low-lying $J=1^{-}$ states, spdf interacting-boson-model
(IBM) calculations have been performed [39]. It has been proposed that the
$p$-boson is related to $\alpha$-cluster configurations. By identifying the
basis states by means of their boson contents $\mid$[ns], [np], [nd],
[nf]$\rangle$, the ratio np/nf permits an assignment of quadrupole-octupole or
$p$-boson character to be associated with the excited states. The results of
these calculations suggest the presence of alpha-cluster modes occurring at
the surface of nuclei with atomic masses just above magic numbers. For
example, in IBM calculations for 144Nd, a remarkable increase of $p$-boson
contributions has been observed for energies up to the neutron-separation
threshold.

#####  3.1.1.5 The M1 Spin-Flip Resonance

Although, overall, much less M1 strength is expected in the region where the
PDR occurs, there should be significant strength due to the spin-flip
resonance. The exact energy of this resonance depends on the underlying shell
structure, but typically it coincides with the onset of the major E1 strength
distribution. Knowledge about this M1 resonance is sparse, and present
parameterizations are based on little available data. The challenges at hand
are the identification of M1 strength within the sea of E1 excited states. In
cases of low fragmentation, this can be done with a state-by-state analysis of
excited-state parities, but at high fragmentation, one has to choose an
average approach. The best way to filter out M1 strength in those energy
regions is the use of photon-scattering with a linearly polarized beam, such
as those available at laser Compton $\gamma$-ray sources.

The impact of such higher-lying M1-excited states has different facets. On the
one hand, they will provide constraints to microscopic calculations, since the
occurrence of spin-flip transitions involving particles moving between
$\ell\pm 1/2$ partner orbitals, usually across a major nuclear shell. In
addition, states excited by a spin-flip transition are those which are
populated through Gamov-Teller transitions in $\beta$ decay. On the other
hand, such 1+ states will directly influence PSFs in even-even nuclei, since
decays between 1- (GDR/PDR) states and the spin-flip states are allowed,
changing the overall shape of the PSFs. More reliable data on the M1 response
can, therefore, serve as a test of parity asymmetry. (Usually the same number
of positive and negative parity states is assumed.)

Again, M1 strength, this time at energies above the scissors mode, can prove
important for neutrino physics. Specifically, there are detectors, such as the
molybdenum-based MOON detector [40], which are in the planning and
construction phases and are expected to serve, not only for the detection of
potential 0$\nu\beta\beta$ decay events, but also for the direct detection of
neutrinos through inverse “$\beta$ decay” by the capture of neutrinos. Another
facet of neutrino detection lies in the excitation of 1+ states by neutrinos
[41]. Hence, the M1 response of relevant nuclei plays an important role in the
characterization of such detectors and also in rate estimates using nuclear
models such as the quasiparticle random-phase approximation.

The beam requirements for carrying out the nuclear structure research using
NRF described in this section are given in Table 3.1.

Table 3.1: Suggested beam parameters for NRF experiments. Parameter | Value
---|---
Energy | 2-20 MeV
Flux ($\gamma$/s) |  109 at 1% FWHM
Polarization | Linear
Diameter | 10 mm on target
Beam Repetition Rate | few MHz
Beam Pulse Width |  $<$ 1.0 ns

####  3.1.2 Photon-Induced Nuclear Fission

Nuclear fission is a highly exothermic and strongly collective nuclear process
in which most of the energy is released through the kinetic energy of the
ejected fission fragments. The evolution of a fissioning system proceeds from
the initial impact of the incident particle through the intermediate saddle
point(s), then through scission, and finally to the configuration of separated
fission fragments. This evolution is governed by a multi-dimensional
potential-energy surface (PES) and by the shell structure of the fragments.
Development of reliable theoretical models of nuclear fission is important for
basic research and for the development of nuclear energy and nuclear security
technologies. For these purposes, data for different types of observables are
needed, including fission product yields over wide ranges of masses and half
lives, as well as the kinetic energy and angular distributions of the emitted
fragments and neutrons.

To first approximation, the fission barrier of a heavy nucleus can be studied
by measuring the fission probability as a function of the excitation energy.
By comparing the experimentally determined fission probabilities to the
results obtained with the WKB approximation, the shape of the fission barrier
(its height and curvature parameter) can be determined.

Significant progress has been made in studying the multiple-humped fission
barrier landscape of actinides, where a strongly deformed deep third minimum
in the potential landscape was established [42, 43]. This is illustrated in
Fig. 3.5, where horizontal dashed lines show transmission resonances in the
superdeformed (SD) second minimum and in the hyperdeformed (HD) third minimum.
One expects that the HD minimum in a cluster description consists of a rather
spherical 132Sn-like component with magic neutron and proton numbers $N=82$
and $Z=50$. Recent theoretical results [44, 45] highlight the role of shell
corrections in the prominence of the third minima for thorium and light
uranium isotopes. There is a clear trend suggesting that lower $N$ values
produce larger neutron shell corrections and, thus, more prominent third
minima. This is in stark disagreement with experimental results. Using the
231Pa(3He,$d$f) reaction, Csige et al. [46] studied sub-barrier fission in
232U and inferred a fission barrier with a well-formed third minimum that does
not agree with the theoretical predictions. Pronounced third minima have also
been inferred to exist through observation of fine structure in the cross
section of transfer-reaction-induced fission or through sub-barrier
photofission cross-section measurements. There is particular interest in
identifying resonances in the third minimum, where very large deformations
cause the GDR to split into two components with a low-lying oscillation along
the long symmetry axis with a typical excitation energy of four to five MeV.
It is expected that these resonances may have a significantly enhanced
$\gamma$-ray width $\Gamma_{\gamma}$ of about 100 eV in the population of the
third minimum, while, for the second minimum, one expects a $\gamma$-ray width
of only about 1 eV. E1 resonances excited from the ground state will have
about the same absolute excitation strength as resonances in the third
minimum, but resonances in the third minimum will be much broader, with a
total width of about 1 keV.

Figure 3.5: Schematic overview of the multiple-humped fission barrier in
actinide isotopes, along with the corresponding nuclear shapes. The lower part
shows a cut through the potential energy surface along the fission path,
revealing an SD second minimum at an axis ratio of 2:1 and an HD third minimum
at an axis ratio of 3:1. The energies and locations of the saddle points and
minima were taken from Ref. [42]. In the upper part, the corresponding nuclear
shapes are displayed as a function of the quadrupole and octupole degrees of
freedom.

