# Fast Neutron Detection with a Segmented Spectrometer

T. J. Langford111Present Address: Yale University, New Haven, CT 06511 USA
Department of Physics, University of Maryland, College Park, MD 20742 USA
Institute for Research in Electronics and Applied Physics, University of
Maryland, College Park, MD 20742 USA  C. D. Bass222Present Address: Le Moyne
College, 1419 Salt Springs Rd, Syracuse, NY 13214 USA National Institute of
Standards and Technology, Gaithersburg, MD 20899 USA  E. J. Beise  H. Breuer
D. K. Erwin  C. R. Heimbach  J. S. Nico

###### Abstract

A fast neutron spectrometer consisting of segmented plastic scintillator and
3He proportional counters was constructed for the measurement of neutrons in
the energy range 1 MeV to 200 MeV. We discuss its design, principles of
operation, and the method of analysis. The detector is capable of observing
very low neutron fluxes in the presence of ambient gamma background and does
not require scintillator pulse-shape discrimination. The spectrometer was
characterized for its energy response in fast neutron fields of 2.5 MeV, 14
MeV, and the results are compared with Monte Carlo simulations. Measurements
of the fast neutron flux and energy response at 120 m above sea-level (39.130∘
N, 77.218∘ W) and at a depth of 560 m in a limestone mine are presented.
Finally, the design of a spectrometer with improved sensitivity and energy
resolution is discussed.

###### keywords:

Capture gating , Fast neutron detection , Low background , Neutron
spectrometer , Underground physics

††journal: Nuclear Instruments and Methods A

##  1 Introduction

To minimize the influence of ambient backgrounds resulting from cosmic rays
and natural radioactivity, many physics researchers site their experiments in
deep underground laboratories [1]. Regardless, there will still exist fast
neutrons that can strike nuclei and produce recoils that may mimic signatures
of rare events. Alternatively, the neutrons may activate radioactive isotopes
in detector material producing background in these low event rate experiments
[2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]. An improved knowledge of the
fast neutron energy spectrum and flux in the varied environments of these
experiments would enable better optimization of efforts to identify and
minimize such backgrounds.

Fast neutrons encountered in underground laboratories arise primarily through
two mechanisms, muon-induced spallation and naturally-occurring radioactivity
in the local environment. Neutrons from radioactivity are predominantly
generated by spontaneous fission in the 238U and 232Th decay chains and
($\alpha$,$n$) reactions in surrounding material. These neutrons typically
have energies of a few MeV and range up to about 15 MeV. Neutrons from the
local environment have fluxes that are related to the uranium and thorium
content of the surrounding rock and are therefore independent of the
laboratory depth. Neutrons of much higher energies (up to a few GeV) are
produced from cosmic-ray muon-induced spallation reactions [15, 16]. These
neutrons range from a few MeV to many GeV, and their flux and spectrum vary
with the amount and composition of the overburden.

For many underground facilities, there is insufficient knowledge of the
underground flux and spectrum at the lower energies, and the situation is
typically poorer at higher energies. The need for understanding fast neutron
backgrounds has outpaced the ability to perform such measurements [16].
Experimenters have become reliant on Monte Carlo modeling as their best
approach to quantifying neutron fluxes as a function of energy [17, 18],
particularly in the high energy regime. In many cases, the models lack
experimental benchmarks with which the results can be verified. The problem
becomes even more complicated when one considers the secondary fast neutrons
that can be created from the initial spallation neutrons. It is clear to the
community of researchers in underground physics that reliable measurements of
both the fluxes and energy spectra are essential to understanding their
backgrounds [19]. A number of collaborations address this concern by measuring
fast neutron parameters directly at their experimental facility [20, 21, 22,
23, 24, 25, 26, 27] and by benchmarking the codes that they use for
simulations [17, 18].

In this paper, we present the design and results of a Fast Neutron
Spectrometer (FaNS-1) whose purpose is to enable energy and flux measurements
of very low levels of fast neutrons with good energy resolution. The
spectrometer consists of segmented plastic scintillator bars and 3He
proportional counters [28, 29]. The proton recoil in the scintillator bars
permits energy reconstruction, and the capture of the delayed, thermalized
neutron improves event identification without requiring pulse-shape
discrimination. It has a dynamic range from approximately 0.5 MeV to 200 MeV,
which allows the determination its energy response to both terrestrial and
cosmic sources. These results can be compared with simulations to help improve
the accuracy their predictions.

In Section 2 the principle of capture gated spectroscopy is briefly reviewed
along with a method to restore energy resolution through segmenting the
detector. The design of the FaNS-1 spectrometer is presented in Section 3
along with the electronics, data acquisition system, and analysis method.
Results from characterization studies performed in 2.5 MeV and and 14 MeV
neutron fields are given in Section 4. In Section 5 a measurement of the fast
neutron response generated at the Earth’s surface is presented. The results
from measuring the fast neutron response in the Kimballton Underground
Research Facility (KURF) [30] are given in Section 6. These measurements are
summarized in Section 7 along with brief description of a second spectrometer
under construction that will have a higher efficiency and a sensitivity over a
broader energy range.

##  2 Neutron Detection with Plastic Scintillator and 3He Proportional
Counters

###  2.1 Capture-gated neutron detection

The detection method employed here is known as capture-gated spectroscopy [31,
32, 33, 34]. In brief, the idea involves using an organic scintillator to
detect fast neutrons through their recoil interaction with protons in the
scintillator. The neutrons that thermalize and are captured produce a signal
indicating that the recoil event was due to a neutron, as illustrated in Fig.
1. This capture serves to discriminate against uncorrelated background events.
It also indicates that there was full deposition of neutron’s energy in the
scintillator bars, thus permitting the reconstruction of the initial neutron
energy.

The FaNS-1 spectrometer uses 3He in proportional counters as the capture agent
although Gd [35] is a common agent and other efforts have focused on the use
of 10B [36, 37] and 6Li [38, 39]. 3He has two important advantages in that the
reaction produces two charged particles in the final state rather than gamma
rays, thus making it easy to identify, and it has a large thermal neutron
cross section. A neutron is captured by a 3He nucleus, resulting in a proton
and a triton

${}^{3}He+n\rightarrow p+t+764\;\textrm{keV.}$ (1)

The energy is shared between the two reaction products. Purely via kinematics,
the proton receives 573 keV and the triton receives 191 keV.

Figure 1: A schematic representation of a fast neutron interacting in a block
of scintillator and captured within a 3He proportional counter. The first few
scatters create recoil protons with enough energy to be measured above a
threshold. After the neutron thermalizes, it diffuses within the medium until
capturing on a 3He nucleus. Segmentation of the detector permits
reconstruction of the incident energy of the neutron, despite the nonlinear
light response.

The use of separate detectors for the proton recoil signals and the neutron
capture signals has the benefit of being able to distinguish between the two
[33]. Because the neutron capture always occurs after the recoil, we look for
signals which have the reverse time order as a measure of the uncorrelated
coincidences between the scintillator and the proportional counters. For the
low rate of capture events in this work, the uncorrelated coincidences are
uniformly distributed in time while the signal from a neutron that thermalizes
and captures occur at short times characterized by the neutron diffusion time
and the scintillator geometry. With the resulting timing spectrum, a proper
background subtraction can be applied to the data. This will be discussed
further in Section 4.

###  2.2 Energy reconstruction and response

When a neutron undergoes thermalization in an organic scintillating medium,
the main mechanism of energy loss is scattering from protons. As it does so,
proton recoils yield light through the scintillation process. For incident
gamma-rays, the light-output response function is linear over a wide range of
energies, but that is not the case for heavy charged particles. The light
output $L$ from a neutron that deposits all of its energy in a single scatter,
$E^{1s}$, is not the same as one that deposits all of its energy through
multiple scatterings. For example, if one considers two scatters with
$E_{a}^{2s}+E_{b}^{2s}=E^{1s}$, one finds specifically that

$L\left(E_{a}^{2s}+E_{b}^{2s}\right)<L\left(E^{1s}\right).$ (2)

This is due to the non-proportional energy-to-light conversion where the ratio
of light to energy decreases with proton energy, leading to a deficit in the
total light collected in multiple scattering events as compared to single
scattering events. For a single scintillator detector, the degree of multiple
scattering is not known and thus reconstructing the kinetic energy of a
stopped neutron leads to poor energy resolution [36].

This problem can be mitigated by separating the occurrence of multiple
scatters into separate segments, as shown in Fig. 1. The energy transfer in
each individual scatter can be reconstructed according to the light response
function of the scintillator and combined to properly reconstruct the energy
of the incident neutron [40, 41]. Ideally, the segments should be small enough
so that on average a neutron makes not more than one interaction per segment,
but one must balance enhanced resolution against detector complexity. In a
practical implementation, the segment size will be such that there is often
more than one scatter within the segment, but the resolution is still
significantly improved when compared with a detector of the same volume but
without segmentation.

