Chromium is a very relevant element regarding criticality safety in nuclear reactors because of its presence in stainless steel, an important structural material. Currently, there are serious ...discrepancies between the different evaluations regarding the neutron capture cross sections of 50Cr and 53Cr, most probably related to the difficulty of reducing and then estimating the very large neutron scattering effects on the shape of the resonances. In this context, there is a recent entry in the Nuclear Energy Agency (NEA) High Priority Request List (HPRL) to measure these reactions between 1 and 100 keV with an accuracy of 8-10%. In response to this request, we have performed a time-of-flight experiment at CERN n_TOF (Switzerland) and a complementary activation experimenton 50Crat30and90keVatCNAHiSPANoS(Spain).Theexperiments are presented herein, together with a discussion on the quality of the preliminary data and the results to be expected.
Neutron capture cross-section measurements are fundamental in the study of astrophysical phenomena, such as the slow neutron capture (s-) process of nucleosynthesis operating in red-giant and massive ...stars. However, neutron capture measurements via the time-of-flight (TOF) technique on key
s
-process nuclei are often challenging. Difficulties arise from the limited mass (∼mg) available and the high sample-related background in the case of the unstable
s
-process branching points. Measurements on neutron magic nuclei, that act as
s
-process bottlenecks, are affected by low (n,γ) cross sections and a dominant neutron scattering background. Overcoming these experimental challenges requires the combination of facilities with high instantaneous flux, such as n_TOFEAR2, with detection systems with an enhanced detection sensitivity and high counting rate capabilities. This contribution reviews some of the latest detector developments in detection systems for (n,γ) measurements at n_TOF, such as i-TED, an innovative detection system which exploits the Compton imaging technique to reduce the dominant neutron scattering background and s-TED, a highly segmented total energy detector intended for high flux facilities. The discussion will be illustrated with results of the first measurement of key the
s
-process branching-point reaction
79
Se(n,γ).
Abstract
Neutron capture cross sections are one of the fundamental nuclear data in the study of the s (slow) process of nucleosynthesis. More interestingly, the competition between the capture and ...the decay rates in some unstable nuclei determines the local isotopic abundance pattern. Since decay rates are often sensible to temperature and electron density, the study of the nuclear properties of these nuclei can provide valuable constraints to the physical magnitudes of the nucleosynthesis stellar environment. Here we report on the capture cross section measurement of two thallium isotopes,
204
Tl and
205
Tl performed by the time-of-flight technique at the n TOF facility at CERN. At some particular stellar s-process environments, the decay of both nuclei is strongly enhanced, and determines decisively the abundance of two s-only isotopes of lead,
204
Pb and
205
Pb. The latter, as a long-lived radioactive nucleus, has potential use as a chronometer of the last s-process events that contributed to final solar isotopic abundances.
Radiative neutron-capture cross sections are of pivotal importance in many fields such as nucle-osynthesis studies or innovative reactor technologies. A large number of isotopes have been measured ...with high accuracy, but there are still a large number of relevant isotopes whose cross sections could not be experimentally determined yet, at least with sufficient accuracy and completeness, owing to limitations in detection techniques, sample production methods or in the facilities themselves.
In the context of the HYMNS (High-sensitivitY Measurements of key stellar Nucleo-Synthesis reactions) project over the last six years we have developed a novel detection technique aimed at background suppression in radiative neutron-capture time-of-flight measurements. This new technique utilizes a complex detection set-up based on position-sensitive radiation-detectors deployed in a Compton-camera array configuration. The latter enables to implement gamma-ray imaging techniques, which help to disentangle true capture events arising from the sample under study and contaminant background events from the surroundings. A summary on the main developments is given in this contribution together with an update on recent experiments at CERN n_TOF and an outlook on future steps.
Capture and total cross section measurements for
94,95,96
Mo have been performed at the neutron time-of-flight facilities, n_TOF at CERN and GELINA at JRC-Geel. The measurements were performed using ...isotopically enriched samples with an enrichment above 95% for each of the
94,95,96
Mo isotopes. The capture measurements were performed at n_TOF using C
6
D
6
detectors and a new sTED detector. The transmission measurements were performed at a 10 m station of GELINA using a
6
Li glass neutron detector. Preliminary results of these measurements are presented.
One of the crucial ingredients for the improvement of stellar models is the accurate knowledge of neutron capture cross-sections for the different isotopes involved in the
s
-,
r
- and
i
- processes. ...These measurements can shed light on existing discrepancies between observed and predicted isotopic abundances and help to constrain the physical conditions where these reactions take place along different stages of stellar evolution.