Photofission measurements enable the selective investigation of extremely
deformed nuclear states in the light actinides and can be utilized to better
understand the landscape of the multiple-humped PES of these nuclei. The
selectivity of these measurements stems from the low and reasonably well-
defined amount of angular momentum transferred during the photoabsorption
process. High-resolution studies can be performed on the mass, atomic number,
and kinetic energy distributions of the fission fragments following the decay
of well-defined initial states in the first, second and third minima of the
PES in the region of the light actinides. The beams available at the next-
generation laser Compton source facilities will enable high fidelity studies
of the PES using transmission resonance spectroscopy, as well as studies of
heavy clusterization in the actinides, and of rare fission processes such as
ternary fission. Each of these opportunities is discussed in this section.

#####  3.1.2.1 Transmission Resonance Spectroscopy in Photofission

The approach to investigating extremely deformed collective and single-
particle nuclear states of the light actinides is based on the observation of
transmission resonances in the prompt fission cross section. Observing
transmission resonances as a function of the excitation energy allows the
identification of the excitation energies of the SD and HD states. Moreover,
the observed states can be ordered into rotational bands, with moments of
inertia proving that the underlying nuclear shape of these states is indeed an
SD or an HD configuration. For the identification of the rotational bands, the
spin information can be obtained by measuring the angular distributions of the
fission fragments. Furthermore, the PES of the actinides can be parameterized
very precisely by analyzing the overall structure of the fission cross
section, and by fitting it with the results of nuclear reaction code
calculations.

So far, transmission resonances in the light actinides have been studied
primarily in light-particle-induced nuclear reactions. These studies do not
benefit from the same selectivity found in photonuclear excitation and,
consequently, they are complicated by a statistical population of the states
in the second (and third) minimum. These measurements have also suffered from
a dominant prompt-fission background. By contrast, the next-generation laser
Compton $\gamma$-ray sources will enable the identification of sub-barrier
transmission resonances with very low integrated cross sections down to
$\Gamma\sigma\approx 0.1$ eVb and in so far unexamined nuclei. The narrow
energy bandwidth of the next generation $\gamma$-ray sources will enable a
significant reduction of the presently dominant background from non-resonant
processes.

Besides exploring the level structure in the second and third minima of the
fission barrier of the light actinides, the harmonicity of the potential
barrier can also be examined, and the parameters of the fission barrier can be
extracted. Such fission barrier parameters are crucial inputs for cross-
section calculations in the thorium-uranium fuel cycle of fourth-generation
nuclear power plants. The selectivity of the photofission measurements allows
high-resolution investigation of fission resonances in photofission in the
second and third minima of the fission barrier. Detailed studies of SD and HD
states via transmission resonance spectroscopy is relevant also for achieving
much cleaner energy production by an efficient transmutation of the long-
lived, and most hazardous radioactive components of nuclear waste, and by
controlling the fission process through using entrance channels via HD states.

#####  3.1.2.2 Photofission as a Probe of Heavy Clusterization in the
Actinides

Theoretical considerations suggest that in a cluster description, the HD
configuration of a light actinide consists of a spherical 132Sn-like component
with magic neutron and proton numbers $N=82$ and $Z=50$, respectively,
complemented by an attached, elongated second cluster of nucleons. Since the
fission-product mass distribution is distinctly determined by the
configuration at the scission point, and the third minimum is very close to
the scission configuration, it is expected that the mass distribution,
following the decay of an HD nucleus, will exhibit a pronounced asymmetric
structure. However, such a dramatic effect of the shell structure has not been
observed so far due to the fact that available studies used particle-induced
fission.

Brilliant quasi-monoenergetic $\gamma$-ray beams will enable high-resolution
investigations of the mass, atomic number, and kinetic energy distributions of
the fission fragments following the decay of states in the first, second and
third minima of the PES in the region of the light actinides. In these
measurements, the heavy clusterization and the predicted cold valleys of the
fission potential can be studied for the first time. Data on the heavy cluster
formation will provide valuable information on the fission dynamics.

#####  3.1.2.3 Investigation of Rare Fission Modes: Ternary Photofission

So far, information on ternary and more exotic fission modes has been deduced
from neutron-induced and spontaneous fission experiments. Ternary particles
are released very close to the scission point, thus providing valuable
information on both the scission-point configuration of the fissioning nucleus
and the dynamics of the fission process. However, ternary photofission has
never been studied due to the very low cross section of the reaction channel.

In such studies, the geometry can be fixed by using polarized $\gamma$-ray
beams, which is a clear advantage over neutron-induced or spontaneous fission
experiments. Moreover, by employing quasi-monoenergetic $\gamma$-ray beams,
excitation-energy correlations of the ternary fission process can be explored
with good resolution. These experiments investigate open problems such as the
mechanism of ternary particle emission, the role of the deformation energy and
of the spectroscopic factor, and the possible formation of heavier clusters.
It will be interesting to measure light-particle decay in photofission and to
search for the predicted enhanced $\alpha$ decay of SD and HD states of the
light actinides.

The availability of brilliant quasi-monoenergetic $\gamma$-ray beams will make
it possible to study the angular distribution of the ternary particles. These,
in turn, will provide important spectroscopic information on the fissioning
system. The beam requirements to carry out the photofission research program
described in this section are given in Table 3.2.

Table 3.2: Suggested beam parameters for the photofission research program. Parameter | Value
---|---
Energy | 5–20 MeV
Flux ($\gamma$/s) |  109 at 3% FWHM
Polarization | Linear and Circular
Diameter | 10 mm on target
Beam Repetition Rate | few MHz
Beam Pulse Width |  $<$ 1.0 ns
Macropulse Width |  1 $\mu$s to 10 s
Macropulse Repetition Rate | 0.1 Hz to 500 kHz

###  3.2 Nuclear Astrophysics

Many questions in astrophysics require a detailed understanding of stars and
stellar properties, thus challenging stellar models to become more
sophisticated, quantitative, and realistic in their predictive power. This in
turn requires more detailed input, such as thermonuclear reaction rates and
opacities, and a concerted effort to validate models through systematic
observations. Consequently, the study of nuclear reactions in the universe
remains at the forefront of nuclear physics and astrophysics research.