Another critical factor in obtaining the correct energy is understanding the
specific light response as a function of the stopped particle energy and type
of scintillator that was used. Much work has been done studying the effective
light response curves for different types of liquid scintillator, including
NE-213333Certain commercial equipment, instruments, or materials are
identified in this paper in order to specify the experimental procedure
adequately. Such identification is not intended to imply recommendation or
endorsement by the National Institute of Standards and Technology, nor is it
intended to imply that the materials or equipment identified are necessarily
the best available for the purpose. and BC-501 [42, 43]. If the specific
energy loss is known for the particular material, it is possible to calculate
the nonlinear light response function [44, 45, 46]. The method is based upon
the concept that the quenching of light stems from recombination of electrons
and ions in the scintillator. For heavy charged particles, which have high
specific energy loss and deposit most of their energy in a very small range,
the quenching is enhanced by the increased density of electrons and ions. Some
pairs recombine, and therefore do not produce light.

A fitting function of the following form was derived in Ref. [45] that
produces the light response of protons

$\displaystyle dL/dx$ $\displaystyle=$ $\displaystyle
S\times(dE/dx)\times[1+kB\times(dE/dx)$ (3)
$\displaystyle\mbox{}+C\times(dE/dx)^{2}]^{-1},$

where $dL$ is the light produced in path length $dx$, $dE/dx$ is the specific
energy loss of a particle with kinetic energy $E$, and $S$ is a scintillation
efficiency determined through gamma calibration. $kB$ and $C$ are adjustable
parameters that were empirically determined to fit light response measurements
in Ref. [47] for NE-102 polyvinyltoluene plastic scintillator (an equivalent
to the BC-400 used in this work). The values obtained are
$kB=0.0085\,\rm{g/cm^{2}\,MeV^{-1}}$ and $C=1\times
10^{-6}\rm\,{(g/cm^{2}\,MeV^{-1})^{2}}$. The specific energy loss used came
from the NIST database for the stopping power and range of protons and alphas
in polyvinyltoluene [48]. This technique provides a smoothly varying function
that covers the broad range of neutron energies used in this work. Recent work
[49] has shown that there may be significant deviations from fits of Ref. [44]
for plastic scintillator at low-energy recoils. This would effect both the
simulated and reconstructed energies in the detector. By measuring and
simulating the detector response to monoenergetic neutron sources, it is
possible to test the validity of this determination for the light response.

For this work, the term “deposited energy” is used to describe the energy
response generated by the stopped neutrons in the FaNS-1 detector. The term is
defined as the neutron kinetic energy that is converted to ionization in the
plastic scintillator. For many neutrons this will represent the full kinetic
energy, but there exist several effects that cause distortions in the measured
spectra. In addition to the nonlinear light output of the scintillator from
protons, there are effects such as a greatly reduced light output for
particles heavier than protons, multiple scattering in a single scintillator,
and scattering in non-scintillating material. Of particular concern are carbon
interactions, elastic and inelastic, which absorb energy without producing
significant light. These reactions combine to widen and distort the measured
peak, generally shifting the response to lower energies. If the these effects
did not contribute to detected events, the resulting energy deposit spectra
would represent the true neutron kinetic energy spectra.

##  3 FaNS-1 Detector

###  3.1 FaNS-1 Design and Data Acquisition System

The design of the detector took advantage of segmentation and capture-gating
to achieve good energy resolution and good background rejection. The detector
consists of six 3He proportional counters and six segments of plastic
scintillator, arranged as illustrated in Fig. 2. Each segment is a 9.0 cm
$\times$ 18.5 cm $\times$ 15.0 cm section of BC-400 scintillator and an
identical section of light guide that is glued to the scintillator. Each
scintillator segment has an active volume of 2.5 L, yielding a total active
volume of 15.0 L. Two 7.62 cm photomultiplier tubes (PMT) are mounted at the
end of each light guide via 5 cm long cylindrical light guides. Each segment
is wrapped in aluminized Mylar and made light tight with black electrical
tape. This optical decoupling prevents crosstalk between the segments. The
segments were repurposed from another experiment [50], and no structural
changes were made to them.

The 3He proportional counters were manufactured by Reuter Stokes and have a
2.54 cm diameter aluminum body and an active length of 46 cm. They contain a
mixture of 4.0 bar of 3He and 1.1 bar of natural krypton, which increases the
stopping power of the gas for heavy, charged particles. This reduces wall
effects where only one of the final state products of the neutron capture
deposits its full energy [51]. The krypton has a negligible effect on the
neutron detection efficiency due to its low interaction cross section.

The detector is housed in two banks, one on either side of the row of six
proportional counters. This arrangement is a convenient geometry to use given
the fixed size of the scintillator segments and proportional counters and the
limited flexibility in their arrangement. The housing ensures that the
detectors will remain in the same location relative to each other and provides
a reproducible configuration. The detector was covered with flexible, 3-mm
thick, boron-loaded silicone rubber to shield from ambient thermal neutrons
that would contribute to the uncorrelated background rate.

Figure 2: An illustration of the FaNS-1 detector (color online). The regions
shaded blue are scintillator and the grey portions are lucite light guide. The
six 3He proportional tubes are red and shown in the center. The PMTs are
mounted to the cylindrical light guides with silicone potting.

The signal from each PMT was sent through an asymmetric splitter circuit,
shown in Fig. 3. The signal is passed through a passive resistor chain that
produces two signals, a full-amplitude signal and an attenuated signal, which
are a factor of nine different in pulse height. This approach allows for both
good resolution at low energy and a large dynamic range [52]. The large
amplitude signal was delayed by about 150 ns, and the two signals for a given
segment were summed in a linear fan-in/fan-out. The two PMT signals from each
segment were summed together to reduce the number of channels needed for
acquisition. The PMT and 3He proportional counter signals were recorded by an
8-channel, 125 MSample/s, 12-bit PCI-based waveform digitizer. Examples of the
resulting traces are shown in Fig. 4.

Figure 3: A block diagram of the trigger electronics. Each PMT signal from a
scintillator segment is asymmetrically split; one signal is delayed and then
summed with the undelayed attenuated signal and input into one channel of the
waveform digitizer. Approximately 11 % of the initial signal is lost through
the splitter box. The data acquisition system (DAQ) is triggered internally by
the 3He signal.

The six 3He proportional counters were biased through preamplifiers and did
not have shaping amplification. All six individual preamplifier signals were
combined in one fan in/out module, and the output went into one channel of the
waveform digitizer. The digitization of the 3He signals allowed the extraction
of risetime and amplitude information, which in turn permitted some
neutron/alpha discrimination [53]. This proved essential for achieving high
sensitivity in environments where the neutron flux was very low. All of the
proportional counters were tested individually and gain matched to within
$5\,\%$.

The data acquisition system triggered on a proportional counter signal, and
typically $400\,\mu$s traces of all channels were digitized. Digitizing all
channels allows us to set the PMT thresholds in software analysis, rather than
hardware. The trigger window includes time before and after a prompt
coincidence (i.e., an event where the PMT and proportional counter signals
occurs nearly simultaneously); this permits a real-time measurement of the
random background of uncorrelated coincidences, which manifest themselves in
events where the proportional counter trigger comes before a PMT signal.
Figure 4 shows a sample event in which a neutron interacts in two segments of
the scintillator before capturing on a 3He nucleus.

Figure 4: Top: A neutron event candidate (color online). The red and black
traces are digitized PMT signals from two scintillator segments, and the blue
trace is the signal from a 3He proportional counter. Note that the
scintillator signals come before the 3He proportional counter signal from the
capture of a thermalized neutron. Bottom: An expanded view of the two separate
scintillator signals. Note that they are coincident in time, indicating that
they most likely arose from the same fast neutron. The delayed full and prompt
attenuated signals are both apparent.

The energy and timing information for each event are extracted from the
digitized signals in software. Both the full and attenuated signals, shown in
Fig. 4, are analyzed. If the full signal saturates the data acquisition, the
pulse height is taken from the attenuated signal. As a safety margin, the
cross-over point from full to attenuated is taken as 80 % of the saturation
point.

###  3.2 Analysis

The digitized waveforms were stored for offline analysis. PMT signals were
integrated to determine the number of photoelectrons detected. Gamma-ray
source calibrations were performed with 137Cs and 60Co and provided the
conversion from photoelectrons into energy. Source availability in the
detector locations limited the ability to calibrate at multiple energies.
During the long running of the detector underground, the PMT gains drifted. To
account for the drift, the data acquisition system was configured to trigger
on any individual scintillator segment for a short period every hour, thus
yielding ambient gamma spectra for each segment. These spectra included
discernible 40K and 208Tl gamma lines from the surrounding rock. The Compton
edge from the 2.6 MeV 208Tl gamma was tracked over time, providing a
continuous calibration reference.

