11institutetext:  1Instituto de Física Corpuscular, CSIC - Universidad de
Valencia, Spain; 2European Organization for Nuclear Research (CERN),
Switzerland; 3Centro de Investigaciones Energéticas Medioambientales y
Tecnológicas (CIEMAT), Spain; 4Laboratori Nazionali di Pavia, Italy; 5Istituto
Nazionali di Fisica Nucleare (INFN), Sezione di Catania, Italy; 6University of
Lodz, Poland; 7University of Manchester, United Kingdom; 8CEA Irfu, Université
Paris-Saclay, France; 9Horia Hulubei National Institute of Physics and Nuclear
Engineering, Romania; 10Department of Physics, Faculty of Science, University
of Zagreb, Zagreb, Croatia; 11University of Santiago de Compostela, Spain;
12Universitat Politècnica de Catalunya, Spain; 13Istituto Nazionale di Fisica
Nucleare, Sezione di Trieste, Italy; 14Osservatorio Astronomico di Trieste,
Italy; 15National Technical University of Athens, Greece; 16Laboratori
Nazionali di Torino, Italy; 17Istituto Nazionale di Fisica Nucleare, Sezione
di Bari, Italy; 18Istituto Nazionale di Fisica Nucleare, Sezione di Bologna,
Italy; 19Universidad de Sevilla, Spain; 20Istituto Nazionale di Fisica
Nucleare, Sezione di Perugia, Italy; 21Istituto Nazionale di Astrofisica -
Osservatorio Astronomico di Teramo, Italy; 22Goethe University Frankfurt,
Germany; 23Instituto Superior Técnico, Lisbon, Portugal; 24Physikalisch-
Technische Bundesanstalt (PTB), Braunschweig, Germany; 25Paul Scherrer
Institut (PSI), Villigen, Switzerland; 26University of Ioannina, Greece;
27Agenzia nazionale per le nuove tecnologie (ENEA), Bologna, Italy; 28Joint
Institute for Nuclear Research (JINR), Dubna, Russia; 29University of Granada,
Spain; 30Laboratori Nazionali di Frascati, Italy; 31European Commission, Joint
Research Centre (JRC), Geel, Belgium; 32University of York, United Kingdom;
33TU Wien, Atominstitut, Wien, Austria; 34Helmholtz-Zentrum Dresden-
Rossendorf, Germany; 35Japan Atomic Energy Agency (JAEA), Tokai-Mura, Japan;
36Charles University, Prague, Czech Republic; 37School of Physics and
Astronomy, University of Edinburgh, United Kingdom; 38Dipartimento di Fisica e
Astronomia, Università di Bologna, Italy; 39Istituto Nazionale di Fisica
Nucleare, Sezione di Legnaro, Italy; 40Dipartimento Interateneo di Fisica,
Università degli Studi di Bari, Italy; 41Consiglio Nazionale delle Ricerche,
Bari, Italy; 42Uppsala University, Sweden; 43Institut Laue Langevin, France;
44Dipartimento di Fisica e Astronomia, Università di Catania, Italy ;
45Leibniz-Institut für Festkörper- und Werkstoffforschung Dresden (IFW) e.V.,
Germany

