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The 72 Ge ( n , ¿ ) cross section was measured for neutron energies up to 300 keV at the neutron time-of-flight facility n _ TOF (CERN), Geneva, for the first time covering energies relevant to heavy-element synthesis in stars. The measurement was performed at the high-resolution beamline EAR-1, using an isotopically enriched 72 Ge O 2 sample. The prompt capture ¿ rays were detected with four liquid scintillation detectors, optimized for low neutron sensitivity. We determined resonance capture kernels up to a neutron energy of 43 keV , and averaged cross sections from 43 to 300 keV . Maxwellian-averaged cross section values were calculated from k T = 5 to 100 keV , with uncertainties between 3.2 % and 7.1 % . The new results significantly reduce uncertainties of abundances produced in the slow neutron capture process in massive stars. Peer Reviewed Article signat per 144 autors/es: M. Dietz ,C. Lederer-Woods, A. Tattersall, U. Battino, F. Gunsing, S. Heinitz, M. Krticka, J. Lerendegui-Marco, R. Reifarth, S. Valenta, O. Aberle, S. Amaducci, J. Andrzejewski, L. Audouin, M. Bacak, J. Balibrea, M. Barbagallo, F. Becvá, E. Berthoumieux, J. Billowes, D. Bosnar, A. Brown, M. Caamaño, F. Calviño, M. Calviani, D. Cano-Ott, R. Cardella, A. Casanovas, F. Cerutti, Y. H. Chen, E. Chiaveri, N. Colonna, G. Cortés, M. A. Cortés-Giraldo, L. Cosentino, L. A. Damone, M. Diakaki,C. Domingo-Pardo, R. Dressler, E. Dupont, I. Durán, B. Fernández-Domínguez, A. Ferrari, P. Ferreira, P. Finocchiaro, V. Furman,K. Göbel, A. R. García, A. Gawlik, S. Gilardoni, T. Glodariu, I. F. Gonçalves, E. González-Romero, E. Griesmayer, C. Guerrero, H. Harada, J. Heyse, D. G. Jenkins, E. Jericha, F. Käppeler, Y. Kadi, D. Kahl, A. Kalamara, P. Kavrigin, A. Kimura, N. Kivel, M. Kokkoris, D. Kurtulgil, E. Leal-Cidoncha, H. Leeb, S. Lo Meo, S. J. Lonsdale, D. Macina, A. Manna, J. Marganiec, T. Martínez, A. Masi, C. Massimi, P. Mastinu, M. Mastromarco, E. A. Maugeri, A. Mazzone, E. Mendoza, A. Mengoni, P. M. Milazzo, F. Mingrone, A. Musumarra, A. Negret, R. Nolte, A. Oprea, N. Patronis, A. Pavlik, J. Perkowski, I. Porras, J. Praena, J. M. Quesada, D. Radeck, T. Rauscher, C. Rubbia, J. A. Ryan, M. Sabaté-Gilarte, A. Saxena, P. Schillebeeckx, D. Schumann, P. Sedyshev, A. G. Smith, N. V. Sosnin, A. Stamatopoulos, G. Tagliente, J. L. Tain, A. Tarifeño-Saldivia, L. Tassan-Got, G. Vannini, V. Variale, P. Vaz, A. Ventura, V. Vlachoudis, R. Vlastou, A. Wallner, S. Warren, C. Weiss, P. J. Woods, T. Wright, P. Žugec Objectius de Desenvolupament Sostenible::7 - Energia Assequible i No Contaminant

Low efficiency total-energy detectors (TEDs) are one of the main tools for neutron capture cross section measurements utilizing the time-of-flight (TOF) technique. State-of-the-art TEDs are based on a CD liquid-scintillation cell optically coupled to a fast photomultiplier tube. The large photomultiplier tube represents yet a significant contribution to the so-called neutron sensitivity background, which is one of the most conspicuous sources of uncertainty in this type of experiments. Here we report on the development of a first prototype of a TED based on a silicon-photomultiplier (SiPM) readout, thus resulting in a lightweight and much more compact detector. Apart from the envisaged improvement in neutron sensitivity, the new system uses low voltage (+28 V) and low current supply (50 mA), which is more practical than the kV supply required by conventional photomultipliers. One important difficulty hindering the earlier implementation of SiPM readout for this type of detector was the large capacitance for the output signal when all pixels of a SiPM array are summed together. The latter leads to long pulse rise and decay times, which are not suitable for time-of-flight experiments. In this work we demonstrate the feasibility of a Schottky-diode multiplexing readout approach, that allows one to preserve the excellent timing properties of SiPMs, hereby paving the way for their implementation in future neutron TOF experiments.