Because plastic scintillator is a low-Z material, the dominant feature in the
gamma energy spectrum is the Compton edge. To achieve an accurate calibration,
the data were compared to a simulated spectrum generated by MCNP5 [54]. The
top plot of Fig. 5 shows the MCNP5 spectrum for a 137Cs source. To account for
detector resolution, a Gaussian smoothing routine was applied, and the
smoothed MCNP5 spectrum was fit the to energy region surrounding the Compton
edge. This process was repeated while varying the smoothing parameters, and
the fit quality was tracked by the resulting $\chi^{2}$. A measured 137Cs is
shown in Fig. 5 and compared with the smoothed MCNP5 spectrum.

Figure 5: Top: The simulated spectrum generated for a 137Cs source placed
above one FaNS-1 scintillator segment. Bottom: The spectrum of deposited
energy from a 137Cs gamma source. The MCNP5 spectrum after smoothing is
overlaid.

The signal from a proportional counter was sent through a charge integrating
preamplifier, therefore the energy of the recorded waveform is simply its
amplitude. The risetime of the preamplifier signal, defined here as the time
from 10 % of the full height to 50 % of the full height, is also extracted.
This typically ranges from 200 ns to 1 $\mu$s. Figure 6 shows the risetime
versus pulse height spectrum of a 3He counter for a long exposure of ambient
background data taken underground (discussed in Section 6). The risetime is a
simple measure of the geometry of the ionization trail of a neutron capture
event in the 3He counter. A dense cluster of charge from an alpha particle,
which has a large specific energy loss, creates a short risetime, while a low
specific energy loss, from an electron, will leave a long track with a low
total energy. Different particles cluster in different regions of risetime
versus energy space, allowing for the rejection of most non-neutron capture
events. A more detailed discussion of the features of the figure is found in
Ref.[53]

Figure 6: The 10 % to 50 % risetime versus energy from all the FaNS-1 3He
proportional counters for the underground data at KURF. The events in the box
correspond to the full energy collection of the neutron capture reaction
products.

In analysis, one searches for the scintillator signal that is coincident
within the $400\;\mu$s timing window of the 3He proportional counter signal.
Events with scintillator signals that precede the 3He signal are considered as
candidate neutron events. However, events which have a helium signal that
precedes a scintillator signal are uncorrelated coincidences and constitute a
flat background in the timing spectrum. This separation of true recoil-capture
events and uncorrelated coincidences allows for the accurate subtraction of
the background. The probability of multiple gamma or neutron events within a
single timing window was small due to the low event rates encountered in these
measurements. For events with multiple signals, the events were discarded with
negligible effect on the response spectra. In principle, the analysis can be
modified to include multiple signals to enable operation in higher rate
fields.

Table 1: The cut parameters for each type of data collected. The second column is the energy threshold for the combined deposition in the plastic scintillators; the third column is the energy acceptance window for the 3He proportional counter; the fourth column is the range of the coincidence timing window relative to a prompt coincidence; and the final column is the range of the rise time window. N/A indicates that the cut was not applicable due to the electronics used for those measurements. Type of measurement | Scintillator energy |  3He energy | Coincidence | Risetime
---|---|---|---|---
| threshold (MeV) | window (MeV) | window ($\mu$s) | window ($\mu$s)
2.5 MeV and 14 MeV | 0.5 | (0.1, 0.9) | ($-100$, $+100$) | N/A
Surface data | 1 | (0.62, 0.9) | ($-100$, $+300$) | N/A
Underground data | 1.1 | (0.69, 0.8) | ($-200$, $+200$) | 0.1-1.0

While the rates encountered in this work were not high, one can make an
estimate of the maximum background fields in which FaNS-1 can operate. For
thermal neutron background, the limitation is primarily from deadtime in the
data acquisition. Because of the large digitization range required for each
trigger, this effect becomes noticeable at rates of around 100/s. For gamma
backgrounds, a problem arises when the rate of false coincidences begins to
outnumber the real coincidences. The detector has a limitation with the
statistical subtraction of random coincidences when the gamma rate causes the
majority of events to have multiple clusters of PMT signals. With the maximum
capture window of 400 $\mu$s, this limiting rate commences at a few 1000/s. We
note that detectors based on capture-gated spectroscopy can minimize both
effects by decreasing the neutron capture time, which can be accomplished in
the design of the detector geometry and the material selection.

Three types of data were collected in this work: the energy response of 2.5
MeV and 14 MeV neutrons and the energy response of fast neutrons at the
Earth’s surface and underground. Analysis cuts were optimized for the data
from the specific type of run, or they changed as a result of improvements to
the hardware. For example, the improved electronics for 3He detection that
were implemented for the underground data runs reduced the noise and improved
the energy resolution, thus permitting a narrower energy window for the 3He
acceptance. Table 1 gives a summary of some of the important cuts for the
different data runs.

##  4 Energy Response to Monoenergetic Neutrons

To understand the performance of the spectrometer, it was important to
characterize it in known fast neutron fields. This was accomplished through a
series of measurements performed at the Californium Neutron Irradiation
Facility (CNIF) at NIST [55]. The CNIF provided monoenergetic neutron
generators at 2.5 MeV and 14 MeV and several 252Cf neutron sources whose
activities were known at the level of 1 % to 2 %. The sources and
monoenergetic neutrons were used to characterize the energy response. Sealed-
tube neutron generators produced the monoenergetic neutrons via the reactions
deuterium-deuterium (D-D) at 2.5 MeV and deuterium-tritium (D-T) at 14 MeV.

One of the beneficial features of the CNIF is that the sources are placed in
the center of a large volume room, thus minimizing the contribution of
neutrons that scatter from the walls and return to the detector (often
referred to as “room return”) [56]. Within the main room of the CNIF, there is
an inner room whose walls, floor, and ceiling are constructed of thin-wall
aluminum that is 5 cm thick and filled with anhydrous borax. This provides
good thermal neutron shielding and shields the detector from neutrons that
thermalize outside the inner structure. During irradiations, the detector was
placed within the inner room near a neutron source, and the resulting fast
neutron field was a combination of an isotropic distribution from the source
and neutron return from the environment, including albedo from boundaries of
the irradiation room.

To obtain the energy response, the spectrometer was irradiated with both 2.5
MeV and 14 MeV neutrons. The data were analyzed as discussed in Section 3.2 to
yield pulse height spectra. To obtain the true energy conversion, the detector
was calibrated for energy using the Compton edges of the gamma rays from 60Co
and 137Cs. This gives a reliable calibration in electron equivalent energy.
The pulse height data from each of the detector sections was then converted to
energy using the light response of Eqn. 3 and added together to give the total
energy of an event. Figure 7 shows the resulting energy response for the 2.5
MeV and 14 MeV neutrons. The agreement between data and simulation indicates
that the light response function of Eqn. 3 reproduces the observed neutron
spectra well. This is an important result because the same function will be
used for determining the energy response of the data for the surface and
underground measurements. In addition, the spectra exhibit well-resolved peaks
without using any unfolding procedure. This highlights a benefit of capture-
gated spectroscopy; with the FaNS-1 detector, one is able to discern directly
the energy response of monoenergetic sources. The low-energy portions of the
plot are populated by room-return neutrons that have lost significant energy
in the surrounding environment.

Figure 7: Pulse height spectra from two monoenergetic neutron generators. Top:
Response from a 2.5 MeV neutron generator. Bottom: Response from a 14 MeV
neutron generator. The MCNPX simulation is overlaid on each plot.

The FaNS-1 spectrometer was modeled with both MCNP5 and MCNPX [54, 57]. MCNP5
was adequate for gamma-ray interactions, but for higher energy neutrons, the
simulations were performed in MCNPX, which extends to higher energies than
MCNP5 and allows tracking of radiation other than neutrons, gamma rays, and
electrons. MCNPX allows some variation in the physics used for the
calculations. For this work, the MCNPX tabular physics were used below 20 MeV,
and the default model physics were used above.

Calculations of the neutron response of the detector used the PTRAC option of
MCNPX, which allows the proton, deuteron, triton, and alpha particles to be
tracked separately. The proton-recoil light response discussed in Section 2.2
was used to convert deposited energy into light on an event-by-event basis.
The light response for deuterons, tritons, 3He, and 4He was derived from the
proton response. The light output $L$ depends mainly on the magnitude of the
differential energy loss, which depends on the velocity and charge of the
particle. Thus, the following scaling was used for particles of equal charge
$Z$ and equal energy per nucleon, $E/A$, but different mass numbers $A1$ and
$A2$

$L\Big{(}A_{2},{E_{2}\over{A_{2}}}\Big{)}=L\Big{(}A_{1},{E_{1}\over{A_{1}}}\Big{)}\times{A_{2}\over{A_{1}}},\
\text{for}\ Z_{1}=Z_{2}.\\\ $ (4)

Numeric filters were developed to analyze the PTRAC files and report the time
between the first collision and capture, whether there was a 3He capture, and
total light produced in individual scintillators. These filters allowed for
the matching of experimental parameters in the simulation.

When environmental scattering (e.g., from floors or walls) was thought to be
significant, the FaNS-1 model was inserted into various geometries. The
effects of gamma rays were also investigated, and the study included
background gamma rays, gamma rays from sources used in calibration, and gamma
rays resulting from neutron interactions. The latter effects were found to be
quite small.