# Advances and new ideas for neutron-capture astrophysics experiments at CERN
n_TOF

C. Domingo-Pardo 11** V. Babiano-Suarez 11 J. Balibrea-Correa 11 L. Caballero
11 I. Ladarescu 11 J. Lerendegui-Marco 11 J.L. Tain 11 A. Tarifeño-Saldivia 11
O. Aberle 22 V. Alcayne 33 S. Altieri 44 S. Amaducci 55 J. Andrzejewski 66 M.
Bacak 22 C. Beltrami 44 S. Bennett 77 A. P. Bernardes 22 E. Berthoumieux 88 M.
Boromiza 99 D. Bosnar 1010 M. Caamaño 1111 F. Calviño 1212 M. Calviani 22 D.
Cano-Ott 33 A. Casanovas 1212 F. Cerutti 22 G. Cescutti 13131414 S.
Chasapoglou 1515 E. Chiaveri 2277 N.M. Chiera 2525 P. Colombetti 1616 N.
Colonna 1717 P. Console Camprini 1818 G. Cortés 1212 M. A. Cortés-Giraldo 1919
L. Cosentino 55 S. Cristallo 20202121 S. Dellmann 2222 M. Di Castro 22 S. Di
Maria 2323 M. Diakaki 1515 M. Dietz 2424 R. Dressler 2525 E. Dupont 88 I.
Durán 1111 Z. Eleme 2626 S. Fargier 22 B. Fernández 1919 B. Fernández-
Domínguez 1111 P. Finocchiaro 55 S. Fiore 2727 V. Furman 2828 F. García-
Infantes 2929 A. Gawlik-Ramiega 66 G. Gervino 1616 S. Gilardoni 22 E.
González-Romero 33 C. Guerrero 1919 F. Gunsing 88 C. Gustavino 3030 J. Heyse
3131 W. Hillman 77 D. G. Jenkins 3232 E. Jericha 3333 A. Junghans 3434 Y. Kadi
22 K. Kaperoni 1515 F. Käppeler †† G. Kaur 88 A. Kimura 3535 I. Knapová 3636
U. Koester 4343 M. Kokkoris 1515 Y. Kopatch 2828 M. Krtička 3636 N. Kyritsis
1515 C. Lederer-Woods 3737 G. Lerner 22 A. Manna 18183838 T. Martínez 33 A.
Masi 22 C. Massimi 18183838 P. Mastinu 3939 M. Mastromarco 17174040 E. A.
Maugeri 2525 A. Mazzone 17174141 E. Mendoza 33 A. Mengoni 2727 P. M. Milazzo
1313 I. Mönch 4545 R. Mucciola 2020 F. Murtas 3030 E. Musacchio-Gonzalez 3939
A. Musumarra 554444 A. Negret 99 A. Pérez de Rada 33 P. Pérez-Maroto 1919 N.
Patronis 2626 J. A. Pavón-Rodríguez 1919 M. G. Pellegriti 55 J. Perkowski 66
C. Petrone 99 E. Pirovano 2424 J. Plaza 33 S. Pomp 4242 I. Porras 2929 J.
Praena 2929 J. M. Quesada 1919 R. Reifarth 2222 D. Rochman 2525 Y. Romanets
2323 C. Rubbia 22 A. Sánchez 33 M. Sabaté-Gilarte 22 P. Schillebeeckx 3131 D.
Schumann 2525 A. Sekhar 77 A. G. Smith 77 N. V. Sosnin 3737 M. Stamati 2626 A.
Sturniolo 1616 G. Tagliente 1717 D. Tarrío 4242 P. Torres-Sánchez 2929 J.
Turko 3434 S. Urlass 343422 E. Vagena 2626 S. Valenta 3636 V. Variale 1717 P.
Vaz 2323 G. Vecchio 55 D. Vescovi 2222 V. Vlachoudis 22 R. Vlastou 1515 T.
Wallner 3434 P. J. Woods 3737 T. Wright 77 R. Zarrella 18183838 P. Žugec  1010
The n_TOF Collaboration

(Received: / Revised version: )

###### Abstract

This article presents a few selected developments and future ideas related to
the measurement of $(n,\gamma)$ data of astrophysical interest at CERN n_TOF.
The MC-aided analysis methodology for the use of low-efficiency radiation
detectors in time-of-flight neutron-capture measurements is discussed, with
particular emphasis on the systematic accuracy. Several recent instrumental
advances are also presented, such as the development of total-energy detectors
with $\gamma$-ray imaging capability for background suppression, and the
development of an array of small-volume organic scintillators aimed at
exploiting the high instantaneous neutron-flux of EAR2. Finally, astrophysics
prospects related to the intermediate $i$ neutron-capture process of
nucleosynthesis are discussed in the context of the new NEAR activation area.

###### pacs:

neutron capture and time-of-flight and s-process and i-process and
nucleosynthesis

##  1 Introduction

The fundamental role of neutron-induced reactions in the formation of the
heavy elements in the universe was already evident in 1948 ABG48 ; Alpher48a ;
Alpher48b ; Alpher48c , although it was probably the first observation of
technetium in S-type stars Merrill52  and the subsequent quantitative theory
of nucleosynthesis BBFH ; Cameron57 , which triggered and guided an enormous
experimental effort, that still prevails today Gibbons67 ; Reifarth05 ;
Kaeppeler11 ; Langanke18 ; Estrade19 ; Massimi22 ; Schatz22 . This article
describes some experimental developments primarily aimed at measuring nuclear
data of interest for nucleosynthesis studies in hydrostatic stages of stellar
evolution, namely asymptotic giant branch (AGB-) and massive-stars Kaeppeler11
; Pignatari10 . These works were carried out at the n_TOF facility, which has
been extensively described in detail elsewhere Colonna18 ; Chiaveri20 ;
Esposito21 . The first topic reported in Sec. 2 is related to the accuracy of
the measurements carried out in neutron time-of-flight (TOF) experiments using
low-efficiency radiation detectors. This is an important subject for
astrophysics because data from many previous measurements still exhibit cross-
section uncertainties that are significantly larger than the few percent
uncertainty attainable from stellar observations or meteorites
analysisKaeppeler11 . The experimental situation is illustrated in Fig.1,
which shows Maxwellian average cross sections (MACS) at $kT=30~{}$ keV and
current uncertainties kadonisv1  for all nuclei involved in $s$-process
nucleosynthesis. As pointed out in several recent sensitivity studies
Neyskens15 ; Cescutti18 ; Cescutti19 ; Nishimura18a ; Nishimura18b , the cross
sections of many isotopes need to be re-measured either with improved accuracy
or over more complete neutron-energy ranges in order to derive reliable
information of astrophysical interest. This is particularly true for the seeds
of the $s$ process around the Fe-Ni region Cescutti18 , whose cross sections
at $kT=30$ keV still show relatively large uncertainties (see bottom panel in
Fig. 1). Following this logic, many neutron-capture experiments were made at
n_TOF over the last 20 years Massimi22  and many more experiments on stable
isotopes will follow in the coming years. The new measurements will benefit,
not only from the enhanced accuracy approach described below in Sec. 2, but
also from new instrumental developments such as those reported in Sec. 3 and
Sec. 4.

