Shedding Light on the Origin of ^{204}Pb, the Heaviest s-Process-Only Isotope in the Solar System

© 2024 The Author(s). Published by the American Physical Society. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0/Asymptotic giant branch stars are responsible for the production of most of the heavy isotopes beyond Sr observed in the solar system. Among them, isotopes shielded from the r-process contribution by their stable isobars are defined as s-only nuclei. For a long time the abundance of ^{204}Pb, the heaviest s-only isotope, has been a topic of debate because state-of-the-art stellar models appeared to systematically underestimate its solar abundance. Besides the impact of uncertainties from stellar models and galactic chemical evolution simulations, this discrepancy was further obscured by rather divergent theoretical estimates for the neutron capture cross section of its radioactive precursor in the neutron-capture flow, ^{204}Tl (t_{1/2}=3.78  yr), and by the lack of experimental data on this reaction. We present the first ever neutron capture measurement on ^{204}Tl, conducted at the CERN neutron time-of-flight facility n_TOF, employing a sample of only 9 mg of ^{204}Tl produced at the Institute Laue Langevin high flux reactor. By complementing our new results with semiempirical calculations we obtained, at the s-process temperatures of kT≈8  keV and kT≈30  keV, Maxwellian-averaged cross sections (MACS) of 580(168) mb and 260(90) mb, respectively. These figures are about 3% lower and 20% higher than the corresponding values widely used in astrophysical calculations, which were based only on theoretical calculations. By using the new ^{204}Tl MACS, the uncertainty arising from the ^{204}Tl(n,γ) cross section on the s-process abundance of ^{204}Pb has been reduced from ∼30% down to +8%/-6%, and the s-process calculations are in agreement with the latest solar system abundance of ^{204}Pb reported by K. Lodders in 2021.Peer reviewe

Measurement of the neutron-induced fission cross section of Th 230 at the CERN n_TOF facility

The neutron-induced fission cross section of 230 Th has been measured at the neutron time-of-flight facility n_TOF located at CERN. The experiment was performed at the experimental area EAR-1 with a neutron flight path of 185 m, using Micromegas detectors for the detection of the fission fragments. The 230 Th(n, f ) cross section was determined relative to the 235 U(n, f ) one, covering the energy range from the fission threshold up to 400 MeV. The results from the present work are compared with existing cross-section datasets and the observed discrepancies are discussed and analyzed. Finally, using the code EMPIRE 3.2.3 a theoretical study, based on the statistical model, was performed leading to a satisfactory reproduction of the experimental results with the proper tuning of the respective parameters, while for incident neutron energy beyond 200 MeV the fission of 230 Th was described by Monte Carlo simulations.This project received funding from the Euratom “Support safe operation of nuclear systems” program 2014–2018 under Grant Agreement No. 847552 (SANDA) and by the funding agencies of the participating institutes. This research is imple- mented through the IKY scholarships program and cofinanced by the European Union (European Social Fund ’ESF) and Greek national funds through the action entitled “Reinforce- ment of Postdoctoral Researchers - 2nd call (MIS 5033021)”, in the framework of the Operational Programme “Human Resources Development Program, Education and Lifelong Learning” of the National Strategic Reference Framework.Article signat per 137 autors/es: V. Michalopoulou, A. Stamatopoulos, M. Diakaki, A. Tsinganis, R. Vlastou, M. Kokkoris, N. Patronis, Z. Eleme, D. Macina, L. Tassan-Got, N. Colonna, E. Chiaveri, A. Ventura, P. Schillebeeckx, J. Heyse, G. Sibbens, G. Alaerts, A. Borella, A. Moens, D. Vanleeuw, O. Aberle, V. Alcayne, S. Amaducci, J. Andrzejewski, L. Audouin, V. Babiano-Suarez, M. Bacak, M. Barbagallo, S. Bennett, E. Berthoumieux, J. Billowes, D. Bosnar, A. Brown, M. Busso, M. Caamaño, L. Caballero, F. Calviño, M. Calviani, D. Cano-Ott, A. Casanovas, F. Cerutti, G. Cortés, M. A. Cortés-Giraldo, L. Cosentino, S. Cristallo, L. A. Damone, P. J. Davies, M. Dietz, C. Domingo-Pardo, R. Dressler, Q. Ducasse, E. Dupont, I. Durán, B. Fernández-Domínguez, A. Ferrari, P. Finocchiaro, V. Furman, K. Göbel, R. Garg, A. Gawlik-Ramiega, S. Gilardoni, I. F. Gonçalves, E. González-Romero, C. Guerrero, F. Gunsing, H. Harada, S. Heinitz, D. G. Jenkins, A. Junghans, F. Käppeler, Y. Kadi, A. Kimura, I. Knapová, Y. Kopatch, M. Krticka, D. Kurtulgil, I. Ladarescu, C. Lederer-Woods, H. Leeb, J. Lerendegui-Marco, S. J. Lonsdale, A. Manna, T. Martínez, A. Masi, C. Massimi, P. Mastinu, M. Mastromarco, E. A. Maugeri, A. Mazzone, E. Mendoza, A. Mengoni, P. M. Milazzo, F. Mingrone, J. Moreno-Soto, A. Musumarra, A. Negret, R. Nolte, F. Ogállar, A. Oprea, A. Pavlik, J. Perkowski, C. Petrone, L. Piersanti, E. Pirovano, I. Porras, J. Praena, J. M. Quesada, D. Ramos-Doval, T. Rauscher, R. Reifarth, D. Rochman, Y. Romanets, C. Rubbia, M. Sabaté-Gilarte, A. Saxena, D. Schumann, A. Sekhar, A. G. Smith, N. V. Sosnin, P. Sprung, G. Tagliente, J. L. Tain, A. Tarifeño-Saldivia, Th. Thomas, P. Torres-Sánchez, J. Ulrich, S. Urlass, S. Valenta, G. Vannini, V. Variale, P. Vaz, D. Vescovi, V. Vlachoudis, A. Wallner, P. J. Woods, T. Wright, and P. Žugec.Postprint (published version