##  5 Surface Measurement

The surface flux and energy spectrum of fast neutrons have been studied
extensively as a function of altitude and latitude [58, 59]. These
measurements typically use Bonner spheres, which rely on unfolding the energy
spectra from multiple detectors and assumptions of the underlying spectrum.
The method of capture-gated spectroscopy provides a path for improvement by
directly recording the energy deposition from incident neutrons. Nevertheless,
it still must address the complications arising from the various energy loss
mechanisms as well as understanding the light response function of the
scintillator over a large energy range.

A simulation of FaNS-1 in an isotropic neutron field was performed to
determine the detector sensitivity. The input energy distribution came from
the work of Ref. [59], which is currently the JEDEC standard cosmogenic
neutron spectrum [60]. The JEDEC spectrum is based on measurements performed
above a hard ground and therefore includes backscattered neutrons. This causes
a significant enhancement of the spectrum for neutrons below 100 MeV.
Therefore, the simulation used an isotropic field of neutrons, without ground,
to avoid double-counting backscattered neutrons. Simulated events must pass
analysis cuts on the deposited energy in each detector segment and time
between neutron scatter and capture. In this way the simulation was matched to
experimental parameters. Two correction factors were applied: one of $0.770\pm
0.015$ that accounts for the efficiency of the energy cut on 3He captures
(discussed in Section 3.2), and second of $0.85\pm 0.15$ that accounts for the
intrinsic efficiency of our 3He proportional counters. With this result, one
may define an average sensitivity $\overline{S}^{surf}=8.1\pm 1.4$ n/(n/cm2)
for cosmogenic neutrons of 1 MeV and above. This sensitivity can be
interpreted as an efficiency weighted by the input energy spectrum.

To measure the surface neutron flux and energy response, the detector was
placed in a steel cargo container on the grounds at NIST. The position of the
detector within the cargo container was approximately six meters away from a
six meter tall building. There was no additional shielding near the detector.
The location was 39.130∘ N, 77.218∘ W at an elevation of 120 meters above sea
level, and the detector was 1.5 m above ground. Data were collected at the end
of May 2010. During two days of operation, the detector was live for 45 hours.

Approximately $1.31\times 10^{5}$ events were recorded during an exposure time
of $\tau=1.62\times 10^{5}$ s, giving a total trigger rate of 0.8 s-1. After
applying coincidence requirements and cuts from Table 1 on the neutron capture
energy in the helium detectors, $1.10\times 10^{4}$ events remained, giving a
post-cut rate of 0.068 s-1. The number of neutrons remaining after subtracting
random coincidences is $N=5133\pm 151$, yielding a final neutron count rate of
0.032 s-1. The timing distribution is shown in Fig. 8. For this data set, the
acquisition time window was extended to improve the coincidence efficiency.
Because the detector operated at a low trigger rate, this did not increase
deadtime.

Figure 8: Top: The timing spectrum from the data run on the surface at NIST
Gaithersburg. The error bars are statistical only. Bottom: The detected
neutron energy response at NIST Gaithersburg after background subtraction.
Overlaid is an MCNPX simulation of the detector’s response to the reported
spectrum from Ref. [59]. The absolute count rate normalization between the
data and simulation has been allowed to float to account for differences in
the environmental conditions between our data and the data of Ref. [59].

Using the same background subtraction and pulse height method as before, we
obtained the neutron energy deposition seen in Fig. 8. The detected neutron
flux above 1 MeV at the surface $\Phi_{n}^{surf}$ using the simulated
sensitivity $\overline{S}^{surf}$ yields

$\displaystyle\Phi_{n}^{surf}$
$\displaystyle=\frac{N}{\tau\times\overline{S}^{surf}}$ $\displaystyle=(3.9\pm
1.1)\times 10^{-3}\,\textrm{\,cm${}^{-2}$ s${}^{-1}$}.$ (5)

Table 2 gives a summary of the statistical and systematic uncertainties that
comprise the total uncertainty. The largest contribution comes from a 20%
uncertainty in the knowledge of the scintillator energy threshold.

Table 2: Summary of correction factors and relative uncertainties for the measured fast neutron fluxes at the surface and underground at KURF. The uncertainties are associated with systematic effects with the exception of those for the counting and simulation statistics. An entry of zero means that no explicit correction was made. | Surface |  | KURF |
---|---|---|---|---
Source | Correction | Relative Uncertainty | Correction | Relative Uncertainty
Energy threshold | 0 | 20% | 0 | 20%
Counting statistics | 0 | 3% | 0 | 21%
3He counter efficiency | 0.85 | 18% | 0.85 | 18%
3He cut efficiency | 0.77 | 2% | 0.77 | 2%
Simulation statistics | 0 | 5% | 0 | 5%
Exposure time | 0 | 1% | 0 | 1%
Total |  | 28% |  | 35%

A comparison between the simulated and measured neutron response is shown in
Figure 8. For this comparison, we let the absolute normalization of the
spectrum [59] float because the detector operated in different environmental
conditions (solar cycle, barometric pressure, humidity, latitude, etc.) that
can effect the absolute neutron flux. The measurement at the surface verifies
that the response of the detector was modeled correctly in MCNPX and that the
spectrum of Ref. [59] is an appropriate choice for the simulation. This gives
confidence that the simulation can be used to infer the neutron flux in a low
neutron flux environment.

##  6 Underground Measurement

After the completion of the calibration runs and the acquisition of the
surface data, the spectrometer was moved to the Kimballton Underground
Research Facility (KURF), located at Lhoist North America’s Kimballton mine in
Ripplemead, VA. The purpose was to measure the ambient fast neutron response
at KURF [61] and also to characterize the detector in a low-background
environment. The facility is located in an active limestone mine at a depth of
1450 meters of water equivalent (mwe) and provides a good low-radioactivity
counting environment [30]. The FaNS-1 detector resided directly on the
concrete floor of the KURF laboratory for all of the measurements discussed in
this work.

The detector commenced operation at KURF in the summer of 2010. In
approximately 2 months of operation with an exposure time of $\tau=3.737\times
10^{6}$ s, 434 events with energies greater than 1.1 MeV were detected. The
data were analyzed in the same manner as the characterization data and the
surface data, and the cut parameters are given in the Table 1. The timing
spectrum for the underground measurement is shown in Fig. 9. Of the total
number of events in the timing spectrum, 272 have positive timing differences,
and 162 have negative timing differences, and thus 110 events are attributed
to neutron capture.

There is a small systematic effect seen in the timing spectrum that is caused
by prompt alpha-gamma coincidences. It occurs when the decay of a radioactive
contaminant from the aluminum body of the 3He counter produces an alpha and
gamma in coincidence [53]. These appear in the spectrum as valid events: a 3He
capture in coincidence with a scintillator signal. To correct for this, the
timing spectrum was rebinned in microsecond increments that revealed an
anomalously high 2-$\mu$s wide peak near $t=0$ attributable to these
coincidences. That peak had 10 events and was subtracted from the 110 neutron
capture events. By making this correction, a small number of real neutron
captures may also have been rejected. We estimate this fraction to be
approximately 2 % and do not make any correction for it. Note that this effect
is also in the surface data, but its contribution is negligible because of the
higher neutron rate.

After the uncorrelated background subtraction and the removal of prompt
events, a total of $N=100\pm 21$ events were observed. The energy response and
timing spectra of these data are shown in Fig. 9. To quantify the detector
response to the underground neutron flux, a simulation was performed using a
uniform distribution of ($\alpha$,n) neutrons incident on FaNS-1. The dominant
source of 1-10 MeV neutrons underground is ($\alpha$,n) interactions in the
surrounding material, but these interactions are complicated and depend on the
isotopic abundance and distribution in the material. Without performing
detailed analysis of the materials composition in the surrounding environment,
a precise spectrum cannot be predicted. Therefore, to approximate the (alpha,
n) spectrum, we used a standard AmBe spectrum [62] for the simulation. As
check on the sensitivity to choice of the input spectrum, we also simulated a
flat spectrum from 1-8 MeV (the range of neutrons detected at KURF) and found
only a small effect on the value of $\overline{S}$.

After applying the same method as in Section 5, we obtain an average
sensitivity to these neutrons of $\overline{S}^{KURF}=15.1\pm 2.9$ n/(n/cm)2
and determine the observed flux above 1.1 MeV at KURF to be

$\displaystyle\Phi_{n}^{KURF}$
$\displaystyle=\frac{N}{\tau\times\overline{S}^{KURF}}$ $\displaystyle=(1.8\pm
0.6)\times 10^{-6}\,\textrm{\,cm${}^{-2}$ s${}^{-1}.$}$ (6)

Table 2 gives a summary of the statistical and systematic uncertainties that
comprise the total uncertainty. This flux is comparable to measurements and
simulations of the ($\alpha$, n) neutron flux in other underground labs [63,
64]. No attempt was made to measure the intrinsic background of the detector,
but based on its material composition and an estimate of the intrinsic
radioactivity, the contribution is expected to be very small.

