N + 7Be Cross-Sections of Astrophysical Interest at the CERN n_TOF Facility

Mario Mastromarco,Massimo Barbagallo,The N_Tof Collaboration,Lucia Damone

doi:10.3390/proceedings2019013012

Abstract

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.

Highlights

Based on the Standard Model of electroweak and strong interactions, the Big Bang Nucleosynthesis (BBN) theory predicts the production of the lightest nuclides such as D, 3 He, 4 He, and 7 Li in the early Universe
These results indicate that the 7 Be(n, α)4 He reaction cannot play any role in the Cosmological Lithium Problem, which worsens by a few percent [16]
The n_TOF data, combined with the cross section extracted from the inverse reaction, have been fitted in Single Level Breit–Wigner approximation (SLBW) by means of an R-matrix code, in order to obtain a smooth cross section all the way from thermal neutron energy to a few MeV, covering in particular the energy region of interest for BBN calculations

Summary

Introduction

Based on the Standard Model of electroweak and strong interactions, the Big Bang Nucleosynthesis (BBN) theory predicts the production of the lightest nuclides such as D, 3 He, 4 He, and 7 Li in the early Universe. The PS repetition rate below 0.8 Hz avoids the overlap of consecutive neutron bunches Both 7 Be(n, α) He and 7 Be(n, p) Li reaction cross-sections have been measured in the second experimental area, n_TOF-EAR2, characterized by an extremely high instantaneous neutron flux of 107 neutrons/pulse, 40 times higher than the one in EAR1 (see Figure 1). These characteristics make EAR2 ideal for measurements on isotopes available in very small amounts and with short half-lives, as is the case of 7 Be. More details on the EAR2 neutron beam can be found in Refs. More details on the EAR2 neutron beam can be found in Refs. [12,13]

The Experimental Setup

Data Analysis

Results

Conclusions