Figure 1: Maxwellian averaged neutron-capture cross section at $kT=30$ keV
(top panel) and their relative uncertainties (bottom panel). Blue colors refer
to nuclei with neutron-shell closures and branching nuclei are displayed in
red. Most values are taken from kadonisv1  (see text for details).

Another topic which focuses many experimental efforts nowadays is the
determination of neutron-capture cross sections on unstable nuclei Kaeppeler11
. In AGB- and massive stars, radioactive nuclei may split the nucleosynthesis
path and yield a local isotopic pattern around the branching nucleus, which is
very sensitive to the physical conditions of the stellar environment.
Therefore, neutron-capture measurements of these nuclei provide stringent
constraints on stellar structure and evolution models. As shown in Fig. 1
several $s$-process branching nuclei have been measured with high accuracy
Kaeppeler11 ; Massimi22 , but there is still a significant number of them that
have not been accessed yet owing to limitations in state-of-the-art detection
systems and sample-production capabilities. Sec. 3 and Sec. 4 describe some
recent technical developments aimed at enhancing detection sensitivity in
neutron-capture experiments, either by means of $\gamma$-imaging or by means
of very-low efficiency detectors. Sec. 5 then describes new ideas at n_TOF
intended to afford direct neutron-capture measurements of interest for more
exotic stellar environments, such as the intermediate $i$-process of
nucleosynthesis Cowan77 . Finally, Sec. 6 summarizes the main conclusions and
future prospects.

##  2 Improved accuracy measurements via MC-aided PHWT

One of the most relevant aspects when dealing with experimental data concerns
the systematic accuracy of the measurement, the proper identification of
experimental uncertainties and their realistic assessment. Therefore, in the
first n_TOF experimental campaign in 2001 a study Tain04  was carried out in
order to address the systematic accuracy attainable with the so-called Pulse-
Height Weighting Technique (PHWT). Originally developed in the sixties at ORNL
in a pioneer work by R.L. Macklin et al. Macklin67 , the PHWT has been
extremely helpful and very extensively used at different laboratories
worldwide for the determination of neutron-capture data of astrophysical
interest Gibbons67 ; Kaeppeler11 . The Total-Energy Detection (TED) principle
in combination with the PHWT allowed one to virtually mimic an ideal Moxon-Rae
detector Moxon63 . However, the new approach was much more flexible in terms
of apparatus and permitted to attain higher efficiency and better detection
sensitivity Gibbons67 . The latter was a key aspect to access neutron-capture
reactions of astrophysical relevance Macklin67 , including also radioactive
isotopes such as the $s$-process branchings 93Zr Macklin85  and 99Tc Winters87
.

An interesting aspect of the TED principle applied with the PHWT is the fact
that, essentially, only the requirement of using low-efficiency $\gamma$-ray
detectors needs to be experimentally fulfilled Macklin67 . This opens up a
wide scope of options in terms of instrumentation, an aspect that has been
also explored and exploited at n_TOF during the last years, as described later
in Sec. 3 and Sec. 4. Obviously, other additional conditions are required for
neutron-TOF experiments, such as fast time-response and low intrinsic
sensitivity to scattered neutrons.

However, for several decades the systematic accuracy attainable with the PHWT
was a topic of controversy and debate. As clearly stated by F. Corvi Corvi88 ,
one of the most puzzling aspects in the eighties was a 20% discrepancy between
capture- and transmission-measurements found for the 1.15 keV resonance in
56Fe($n,\gamma$). At that time, this was quoted as ”one of the four major
outstanding neutron data problems in the field of fission reactor neutronics”
Coates83 . The 1.15 keV resonance in 56Fe represents indeed an ideal case for
testing the accuracy of the technique because the capture TOF experiment is
mainly sensitive to the neutron width $\Gamma_{n}$, which is accurately known
from transmission measurements Perey86 .