Nuclear reactions generate the energy in stars and are responsible for the
synthesis of the elements. When stars eject part of their matter through
various means, they enrich the interstellar medium with their nuclear ashes
and thereby provide the building blocks for the birth of new stars, of
planets, and of life itself. Element synthesis and nuclear energy generation
in stars are the two primary research topics in nuclear astrophysics. Both
require accurate knowledge of charged-particle- and neutron-induced nuclear
reactions that take place in the hot stellar plasma. It is remarkable how the
quantum mechanical nature of atomic nuclei influence the macroscopic
properties of stars.

The DOE report The 2015 Long Range Plan for Nuclear Science: Reaching for the
Horizon organizes nuclear astrophysics into five broad topical areas: (1) the
origin of the elements, (2) the life of stars, (3) the death of stars, (4) the
matter of neutron stars, and (5) connections: dark matter, QCD phase diagram,
weak interactions and neutrinos. The beams available at the next-generation
laser Compton $\gamma$-ray source facilities will enable measurements that
contribute to the first four topical areas. The main opportunities are for
cross-section measurements of ($\gamma$,$\gamma$’) nuclear resonance
florescence (NRF) processes and ($\gamma$,particle) reactions. The NRF
measurements provide important information for determining photon strength
functions (PSFs), $\gamma$-ray transition probabilities, and nuclear structure
spectroscopic information, all of which are inputs to nuclear astrophysics
reaction-network calculations. The ($\gamma$,particle) reaction measurements
provide data that are important input for $\gamma$-ray-induced reactions on
stable nuclei in stars and also for the time reverse of particle capture on
unstable nuclei. These measurements are particularly relevant for $p$-process,
$s$-process, and $r$-process nucleosynthesis. The most important contribution
to nuclear astrophysics that will come from opportunities created by the beam
capabilities of a next-generation laser Compton $\gamma$-ray source is the
measurement of the 16O($\gamma$,$\alpha$)12C reaction cross section as a means
for determining the cross section for the ${12}$C($\alpha$,$\gamma$)16O
reaction at center-of-mass energies important for carbon burning in massive
stars using time reversal invariance. Some examples of research opportunities
at the next-generation laser Compton $\gamma$-ray source are outlined below.
Table 3.3 gives the suggested beam parameters for the nuclear astrophysics
program.

Table 3.3: Suggested beam parameters for the nuclear astrophysics program. The beam parameters required to carryout the 16O($\gamma$, $\alpha$)12C measurements are the most demanding in this research area. In addition to the listed beam attributes, another important requirement is that the beam on target have a very small bremsstrahlung component, in order to minimize backgrounds caused by $\gamma$-ray-induced particle production. Parameter | Value
---|---
Energy | 2–20 MeV
Flux ($\gamma$/s) |  1011 at 1% FWHM
Polarization | Linear and circular
Diameter | 10 mm on target
Beam Repetition Rate | few MHz
Beam Pulse Width |  $<1.0$ ns

####  3.2.1 The Origin of the Elements: $p$-Process and $s$-Process
Nucleosynthesis

Models of $p$-process and $s$-process nucleosynthesis require reliable
($\gamma$,$n$), ($\gamma$,$p$) and ($\gamma$,$\alpha$) reaction cross sections
on hundreds of stable and unstable nuclei. The proton-rich nuclei cannot be
produced by neutron capture reactions. Complete network calculations on
$p$-process nucleosynthesis include several hundred isotopes and the
corresponding reaction rates. Theoretical predictions of the rates, normally
in the framework of the Hauser-Feshbach theory, are necessary for modeling the
nucleosynthesis reaction network. The reliability of these calculations should
be tested experimentally, especially at rate-limiting paths in the network.
Different approaches are available and necessary to improve the experimental
data base for the $p$-process. The ($\gamma$,$n$) cross sections in the energy
regime of the GDR have already been measured extensively (see, e.g., [47]).
More recently, substantial effort has be devoted to using beams with a
continuous bremsstrahlung spectrum to determine the reaction rates without any
assumptions on the shape of the cross section’s energy dependence in the
astrophysically relevant energy region, close to the reaction threshold [48,
49, 50]. A determination of the reaction rates by an absolute cross section
measurement is also possible using monoenergetic photon beams produced by a
laser Compton $\gamma$-ray source [51]. The beam capabilities at next-
generation laser Compton $\gamma$-ray sources will open the possibility of
higher accuracy ($\gamma$,$n$) cross-section measurements and measurements on
nuclei with low natural abundances, for which the amount of target material
will be small.

In contrast, the experimental knowledge about the ($\gamma$,$p$) and
($\gamma$,$\alpha$) reactions in the corresponding Gamow window is smaller. In
fact, the experimental data is based on the observation of the time reversal
($p$,$\gamma$) and ($\alpha$,$\gamma$) cross sections, respectively [52, 53,
54, 55, 56] for the proton-rich nuclei with mass numbers around 100. Due to
the difficulties concerning the experimental accessibility of the
($\gamma$,$\alpha$) reaction rates, a method using elastic $\alpha$ scattering
has been established [57, 58]. It would be a tremendous improvement in the
quality of the database to measure these rates directly using photon beams.
Having a significant impact on this database will require a program of
systematic measurements on a broad range of nuclei. This type of program would
be made possible by a next-generation laser Compton $\gamma$-ray source.

Figure 3.6: The $s$-process reaction path in the Ni-Cu-Zn region. During He-
core burning (yellow), the temperature and neutron density are low compared to
those during C-shell burning (red). Therefore, the unstable nucleus 63Ni
either experiences decay or radiative neutron capture and acts as a branch-
point nucleus. The isotopic abundance pattern of copper and zinc depends
strongly on the stellar neutron-capture rate of 63Ni. This figure is adapted
from Ref. [59].