Neutron capture cross sections are fundamental in the study of the slow neutron capture process of nucleosynthesis, also known as the s-process, which produces half of the observed solar system abundances of elements heavier than iron. Some nuclei along the nucleosynthesis chain are unstable, and there the capture process competes with the decay process, creating a split in the nucleosynthesis path. The nuclear properties of some of these radionuclides change with the conditions of the stellar environment, a fact that influences the local abundance pattern. 204Tl is a very interesting branching point, because it is shielded from any contribution from other nucleosynthesis processes. The result is that both 204Tl and its stable daughter isotope 204Pb are only produced by the s-process. Hence, by competing with the beta decay, the capture cross section of 204Tl crucially determines the final abundance of 204Pb. A faithful prediction of the solar abundances of s-only isotopes, like 204Pb, is one of the key accuracy tests for modern stellar nucleosynthesis calculations. However, until the present work, due to the challenges of performing a capture measurement on 204Tl, there was no experimental data of its cross section. Thus, large uncertainties existed in its capture cross section, which hampered a more accurate and precise knowledge of the predicted s-process production of 204Pb. By affecting the abundance of 204Pb, the cross section of 204Tl(n,¿) also influences the ratio of abundances 205Pb/204Pb. 205Pb is also produced only by the s-process, and it is radioactive, with a long half-life of 17.2 My. Therefore, the ratio of abundances of 205Pb/204Pb has the potential to be used as a chronometer of the s-process. In the year 2013, a sample enriched up to a few percent in 204Tl was produced by neutron irradiation of a 203Tl seed sample at the high thermal neutron flux nuclear reactor of the ILL, in Grenoble (France). Two years later, the 204Tl enriched sample was employed to measure, for the first time, the capture cross section of 204Tl at the n TOF time-of-flight facility at CERN. The measurement was possible thanks to the unique features of this facility, in particular, its high instant neutron flux low background levels. The measurement was performed employing the well-established total energy detection technique (TED), which offers a very low neutron sensibility, and low levels of background, compared to other methods like the Total Absorption technique. The main challenges for the 204Tl measurement were the very high background due to the activity of the sample, the very low amount of material, and the limited knowledge of the geometry of the sample. Such difficulties required the adoption of specific solutions during the measurement and the posterior data analysis. Related to this, several sources of systematic error were evaluated by means of Monte Carlo simulations. The complications with the 204Tl sample geometry required to apply an in-sample normalization procedure. For this purpose, an ancillary capture measurement on a 203Tl sample was also performed in the same experimental campaign. As a stable nuclide, most of the sources of systematic error could be kept under control. This allowed for an accurate R-matrix analysis of the most relevant capture levels in the resolved resonance region of 203Tl, including the first ever measurement under 3 keV of neutron energy. As a result, the present work has contributed, as well, to improve the 203Tl stellar capture cross section in the 8 to 25 keV neutron energy range. With the improved 203Tl(n,gamma) cross section, an R-matrix analysis of several 204Tl resonances was made possible. These results were employed to experimentally constrain the 204Tl stellar cross section at low energies, and setting additional limits to the stellar cross section predicted by nuclear data evaluations at s-process temperatures. Les seccions eficaces de captura de neutrons són fonamentals en l'estudi del procés lent de nucleosíntesi estel·lar dels elements, també conegut com a procés s (de l'anglès "slow"). El procés s és responsable de la producció d'aproximadament la meitat de les abundàncies solars dels elements més pesats que el ferro. Alguns nuclis en la cadena de nucleosíntesi són inestables, i en aquests el procés de captura competeix amb el procés de desintegració, el que dóna lloc a una ramificació del camí de producció dels elements. Les propietats nuclears d'alguns d'aquests núclids canvien amb les condicions de l'entorn estel·lar on són produïts, el que influencia el patró d'abundàncies local. El 204Tl és un punt de ramificació molt interessant, perquè està protegit de contribucions d'altres processos de nucleosíntesi. El resultat és que tant el 204Tl com el seu producte de desintegració 204Pb només són produïts pel procés s. Per tant, degut que competeix amb el procés de desintegració, la secció eficaç de captura neutrònica del 204Tl és clau per fixar l'abundància final de 204Pb. Una predicció cartera de l'abundància dels núclids només produïts pel procés s és una de les proves de precisió dels models de càlculs de nucleosíntesi estel·lar. Tot i això, fins el present treball, no hi havia dades de la seva secció eficaç degut al reptes en la realització d'una mesura de captura en el 204Tl. Això feia que hi hagués una incertesa important en la secció eficaç estel·lar, la qual cosa dificultava una predicció precisa de la producció de 204Pb pel procés s. De la mateixa manera, la secció eficaç de 204Tl(n,y) també afecta la relació d'abundàncies 205Pb/204Pb. Degut al fet que el 205Pb és radioactiu, amb un període de semidesintegració de 17,2 Ma, la dita relació té el potencial de ser usada com a cronòmetre del procés s. L'any 2013, es va produir una mostra enriquida en 204Tl per irradiació neutrònica d'una llavor de 203Tl al reactor nuclear del ILL de Grenoble (França). Dos anys després, la mostra enriquida es va fer servir per mesurar, per primera vegada, la secció eficaç de captura del 204Tl utilitzant la tècnica de temps de vol a l'experiment n_TOF del CERN. La mesura va ser possible gràcies a les característiques úniques d'aquesta instal·lació, en particular, l'elevat flux instantani de neutrons i els nivells baixos de fons. La mesura es va dur a terme mitjançant la tècnica de Detecció Total d'Energia (TED, en anglès), que té com a avantatges una sensibilitat de detecció de neutrons molt baixa, i una baixa taxa de detecció de fons. Els principals reptes en la mesura de 204Tl(n,y) van ser la gran quantitat de senyals de fons causats per l'activitat de la mostra, la baixa quantitat de material disponible, i el coneixement limitat de la geometria de la mostra després del procés d'irradiació. Totes aquestes dificultats han requerit l'adopció de solucions específiques durant la mesura i l'anàlisi de dades posterior. Les complicacions amb la geometria de la mostra van fer necessari aplicar un mètode d'autonormalització. Per aquest propòsit es va dur a terme una mesura auxiliar de captura en 203Tl durant la mateixa campanya experimental. Com que el 203Tl es estable, els principals errors sistemàtics de mesura es van poder mantenir baixos. Això ha permès fer un anàlisi de tipus R-matrix de les principals ressonàncies de captura neutrònica del 203Tl, incloent la primera mesura dels nivells per sota de 3 keV. Això ha contribuït a millorar la secció eficaç de captura estel·lar del 203Tl en el rang entre 8 keV i 25 keV d'energia tèrmica dels neutrons. La secció eficaç millorada de la reacció 203Tl(n,y) ha permès realitzar un anàlisi de diverses ressonàncies de captura del 204Tl. Els resultats s¿han utilitzat per fitar la secció eficaç estel·lar del 204Tl a energies de neutrons baixes, i així limitar els valors possibles a la secció transversal estel·lar predits per les avaluacions de dades nuclears a temperatures del procés s.

