Li7(p,n)Be7 cross section from threshold to 1960 keV and precise measurement of the Au197(n,γ) spectrum-averaged cross section at 30 keV

Abstract

Background: The $^{7}\mathrm{Li}(p,n)^{7}\mathrm{Be}$ reaction is one of the most used nuclear reaction for accelerator-based neutron sources. There are few experimental cross section data in the double-value energy region and they are discrepant, as are the reaction yields.Purpose: We derive the $^{7}\mathrm{Li}(p,n)^{7}\mathrm{Be}$ reaction cross section, and measure with small uncertainty the $^{197}\mathrm{Au}(n,\ensuremath{\gamma})^{198}\mathrm{Au}$ spectrum-averaged cross section at neutron energy around 30 keV.Method: By irradiating Li metal targets over the proton energy range of 1879 to 1960 keV, thick target yields were measured using the generated $^{7}\mathrm{Be}$ activity. Based on the theoretical description of the reaction yield, accelerator parameters and reaction cross sections are derived. Gold foils were activated with the neutron field generated by the $(p,n)$ reaction on a Li target at a proton energy of about a half keV above the reaction threshold.Results: The thick target yield is well reproduced when the Breit-Wigner single-resonance formula for $s$-wave particles is used to describe the reaction cross section. The ratio between neutron and proton widths was found to be equal to ${\mathrm{\ensuremath{\Gamma}}}_{n}/{\mathrm{\ensuremath{\Gamma}}}_{p}=5.4\sqrt{1\ensuremath{-}{T}_{th}/{T}_{p}}$. The detailed balance principle is used to obtain the cosmologically important time-reversed $^{7}\mathrm{Be}(n,p)^{7}\mathrm{Li}$ reaction cross section. The measured $^{197}\mathrm{Au}(n,\ensuremath{\gamma})^{198}\mathrm{Au}$ spectrum-averaged cross section agrees with the value calculated from the ENDF/B-VIII.0 library.Conclusions: We demonstrated the feasibility of deriving the $^{7}\mathrm{Li}(p,n)^{7}\mathrm{Be}$ reaction cross section from the thick target yield. Using the ratio between neutron and proton widths obtained in this work reduces the uncertainty in calculating the reaction cross section to a factor of 2.3.