We report high-resolution measurements of the ^94Mo(n,γ)^95Mo cross section in the neutron energy range from a few eV up to about 250 keV, performed at the n_TOF facility (CERN), and of the ^94Mo(n,tot) cross section up to 32 keV, measured at GELINA (JRC Geel). A combined R-matrix analysis of capture and transmission data yields significantly improved neutron-resonance parameters for ^94Mo. A total of 186 resonances were observed in this analysis of which 127 reported here for the first time. The resulting Maxwellian-averaged cross section at stellar temperatures relevant to the slow neutron-capture process (s-process) is approximately 25

Isotopic measurements of presolar silicon carbide grains from dying stars have revealed a puzzling overabundance of ^94Mo that stellar nucleosynthesis models have failed to reproduce for two decades. This discrepancy challenged our understanding of the slow neutron-capture process (s-process) that forges approximately half of the elements heavier than iron. The key uncertainty lies at ^94Nb, a radiactive branching point where competition between neutron capture and beta decay governs the ^94Mo production, yet the neutron-capture cross section had never been measured. Here we report the first experimental determination of the ^94Nb(n,γ)^95Nb cross section important for Mo isotopic abundances. The measurement was enabled by a coordinated effort involving high-purity target preparation at Institute of Solid State and Materials Research (IFW) Dresden, radioactive sample production at the Institut Laue-Langevin (ILL) Grenoble, radiochemical characterization at Paul Scherrer Institute (PSI) Villigen, and the Time-of-Flight CERN n_TOF facility using for the first time segmented total-energy detectors. Incorporation of the resulting Maxwellian-averaged cross section into fully coupled nucleosynthesis models of low-mass asymptotic giant branch (AGB) stars brings them into agreement with the presolar grain data. These results remove a major nuclear-physics input uncertainty at the ^94Nb branching point and provide a firmer foundation for understanding the origin of ^94Mo in the solar system.

Neutron-capture reactions provide essential nuclear-physics input for modeling the synthesis of heavy elements in stars. The growing precision of stellar spectroscopy and isotopic measurements in presolar SiC grains now demands cross sections with improved accuracy over the full energy range, and access to unstable nuclei relevant to slow (s-) process branchings and the intermediate (i-) process. This article reviews recent progress in direct neutron-capture measurements, focusing on time-of-flight (TOF) experiments at CERN n_TOF and complementary activation techniques. Substantial advances have been achieved for stable s-only and bottleneck isotopes, significantly improving constraints on s-process models. In parallel, the combination of high instantaneous neutron fluxes and advanced detector systems has facilitated first-time measurements of several radioactive branching-point nuclei. Feasibility studies, however, reveal current limitations related to sample availability, background conditions, and restricted energy coverage. In this context, the complementarity between TOF and activation emerges as a central strategy. Future developments, including high-flux facilities and novel inverse-kinematics experiments in ion storage rings, are expected to extend the boundaries of neutron-capture measurements, overcoming current limitation

This article presents a review about the main CERN n_TOF contributions to the field of neutron-capture experiments of interest for s-process nucleosynthesis studies over the last 25 years, with a special focus on the measurement of radioactive isotopes. A few recent capture experiments on stable isotopes of astrophysical interest are also discussed. Results on s-process branching nuclei are appropriate to illustrate how advances in detection systems and upgrades in the facility have enabled increasingly challenging experiments and, as a consequence, have led to a better understanding and modeling of the s-process mechanism of nucleosynthesis. New endeavors combining radioactive-ion beams from ISOLDE for the production of radioisotopically pure samples for activation experiments at the new NEAR facility at n_TOF are briefly discussed. On the basis of these new exciting results, also current limitations of state-of-the-art TOF and activation techniques will be depicted, thereby showing the pressing need for further upgrades and enhancements on both facilities and detection systems. A brief account of the potential technique based on inverse kinematics for direct neutron-capture measurements is also presented.