Figure 9: Top: The timing spectrum from the 2 month exposure at KURF. Bottom:
The detected neutron energy response at KURF after background subtraction. The
error bars in both plots are statistical only.

One may obtain an estimate of the total muon-induced neutron flux using the
method of Ref. [16]. Their formula for the neutron flux as a function of depth
is

$\phi_{n}=P_{0}(P_{1}/h_{0})e^{-h_{0}/P_{1}},$ (7)

where $h_{0}$ is the vertical depth in kilometer of water equivalent
(km.w.e.). The fit parameters are $P_{0}=(4\pm 1.1)\times 10^{-7}$ cm-2s-1 and
$P_{1}=(0.86\pm 0.05)$ km.w.e. At the depth of KURF, one obtains an expected
total muon-induced neutron flux of $3.4\times 10^{-8}$ cm-2s-1. The authors
predict that 22% of these neutrons are above 10 MeV, which yields a flux above
10 MeV of $7.5\times 10^{-9}$ cm-2s-1.

No events were observed with energies greater than 10 MeV during the period of
operation of FaNS-1 at KURF. Other work has shown that muon-induced neutrons
above 10 MeV should have a similar shape to cosmogenic neutrons at the surface
although with a substantially reduced flux. One can estimate an average
sensitivity to these neutrons from a simulation of the cosmogenic neutron
spectrum. For FaNS-1 we determine an average sensitivity for muon-induced
neutrons with energies above 10 MeV to be $\overline{S}^{KURF}=3.5\pm 0.7$
n/(n/cm2). Using that result and a value for $N^{KURF}=3$ at the Poisson 95%
confidence level, Eqn. 6 yields an upper limit of the neutron flux above 10
MeV of

$\displaystyle\Phi_{n}^{KURF}(>10\,{\rm MeV})\leq 2.3\times
10^{-7}~{}\textrm{\,cm${}^{-2}$ s${}^{-1}$}.$ (8)

This upper limit is considerably higher than the expected rate calculated from
Eqn. 7. If one extrapolates from our simulated sensitivity, one would expect
FaNS-1 to observe only about one muon-induced neutron per year of operation at
KURF. Such a measurement is not practical using FaNS-1 at KURF, and therefore
one must consider moving to a shallower depth or constructing a significantly
larger detector to study the cosmically-induced part of the energy spectrum.

##  7 Summary and Outlook

We constructed a fast neutron spectrometer consisting of plastic scintillator
and 3He proportional counters. The detection principle uses segmentation to
improve energy resolution and capture-gated spectroscopy to reduce
backgrounds. The spectrometer is capable of observing very low neutron fluxes
in the presence of ambient gamma background, and this technique does not
require scintillator pulse-shape discrimination. Using monoenergetic neutron
generators and calibrated neutron sources, we characterized the neutron
response of the detector. The results compare very favorably with simulations
performed with MCNP. Additional work is required to extract the incident
kinetic energy of the incident neutron from the measured detector response.

A measurement of the surface neutron flux at NIST in Gaithersburg, MD was made
of $(3.9\pm 1.1)\times 10^{-3}$ cm-2 s-1 for energy above 1 MeV. The data
agree reasonably well with a simulated spectrum based on another measurement.
The spectrometer was subsequently installed at KURF and measured the ambient
fast neutron flux above 1.1 MeV. The measurement yielded a result of $(1.8\pm
0.6)\times 10^{-6}$ cm-2 s-1.

It is important to understand the flux and energy spectrum of muon-induced
fast neutrons. The depth of KURF and the efficiency of FaNS-1 detector make it
impractical to improve on these measurements. Improvement would come from
performing the measurements at a shallower depth, and increasing both the
efficiency and energy range of the spectrometer. Toward that end, this
collaboration has developed a second detector, FaNS-2, that has an energy
range of 1 MeV to 1 GeV. The detector uses the same principles of capture-
gated spectroscopy and segmentation as FaNS-1 to achieve good background
rejection and energy resolution. It is a larger volume detector composed of 16
bars of plastic scintillator with 21 3He proportional counters interspersed
among them. The geometry was optimized using MCNPX simulations. Its
sensitivity to cosmogenic neutrons is approximately 90 n/(n/cm2) [29] versus
8.1 n/(n/cm2) for FaNS-1.

This increased sensitivity along with deploying the detector at shallower
depth should permit the acquisition of an energy spectrum with significantly
higher statistics. Two of the main challenges will be understanding the
sources of background at high energy where the event rate is low and
determining the light response function of the scintillator over the large
energy range.

##  8 Acknowledgments

We thank Vladimir Gavrin and Johnrid Abdurashitov of the Institute for Nuclear
Research - Russian Academy of Sciences for useful discussions. We acknowledge
the NIST Center for Neutron Research for the loan of the 3He proportional
counters used in this work. We also acknowledge KURF and Lhoist North America,
especially Mark Luxbacher, for providing us access to the underground site and
logistical support. The research has been partially supported by NSF grant
0809696. T. Langford acknowledges support under the National Institute for
Standards and Technology American Recovery and Reinvestment Act Measurement
Science and Engineering Fellowship Program Award 70NANB10H026 through the
University of Maryland.