The heavy elements above the so-called iron peak are mainly produced in
neutron capture processes: the $r$ process (r: rapid neutron capture) deals
with high neutron densities, well above 1020 cm-3, and temperatures of around
2 to 3 GK. It is thought to occur in explosive scenarios such as supernovae
[60, 61] and was recently verified in neutron star merger via gravity wave
observations [62]. In contrast, the average neutron densities during
$s$-process nucleosynthesis (s: slow neutron capture) are rather small (around
108 cm-3), so that the neutron capture rate $\lambda_{n}$ is normally well
below the $\beta$-decay rate $\lambda_{\beta}$, and the reaction path is
therefore close to the valley of $\beta$ stability [63, 64, 65]. However,
during the peak neutron densities, branching occurs at unstable isotopes with
half-lives as low as several days. The half-lives of these branch points are
normally known with high accuracy, at least under laboratory conditions, and
rely on theory only for the extrapolation to stellar temperatures [66].
However, their neutron capture cross sections are accessible to direct
experiments only in special cases. For this reason, nuclear astrophysics
reaction-network simulations currently rely heavily on Hauser-Feshbach (HF)
model calculations of the critical neutron capture cross sections. The
difficulty is that there are many examples where the results of HF model
calculations differ substantially from measurements (see, e.g., Ref. [67]).
Thus, experimental constraints on the theoretical predictions of these branch
points are needed. Laser Compton $\gamma$-ray sources enable measurements of
the inverse ($\gamma$,$n$) reaction, which can also provide information about
the stellar enhancement factors (SEFs). The SEF accounts for the difference in
the neutron-capture cross section measured in the laboratory and the effective
cross section in the stellar environment. Because nuclei in stars spend much
of the time in excited states, capture reactions mostly occur on excited
states, not the ground state. In addition to determining the cross sections
and the SEFs directly from ($\gamma$,$n$) cross-section measurements, the SEFs
can be calculated using photon strength functions determined from NRF
measurements. The importance of each nucleus and reaction in the $s$-process
network must be analyzed on its own merits. An example of the type of
information that can be obtained using linearly polarized mono-energetic
photon beams is illustrated in the proposed NRF measurement on 64Ni at
HI$\gamma$S.

The 64Ni nucleus is the product of neutron capture on 63Ni, which is a branch-
point nucleus. Figure 3.6 illustrates the situation during He-core burning
(yellow) and C-shell burning (red) of a massive star, where the weak component
of the $s$-process originates [68]. As the branching is very sensitive to the
63Ni($n$,$\gamma$) cross section, a measurement was performed at CERN using a
radioactive 63Ni target [59, 69]. However, the reaction rate in the hot
environment of the stellar plasma differs significantly from the measured
value because excited states are populated [70]. In the case of
63gsNi($n$,$\gamma$), the stellar rate is still around 90% at He-core burning
temperatures, but it drops to around 40% at the higher temperature in the
C-shell burning phase [59]. Hauser-Feshbach model calculations are required to
account for the stellar enhancement [71]. These calculations rely on the
photon strength function in 64Ni, which can be deduced from photoabsorption
cross sections and the decay properties of low-spin states. The linearly
polarized monoenergetic beams available at laser Compton $\gamma$-ray sources
enable model-independent determination of photoabsorption cross sections as a
function of the excitation energy, and consequently, a determination of the
photon strength functions (see, e.g., [72, 73, 74, 16, 15]).

####  3.2.2 The Life of Stars: The 12C($\alpha$,$\gamma$) Reaction Cross
Section

Late-stage red giant stars produce energy in their interiors via helium
burning. In first generation stars, helium burning proceeds through the
3$\alpha$ process and then 12C($\alpha$,$\gamma$)16O, while in later
generation stars, $\alpha$ captures also involve the various CNO seed nuclei.
In both cases, the 12C($\alpha$,$\gamma$)16O reaction helps to regulate the
efficiency of helium burning in massive stars (those with masses greater than
the suns). It also determines core mass, temperature and density during the
latter stages of stellar evolution, and ultimately the mass of the iron core
in the incipient supernova. In addition, the carbon-to-oxygen ratio (C/O)
influences the abundances of elements produced in the ensuing explosion. After
several decades of effort, the uncertainty in the rate of the
$\alpha$-particle capture reaction on 12C is still large enough to
substantially limit our understanding of the latter stages of stellar
evolution. To resolve this problem, the astrophysical S factor for the
12C($\alpha$,$\gamma$)16O reaction must be determined to an accuracy better
than about 10% in the Gamow window ($E_{cm}=300$ keV), which is indicated by
the vertical band in Fig. 3.7 [75].

Figure 3.7: Plot of total S-factor data (filled-in circles) [76] for
12C($\alpha$,$\gamma$)16O compared with E1 (open triangles) and E2 (open
squares) $\gamma$-ray measurements [77] and the $E_{x}=6.05$ MeV cascade data
(open circles) [78]. The solid line represents the sum of the single
amplitudes of an R-matrix fit [79], while the dotted and dashed lines are the
E1 and E2 amplitudes, respectively. In addition, the R-matrix fit of [78] to
their cascade data is shown as the dot-dashed line. The latter component is
not included in the sum and might explain the high yield in the S-factor data
between the resonances. This figure is adapted from Ref. [80]. Figure 3.8:
Plot of the cross section of the 16O($\gamma$,$\alpha$)12C reaction as a
function the system center-of-mass energy (bottom axis) and the incident
$\gamma$-ray beam energy (top axis). The green hash shows the region where
data have been collected at HI$\gamma$S using the optical time-projection
chamber (TPC). The blue filled bar is the lowest energy region experimentally
accessible at HI$\gamma$S using current target and detector technologies. The
energy region indicated by the red brick filled area is what would be possible
with an intensity upgrade of HI$\gamma$S.

As shown in Fig. 3.7, the cross section has been measured at various levels of
precision down to a center-of-mass (cm) energyof 1.2 MeV; then it must be
extrapolated down to 300 keV, the energy needed for stellar reaction-rate
calculations. One of the major uncertainties in performing the extrapolation
arises from the presence of several resonances which contribute to the cross
section at energies around $E_{cm}=0.75$ MeV. Above $E_{cm}=1$ MeV, the
elastic scattering and capture reactions are dominated by a broad 1- resonance
at an excitation energy in 16O of $E_{x}=9.59$ MeV ($E_{cm}=2.43$ MeV) and a
narrow 2+ state at $E_{x}=9.85$ MeV ($E_{cm}=2.70$ MeV). However, a 1- state
at $E_{x}=7.12$ MeV, just 42 keV below threshold, determines the capture cross
section in the astrophysically relevant energy region, including its
interference with the higher lying 1- and 2+ states. In addition, broad high-
lying states and direct processes produce a coherent background that affects
the energy dependence of the cross section and thereby the extrapolation.