In order to tackle this challenge and eventually develop a general and
reliable methodology for the analysis of capture data with the PHWT, at n_TOF
we carried out a detailed Monte-Carlo study Tain02  followed by a series of
systematic measurements Tain04 . The latter involved the use of two different
C6D6 detectors and iron samples of three different thicknesses (from 0.5 mm to
2 mm). The general conclusions of this work were essentially two. First, it
was understood that the only reliable methodology to apply the PHWT accurately
was by means of detailed and realistic Monte Carlo (MC) simulations of the
experimental set-up for the determination of the weighting function (WF),
which included also a specific simulation for every particular sample used in
the capture experiments. Thus, at variance with the original approach
Macklin67  and later works Yamamuro76 ; Corvi88 , there is not such a thing
like ”The weighting function of the C6F6 scintillation detector” Yamamuro76 ;
Mizuno99 ; Katabuchi17  or a unique ”experimental WF” Corvi88 . Instead, a WF
needs to be calculated for each capture set-up and for each specific sample
measured in the TOF experiment Tain04 . For relatively thick samples a
resonance-dependent WF may be needed in order to account for the different
$\gamma$-ray emission and absorption profiles across the sample thickness
Borella07 . This effect was relevant, for example, in the measurement of
197Au($n,\gamma$) Massimi10  or 232Th($n,\gamma$) Gunsing12 . Self-shielding
effects can also play an important role for some samples or resonances and,
therefore, the methodology developed in Ref. Borella07  has been included in
the R-matrix analysis code REFIT refit . For a recent review on the analysis
techniques for neutron induced reaction cross-section data the reader is
referred to Ref. Schillebeeckx12 .

The fact that the WF and the PHWT accuracy is so dependent on so many
experimental details reflects also the level of sensitivity in these
measurements, where small changes in the experimental conditions can be
quickly reflected in the acquired capture data. In some sense, the new MC-
aided approach represented a change of paradigm in the analysis of neutron-
capture data using the PHWT, which has been adopted by the scientific
community Borella07 ; Ren20 . It is worth to emphasize that the work reported
in Corvi88  and references therein, although did not provide a final solution
to this problem, it had a crucial relevance towards understanding its origin.
It is worth recalling also that MC simulations using the EGS-transport code
were applied in ORNL already in 1988 Perey88 . However, the latter work still
proposed a single WF for all capture experiments regardless of the sample
characteristics.

The second aspect found in Tain04  to be of relevance for the accuracy of the
PHWT was related to the signature of nuclear-structure effects in the response
functions measured with the C6D6 detectors. In general, differences are found
between the capture-cascade spectrum of the sample under study, and the one
used as reference, commonly 197Au($n,\gamma$). The methodology proposed in
Ref. Tain04  to account for this effect involves the MC simulation of the full
capture cascade for both studied- and reference-samples, and then determine a
yield correction factor. Because of the interplay with the nuclear-structure
effects, the correction factor may even change from one capture-resonance to
another, depending on the level spin and parity Domingo06a ; Domingo06b ;
Domingo07a . The main contributions to the yield-correction factor arise from
the different number of counts missing under the detection threshold
(typically 150-200 keV), $\gamma$-ray summing effects, angular-distribution
effects Domingo06b ; Domingo07b , conversion-electrons and, if present,
isomeric-states Domingo06a . References quoted represent examples, where such
correction factors were crucial to keep the systematic uncertainty within the
level of 2-3% RMS. Finally, this result also highlights the relevance of
suitable computing codes and libraries Agostinelli03 , methods and models
Becvar98 ; Tain07 ; Valenta17 ; Moreno22  for simulating the cascade of prompt
$\gamma$-rays in neutron-capture experiments.

##  3 Background suppression via $\gamma$-ray imaging

As discussed in the preceding section, one of the most striking features of
the TED principle is related to its versatility, namely enabling the use of
almost any detection system with efficiency low enough to satisfy

$\varepsilon^{c}=1-\prod^{N}_{j=1}(1-\varepsilon^{\gamma}_{j})\simeq\sum^{N}_{j=1}\varepsilon_{j}^{\gamma}.$
(1)

Here, $N$ is the number of emitted $\gamma$-rays, $\varepsilon^{c}$ represents
the capture-detection probability and $\varepsilon^{\gamma}$ the $\gamma$-ray
detection efficiency. In addition, the efficiency-energy proportionality,
$\varepsilon^{\gamma}_{j}\propto E^{\gamma}_{j}$, required to attain the total
cascade-energy response $\varepsilon^{c}\propto E^{c}$ can be achieved by
means of the PHWT Macklin67 . As mentioned before, the detector response
function needs to be also suitable for neutron-TOF experiments. Aiming at
reducing neutron-induced backgrounds in the detector itself, organic C6F6
detectors were used in the first experiments Macklin67 ; Corvi88 , which were
later replaced by C6D6 further optimized by means of C-fiber encapsulations
and other improvements Plag03 ; Tain04 .

