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تسجيل مستخدم جديدThe 7Be(n,alpha)4He reaction and the Cosmological Lithium Problem: measurement of the cross section in a wide energy range at n_TOF (CERN)
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The energy-dependent cross section of the 7Be(n,alpha)4He 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 7Be 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 7Be, and a specifically designed experimental setup. Coincidences between the two alpha-particles have been recorded in two Si-7Be-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 60s at a nuclear reactor. The energy dependence here reported 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 to a minor role of this reaction in BBN, leaving the long-standing Cosmological Lithium problem unsolved.
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We report on the measurement of the $^{7}$Be($n, p$)$^{7}$Li cross section from thermal to approximately 325 keV neutron energy, performed in the high-flux experimental area (EAR2) of the n_TOF facility at CERN. This reaction plays a key role in the
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We report on the measurement of the $^{7}$Be($n, p$)$^{7}$Li cross section from thermal to approximately 325 keV neutron energy, performed in the high-flux experimental area (EAR2) of the n_TOF facility at CERN. This reaction plays a key role in the
lithium yield of the Big Bang Nucleosynthesis (BBN) for standard cosmology. The only two previous time-of-flight measurements performed on this reaction did not cover the energy window of interest for BBN, and showed a large discrepancy between each other. The measurement was performed with a Si-telescope, and a high-purity sample produced by implantation of a $^{7}$Be ion beam at the ISOLDE facility at CERN. While a significantly higher cross section is found at low-energy, relative to current evaluations, in the region of BBN interest the present results are consistent with the values inferred from the time-reversal $^{7}$Li($p, n$)$^{7}$Be reaction, thus yielding only a relatively minor improvement on the so-called Cosmological Lithium Problem (CLiP). The relevance of these results on the near-threshold neutron production in the p+$^{7}$Li reaction is also discussed.
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