Direct measurements of the 12C($\alpha$,$\gamma$)16O cross section at energies
below $E_{cm}=2$ MeV have been attempted for over 30 years. The major
difficulty encountered in these experiments is the intense neutron background
arising from the 13C($\alpha$,$n$) reaction. This background tends to swamp
the $\gamma$-ray detector. The $\gamma$-ray beams produced by laser Compton
sources have a narrow energy spread and thus offer an alternative technique
for measuring the cross section for $\alpha$ capture on 12C at energies
important to astrophysics as discussed above. The principle of detailed
balance allows the determination of the ($\alpha$,$\gamma$) cross section from
the measurement of the cross section for the time reversed ($\gamma$,$\alpha$)
reaction. An added advantage of using the ($\gamma$,$\alpha$) reaction over
the direct reaction is that the cross section in the Gamma window is enhanced
by about a factor of 50 due to detailed balance. This experimental concept has
been demonstrated at HI$\gamma$S using a CO2 gas-filled optical time
projection chamber (TPC) [81, 82]. The calculated cross section for the
16O($\gamma$,$\alpha$)12 reaction is shown in Fig. 3.8. An upgrade of
HI$\gamma$S to increase the $\gamma$-ray beam flux on target by a factor of
about fifty in the energy region important for this reaction, would enable
meaningful measurements in the red shaded energy range ($\langle
E_{cm}\rangle=1.3$ MeV) in the figure. The next-generation laser Compton
sources could potentially deliver more than an additional factor of 500 in
beam flux on target relative to HI$\gamma$S, thereby enabling measurements
down to about $\langle$E${}_{cm}\rangle=700$ keV. This measurement should be
performed at next-generation laser Compton $\gamma$-ray source facilities
using a variety of experimental techniques, such as gas TPCs (both optical and
charge readout), silicon strip detectors with thin targets, and total cross-
section measurements using super-heated high-purity water detectors (e.g., as
in Ref. [83]).

####  3.2.3 The Death of Stars: $r$-Process Nucleosynthesis

More than half of the heavy nuclei with $A>120$ are produced by $r$-process
nucleosynthesis. The $r$-process involves nuclear reactions driven by rapid
neutron capture, where the neutron capture rate is faster than the competing
beta decay. The likely environments for the $r$-process are type-II supernovae
and merging neutron stars. Simulations of $r$-process nucleosynthesis require
reliable ($n$,$\gamma$) reaction cross sections on hundreds of stable and
unstable nuclei. Theoretical predictions of the rates, normally in the
framework of Hauser-Feshbach theory, are necessary for modeling neutron
capture on unstable nuclei. The reliability of these calculations should be
tested experimentally, especially at rate-limiting paths in the network. The
($\gamma$,$n$) time-reversed photodisintegration reaction offers a mechanism
for determining the neutron capture cross section and level density of
radioactive nuclei. Next-generation laser Compton $\gamma$-ray sources will
enable measurements on isotopes with low natural abundances and thus very
limited sample sizes.

####  3.2.4 Neutron Stars: the EOS of Neutron-Rich Matter

Obtaining information about the detailed structure of the crust of a neutron
star is an important open challenge in astrophysics. The crust is composed of
non-uniform neutron-rich solid matter that is about 1 km thick and located
above a liquid core [84, 85]. The inner crust comprises the region from the
density at which neutrons drip from nuclei to the inner edge separating the
solid crust from the homogeneous liquid core. While the density at which
neutrons drip from nuclei is well determined, the transition density at the
inner edge is much less certain because of insufficient knowledge of the
equation of state (EOS) of neutron-rich nuclear matter. Measurements of
collective responses of neutron-rich nuclei provide constraints on the nuclear
EOS. The monoenergetic and linearly polarized beams at laser Compton
$\gamma$-ray sources enable unique measurements of dipole and quadrupole
excitations. Those most relevant to exploring the EOS are studies of the pygmy
dipole resonance (PDR) on nuclei at the neutron rich end of an isotope chain
using nuclear resonance florescence (NRF) and determination of the centroid
energy and width of the isovector giant quadrupole resonance (IVGQR) via
Compton scattering [86]. The underlying structure of the PDR is often
interpreted as an oscillation of less bound valence neutrons forming a neutron
skin [87, 88]. However, the true nature of the PDR is a matter of ongoing
discussion. Furthermore the PDR and the E1 strength in general have a direct
connection to the neutron skin of nuclei and the symmetry energy of nuclear
matter [89, 90].

Figure 3.9: Constraints of the symmetry energy at saturation $J$ and slope
parameter $L$, obtained from a comparison of relativistic nuclear energy
density functional results and data on anti-analog giant dipole resonance
(AGDR) [91] and IVGQR [86] excitation energies in 208Pb; the dipole
polarizability $\alpha$D of 208Pb [92]; and the PDR energy-weighted strength
in 68Ni [93] and 130,132Sn [89]. This figure is taken from Ref. [94].

###  3.3 Hadronic Parity Violation

The capabilities of a next generation laser Compton $\gamma$-ray source,
combined with current and expected advances in theory and lattice
calculations, provide the opportunity to make significant progress in our
understanding of hadronic parity violation (PV). Parity violation in nuclear
processes can probe the standard model (SM) and point towards possible SM
extensions. In the SM sector, hadronic PV provides an important probe of two
phenomena that are not well understood: neutral-current nonleptonic weak
interactions and nonperturbative strong dynamics. While PV is well understood
in quark-quark weak interactions, it is ultimately the interplay of different
forces at different length scales in the SM that is responsible for hadronic
PV phenomena. Neutral-current interactions are suppressed in flavor-changing
hadronic decays, making hadronic PV (HPV) between nucleons the best place to
study neutral-current effects. Because parity violating nucleon-nucleon (NN)
interactions are the manifestation of the interplay of nonperturbative strong
effects and the short-range weak interactions between quarks, they are
sensitive to short-distance quark-quark correlations inside the nucleon.
Quark-quark and NN weak interactions also induce parity-odd effects in
electron scattering [95, 96, 97, 98], nuclear decays [99], compound nuclear
resonances [100, 101], and atomic structure, where they are the microscopic
source for nuclear anapole moments [102, 103, 104, 105]. The comparison
between NN weak amplitudes in few-nucleon systems and heavy nuclei can also
test the statistical theory of parity violation in compound nuclei, which will
become useful in the future for the interpretation of time reversal violation
experiments in the transmission of polarized neutrons through polarized
nuclear targets on p-wave compound nuclear resonances [107].

The NN weak interactions are therefore worth understanding both for their own
sake and for other areas of nuclear, particle, and atomic physics. Agreement
between theory and experiment in this area pushes the “complexity” frontier of
the SM, as it requires a quantitative understanding of strong,
electromagnetic, and weak interactions in the strongly-interacting regime of
QCD all acting in the same system. This research program will also stretch the
technology of lattice gauge theory calculations [108] to its computational
limits.