Apart from organic scintillation detectors, a NaI(Tl) spectrometer has been
used at ANNRI J-PARC Mizuno99 ; Igashira09 ; Katabuchi14 ; Katabuchi17 , which
actually demonstrates that it is possible to extend the TED principle to very
different types of detection systems. Exploiting further this aspect, a new
approach has been investigated at n_TOF, which applies $\gamma$-ray imaging
techniques to discriminate spatially localized $\gamma$-ray background sources
Domingo16 . This concept seems particularly interesting for the measurement of
samples with a small neutron-capture cross section, where neutrons scattered
in the sample and subsequently captured in the walls of the experimental hall
dominate the background level, instead of neutrons captured directly in the
detectors themselves. This situation is depicted in Fig.2-top, which shows
that in the keV neutron-energy region of astrophysical interest the background
may be rather dominated by neutrons captured in the walls of the experimental
hall, rather than in the detectors themselves Zugec14 . The impact of this
background is illustrated in Fig. 2-bottom with the measurement of
93Zr($n,\gamma$) performed at n_TOF Tagliente13 . As indicated in Ref.
Neyskens15 , improving the cross-section measurement could help to constrain
even more the thermal conditions in AGB stars.

Figure 2: (Top panel) MC simulation Zugec14  of the neutron sensitivity, which
shows the C6D6-response to neutron-induced $\gamma$-ray background in the
walls of the experimental hall. In practice, the resonant structure in the 1
keV-100 keV neutron-energy range is suppressed due to the loss of time-energy
correlations for the scattered neutrons (see Ref. Zugec16  for details).
(Bottom panel) Capture yield of 93Zr($n,\gamma$) Tagliente13 , which shows the
limiting effect of the background level in the keV neutron-energy range.

First attempts to apply $\gamma$-ray imaging techniques for background
suppression in neutron-capture experiments at n_TOF employed a pin-hole
$\gamma$-camera with a bulky lead collimator attached to a position-sensitive
radiation detector Perez16 . This work actually demonstrated for the first
time the possibility to incorporate imaging techniques in neutron-TOF
experiments, although improvements were rather limited owing to the additional
background induced by neutrons in the massive collimator itself. The problems
ascribed to the use of a massive collimator could be fully overcome by means
of an alternative technique based on electronic collimation, originally
developed for $\gamma$-ray astronomy Schoenfelder73 ; Wilderman98 . This new
approach based on the Compton imaging technique Domingo16  has been developed
in the framework of the ERC-project HYMNS hymns  during the last years at CERN
n_TOF. Compton imaging is based on the use of two or more planes of radiation
detectors with both energy- and position-sensitivity operated in time-
coincidence. In this way, when a $\gamma$-ray undergoes interaction in several
detectors the Compton scattering law can be applied in order to infer
information on the incoming radiation direction. Several technical
developments were necessary in order to adapt existing technologies to the
field of neutron-capture measurements. These developments were mainly related
to the need of achieving good enough energy resolution with SiPMs and large
monolithic crystals Olleros18 , high spatial resolution and linearity that are
challenging due to the big size of the scintillation crystals Babiano19 ;
Balibrea21  and implementing a customized dynamic electronic-collimation
method for enhanced performance in the Compton imaging Babiano20 .

Proof-of-principle experiments Babiano21  have been performed at n_TOF with a
prototype of a Total-Energy Detector with imaging capability, called i-TED.
These measurements show a significant background reduction in the keV neutron-
energy range of interest for astrophysics, when compared to state-of-the-art
C6D6 detectors.

Fig. 3 shows a picture of the final i-TED system for ($n,\gamma$) experiments,
which consists of an array of four large-solid angle Compton cameras in a
close configuration around the capture sample. Every Compton module comprises
5 inorganic scintillation crystals, each of them with a size of 50$\times$50
mm2. The front scatter position-sensitive detector has a thickness of 15 mm,
whereas the four crystals in the rear absorber plane have a thickness of 25
mm. The modules have been designed in order to maximize detection efficiency,
while minimizing neutron-sensitivity in the detectors themselves. To
accomplish the latter goal LaCl3(Ce) was preferred versus other options, owing
to the relatively small integral capture cross section of Chlorine, and the
small contribution of resonances in the keV-energy range of relevance. The
Compton modules are supplemented with 6Li neutron-absorber pads of 20 mm
thicknes for reducing further the intrinsic neutron sensitivity of the array
(see Fig. 3).

Figure 3: Photograph of the i-TED array during a calibration measurement for
the 79Se($n,\gamma$) experiment in 2022 at CERN n_TOF EAR1.

Pixelated silicon photomultipliers (SiPMs) are used for the readout of the 20
inorganic crystals, leading to a total number of 1280 readout channels. To
cope with this large number a dedicated acquisition system based on ASIC
TOFPET2 modules petsys  was implemented and adapted to this type of
experiments. For further details the reader is referred to Refs. Babiano20 ;
Babiano21  and references therein.