## References

  * [1] J. Formaggio, C. Martoff, Backgrounds to Sensitive Experiments Underground, Annu. Rev. Nucl. Part. Sci. 54 (2004) 361.
  * [2] R. J. Gaitskell, Direct Detection of Dark Matter, Annu. Rev. Nucl. Part. Sci. 54 (2004) 315.
  * [3] G. Angloher, M. Bauer, I. Bavykina, A. Bento, C. Bucci, C. Ciemniak, G. Deuter, F. von Feilitzsch, D. Hauff, P. Huff, C. Isaila, J. Jochum, M. Kiefer, M. Kimmerle, J.-C. Lanfranchi, F. Petricca, S. Pfister, W. Potzel, F. Pröst, F. Reindl, S. Roth, K. Rottler, C. Sailer, K. Schäffner, J. Schmaler, S. Scholl, W. Seidel, M. Sivers, L. Stodolsky, C. Strandhagen, R. Strauß, A. Tanzke, I. Usherov, S. Wawoczny, M. Willers, A. Zöller, Results from 730 kg days of the CRESST-II Dark Matter search, The European Physical Journal C 72 (4).
  * [4] E. Aprile, M. Alfonsi, K. Arisaka, F. Arneodo, C. Balan, L. Baudis, B. Bauermeister, A. Behrens, P. Beltrame, K. Bokeloh, E. Brown, G. Bruno, R. Budnik, J. M. R. Cardoso, W.-T. Chen, B. Choi, D. Cline, A. P. Colijn, H. Contreras, J. P. Cussonneau, M. P. Decowski, E. Duchovni, S. Fattori, A. D. Ferella, W. Fulgione, F. Gao, M. Garbini, C. Ghag, K.-L. Giboni, L. W. Goetzke, C. Grignon, E. Gross, W. Hampel, F. Kaether, A. Kish, J. Lamblin, H. Landsman, R. F. Lang, M. Le Calloch, C. Levy, K. E. Lim, Q. Lin, S. Lindemann, M. Lindner, J. A. M. Lopes, K. Lung, T. Marrodán Undagoitia, F. V. Massoli, A. J. Melgarejo Fernandez, Y. Meng, A. Molinario, E. Nativ, K. Ni, U. Oberlack, S. E. A. Orrigo, E. Pantic, R. Persiani, G. Plante, N. Priel, A. Rizzo, S. Rosendahl, J. M. F. dos Santos, G. Sartorelli, J. Schreiner, M. Schumann, L. Scotto Lavina, P. R. Scovell, M. Selvi, P. Shagin, H. Simgen, A. Teymourian, D. Thers, O. Vitells, H. Wang, M. Weber, C. Weinheimer, Dark Matter Results from 225 Live Days of XENON100 Data, Phys. Rev. Lett. 109 (2012) 181301.
  * [5] R. Agnese, J. Anderson, A. M. Asai, D. Balakishiyeva, R. Basu Thakur, A. Bauer, D. J. Billard, A. Borgland, A. Bowles, M. D. Brandt, L. Brink, P. R. Bunker, B. Cabrera, O. Caldwell, D. G. Cerdeno, D. H. Chagani, J. Cooley, B. Cornell, H. Crewdson, C. P. Cushman, M. Daal, P. C. F. Di Stefano, T. Doughty, L. Esteban, S. Fallows, E. Figueroa-Feliciano, L. Godfrey, G. R. Golwala, S. J. Hall, R. Harris, H. A. Hertel, S. T. Hofer, D. Holmgren, L. Hsu, E. Huber, M. A. Jastram, O. Kamaev, B. Kara, H. Kelsey, M. A. Kennedy, M. Kiveni, K. Koch, B. Loer, E. Lopez Asamar, R. Mahapatra, V. Mandic, C. Martinez, A. McCarthy, K. N. Mirabolfathi, A. Moffatt, R. C. Moore, D. P. Nadeau, H. Nelson, R. K. Page, R. Partridge, M. Pepin, A. Phipps, K. Prasad, M. Pyle, H. Qiu, W. Rau, P. Redl, A. Reisetter, Y. Ricci, T. Saab, B. Sadoulet, J. Sander, K. Schneck, W. Schnee, R. S. Scorza, B. Serfass, B. Shank, D. Speller, N. Villano, A. B. Welliver, H. Wright, D. S. Yellin, J. Yen, J. A. Young, B. J. Zhang, Search for Low-Mass Weakly Interacting Massive Particles Using Voltage-Assisted Calorimetric Ionization Detection in the SuperCDMS Experiment, Phys. Rev. Lett. 112 (2014) 041302.
  * [6] S. Akerib, D. M. Araújo, H. X. Bai, J. Bailey, A. J. Balajthy, S. Bedikian, E. Bernard, A. Bernstein, A. Bolozdynya, A. Bradley, D. Byram, B. Cahn, S. C. Carmona-Benitez, M. C. Chan, J. Chapman, J. A. Chiller, A. C. Chiller, K. Clark, T. Coffey, A. Currie, A. Curioni, S. Dazeley, L. de Viveiros, A. Dobi, J. Dobson, M. Dragowsky, E. E. Druszkiewicz, B. Edwards, H. Faham, C. S. Fiorucci, C. Flores, J. Gaitskell, R. M. Gehman, V. C. Ghag, R. Gibson, K. M. Gilchriese, C. Hall, M. Hanhardt, A. Hertel, S. M. Horn, Q. Huang, D. M. Ihm, G. Jacobsen, R. L. Kastens, K. Kazkaz, R. Knoche, S. Kyre, R. Lander, A. Larsen, N. C. Lee, S. Leonard, D. T. Lesko, K. A. Lindote, I. Lopes, M. A. Lyashenko, C. Malling, D. R. Mannino, N. McKinsey, D. D.-M. Mei, J. Mock, M. Moongweluwan, J. Morad, M. Morii, A. Murphy, C. Nehrkorn, H. Nelson, F. Neves, A. Nikkel, J. A. Ott, R. M. Pangilinan, D. Parker, P. K. Pease, E. K. Pech, P. Phelps, L. Reichhart, T. Shutt, C. Silva, W. Skulski, J. Sofka, C. N. Solovov, V. P. Sorensen, T. Stiegler, K. O’Sullivan, J. Sumner, T. R. Svoboda, M. Sweany, M. Szydagis, D. Taylor, B. Tennyson, R. Tiedt, D. M. Tripathi, S. Uvarov, R. Verbus, J. N. Walsh, R. Webb, T. White, J. D. White, S. Witherell, M. M. Wlasenko, F. Wolfs, M. Woods, C. Zhang, First Results from the LUX Dark Matter Experiment at the Sanford Underground Research Facility, Phys. Rev. Lett. 112 (2014) 091303.
  * [7] J. N. Abdurashitov, V. N. Gavrin, V. V. Gorbachev, P. P. Gurkina, T. V. Ibragimova, A. V. Kalikhov, N. G. Khairnasov, T. V. Knodel, I. N. Mirmov, A. A. Shikhin, E. P. Veretenkin, V. E. Yants, G. T. Zatsepin, T. J. Bowles, S. R. Elliott, W. A. Teasdale, J. S. Nico, B. T. Cleveland, J. F. Wilkerson, Measurement of the solar neutrino capture rate with gallium metal. III. Results for the 2002-2007 data-taking period, Phys. Rev. C 80 (2009) 015807.
  * [8] D. McKinsey, K. Coakley, Neutrino detection with CLEAN, Astropart. Phys. 22 (2005) 355.
  * [9] B. Aharmim, et al., Determination of the $\nu_{e}$ and total 8B solar neutrino fluxes using the Sudbury Neutrino Observatory Phase I data set, Phys. Rev. C 75 (2007) 045502.
  * [10] S. R. Elliott, P. Vogel, Double beta decay, Annu. Rev. Nucl. Part. Sci. 52 (2002) 115.
  * [11] C. Arnaboldi, F. A. III, J. Beeman, M. Barucci, M. Balata, C. Brofferio, C. Bucci, S. Cebrian, R. Creswick, S. Capelli, L. Carbone, O. Cremonesi, A. de Ward, E. Fiorini, H. Farach, G. Frossati, A. Giuliani, D. Giugni, P. Gorla, E. Haller, I. Irastorza, R. McDonald, A. Morales, E. Norman, P. Negri, A. Nucciotti, M. Pedretti, C. Pobes, V. Palmieri, M. Pavan, G. Pessina, S. Pirro, E. Previtali, C. Rosenfeld, A. Smith, M. Sisti, G. Ventura, M. Vanzini, L. Zanotti, CUORE: a cryogenic underground observatory for rare events, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 518 (3) (2004) 775 – 798.
  * [12] C. Aalseth, M. Amman, F. A. III, H. Back, A. Barabash, P. Barbeau, M. Bergevin, F. Bertrand, M. Boswell, V. Brudanin, W. Bugg, T. Burritt, M. Busch, G. Capps, Y.-D. Chan, J. Collar, R. Cooper, R. Creswick, J. Detwiler, J. Diaz, P. Doe, Y. Efremenko, V. Egorov, H. Ejiri, S. Elliott, J. Ely, J. Esterline, H. Farach, J. Fast, N. Fields, P. Finnerty, B. Fujikawa, E. Fuller, V. Gehman, G. Giovanetti, V. Guiseppe, K. Gusey, A. Hallin, G. Harper, R. Hazama, R. Henning, A. Hime, E. Hoppe, T. Hossbach, M. Howe, R. Johnson, K. Keeter, M. Keillor, C. Keller, J. Kephart, M. Kidd, A. Knecht, O. Kochetov, S. Konovalov, R. Kouzes, L. Leviner, J. Loach, P. Luke, S. MacMullin, M. Marino, R. Martin, D.-M. Mei, H. Miley, M. Miller, L. Mizouni, A. Meyers, M. Nomachi, J. Orrell, D. Peterson, D. P. II, A. W. P. Poon, G. Prior, J. Qian, D. Radford, K. Rielage, R. Robertson, L. Rodriguez, K. Rykaczewski, H. Salazar, A. Schubert, T. Shima, M. Shirchenko, D. Steele, J. Strain, G. Swift, K. Thomas, V. Timkin, W. Tornow, T. V. Wechel, I. Vanyushin, R. Varner, K. Vetter, J. Wilkerson, B. Wolfe, W. Xiang, E. Yakushev, H. Yaver, A. Young, C.-H. Yu, V. Yumatov, C. Zhang, S. Zimmerman, Astroparticle physics with a customized low-background broad energy germanium detector, Nucl. Instrum. Meth. A 652 (2011) 692–695.