This section explores possibilities for measuring hadronic PV in few nucleon-
systems and in the mixing of parity doublet states in light nuclei using
circularly polarized high-intensity photon beams.

####  3.3.1 Hadronic Parity Violation in Few-Nucleon Systems

Weak NN amplitudes at low energy are suppressed by six to seven orders of
magnitude compared to strong NN amplitudes and are therefore difficult to
observe. While these effects can be amplified by several orders of magnitude
by nuclear dynamics in complex systems, the unambiguous extraction of the weak
interactions among nucleons requires parity violating measurements in very
light nuclei, where theory is under good control. At the moment this includes
the deuteron, the triton, and 3He, with the four-nucleon system in the
pipeline. These very light systems can be calculated using two- and three-
nucleon interactions, which in turn are understood in terms of effective field
theories (EFTs) that systematically incorporate the symmetries of QCD. The EFT
approach can consistently be applied to strong and weak nucleon interactions
as well as to external currents. There is also growing optimism that parity-
odd effects in $A>4$ nuclei can be calculated. We are at a juncture in the
development of theory where PV may soon be understood in terms of QCD dynamics
even in many body nuclei. Ongoing programs [109] are pursuing the matching of
lattice QCD to EFTs, which are used as input into many-body calculations of
heavier nuclei. Both SM and beyond-the-SM physics can be treated in this way.

At leading order in the EFT power counting, and at very low photon energies
(below 10 MeV), there are five parity violating low-energy constants (LECs)
that parameterize the short-distance physics [110, 111, 112].111At higher
energies, pions have to be treated as active degrees of freedom and this
increases the number of LECs. Parity violation in chiral EFT has been
considered recently in Refs. [113, 114, 115, 116]. The values of these LECs
cannot be determined within the EFT framework but have to be extracted from
experiment or ultimately from calculations involving nonperturbative QCD.
Recent work shows that upon combining the EFT approach with the large-$N_{c}$
expansion of QCD, the number of independent parity-violating LECs reduces to
two at leading order [117]. One of these is the $\Delta I=2$ isotensor
coupling, which is poorly constrained from existing measurements. Combining
the existing data for $\vec{p}+p$ scattering (which constrains the remaining
leading-order isoscalar piece) with the results of a measurement of the
$\Delta I=2$ contribution would therefore constrain all terms at leading order
in this combined EFT and large-$N_{c}$ expansion [118].

An important experimental constraint on the isovector partity violating
interactions comes from the parity violating asymmetry from polarized neutron
capture on the proton, $\vec{n}+p\to D+\gamma$, recently measured by the
NPDGamma collaboration at the Spallation Neutron Source (SPS) at ORNL [119].
In the next few years we expect to possess two additional pieces of
experimental information on the NN weak interaction in few-nucleon systems.
The analysis of parity violation in
$\vec{n}+{}^{3}\text{He}\to{}^{3}\text{H}+p$ completed recently at the
Spallation Neutron Source at ORNL will be published, and another experiment on
parity violation in $\vec{n}+{}^{4}\text{He}$ spin rotation [120] will be
performed at the new NG-C neutron beam line at the NIST Center for Neutron
Research (NCNR). However, these will yield scant experimental information on
the elusive $\Delta I=2$ component of the NN weak interaction. Fortunately the
$\Delta I=2$ operator is the easiest target for a quantitative lattice
calculation from the SM because it does not receive contributions from
disconnected diagrams. The isotensor operator is scheduled to be calculated on
the lattice in the next few years [121, 122]. Lattice constraints on other NN
weak amplitudes are many years out, at least at physical quark masses.

The cleanest experimental channel for extracting the $\Delta I=2$ component in
few-body PV is P-odd deuteron photodisintegration near threshold using the
helicity-dependent photodisintegration cross section for circularly polarized
photons. Limits from previous attempts exist from Chalk River [123] and from
the reverse reaction [124], but did not resolve a nonzero value. Experiments
to measure this observable have been proposed in the past at JLab [125, 126],
SPRING-8, and the Shanghai Synchrotron, but they have not yet been realized.
It was therefore listed in the last NSAC Long-Range plan as a priority NN weak
interaction measurement for the future. In addition, it is possible to
contemplate other parity violation experiments involving helicity-dependent
reactions with circularly polarized photons in few-nucleon systems, such as
the breakup of three-nucleon systems. We emphasize the importance of a
$\vec{\gamma}d\rightarrow pn$ parity-violation measurement because of the
unique information it will provide about the weak interaction in hadronic
systems and the state of EFT and lattice calculations.

The parity-violating asymmetry in deuteron photodisintegration, including its
energy dependence, has been considered in pionless EFT in Refs. [127, 128].
The next-to-leading order results of Ref. [128] were combined with different
model estimates for the LECs to determine the expected size of the parity-
violating asymmetry $A_{L}^{\gamma}$, to estimate a figure of merit (FOM), and
to extract the best energy regime for performing the experiment.
$A_{L}^{\gamma}$ is computed as the ratio of the difference in the
photodisintegration cross section measured with positive ($h=+1$) and negative
($h=-1$) beam helicity divided by the unpolarized cross section, which is the
sum of the positive and negative helicity cross sections. The beam helicity is
$h=+1$ when the polarization vector of the circularly polarized beam is in the
direction of the beam momentum vector. $A_{L}^{\gamma}$ for the
photodisintegration of the deuteron computed using various parameterizations
of the hadronic weak coupling constants is plotted as a function of photon
energy in Fig. 3.10. The FOM quantity that is optimized for obtaining the best
statistical accuracy per hour of beam time is FOM =
($A_{L}^{\gamma}$)${}^{2}\sigma$, where $\sigma$ is the unpolarized
photodisintegration cross section. The FOM computed for various
parameterizations of the hadronic weak coupling constants are plotted as a
function of photon energy in Fig. 3.11. This analysis indicates that the
experiment should ideally be performed in the photon energy region between
2.26 and 2.30 MeV. In this energy window, the average cross section and
$A_{L}^{\gamma}$ are 600 $\mu$b and 4 $\times$ 10-7, respectively.