^50Cr and ^53Cr are very relevant in criticality safety benchmarks related to nuclear reactors. The discrepancies of up to 30

The U-235(n, f) reaction cross section was measured relative to neutron-proton elastic scattering for the first time in the energy region from 10 MeV to 440 MeV at the CERN n_TOF facility, extending the upper limit of the only previous measurement in the literature by more than 200 MeV. For neutron energies below 200 MeV, our results agree within one standard deviation with data in literature. Above 200 MeV, the comparison of model calculations to our data indicates the need to introduce a transient time in neutron-induced fission to allow the simultaneous description of (n, f) and (p, f) reactions.

The n_TOF facility at CERN has undergone a major upgrade after the installation of a new spallation target, designed to improve the features of both neutron beamlines at the experimental areas 1 and 2 (EAR1 and EAR2) and the commissioning of a new experimental area (NEAR). Due to improved coupling of the spallation target with the EAR2 beamline, the upgrade resulted in a significantly increased neutron flux and improved neutron energy resolution. This paper presents the results of the commissioning phase that followed to characterise the EAR2 neutron beamline and validate the FLUKA Monte Carlo simulations of the facility. The main features of the neutron beam, namely the neutron flux, spatial profile and energy resolution, are evaluated and compared to the previous target. The neutron flux presents a general increase of 20

The design of fast reactors burning MOX fuels requires accurate capture and fission cross sections. For the particular case of neutron capture on 242Pu, the NEA recommends that an accuracy of 8-12 energy region (2 keV-500 keV) compared to their estimation of 35 current uncertainty. Integral irradiation experiments suggest that the evaluated cross section of the JEFF-3.1 library overestimates the 242Pu(n,γ) cross section by 14 addition, the last measurement at LANSCE reported a systematic reduction of 20-30 data sets. In the present work this cross section has been determined up to 600 keV in order to solve the mentioned discrepancies. A 242Pu target of 95(4) mg enriched to 99.959 capture cross section of 242Pu has been obtained between 1 and 600 keV with a systematic uncertainty (dominated by background subtraction) between 8 and 12 reducing the current uncertainties of 35 by the NEA in a large energy range. The shape of the cross section has been analyzed in terms of average resonance parameters using the FITACS code as implemented in SAMMY, yielding results compatible with our recent analysis of the resolved resonance region.The results are in good agreement with the data of Wisshak and Käppeler and on average 10-14 keV, which helps to achieve consistency between integral experiments and cross section data. At higher energies our results show a reasonable agreement within uncertainties with both ENDF/B-VII.1 and JEFF-3.2. Our results indicate that the last experiment from DANCE underestimates the capture cross section of 242Pu by as much as 40

Neutron capture cross-section measurements are fundamental in the study of the slow neutron capture (s-) process of nucleosynthesis and for the development of innovative nuclear technologies. One of the best suited methods to measure radiative neutron capture (n,$\gamma$) cross sections over the full stellar range of interest for all the applications is the time-of-flight (TOF) technique. Overcoming the current experimental limitations for TOF measurements, in particular on low mass unstable samples, requires the combination of facilities with high instantaneous flux, such as the CERN n_TOF facility, with detection systems with an enhanced detection sensitivity and high counting rate capabilities. This contribution presents a summary about the recent highlights in the field of (n,$\gamma$) measurements at n_TOF. The recent upgrades in the facility and in new detector concepts for (n,\g) measurements are described. Last, an overview is given on the existing limitations and prospects for TOF measurements involving unstable targets and the outlook for activation measurements at the brand new high-flux n_TOF-NEAR station.