  * [13] N. Ackerman, B. Aharmim, M. Auger, D. J. Auty, P. S. Barbeau, K. Barry, L. Bartoszek, E. Beauchamp, V. Belov, C. Benitez-Medina, M. Breidenbach, A. Burenkov, B. Cleveland, R. Conley, E. Conti, J. Cook, S. Cook, A. Coppens, I. Counts, W. Craddock, T. Daniels, M. V. Danilov, C. G. Davis, J. Davis, R. deVoe, Z. Djurcic, A. Dobi, A. G. Dolgolenko, M. J. Dolinski, K. Donato, M. Dunford, W. Fairbank, J. Farine, P. Fierlinger, D. Franco, D. Freytag, G. Giroux, R. Gornea, K. Graham, G. Gratta, M. P. Green, C. Hägemann, C. Hall, K. Hall, G. Haller, C. Hargrove, R. Herbst, S. Herrin, J. Hodgson, M. Hughes, A. Johnson, A. Karelin, L. J. Kaufman, T. Koffas, A. Kuchenkov, A. Kumar, K. S. Kumar, D. S. Leonard, F. Leonard, F. LePort, D. Mackay, R. MacLellan, M. Marino, Y. Martin, B. Mong, M. Montero Díez, P. Morgan, A. R. Müller, R. Neilson, R. Nelson, A. Odian, K. O’Sullivan, C. Ouellet, A. Piepke, A. Pocar, C. Y. Prescott, K. Pushkin, A. Rivas, E. Rollin, P. C. Rowson, J. J. Russell, A. Sabourov, D. Sinclair, K. Skarpaas, S. Slutsky, V. Stekhanov, V. Strickland, M. Swift, D. Tosi, K. Twelker, P. Vogel, J.-L. Vuilleumier, J.-M. Vuilleumier, A. Waite, S. Waldman, T. Walton, K. Wamba, M. Weber, U. Wichoski, J. Wodin, J. D. Wright, L. Yang, Y.-R. Yen, O. Y. Zeldovich, Observation of Two-Neutrino Double-Beta Decay in Xe136 with the EXO-200 Detector, Phys. Rev. Lett. 107 (2011) 212501.
  * [14] M. Agostini, M. Allardt, E. Andreotti, A. M. Bakalyarov, M. Balata, I. Barabanov, M. B. Heider, N. Barros, L. Baudis, C. Bauer, N. Becerici-Schmidt, E. Bellotti, S. Belogurov, S. T. Belyaev, G. Benato, A. Bettini, L. Bezrukov, T. Bode, V. Brudanin, R. Brugnera, D. Budjáš, A. Caldwell, C. Cattadori, A. Chernogorov, F. Cossavella, E. V. Demidova, A. Denisov, A. Domula, V. Egorov, R. Falkenstein, A. D. Ferella, K. Freund, F. Froborg, N. Frodyma, A. Gangapshev, A. Garfagnini, S. Gazzana, P. Grabmayr, V. Gurentsov, K. Gusev, K. K. Guthikonda, W. Hampel, A. Hegai, M. Heisel, S. Hemmer, G. Heusser, W. Hofmann, M. Hult, L. V. Inzhechik, L. Ioannucci, J. J. Csáthy, J. Jochum, M. Junker, S. Kianovsky, I. V. Kirpichnikov, A. Kirsch, A. Klimenko, K. T. Knöpfle, O. Kochetov, V. N. Kornoukhov, V. Kuzminov, M. Laubenstein, A. Lazzaro, V. I. Lebedev, B. Lehnert, H. Y. Liao, M. Lindner, I. Lippi, X. Liu, A. Lubashevskiy, B. Lubsandorzhiev, G. Lutter, A. A. Machado, B. Majorovits, W. Maneschg, I. Nemchenok, S. Nisi, C. O’Shaughnessy, L. Pandola, K. Pelczar, L. Peraro, A. Pullia, S. Riboldi, F. Ritter, C. Sada, M. Salathe, C. Schmitt, S. Schönert, J. Schreiner, O. Schulz, B. Schwingenheuer, E. Shevchik, M. Shirchenko, H. Simgen, A. Smolnikov, L. Stanco, H. Strecker, M. Tarka, C. A. Ur, A. A. Vasenko, O. Volynets, K. von Sturm, M. Walter, A. Wegmann, M. Wojcik, E. Yanovich, P. Zavarise, I. Zhitnikov, S. V. Zhukov, D. Zinatulina, K. Zuber, G. Zuzel, Measurement of the half-life of the two-neutrino double beta decay of 76 Ge with the GERDA experiment, Journal of Physics G: Nuclear and Particle Physics 40 (3) (2013) 035110.
  * [15] J. Delorme, M. Ericson, T. Ericson, P. Vogel, Pion and neutron production by cosmic-ray muons underground, Phys. Rev. C 52 (1995) 2222–2230.
  * [16] D.-M. Mei, A. Hime, Muon-induced background study for underground laboratories, Phys. Rev. D 73 (2006) 053004.
  * [17] H. Wulandari, J. Jochum, W. Rau, F. von Feilitzsch, Neutron Background Studies for the CRESST Dark Matter Experiment, ArXiv High Energy Physics - Experiment e-printsarXiv:arXiv:hep-ex/0401032.
  * [18] M. G. Marino, J. A. Detwiler, R. Henning, R. A. Johnson, A. G. Schubert, J. F. Wilkerson, Validation of spallation neutron production and propagation within Geant4, Nucl. Instrum. Meth. A 582 (2007) 611–620.
  * [19] V. E. Guiseppe, S. R. Elliott, N. E. Fields, D. Hixon, Fast-Neutron Activation of Long-Lived Nuclides in Natural Pb, ArXiv Nuclear Physics - Experiment e-printsarXiv:arXiv:1209.4412v1.
  * [20] J. N. Abdurashitov, V. N. Gavrin, A. V. Kalikhov, V. L. Matushko, A. A. Shikhin, V. E. Yants, O. S. Zaborskaia, High-sensitive spectrometer of fast neutrons and the results of fast neutron background flux measurements at the Gallium-Germanium Solar Neutrino Experiment, Nucl. Instrum. Meth. A 476 (2002) 322 – 326.
  * [21] A. Bonardi, M. Aglietta, G. Bruno, W. Fulgione, A. A. B. Machado, Direct measurement of the atmospheric neutron flux in the energy range 10 - 500 MeV, Astropart. Phys. 34 (2010) 225 – 229.
  * [22] R. Cooper, L. Garrison, H.-O. Meyer, T. Mikev, L. Rebenitsch, et al., SciBath: A Novel Tracking Detector for Measuring Neutral Particles Underground , Proceedings, Meeting of the Division of the American Physical Society, DPF 2011, Providence, USA, August 9-13, 2011arXiv:1110.4432.
  * [23] G. Bellini, J. Benziger, D. Bick, S. Bonetti, M. B. Avanzini, B. Caccianiga, L. Cadonati, F. Calaprice, C. Carraro, A. Chavarria, A. Chepurnov, D. D’Angelo, S. Davini, A. Derbin, A. Etenko, F. von Feilitzsch, K. Fomenko, D. Franco, C. Galbiati, S. Gazzana, C. Ghiano, M. Giammarchi, M. Goeger-Neff, A. Goretti, E. Guardincerri, S. Hardy, A. Ianni, A. Ianni, M. Joyce, V. Kobychev, Y. Koshio, D. Korablev, G. Korga, D. Kryn, M. Laubenstein, C. Lendvai, T. Lewke, E. Litvinovich, B. Loer, F. Lombardi, P. Lombardi, L. Ludhova, I. Machulin, S. Manecki, W. Maneschg, G. Manuzio, Q. Meindl, E. Meroni, L. Miramonti, M. Misiaszek, D. Montanari, V. Muratova, L. Oberauer, M. Obolensky, F. Ortica, M. Pallavicini, L. Papp, L. Perasso, S. Perasso, A. Pocar, R. S. Raghavan, G. Ranucci, A. Razeto, A. Re, A. Romani, D. Rountree, A. Sabelnikov, R. Saldanha, C. Salvo, S. Schönert, H. Simgen, M. Skorokhvatov, O. Smirnov, A. Sotnikov, S. Sukhotin, Y. Suvorov, R. Tartaglia, G. Testera, D. Vignaud, R. B. Vogelaar, J. Winter, M. Wojcik, A. Wright, M. Wurm, J. Xu, O. Zaimidoroga, S. Zavatarelli, G. Zuzel, Muon and cosmogenic neutron detection in Borexino, Journal of Instrumentation 6 (05) (2011) P05005.
  * [24] C. Zhang, D.-M. Mei, P. Davis, B. Woltman, F. Gray, Measuring fast neutrons with large liquid scintillation detector for ultra-low background experiments, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 729 (2013) 138 – 146.
  * [25] D. Jordan, J. Tain, A. Algora, J. Agramunt, C. Domingo-Pardo, M. Gomez-Hornillos, R. Caballero-Folch, G. Cortés, D. Cano-Ott, E. Mendoza, I. Bandac, A. Bettini, L. Fraile, C. Domingo, Measurement of the neutron background at the Canfranc Underground Laboratory {LSC}, Astroparticle Physics 42 (0) (2013) 1 – 6.
  * [26] H. Park, J. Kim, Y. Hwang, K.-O. Choi, Neutron spectrum at the underground laboratory for the ultra low background experiment, Appl. Radiat. Isot. 81 (2013) 302 – 306.
  * [27] A. N. Villano, R. Bunker, P. Cushman, Measurements of cosmic-ray correlated events at the Soudan underground laboratory, AIP Conference Proceedings 1549 (1) (2013) 177–180.
  * [28] T. Langford, E. Beise, H. Breuer, D. Erwin, C. Bass, C. Heimbach, J. Nico, A fast neutron spectrometer for underground science, Bull. Am. Phys. Soc. 55.
  * [29] T. J. Langford, Measurement of the surface and underground fast neutron spectra with the UMd/NIST fast neutron spectrometers, Ph.D. thesis, University of Maryland (2013).