Shedding Light on the Origin of 204^{204}Pb , the Heaviest s-Process–Only Isotope in the Solar System

Asymptotic giant branch stars are responsible for the production of most of the heavy isotopes beyond Sr observed in the solar system. Among them, isotopes shielded from the r-process contribution by their stable isobars are defined as s-only nuclei. For a long time the abundance of $^{204}$Pb, the heaviest s-only isotope, has been a topic of debate because state-of-the-art stellar models appeared to systematically underestimate its solar abundance. Besides the impact of uncertainties from stellar models and galactic chemical evolution simulations, this discrepancy was further obscured by rather divergent theoretical estimates for the neutron capture cross section of its radioactive precursor in the neutron-capture flow, $^{204}$Tl (t1=2 ¼ 3.78 yr), and by the lack of experimental data on this reaction. We present the first ever neutron capture measurement on $^{204}$Tl, conducted at the CERN neutron time-of-flight facility n_TOF, employing a sample of only 9 mg of $^{204}$Tl produced at the Institute Laue Langevin high flux reactor. By complementing our new results with semiempirical calculations we obtained, at the s-process temperatures of kT ≈ 8 keV and kT ≈ 30 keV, Maxwellian-averaged cross sections (MACS) of 580(168) mb and 260(90) mb, respectively. These figures are about 3% lower and 20% higher than the corresponding values widely used in astrophysical calculations, which were based only on theoretical calculations. By using the new 204Tl MACS, the uncertainty arising from the ^{204}$Tl(n, γ) cross section on the s-process abundance of 204Pb has been reduced from ∼30$^{204}$Pb reported by K. Lodders in 2021

Be-7(n,alpha)He-4 reaction and the cosmological lithium problem: measurement of the cross section in a wide energy range at n_TOF at CERN

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

Shedding Light on the Origin of <math display="inline"><mrow><mmultiscripts><mrow><mi>Pb</mi></mrow><mprescripts/><none/><mrow><mn>204</mn></mrow></mmultiscripts></mrow></math>, the Heaviest <math display="inline"><mrow><mi>s</mi></mrow></math>-Process–Only Isotope in the Solar System

International audienceAsymptotic giant branch stars are responsible for the production of most of the heavy isotopes beyond Sr observed in the solar system. Among them, isotopes shielded from the r-process contribution by their stable isobars are defined as s-only nuclei. For a long time the abundance of Pb204, the heaviest s-only isotope, has been a topic of debate because state-of-the-art stellar models appeared to systematically underestimate its solar abundance. Besides the impact of uncertainties from stellar models and galactic chemical evolution simulations, this discrepancy was further obscured by rather divergent theoretical estimates for the neutron capture cross section of its radioactive precursor in the neutron-capture flow, Tl204 (t1/2=3.78  yr), and by the lack of experimental data on this reaction. We present the first ever neutron capture measurement on Tl204, conducted at the CERN neutron time-of-flight facility n_TOF, employing a sample of only 9 mg of Tl204 produced at the Institute Laue Langevin high flux reactor. By complementing our new results with semiempirical calculations we obtained, at the s-process temperatures of kT≈8  keV and kT≈30  keV, Maxwellian-averaged cross sections (MACS) of 580(168) mb and 260(90) mb, respectively. These figures are about 3% lower and 20% higher than the corresponding values widely used in astrophysical calculations, which were based only on theoretical calculations. By using the new Tl204 MACS, the uncertainty arising from the Tl204(n,γ) cross section on the s-process abundance of Pb204 has been reduced from ∼30% down to +8%/-6%, and the s-process calculations are in agreement with the latest solar system abundance of Pb204 reported by K. Lodders in 2021