The energy-dependent cross section of the Be7(n,alpha)He4 reaction, of interest for the so-called cosmological lithium problem in big bang nucleosynthesis, has been measured for the first time from 10 meV to 10 keV neutron energy. The challenges posed by the short half-life of Be7 and by the low reaction cross section have been overcome at n_TOF thanks to an unprecedented combination of the extremely high luminosity and good resolution of the neutron beam in the new experimental area (EAR2) of the n_TOF facility at CERN, the availability of a sufficient amount of chemically pure Be7, and a specifically designed experimental setup. Coincidences between the two alpha particles have been recorded in two Si-Be7-Si arrays placed directly in the neutron beam. The present results are consistent, at thermal neutron energy, with the only previous measurement performed in the 1960s at a nuclear reactor. The energy dependence reported here clearly indicates the inadequacy of the cross section estimates currently used in BBN calculations. Although new measurements at higher neutron energy may still be needed, the n_TOF results hint at a minor role of this reaction in BBN, leaving the long-standing cosmological lithium problem unsolved.

The neutron time of flight (n_TOF) facility at CERN is a spallation source characterized by a white neutron spectrum. The innovative features of the facility, in the two experimental areas, (20 m and 185 m), allow for an accurate determination of the neutron cross section for radioactive samples or for isotopes with small neutron capture cross section, of interest for Nuclear Astrophysics. The recent results obtained at n_TOF facility are presented.

Monte Carlo (MC) simulations are an essential tool to determine fundamental features of a neutron beam, such as the neutron flux or the ¿ -ray background, that sometimes can not be measured or at least not in every position or energy range. Until recently, the most widely used MC codes in this field had been MCNPX and FLUKA. However, the Geant4 toolkit has also become a competitive code for the transport of neutrons after the development of the native Geant4 format for neutron data libraries, G4NDL. In this context, we present the Geant4 simulations of the neutron spallation target of the n TOF facility at CERN, done with version 10.1.1 of the toolkit. The first goal was the validation of the intra-nuclear cascade models implemented in the code using, as benchmark, the characteristics of the neutron beam measured at the first experimental area (EAR1), especially the neutron flux and energy distribution, and the time distribution of neutrons of equal kinetic energy, the so-called Resolution Function. The second goal was the development of aMonte Carlo tool aimed to provide useful calculations for both the analysis and planning of the upcoming measurements at the new experimental area (EAR2) of the facility.