A schematic diagram of the concept for the experiment setup is shown in Fig.
3.12. The neutrons from the photodisintegration reaction in the liquid
deuterium (LD) target are detected in an array of 3He ionization tubes
embedded in a cylindrical polyethylene moderator that surrounds the target.
The background due to interactions of scattered $\gamma$ rays in the 3He
ionization tubes is determined using an array of 4He ionization tubes located
outside the inner layers of 3He ionization tubes. About 75% of the $\gamma$
rays incident on the 21-cm long LD target are transmitted through the target.
The $\gamma$-ray beam is collimated to a diameter of 10 mm, and the inner
diameter of the 3He ionization tube array is 6 cm. The ionization tubes are 50
cm long and centered about the LD target. With this target thickness, detector
geometry, and a $\gamma$-ray beam intensity of 1010 $\gamma$/s on target in
the above energy window, a statistical accuracy of $\pm$10-7 can be obtained
with about 9000 hours of beam time.

Figure 3.10: Plot of the calculated $A_{L}^{\gamma}$ for parity violating
photodisintegration of the deuteron as a function of photon energy. The lowest
photon energy on the plot is the breakup threshold value corresponding to the
deuteron binding energy. This figure is adapted from Ref. [128]. The curves
represent next-to-leading-order EFT calculations using three different sets of
hadronic weak coupling constants, which are labeled DDH, DDH-adj, and Bowman.
DDH represents the DDH "best values" [129]. DDH-adj refers to a set in which
two combinations of $\rho$ and $\omega$ couplings are fit to data on the
$\vec{p}p$ longitudinal asymmetry, while the remaining couplings take the DDH
"best values" [130]. The set labeled Bowman is obtained by fitting the parity
violating couplings to a variety of available data [131]. The optimum energy
window for the measurement is indicated by the rectangle.  Figure 3.11: Plot
of the calculated figure of merit for PV photodisintegration of the deuteron
as a function of photon energy. The lowest photon energy on the plot is the
breakup threshold value corresponding to the deuteron binding energy. This
figure is adapted from Ref. [128]. The curves follow the legend given in Fig.
3.10. The optimum energy window for the measurement is indicated by the
rectangle.  Figure 3.12: Schematic diagram of the experimental concept for the
$A_{L}^{\gamma}$ measurement for PV photodisintegration of the deuteron.

The technical demands to minimize $\gamma$-ray helicity-dependent changes in
the phase space of the $\gamma$-ray beam are comparable to those encountered
at the polarized electron injector section at JLab in the parity violating
electron scattering. Developing the capability for delivering polarized
$\gamma$-ray beams to targets with the stability and precision required to
perform 10-7 asymmetry measurements will demand focused and dedicated
resources for R&D work and implementation of the supporting systems. Suggested
beam parameters for studying PV in deuteron photodisintegration are shown in
Table 3.4.

Table 3.4: Suggested beam parameters for the $A_{L}^{\gamma}$ measurement in PV deuteron photodisintegration. Parameter | Value
---|---
Energy | 2.25-2.30 MeV
Flux | $10^{10}$
$\Delta$E/E | 0.001 to 0.01 FWHM
Polarization | Circular
Diameter | 10 mm on target
Time Structure | 10 Hz polarization flip

####  3.3.2 Hadronic Parity Violation in Light Nuclei

The size of the expected parity violating asymmetries from reactions in few-
nucleon systems induced with photons at MeV energies are generally around
several times 10-7. For such experiments, achieving the required systematic
accuracy is a formidable challenge, especially at the initial ramp-up stage of
a NGLCGS facility. This motivates us to look for parity violating asymmetries
that are larger, and therefore easier to measure, even if the interpretation
is not as theoretically clean as in few-nucleon systems. Certain light nuclei
possess doublets with narrow energy spacings in the 10 to 100 keV range
between levels of opposite parity. Parity-odd asymmetries can be amplified by
several orders of magnitude by the interference of such parity doublets states
involved in photon-induced reactions. There are a half dozen such nuclei (some
of which were investigated using charged-particle beams a few decades ago)
[99] whose parity-odd asymmetries could be measured using the ultra-high
intensity mono-energetic polarized photon beam at the NGLCGS. To develop the
technical infrastructure and scintific expertise among the staff, a parity
violation program at such facilities should start with an asymmetry
measurement on parity doublets in nuclei, where the parity violating assymetry
is several orders of magnitude larger than that in few-nucleon systems. The
experiment could measure either the helicity dependence in the total cross
section using transmission or the asymmetry of the fluorescence from the
doublet using a HPGe detector array. Examples of candidate nuclei and
estimated amplification factors for parity violating asymmetries in nuclear
resonance fluorescence are described by Titov, Fuji, Waral, and Kawase [132].

Either parity-odd asymmetry can be calculated in a simple way in terms of the
energies and widths of the states and the parity-odd matrix elements of
interest. Parity violation in states which are particle unstable will be
difficult to handle theoretically. However, as the theoretical tools to
calculate parity violation in these nuclei improve, we can identify one or
more candidate nuclei for a focused effort involving both theory and
experiment.

Table 3.5: Suggested beam parameters for parity violiaying asymmetry measurements in parity doublets. Parameter | Value
---|---
Energy | 1-20 MeV
Flux |  109 at 0.1-1% FWHM
Polarization | Circular
Diameter | 10 mm on target
Time Structure | 10 Hz polarization flip

Although the parity violating asymmetry is enhanced in the parity doublet
systems, the photonuclear cross section is small compared to Compton and pair
production cross sections in the energy range of interest below the giant
dipole resonance. The beam requirements are therefore still challenging, as
shown in Table 3.5.

###  3.4 Nuclear and Homeland Security

A next-generation Compton $\gamma$-ray beam facility can contribute to the R&D
of techniques and technologies for applications in homeland security, nuclear
safeguards, and medicine. The facility should have the infrastructure that
supports measurements that contribute to the nuclear databases important in
supporting the development of technologies and techniques in the above areas.
Equally important, the facility should have a target area that is equipped for
evaluating concepts for $\gamma$-ray beam interrogation of cargo, nuclear
fuel, and special assemblies.

Safeguarding the nation against evolving threats that involve the use of
special nuclear materials (SNM) requires continued innovations in the
procedures and technologies used to inspect cargo at ports of entry into the
United States. Research frontier areas include the development of systems for
$\gamma$-ray and neutron-beam interrogation of cargo for shielded SNM, new
cost-effective materials for $\gamma$-ray and neutron detection, and improved
techniques and supporting technologies for applications of nuclear forensics
in the analysis of interdicted materials. The beam capabilities and technical
infrastructure of next generation $\gamma$-ray beam facilities should support
the following research: (1) evaluation of $\gamma$-ray beam interrogation
techniques and technology concepts, (2) filling gaps in the photonuclear
reaction database needed in methods for identification of SNM, including the
development and maintenance of online databases, and (3) testing,
characterizing, and calibrating $\gamma$-ray and neutron detectors. The
infrastructure for photonuclear data measurements should be optimized for
nuclear resonance fluorescence (NRF), photo-neutron reactions, and
photofission.