In the particular case of the radioactive
94
Nb, the
94
Nb(
n
,
γ
) cross-section could play a role in the determination of the
s
-process production of
94
Mo in AGB stars, which presently cannot be reproduced by state-of-the-art stellar models. There are no previous
94
Nb(
n
,
γ
) experimental data for the resolved and unresolved resonance regions mainly due to the difficulties in producing highquality samples and also due to limitations in conventional detection systems commonly used in time-of-flight experiments.
Motivated by this situation, a first measurement of the
94
Nb(
n
,
γ
) reaction was carried out at CERN n_TOF, thereby exploiting the high luminosity of the EAR2 area in combination with a new detection system of small-volume C6D6-detectors and a high quality
94
Nb-sample. The latter was based on hyper-pure 93Nb material activated at the high-flux reactor of ILL-Grenoble. An innovative ring-configuration detection system in close geometry around the capture sample allowed us to significantly enhance the signal-to-background ratio. This set-up was supplemented with two conventional C
6
D
6
-detectors and a highresolution LaCl
3
(Ce)-detector, which will be employed for addressing reliably systematic effects and uncertainties.
At the current status of the data analysis, 18 resonance in
94
Nb+
n
have been observed for the first time in the neutron energy range from thermal up to 10 keV.
Neutron-capture reactions on gadolinium isotopes play an important role in several fields of physics,in particular in nuclear Astrophysics for the understanding of the nucleosynthesis of heavy ...elements (beyond iron) in stars via the s- and r-processes 1 and in nuclear technology. Another important application of gadolinium is linked to the production of terbium, that offers a set of clinically interesting isotopes for theranostics, characterized by complementary physical decay characteristics. In particular, the low-energy β −emitter terbium-161 is very similar to lutetium-177 in terms of half-life (6.89 d), β − energy and chemical properties. Being a significant emitter of conversion/Auger electrons, greater therapeutic effect can therefore be expected in comparison to Lu-177 2, 3. For this reason, in the last decade, the study of the neutron capture reaction
160
Gd(n,γ)
161
Gd and the subsequent β − decay in terbium-161 is getting particular attention. As the nuclear data on the Gd-160 neutron capture reaction are quite scarce and inconsistent, a new measurement of the capture cross section of Gd-160 at the CERN neutron Time-Of-Flight facilty was performed in order to provide high resolution, high-accuracy data on this important reaction, in the energy range from thermal to hundreds of keV. In this contribution, the preliminary results of the n_TOF measurement are presented.
Neutron capture cross sections are one of the fundamental nuclear data in the study of the s (slow) process of nucleosynthesis. More interestingly, the competition between the capture and the decay ...rates in some unstable nuclei determines the local isotopic abundance pattern. Since decay rates are often sensible to temperature and electron density, the study of the nuclear properties of these nuclei can provide valuable constraints to the physical magnitudes of the nucleosynthesis stellar environment. Here we report on the capture cross section measurement of two thallium isotopes, 204Tl and 205Tl performed by the time-of-flight technique at the n TOF facility at CERN. At some particular stellar s-process environments, the decay of both nuclei is strongly enhanced, and determines decisively the abundance of two s-only isotopes of lead, 204Pb and 205Pb. The latter, as a long-lived radioactive nucleus, has potential use as a chronometer of the last s-process events that contributed to final solar isotopic abundances.
Radiative neutron capture cross section measurements are of fundamental importance for the study of the slow neutron capture (s-) process of nucleosynthesis. This mechanism is responsible for the ...formation of most elements heavier than iron in the Universe. Particularly relevant are branching nuclei along the s-process path, which are sensitive to the physical conditions of the stellar environment. One such example is the branching at 79Se (3.27 × 105 y), which shows a thermally dependent β-decay rate. However, an astrophysically consistent interpretation requires also the knowledge of the closest neighbour isotopes involved. In particular, the 80Se(n,γ) cross section directly affects the stellar yield of the "cold" branch leading to the formation of the s-only 82Kr. Experimentally, there exists only one previous measurement on 80Se using the time of flight (TOF) technique. However, the latter suffers from some limitations that are described in this presentation. These drawbacks have been significantly improved in a recent measurement at CERN n TOF. This contribution presents a summary of the latter measurement and the status of the data analysis.
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