  * [30] P. Finnerty, S. MacMullin, H. O. Back, R. Henning, A. Long, K. T. Macon, J. Strain, R. M. Lindstrom, R. B. Vogelaar, Low-Background gamma counting at the Kimballton Underground Research Facility, Nucl. Instrum. Meth. A 642 (2010) 65 – 69.
  * [31] D. Drake, W. Feldman, C. Hurlbut, New electronically black neutron detectors, Nucl. Instrum. Meth. A 247 (1986) 576 – 582.
  * [32] J. Czirr, G. L. Jensen, A neutron coincidence spectrometer, Nucl. Instrum. Meth. A 284 (1989) 365 – 369.
  * [33] J. N. Abdurashitov, V. N. Gavrin, G. D. Efimov, A. V. Kalikhov, A. A. Shikhin, V. E. Yants, Instrum. and Exp. Tech. 40 (1997) 741.
  * [34] J. Czirr, D. B. Merrill, D. Buehler, T. K. McKnight, J. L. Carroll, T. Abbott, E. Wilcox, Capture-gated neutron spectrometry, Nucl. Instrum. Meth. A 476 (2002) 309 – 312.
  * [35] I. Pawełczak, J. Tõke, E. Henry, M. Quinlan, H. Singh, W. Schröder, NSTAR - A capture gated plastic neutron detector, Nucl. Instrum. Meth. A 629 (2011) 230–238.
  * [36] T. Aoyama, K. Honda, C. Mori, K. Kudo, N. Takeda, Energy response of a full-energy-absorption neutron spectrometer using boron-loaded liquid scintillator BC-523, Nucl. Instrum. Meth. A 333 (1993) 492–501.
  * [37] S. Normand, B. Mouanda, S. Haan, M. Louvel, Study of a new boron loaded plastic scintillator (revised), IEEE Trans. Nucl. Sci. 49 (2002) 1603 – 1608.
  * [38] B. M. Fisher, J. N. Abdurashitov, K. J. Coakley, V. N. Gavrin, D. M. Gilliam, J. S. Nico, A. A. Shikhin, A. K. Thompson, D. F. Vecchia, V. E. Yants, Nucl. Instrum. Meth. A 646 (2011) 126.
  * [39] C. D. Bass, E. J. Beise, H. Breuer, C. R. Heimbach, T. J. Langford, J. S. Nico, Appl. Radiat. Isot. 77 (2013) 130.
  * [40] J. N. Abdurashitov, V. N. Gavrin, A. V. Kalikhov, V. L. Matushko, A. A. Shikhin, V. E. Yants, O. S. Zaborskaia, J. M. Adams, J. S. Nico, A. K. Thompson, A High Resolution, Low Background Fast Neutron Spectrometer, Nucl. Instrum. Meth. A 476 (2002) 318.
  * [41] N. S. Bowden, P. Marleau, J. T. Steele, S. Mrowka, G. Aigeldinger, W. Mengesha, Improved fast neutron spectroscopy via detector segmentation, Nucl. Instrum. Meth. A 609 (2009) 32–37.
  * [42] N. Nakao, T. Nakamura, M. Baba, Y. Uwamino, N. Nakanishi, H. Nakashima, S. ichi Tanaka, Measurements of response function of organic liquid scintillator for neutron energy range up to 135 MeV, Nucl. Instrum. Meth. A 362 (1995) 454 – 465.
  * [43] V. V. Verbinski, W. R. Burrus, T. A. Love, W. Zobel, N. W. Hill, R. Textor, Calibration of an organic scintillator for neutron spectrometry, Nucl. Instrum. Meth. 65 (1968) 8 – 25.
  * [44] J. B. Birks, Scintillations from Organic Crystals: Specific Fluorescence and Relative Response to Different Radiations, Proceedings of the Physical Society. Section A 64 (1951) 874.
  * [45] R. Craun, D. Smith, Analysis of response data for several organic scintillators, Nucl. Instrum. Meth. 80 (1970) 239 – 244.
  * [46] G. O’Rielly, N. Kolb, R. Pywell, The response of plastic scintillator to protons and deuterons, Nucl. Instrum. Meth. A 368 (1996) 745 – 749.
  * [47] R. Madey, F. M. Waterman, A. R. Baldwin, J. N. Knudson, J. D. Carlson, J. Rapaport, The response of NE-228A, NE-228, NE-224, and NE-102 scintillators to protons from 2.43 to 19.55 MeV, Nucl. Instrum. Meth. 151 (1978) 445 – 450.
  * [48] M. J. Berger, J. S. Coursey, M. A. Zucker, J. Chang, ESTAR, PSTAR, and ASTAR: Computer Programs for Calculating Stopping-Power and Range Tables for Electrons, Protons, and Helium Ions (version 1.2.3), Available: http://physics.nist.gov/Star National Institute of Standards and Technology, Gaithersburg, MD. (2005).
  * [49] L. Reichhart, D. Akimov, H. M. Araújo, E. J. Barnes, V. A. Belov, A. A. Burenkov, V. Chepel, A. Currie, L. DeViveiros, B. Edwards, V. Francis, C. Ghag, A. Hollingsworth, M. Horn, G. E. Kalmus, A. S. Kobyakin, A. G. Kovalenko, V. N. Lebedenko, A. Lindote, M. I. Lopes, R. Lüscher, P. Majewski, A. J. Murphy, F. Neves, S. M. Paling, J. Pinto da Cunha, R. Preece, J. J. Quenby, P. R. Scovell, C. Silva, V. N. Solovov, N. J. T. Smith, P. F. Smith, V. N. Stekhanov, T. J. Sumner, C. Thorne, R. J. Walker, Quenching factor for low-energy nuclear recoils in a plastic scintillator, Phys. Rev. C 85 (2012) 065801.
  * [50] B. A. Raue, T. A. Greco, M. G. Khayat, F. Adimi, B. v. d. Brandt, H. Breuer, T. Chang, N. S. Chant, H. Chen, T. A. Dooling, A. P. Dvoredsky, B. S. Flanders, T. Gu, J. P. Haas, P. Hautle, J. Huffman, J. J. Kelly, A. Klein, K. Koch, J. A. Konter, A. I. Kovalev, G. S. Kyle, J. J. Lawrie, Z. Lin, S. Mango, P. Markowitz, R. Meier, T. Payerle, S. Ritt, P. G. Roos, M. Wang, Analyzing powers for ${}^{1}\rightarrow{}({\mathrm{\pi{}}}^{+},{\mathrm{\pi{}}}^{+}p){\mathit{T}}_{\mathrm{\pi{}}}=165$ and 240 MeV , Phys. Rev. C 53 (1996) 1005–1008.
  * [51] G. F. Knoll, Radiation Detection and Measurement, 2nd Edition, Wiley, New York, 1989.
  * [52] H. Breuer, Splitter-delay-attenuator-summer module for the fast neutron detector system, internal report, University of Maryland (2013).
  * [53] T. J. Langford, C. D. Bass, E. J. Beise, H. Breuer, D. K. Erwin, C. R. Heimbach, J. S. Nico, Nucl. Instrum. Meth. A 717 (2013) 51.
  * [54] X-5 Monte Carlo Team, MCNP - A General Monte Carlo N-Particle Transport Code, Version 5, LANL Report LA-UR-03-1987, Los Alamos (2008).
  * [55] J. A. Grundl, V. Spiegel, C. M. Eisenhauer, H. T. Heaton II, D. M. Gilliam, J. Bigelow, A Californium-252 Fission Spectrum Irradition Facility for Neutron Reaction Rate Measurements, Nucl. Technol. 32 (1977) 315–319.
  * [56] C. M. Eisenhauer, Review of scattering corrections for calibration of neutron instruments, Rad. Prot. Dos. 28 (1989) 253.
  * [57] D. B. Pelowitz, et al., MCNPX 2.7.0 Extensions, LANL Report LA-UR-11-02295, Los Alamos (2011).
  * [58] P. Goldhagen, M. Reginatto, T. Kniss, J. W. Wilson, R. C. Singleterry, I. W. Jones, W. V. Steveninck, Measurement of the energy spectrum of cosmic-ray induced neutrons aboard an ER-2 high-altitude airplane, Nucl. Instrum. Meth. A 476 (2002) 42 – 51.
  * [59] M. S. Gordon, P. Goldhagen, K. P. Rodbell, T. H. Zabel, H. H. K. Tang, J. M. Clem, P. Bailey, Measurement of the flux and energy spectrum of cosmic-ray induced neutrons on the ground, IEEE Trans. Nucl. Sci. 51 (2004) 3427 – 3434\.
  * [60] Measurement and Reporting of Alpha Particle and Terrestrial Cosmic Ray-Induced Soft Errors in Semiconductor Devices, Tech. Rep. JESD89A, JEDEC Solid State Technology Association (2006).
  * [61] A. Best, J. Görres, A. Long, K. Smith, E. Stech, M. Wiescher, Neutron background characterization of deep underground laboratories, Bull. Am. Phys. Soc. 57 (2012) 1.
  * [62] International Atomic Energy Agency, Compendium of Neutron Spectra and Detector Responses for Radiation Protection Purposes, IAEA Technical Report Series No. 403, Vienna 2001.
  * [63] V. Chazal, R. Brissot, J. F. Cavaignac, B. Chambon, M. de Jesus, D. Drain, Y. Giraud-Heraud, C. Pastor, A. Stutz, L. Vagneron, Neutron background measurements in the underground laboratory of Modane, Astropart. Phys. 9 (1998) 163 – 172.
  * [64] M. J. Carson, J. C. Davies, E. Daw, R. J. Hollingworth, V. A. Kudryavtsev, T. B. Lawson, P. K. Lightfoot, J. E. McMillan, B. Morgan, S. M. Paling, M. Robinson, N. J. C. Spooner, D. R. Tovey, Neutron background in large-scale xenon detectors for dark matter searches, Astropart. Phys. 21 (2004) 667 – 687\.