The i-TED array has been recently applied for the first measurement of the
79Se($n,\gamma$) capture cross section at n_TOF EAR1 Lerendegui21 . A 79Se
sample was produced by high-fluence neutron irradiation in the V4 beam tube of
the ILL reactor in Grenoble. To this aim, an eutectic PbSe-alloy sample was
prepared at the Paul Scherrer Institut (PSI) in Switzerland, which allowed one
to overcome the difficulties ascribed to the low melting point of selenium
Chiera22 . The measurement with the i-TED array in EAR1 was intended to reduce
the large scattered-neutron background arising from the large lead content in
the sample, 2.8 g. Furhter, the final PbSe sample had an activity of 5 MBq of
75Se and 1.6 MBq of 60Co. Therefore, this sample was also measured at the EAR2
station with the set-up described in the following section. 79Se is an
important $s$-process branching nucleus, which is particularly well suited to
constrain the thermal conditions of the $s$-process in the weak $s$ process
Walter86 ; Kaeppeler11 ; Cescutti19 . Once fully analyzed, the results of this
experiment will help to constrain the thermal conditions during core He-
burning and shell C-burning in massive stars.

##  4 Small-volume C6D6 detectors with high count-rate capability

In situations where the background in the experiment is dominated by the decay
radioactivity of the sample itself it may become more convenient to exploit
the high instantaneous neutron-flux of the EAR2 measuring station. In this
way, the relative contribution of the sample radioactivity is minimized with
respect to the radiative-capture channel of interest. The large instantaneous
neutron-flux of n_TOF EAR2 Lerendegui16  is particularly well suited for these
challenging cases. As described in Koehler00 , the high peak flux becomes one
of the most important features when measuring radioactive samples because it
allows for a reduction of the sample-activity background contribution relative
to the time-interval where the neutron capture yield is measured. However, in
order to exploit the large peak-neutron flux one requires also radiation
detectors with a fast time-response and a high count-rate capability. State-
of-the-art C6D6 detectors with a volume of $\sim$1 l Plag03  have a relatively
large efficiency, which in turn requires a large sample-detector distance to
avoid excessive signal pile-up and dead time arising from the high count-rate
conditions of about 1 MHz, or more. Also, the i-TED system described in the
preceding section is presently limited to count rates of about 500 kHz petsys
, owing to the ASIC-readout scheme implemented to cope with the 1280 readout
channels of the twenty position-sensitive detectors. It is expected that
future developments will help to enhance the ASIC event-rate capability and
possibly, make the i-TED system also useful for measurements in the high-flux
conditions of EAR2.

To overcome the count-rate limitations of conventional C6D6 detectors an array
of nine small-volume (49 ml) C6D6 detectors Alcayne22 , was implemented in a
compact-ring configuration Balibrea22b  around the capture sample in EAR2 as
shown in Fig. 4. The main advantage of this innovative setup is that the small
detection volume allows one to place the detectors much closer to the capture
sample under study, and thus enhance also the efficiency for true capture
$\gamma$-rays and increase the signal-to-background ratio (SBR) with respect
to previous set-ups based on larger C6D6 detectors placed further away from
the beam-line. The improvement in SBR is shown in the bottom panel of Fig. 4,
which shows an enhanced signal-to-background ratio for the 197Au($n,\gamma$)
reaction over most of the energy range when measured with the small-volume
C6D6 detectors.

Figure 4: (Top) Photograph of the capture setup based on an array of small-
volume C6D6 detectors used for the 94Nb($n,\gamma$) experiment in 2022 at CERN
n_TOF EAR2. (Bottom) Capture-spectra for 197Au($n,\gamma$) measured with a
conventional large C6D6 detector and with a small volume C6D6 detector in
EAR2. Both spectra have been normalized to the peak of the 4.9 eV resonance.

The set-up shown in Fig. 4 was used in the 2022 campaign for the measurement
of the 94Nb($n,\gamma$) cross section Balibrea21 . The 94Nb sample used for
this TOF experiment was produced by high-fluence neutron irradiation of
hyperpure niobium samplesMoe00  in the V4 tube of the ILL-Grenoble reactor.
The final sample contained a total amount of $9\times 10^{18}$ atoms with an
activity dominated (10 MBq) by the $\beta$-decay of 94Nb (2$\times 10^{4}$ y).
The results from this experiment are expected to shed light on isotopic
anomalies observed in pre-solar SiC grains Lugaro03 , which apparently require
an unexpectedly large $s$-process contribution to the abundance of 94Mo.

##  5 New astrophysics prospects at NEAR using GEAR and CYCLING

The combination of neutron-TOF with activation measurements, when feasible,
may deliver complementary and more accurate information on a specific cross
section (see Table II in Ref. Kaeppeler11 ). When applicable, the activation
technique shows an unsurpassed sensitivity for the measurement of minuscule
sample quantities, as it has been demonstrated for samples of only $\sim
10^{14}$ to $10^{15}$ atoms Reifarth03 ; Uberseder07 .

Following this logic, one of the most recent efforts at n_TOF concerns the
development of the neutron-activation station NEAR Ferrari22 ; Patronis22 ,
aiming at exploiting the large neutron-fluxes in the proximity of the
spallation target. Preliminary MC calculations Mengoni20  show the possibility
of using suitable filters and moderation materials for producing quasi-
Maxwellian neutron-energy spectra over a broad range between a few and several
hundreds keV. A detailed description of the new NEAR installation will be
reported in Ref. Patronis22  and preliminary flux characterization
measurements have been carried out recently Stamati22 . Many of the latter
measurements have been carried out at the Gamma-ray spectroscopy Experimental
ARea (GEAR) of n_TOF, which is based on a CANBERRA HPGe detector GR5522
supplemented with convenient shielding Patronis22 . This station is available
for conventional neutron-activation measurements where $\gamma$-rays from the
decay of the activation products with half-lives longer than a few hours are
measured.

