The Interaction of Neutrons with ⁷Be at BBN Temperatures: Lack of Standard Nuclear Solution to the “Primordial ⁷Li Problem”

¹ ‘LNS at Avery Point, University of Connecticut, Groton, CT 06340, USA
² Department of Particle Physics, Weizmann Inst. of Science, Rehovot 76100, Israel
³ Department of Physics, San Diego State University, San Diego, CA 92182-1233, USA
⁴ ISOLDE, CERN, CH-1211 Geneva, Switzerland
⁵ Faculty of Engineering, Bar Ilan University, Ramat Gan 52900, Israel
⁶ Bio-Imaging Unit, Institute for Life Sciences, Hebrew University, Jerusalem 91904, Israel

^* Communication author: moshe.gai@uconn.edu
^† Deceased

Published online: 14 January 2020

Abstract

We report the first measurement of alpha-particles from the interaction of neutrons with ⁷Be at “temperatures” of Big Bang Nucleosynthesis (BBN). We measured the Maxwellian averaged cross sections (MACS), with neutron beams produced by the LiLiT at the SARAF in Israel (with kT = 49.5 keV hence 0.57 GK). In addition, we measured the cross section of the ⁷Be(n,p) reaction, which is in excellent agreement with the recent measurement of the n_TOF collaboration, further substantiating our method as a demonstration of “proof of principle”. The cross section for the ⁷Be(n,ga) and the ⁷Be(n,a) reaction measured in the “BBN window” is considerably smaller than compiled by Wagoner in 1969 and used today in Big Bang Nucleosynthesis (BBN). We also rule out a hitherto unknown resonance in ⁸Be at the BBN window, that was conjectured as a possible standard nuclear physics solution to the “Primordial ⁷Li Problem”. Together with previous results, we deduce a new Wagoner-like Rate for the destruction of ⁷Be by neutrons which is based on all current measured data. We conclude the lack of a standard nuclear solution to the “Primordial ⁷Li Problem”. Our upper limit on the cross sections for the high energy alpha-particles is in agreement with recent measurement of the n_TOF collaboration, but it is considerably smaller than the p-wave extrapolation of the Kyoto collaboration. We measured the alpha-particles from the ⁷Be(n,gi)⁸Be*(3.03 MeV) reaction, which is considerably larger than a previous s-wave estimate. Hence, in contrast, we conclude s-wave dominance at BBN energies, as would be expected due to the broad (122 keV) low lying 2” state at En = 10 keV.