The 235U(n,f) cross section was measured in a wide energy range (25 meV - 170 keV) at the n_TOF facility at CERN, relative to 6Li(n,t) and 10B(n,alpha) standard reactions, with high resolution and accuracy, with a setup based on a stack of six samples and six silicon detectors placed in the neutron beam. In this paper we report on the results in the region between thermal and 10 keV neutron energy. A resonance analysis has been performed up to 200 eV, with the code SAMMY. The resulting fission kernels are compared with the ones extracted on the basis of the resonance parameters of the most recent major evaluated data libraries. A comparison of the n_TOF data with the evaluated cross sections is also performed from thermal to 10 keV neutron energy for the energy-averaged cross section in energy groups of suitably chosen width. A good agreement is found in average between the new results and the latest evaluated data files ENDF-B/VIII and JEFF-3.3, as well as with respect to the IAEA reference files. However, some discrepancies are still present in some specific energy regions. The new dataset here presented, characterized by unprecedented resolution and accuracy, can help improving the evaluations in the Resolved Resonance Region and up to 10 keV, and reduce the uncertainties that affect this region.

One of the possible explanations of the so-called Cosmological Lithium Problem (CLIP), i.e., the large overestimate of the primordial 7 Li abundance by the standard Big Bang Nucleosynthesis theory (BBN), is related to an incorrect underestimation of the 7 Be destruction rate by neutron-induced reactions. To verify this possibility, the n + 7 Be reactions have been investigated at n_TOF (CERN) in a wide neutron energy range, taking advantage of the new high-flux experimental area (EAR2) and specifically developed experimental setups. The 7 Be(n, α ) 4 He cross section, measured for the first time from thermal to 10 keV neutron energy, was found consistent with previous estimates. A 10% increase of the 7 Be destruction rate was instead determined on the basis of the 7 Be(n, p) 7 Li cross section measured at n_TOF from thermal to 300 keV neutron energy, a value clearly insufficient to provide a solution to the Cosmological Lithium Problem. Combined together, the two measurements finally rule out neutron-induced reactions on 7 Be as a possible explanation of the CLIP.

The \(^{16}\)O(n, \({\alpha }\))\(^{13}\)C reaction, as the inverse reaction of the astrophysically important \(^{13}\)C(\({\alpha }\), n)\(^{16}\)O reaction, is proposed to be measured at the neutron time-of-flight (n_TOF) facility of CERN. To this purpose, a Double Frisch Grid Ionization Chamber (DFGIC) containing the oxygen atoms as a component in the counting gas has been developed and a prototype was constructed at Helmholtz-Zentrum Dresden-Rossendorf(HZDR), in Germany. The first in-beam tests of the detector have been performed in November 2017 in the first (EAR1) and in April 2018 in the second (EAR2) experimental areas of the n_TOF at facility.

A considerable amount of (n,\(\gamma \)) reactions has been studied, so far, at the neutron time-of-flight facility n_TOF at CERN. The experimental program aims at determining and improving cross sections for a number of isotopes relevant to s-process nucleosynthesis. A brief summary of some physical cases related to the s-process nucleosyntheis is presented in this work together with ongoing experiments and challenging future programs.

One of the most puzzling problems in Nuclear Astrophysics is the “Cosmological Lithium Problem”, i.e the discrepancy between the primordial abundance of \(^{7}\)Li observed in metal poor halo stars (Asplund et al. in Astrophys J 644:229–259, 2006, [1]), and the one predicted by Big Bang Nucleosynthesis (BBN). One of the reactions that could have an impact on the problem is \(^{7}\)Be(n,p)\(^{7}\)Li. Despite of the importance of this reaction in BBN, the cross-section has never been directly measured at the energies of interest for BBN. Taking advantage of the innovative features of the second experimental area at the n\(\_\)TOF facility at CERN (Sabate-Gilarte et al. in Eur Phys J A 53:210, 2017, [2]; Weiss et al. in NIMA 799:90, 2015, [3]), an accurate measurement of \(^{7}\)Be(n,p) cross section has been recently performed at n\(\_\)TOF, with a pure \(^{7}\)Be target produced by implantation of a \(^{7}\)Be beam at ISOLDE. The mesurement started in April 2016 and lasted for two months. The experimental procedure, the setup used in the measurement and the results obtained so far will be here presented.