Because of the low duty cycle the average neutron fluence attainable at NEAR
is expected to be comparable to the one available in the past at FZK
Reifarth03  or currently at other activation facilities Alzubaidi16 ;
Fernandez20 . However, one of the unique features at NEAR will be the
possibility to perform activation measurements on small samples of highly
isotopically enriched (or even pure) material, which can be produced in
sufficient quantities at the nearby ISOLDE Ballof2020  and MEDICIS facilities
Gadelshin20 . In addition to the GEAR station, there is another planned
station for fast cyclic-activation measurements at NEAR called CYCLING
Lerendegui22 . The fast-cyclic activation technique was pioneered at FZK-
Karslruhe Beer94 , where it was applied to measure the neutron-capture cross
section of several nuclides of relevance for nucleosynthesis in AGB stars,
such as 107,109Ag($n,\gamma$) Beer94 , 26Mg($n,\gamma$)Mohr98 ,
50Ti($n,\gamma$)Sedyshev99  and 19F($n,\gamma$)Uberseder07 . It is worth
noting that measurements on isotopes with activation products with half-lives
as short as $\sim 11$ s (20F) are accessible with this technique. The CYCLING
station will enable the repetition of a short irradiation, followed by a rapid
transport to a detector, where the measurement of the decay will take place
and subsequently transported back to the irradiation position. This process is
repeated for a number of cycles thus enhancing counting statistics and signal-
to-background ratio for short-lived nuclei.

Thus, with the future combination of ISOLDE and GEAR-CYCLING it may become
possible to access also direct neutron-capture measurements on several
unstable nuclei of interest for the study of $s$-process branchings, and also
for the more exotic intermediate $i$-process of nucleosynthesis Cowan77 . The
$i$ process involves neutron capture at neutron densities of $10^{13}–10^{16}$
cm-3, in between the $s$ and $r$ processes. Recently, the $i$ process
attracted significant interest because it might explain the abundance pattern
of a special kind of Carbon-Enhanced Metal-Poor stars (CEMPs), called CEMP-s/r
Hampel2016 . The site of the $i$-process has been identified as the very late
thermal pulse H-ingestion of post-AGB stars. Recent studies show also the
relevance of this mechanism for the early generation of stars Heger02 ;
Frebel05 .

One case of interest is neutron capture on 135Cs (t1/2 = 2 Myr). The stellar
neutron-capture rate of 135Cs is relevant for the interpretation of the
$s$-process branching at 134Cs (t1/2 = 2 yr) Kaeppeler11 ; Patronis04  and
also for $i$-process nucleosynthesis, as discussed latter.

A suitable sample of 135Cs could be ion-implanted at ISOLDE. After,
characterization and activation at NEAR the decay of the activation product,
136Cs (t1/2=13 d), could be measured at the GEAR station or any other low-
background laboratory. The neutron capture of 135Cs at $kT=25$ keV has been
already measured at FZK Patronis04  and therefore this measurement could be a
good benchmark case for the performance of the new installation. In addition,
at NEAR the MACS could be also completed for other neutron energy ranges
around $kT=8$ keV and $kT=90$ keV, where presently there is no experimental
information available.

In the high neutron fluxes characteristic of the $i$-process it has been found
Bertolli2013  that variations in the neutron-capture rates of some specific
radioactive isotopes around the $N=82$ neutron-shell closure could affect
elemental ratio predictions, involving the benchmark (observable) elements Ba,
La and Eu Bertolli2013 . Some of the involved reactions, such as
137Cs($n,\gamma$) may become accessible at NEAR. Commercial samples of 137Cs
(t1/2=30 yr) are available and could be used for this measurement. A sample of
about $2\times 10^{14}$ atoms and an activity of less than 200 kBq (662 keV
$\gamma$-rays) could be a suitable option. Capture on 137Cs leads either
directly or via the detour of the shorter-lived 138mCs ($t_{1/2}=3$ m) to the
activation product 138gCs ($t_{1/2}=33$ m) that emits a significant
$\gamma$-ray intensity at 1.4 MeV. Owing to the short half-life it could be
best measured at the CYCLING station.