Each nucleus with $Z>2$ is characterized by unique excited states. These
states have very narrow widths (rarely more than a fraction of an eV) and can
be populated by absorption of photons of the appropriate energy. When such an
excited state decays, characteristic $\gamma$-rays are emitted with unique and
well-defined energies. The NRF method of isotope identification, shown in Fig.
3.13, is based on these observations. The main advantage of this method over
other beam-based techniques is that both the excitation and the de-excitation
processes proceed via the electromagnetic interaction, which is well
understood. Because of the low angular momentum transferred in photonuclear
reactions, photon-induced nuclear excitations are mostly electric dipole (E1),
magnetic dipole (M1), and to a lesser extent electric quadruple (E2)
transitions from the ground state. If the excited state is unbound to emission
of strongly interacting particles (e.g., neutrons, protons, or alpha
particles), the photon decay is not strong enough to be observed in
competition with particle decay channels. Hence, most of the useful states are
positioned below the particle separation energies of around 6 to 8 MeV, and
they generally have ground state radiative widths ($\Gamma_{\gamma}$) of 10 to
100 meV.

There are two main approaches to identifying nuclei using NRF: (1) detection
of scattered characteristic $\gamma$-rays, and (2) resonance absorption or the
observation of flux removal from the $\gamma$-ray beam by resonance nuclear
scattering. In the latter method, resonance absorption creates notches in the
energy spectrum of the transmitted beam at energies characteristic of the
nuclei encountered by the beam. Bertozzi and Ledoux proposed this method for
applications in scanning seagoing cargo containers [133]. Detection of these
notches provides the signature for identifying the isotopes through which the
beam has passed. The application of the notch technique with nearly mono-
energetic $\gamma$-ray beams has been studied recently [134], and a recent
measurement performed using the $\gamma$-ray beam at HI$\gamma$S demonstrates
that for $\gamma$-ray beams with a small energy spread, such as the 5% FWHM at
HI$\gamma$S, the probability of notch refilling due to small-angle scattering
is likely to be negligible for most cargo inspection scenarios [135]. Beams at
a next-generation laser Compton $\gamma$-ray source will enable expansion of
the experimental studies of the notch technique reported by Hagmann et
al.[135]. The goals of such work would include: (1) comparison of signal-to-
background ratio of the notch technique to that for detection of scattered
$\gamma$-rays for high $Z$ nuclei and a variety of cargo configurations; (2)
measurements of effective NRF cross sections for selected isotopes; (3)
assessments of performance quantities for the notch technique for simple cargo
arrangements, in order to benchmark computer models of the setup; and (4)
evaluations of the performance of the notch technique implemented with low-
resolution high-efficiency $\gamma$-ray detectors, such as LaBr3
scintillators. The performance quantities include the signal-to-background
ratio, the number of signal counts per 109 incident $\gamma$-rays, and the
radiation levels in the proximity of the cargo.

Figure 3.13: Schematic diagram of the NRF technique. First the incident photon
is absorbed by a nucleus at certain resonance energies characterized by the
parameters $E_{i}$, Jπ, $\Gamma_{i}$. Then the excited state quickly re-emits
$\gamma$-rays at the same or a different energy in a decay pattern that is
unique for each type of nucleus. The MeV energy scale is high enough to
penetrate heavily shielded containers, thus permitting non-destructive,
unambiguous isotope identification and quantitative mass determination.

The driving principle of the notch technique is the analysis of the energy
distribution of the beam transmitted through the sample material. The
resonance absorption of the $\gamma$-rays by the nuclei in a sample material
creates notches in the transmitted beam at the resonance energies. Because the
width of the notch (or absorption peak) in the beam-energy profile is
typically less than 1 eV, direct detection of the notch is not possible with
current $\gamma$-ray-detector technology, but the natural line width of
$\gamma$-ray transitions in nuclei is used to overcome this technical
challenge [133]. The comparison of the beam-flux normalized NRF spectrum on a
target measured using the beam transmitted through the cargo sample with that
measured with the sample removed gives a measure of the resonance absorption
of the cargo material for the nuclei in the downstream target. The
$\gamma$-ray beam flux transmitted through the sample at the energy $E_{r}$ of
a resonance in a nucleus in the sample is given by

$\phi=\phi_{0}\,exp(-\sigma_{eff}\,\rho x).\\\ $ (3.1)

In this equation $\phi_{0}$ is the $\gamma$-ray beam flux incident on the
sample at energy $E_{r}$, $\phi$ is the transmitted beam flux at energy
$E_{r}$, $\sigma_{eff}$ is the effective nuclear resonance absorption cross
section, $\rho$ is the density of the nuclei associated with the resonance in
the sample, and $x$ is the thickness of the sample. The quantity
$\sigma_{eff}$ includes effects from Doppler broadening caused by the thermal
motion of the nuclei in the sample.

A schematic diagram of the experimental setup for the notch-technique
evaluation measurements is shown in Fig. 3.14. The time structure of the beam
enables the backgrounds uncorrelated with the beam to be measured while
collecting beam-induced reaction data and determining the velocity of neutrons
emitted from the target via time-of-flight measurements. The beam-shaping
collimator shields the experimental setup from the radiation produced by the
beam using lead and concrete walls indicated in the drawing. The standard
collimator diameter in these measurements will be around 25 mm (1 inch). These
studies will be performed using circularly polarized $\gamma$-ray beams. At
$\gamma$-ray beam energies below 7 MeV, the collimator material should be
lead, and at higher energies the collimator should be made of aluminium to
reduce neutron production via the ($\gamma$,$n$) reaction on the collimator.
The sample materials being studied are placed between the steel plates in the
area labeled cargo in the diagram. The energy spectrum of the beam transmitted
through the sample should be analyzed using the NRF measurement setup in the
area labeled inspection unit. The cargo arrangement will nominally contain
targets that are one to several cm thick. The analysis foils will normally be
a few mm thick. The collimated beam flux before the cargo will be monitored
e.g., with a thin plastic scintillator paddle. The absolute beam flux can be