The aim of this work is to provide a precise and accurate measurement of the ${}^{238}U$($n$,$\gamma$) reaction cross section in the energy region from 1 eV to 700 keV. This reaction is of fundamental importance for the design calculations of nuclear reactors, governing the behavior of the reactor core. In particular, fast reactors, which are experiencing a growing interest for their ability to burn radioactive waste, operate in the high energy region of the neutron spectrum. In this energy region most recent evaluations disagree due to inconsistencies in the existing measurements of up to 15%. In addition, the assessment of nuclear data uncertainty performed for innovative reactor systems shows that the uncertainty in the radiative capture cross section of ${}^{238}U$ should be further reduced to 1-3% in the energy region from 20 eV to 25 keV. To this purpose, addressed by the Nuclear Energy Agency as a priority nuclear data need, complementary experiments, one at the GELINA and two at the n_TOF facility, were proposed and carried out within the 7th Framework Project ANDES of the European Commission. The results of one of these ${}^{238}U$($n$,$\gamma$) measurements performed at the n_TOF CERN facility are presented in this work. The $\gamma$-ray cascade following the radiative neutron capture has been detected exploiting a setup of two C$_6$D$_6$ liquid scintillators. Resonance parameters obtained from this work are on average in excellent agreement with the ones reported in evaluated libraries. In the unresolved resonance region, this work yields a cross section in agreement with evaluated libraries up to 80 keV, while for higher energies our results are significantly higher.

The characteristics of the neutron beam at the new n_TOF-EAR2 facility have been simulated with the Geant4 code with the aim of providing useful data for both the analysis and planning of the upcoming measurements. The spatial and energy distributions of the neutrons, the resolution function and the in-beam \( \gamma\)-ray background have been studied in detail and their implications in the forthcoming experiments have been discussed. The results confirm that, with this new short (18.5m flight path) beam line, reaching an instantaneous neutron flux beyond 105n/μs/pulse in the keV region, n_TOF is one of the few facilities where challenging measurements can be performed, involving in particular short-lived radioisotopes.

Neutron production and transport in the spallation target of the n_TOF facility at CERN has been simulated with GEANT4. The results obtained with different models of high-energy nucleon-nucleus interaction have been compared with the measured characteristics of the neutron beam, in particular the flux and its dependence on neutron energy, measured in the first experimental area. The best agreement at present, within 20% for the absolute value of the flux, and within few percent for the energy dependence in the whole energy range from thermal to 1 GeV, is obtained with the INCL++ model coupled with the GEANT4 native de-excitation model. All other available models overestimate by a larger factor, of up to 70%, the n_TOF neutron flux. The simulations are also able to accurately reproduce the neutron beam energy resolution function, which is essentially determined by the moderation time inside the target/moderator assembly. The results here reported provide confidence on the use of GEANT4 for simulations of spallation neutron sources.

The neutron time-of-flight facility n_TOF is operating at CERN (Switzerland) since 2001, having started in 2009 a new campaign with an upgraded spallation target. The ambitious program carried out includes a large number of experiments in nuclear technology, astrophysics, basic physics, detector development and medical applications. This paper is devoted to the physics program at n_TOF and the measurements performed between 2009 and 2012. Special attention is given to those experiments that have been most challenging, are more important for a particular field, have reached unprecedented levels of accuracy, or have been carried out for the first time ever.

The neutron-induced fission cross-section of 233U has been measured at the CERN n_TOF facility relative to the standard fission cross-section of 235U between 0.5 and 20MeV. The experiment was performed with a fast ionization chamber for the detection of the fission fragments and to discriminate against \( \alpha\) -particles from the natural radioactivity of the samples. The high instantaneous flux and the low background of the n_TOF facility result in data with uncertainties of \( \approx\) 3% , which were found in good agreement with previous experiments. The high quality of the present results allows to improve the evaluation of the 233U (n,f) cross-section and, consequently, the design of energy systems based on the Th/U cycle.