As reported in Ref. Choplin21 , an AGB star experiencing $s$\- or $i$-process
nucleosynthesis would show very different isotopic fractions which, although
challenging, could be inferred from observations. Thus, several isotopes of
Ba, Nd, Sm and Eu may be used as tracers of $i$-process nucleosynthesis. For
example, under $i$-process conditions the final abundance of 137Ba is larger
than that of 138Ba. 138Ba, with $N=82$, has a very small neutron-capture cross
section, acting as a bottleneck and therefore being copiously produced by the
$s$ process. The relatively large $i$-process abundance of 137Ba is due to the
decay of 137Cs which, at variance with the $s$ process, can be easily reached
in $i$-process conditions. Therefore, the aforementioned 135Cs($n,\gamma$) and
137Cs($n,\gamma$) cross section measurements could provide a valuable input
information for $i$-process models and observations. In addition, the
measurement of the intermediate 136Cs($n,\gamma$) may become feasible,
assuming that a sample with sufficient mass could be produced at ISOLDE and
later activated at NEAR. After the neutron activation and sufficient waiting
time to let the 136Cs (t1/2=13 d) in the sample decay, one could measure the
activity of the activation product 137Cs (t1/2=30 yr) at the GEAR station.
Other similar cases related to the $i$-process tracers discussed in Ref.
Choplin21  might be also accessible at NEAR, such as neutron capture on 144Ce
(t1/2=285 d) leading to 145Ce (t1/2=3 m). However, the feasibility with
CYCLING needs to be studied in detail owing to the $\gamma$-ray activity from
neighbouring decays (mainly 144Pr).

Finally, there are many other neutron-capture reactions of interest for the
$i$-process, such as neutron capture on 66Ni (t1/2=55 h), which represents one
of the major bottle-necks in $i$-process models McKay2020  or neutron capture
on 72Zn (t1/2=46 h) that determines the $i$-process abundance of Ge McKay2020
. However, in these cases activation measurements become prohibitive due to
the large sample activity, which typically exceeds 100 MBq for sample
quantities of about 1012-13 atoms. For this reason, indirect methods such as
surrogate reactions using storage rings Perez20 ; Jurado21 , may represent the
most promising alternative to obtain experimental information and to constrain
the stellar environments.

##  6 Summary and outlook

This article has presented a few technical contributions of n_TOF to the field
of neutron-capture experiments of astrophysical interest. These works have
been key, on the one hand, to address the accuracy of the measurements, and
even enhance the systematic precision for this type of studies Tain04 , an
aspect which is closely connected with the 4-5% systematic error commonly
required for reliable astrophysical interpretation of observational data or
meteorites analysis Kaeppeler11 ; Cescutti19 ; Nishimura18b . Although
historically, a large effort has been invested in reducing the intrinsic
neutron-sensitivity of the detection apparatus, detailed MC calculations
Zugec14  showed that, in many situations, the background level is dominated by
scattered neutrons, which are captured in the surroundings of the detectors,
rather than in the detection system itself. In this respect, a novel i-TED
detection system Domingo16  based on $\gamma$-ray imaging has been developed,
which allows one to attain a significant improvement in signal-to-background
ratio for such specific cases in the keV-energy range of astrophysical
interest Babiano21 . This system has been employed at CERN n_TOF for the first
measurement of the 79Se($n,\gamma$) cross section, which is one of the main
branching points in the weak $s$ process Kaeppeler11 . Further, for the
measurement of highly-radioactive samples, such as the one of 94Nb described
in Sec. 4, a new array of very small-volume C6D6 detectors was developed and
implemented, which enabled also for a significant improvement in terms of
signal-to-background ratio with respect to state-of-the-art C6D6 detectors.
This measurement, carried out also in 2022 at CERN n_TOF, will help to shed
light on isotopic Mo-anomalies observed in pre-solar SiC grains Lugaro03 .
Future ideas and proposals at n_TOF are related to the new NEAR experimental
area for exploiting also the neutron-activation technique in measurements of
astrophysical interest. In this respect, current efforts to design a station
for fast cyclic activation measurements (CYCLING) have been also presented.
This installation could help to access for the first time to direct neutron-
capture cross sections on radioactive isotopes which are of great interest for
the intermediate process of nucleosynthesis.

## Acknowledgment

For the purpose of open access, the author has applied a Creative Commons
Attribution (CC BY) licence to any Author Accepted Manuscript version arising
from this sub- mission. Part of this work has been carried out in the
framework of a project funded by the European Research Council (ERC) under the
European Union’s Horizon 2020 research and innovation programme (ERC
Consolidator Grant project HYMNS, with grant agreement No. 681740). The
authors acknowledge support from the Spanish Ministerio de Ciencia e
Innovación under grants PID2019-104714GB-C21, FPA2017-83946-C2-1-P,
FIS2015-71688-ERC, FPA2016-77689-C2-1-R, RTI2018-098117-B-C21, CSIC for
funding PIE-201750I26, European H2020-847552 (SANDA) and by funding agencies
of participating institutes.

This article belongs to a series of articles devoted to the memory of Franz
Käppeler. The present work contains some of the developments where he was
involved or witnessed and, some other contributions which came up more
recently. In any case, all of them have undoubtedly benefited from the
motivation and creativity that Franz inspired in all of us. Thank you Franz.

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