Li7(p,n)Be7 cross section from threshold to 1960 keV and precise measurement of the

Guido Martín-Hernández

¹

,

P. Mastinu

²

,

E. González

³

et al.

Abstract

Background: The 7 Li(p, n) 7 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 Li(p, n) 7 Be reaction cross section, and measure with small uncertainty the 197 Au(n, γ ) 198 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 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 n / p = 5.4 1 − T th /T p . The detailed balance principle is used to obtain the cosmologically important time-reversed 7 Be(n, p) 7 Li reaction cross section. The measured 197 Au(n, γ ) 198 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 Li(p, n) 7 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.

Work on the neutron standards since the completion of the standards evaluation

“…Several recent experiments are in very good agreement with the standards values (e.g. [28] [29], [30], [31] ).…”

Hierarchical Bayesian Thermonuclear Rate for the $^7$Be(n,p)$^7$Li Big Bang Nucleosynthesis Reaction

“…We also adopted the results of two 7 Li(p,n) 7 Be measurements (Gibbons & Macklin 1959;Martín-Hernández et al 2019), and transformed the data to 7 Be(n,p) 7 Li cross sections using the reciprocity theorem (see Appendix A.1). We only considered data for proton laboratory energies of E p ≤ 2371 keV (or neutron center-ofmass energies of E7 Be+n ≤ 420 keV), since at higher energies the neutron channel to the first excited 7 Be state at 429 keV is open.…”

“…The green inverted triangles and orange squares show the non-thermal data of Koehler et al (1988) and Damone et al (2018), respectively. The blue triangles and purple circles correspond to the transformed 7 Li(p,n) 7 Be cross sections of Gibbons & Macklin (1959) and Martín-Hernández et al (2019), respectively. The red data points at thermal neutron energy (E c.m.…”

“…The single data points of Červená et al (1989) and Tomandl et al (2019) at thermal neutron energy provide only information on the absolute cross section. The data sets of Gibbons & Macklin (1959) and Martín-Hernández et al (2019) were also split into two parts, one part containing all data points, except one, providing only relative cross section information, and one part containing a single data point (here arbitrarily chosen at the lowest measured energy) that provides only information on the absolute normalization. Since the thermal and non-thermal data of Damone et al (2018) share the same absolute normalization, we treated their reported thermal cross section as an absolute cross section, and all other data points as relative cross sections.…”

“…The index i = 1, ..., 160 labels individual data points, j = 1, ..., 4 denotes the relative data sets with information on the energy dependence only (Gibbons & Macklin 1959;Koehler et al 1988;Martín-Hernández et al 2019;Damone et al 2018), k = 1, ..., 6 labels the absolute data sets with information on the cross section normalization only (Koehler et al 1988;Damone et al 2018;Gibbons & Macklin 1959;Martín-Hernández et al 2019;Červená et al 1989;Tomandl et al 2019), and r = 1, ..., 7 denotes the resonances. The symbols have the following meaning: measured energy (E exp r ) and measured reduced cross section (S exp ); true resonance energy (E r ); the true reduced cross section (S) is calculated from the cross section, σ np , according to Equation (1), using the R-matrix parameters (E r , γ 2 n;r , γ 2 p;r , a n , a p ).…”