Study of the photon strength functions and level density in the gamma decay of the n + 234U reaction

The accurate calculations of neutron-induced reaction cross sections are relevant for many nuclear applications. The photon strength functions and nuclear level densities are essential inputs for such calculations. These quantities for 235U are studied using the measurement of the gamma de-excitation cascades in radiative capture on 234U with the Total Absorption Calorimeter at n_TOF at CERN. This segmented 4π gamma calorimeter is designed to detect gamma rays emitted from the nucleus with high efficiency. This experiment provides information on gamma multiplicity and gamma spectra that can be compared with numerical simulations. The code DICEBOXC is used to simulate the gamma cascades while GEANT4 is used for the simulation of the interaction of these gammas with the TAC materials. Available models and their parameters are being tested using the present data. Some preliminary results of this ongoing study are presented and discussed

New reaction rates for the destruction of 7Be during big bang nucleosynthesis measured at CERN/n_TOF and their implications on the cosmological lithium problem

New measurements of the 7Be(n,α)4He and 7Be(n,p)7Li reaction cross sections from thermal to keV neutron energies have been recently performed at CERN/n_TOF. Based on the new experimental results, astrophysical reaction rates have been derived for both reactions, including a proper evaluation of their uncertainties in the thermal energy range of interest for big bang nucleosynthesis studies. The new estimate of the 7Be destruction rate, based on these new results, yields a decrease of the predicted cosmological 7Li abundance insufficient to provide a viable solution to the cosmological lithium problem

Status and perspectives of the neutron time-of-flight facility n_TOF at CERN

Since the start of its operation in 2001, based on an idea of Prof. Carlo Rubbia [1], the neutron time of-flight facility of CERN, n_TOF, has become one of the most forefront neutron facilities in the world for wide-energy spectrum neutron cross section measurements. Thanks to the combination of excellent neutron energy resolution and high instantaneous neutron flux available in the two experimental areas, the second of which has been constructed in 2014, n_TOF is providing a wealth of new data on neutron-induced reactions of interest for nuclear astrophysics, advanced nuclear technologies and medical applications. The unique features of the facility will continue to be exploited in the future, to perform challenging new measurements addressing the still open issues and long-standing quests in the field of neutron physics. In this document the main characteristics of the n_TOF facility and their relevance for neutron studies in the different areas of research will be outlined, addressing the possible future contribution of n_TOF in the fields of nuclear astrophysics, nuclear technologies and medical applications. In addition, the future perspectives of the facility will be described including the upgrade of the spallation target, the setup of an imaging installation and the construction of a new irradiation area

The CERN n_TOF facility: a unique tool for nuclear data measurement

he study of the resonant structures in neutron-nucleus cross-sections, and therefore of the compound-nucleus reaction mechanism, requires spectroscopic measurements to determine with high accuracy the energy of the neutron interacting with the material under study. To this purpose, the neutron time-of-flight facility n_TOF has been operating since 2001 at CERN. Its characteristics, such as the high intensity instantaneous neutron flux, the wide energy range from thermal to few GeV, and the very good energy resolution, are perfectly suited to perform high-quality measurements of neutron-induced reaction cross sections. The precise and accurate knowledge of these cross sections plays a fundamental role in nuclear technologies, nuclear astrophysics and nuclear physics. Two different measuring stations are available at the n_TOF facility, called EAR1 and EAR2, with different characteristics of intensity of the neutron flux and energy resolution. These experimental areas, combined with advanced detection systems lead to a great flexibility in performing challenging measurement of high precision and accuracy, and allow the investigation isotopes with very low cross sections, or available only in small quantities, or with very high specific activity. The characteristics and performances of the two experimental areas of the n_TOF facility will be presented, together with the most important measurements performed to date and their physics case. In addition, the significant upcoming measurements will be introduced

First measurement of 72Ge(n,γ) at n_TOF

The slow neutron capture process (s-process) is responsible for producing about half of the elemental abundances heavier than iron in the universe. Neutron capture cross sections on stable isotopes are a key nuclear physics input for s-process studies. The 72Ge(n, γ) cross section has an important influence on production of isotopes between Ge and Zr during s-process in massive stars and therefore experimental data are urgently required. 72Ge(n, γ) was measured at the neutron time-of-flight facility n_TOF (CERN) for the first time at stellar energies. The measurement was performed using an enriched 72GeO2 sample at a flight path of 185m with a set of liquid scintillation detectors (C6D6). The motivation, experiment and current status of the data analysis are reported