The Role of 14-MeV Neutrons in Light Element Nucleosynthesis

V Valkovic,D Sudac,J Obhodas

doi:10.1109/tns.2018.2847743

Abstract

The 14-MeV neutron-induced nuclear reactions which might contribute to better understanding of the so-called “cosmological lithium problem” have been discussed. The big bang nucleosynthesis theory predicted 7Li overestimated value could be lowered by 2n-induced reactions on Li isotopes and/or 7Be destruction via resonance in the d + 7Be → 9B process. Proposals for 6Li + 2n and 7Li + 2n reactions’ cross section evaluations and measurements are discussed in some details. In addition, we discuss also a preliminary measurements of n–n coincidences from the 10B(n,2n) 9B reaction with two neutron detectors placed outside the cone of 14-MeV tagged neutron beam. From the time-of-flight measurements and known geometry, we tried to deduce the neutron energies and calculate the 9B missing mass spectrum. Improvements for better measurements are suggested.

Highlights

THE 14 MeV neutrons are produced in the 2H+3H→4He+n nuclear reaction, (Q=17.590 MeV)
We propose Li-isotopes destruction by the following series of nuclear reactions: (i) 6Li+2n→8Li→8Be+e+ῡ followed by 8Be→2α. (ii) 7Li+2n→9Li→9Be+e+ῡ followed by 9Be→n+2α Where the 2n symbol represent neutron-neutron final state interaction as observed in the n+d→p+n+n reaction for En = 14 MeV bombarding energy. (Nuclear reactions 1H+2H→ 3H+e++νe and/or 2H+2H→3H+1H can than lead to 14 MeV neutron producing reaction 2H+3H→4He+n)
The neutron-neutron scattering length of ann = -21.7 F deduced from the measured proton spectrum from the n+d→p+n+n reaction at En = 14 MeV bombarding energy allows the two neutrons to interact with target nucleus as a single projectile

Summary

INTRODUCTION

THE 14 MeV neutrons are produced in the 2H+3H→4He+n nuclear reaction, (Q=17.590 MeV). One of the most important unresolved problem in nuclear astrophysics is so-called “cosmological lithium problem” It refers to the large discrepancy between the abundance of primordial 7Li predicted by the standard BBN theory and the value inferred from the so called “Spite plateau” in halo stars. During BBN, in the period of transition from quark-gluon plasma to baryonic matter, probability of 2n formation [n-n fsi or n-n bound state, if existed at that time] was increased This would have led to Li distraction via reactions (i) and (ii). Since the primordial 7Li is mainly produced by β-decay of 7Be (t1⁄2=53.2 days) the abundance of 7Li is essentially determined by the production and destruction of 7Be. The neutron induced reactions can play a role in the destruction of 7Be, in particular the 7Be(n,α)4He reaction in the energy range of interest for BBN, in particular between 20 and. Particular attention has been paid to the reactions involving 9B compound nuclear system d+7Be→9B

THE 2N-INDUCED NUCLEAR REACTIONS

Did not find

Experimental set-up

Experimental results

CONCLUSION