Development of a low-background neutron detector array

Abstract

A low-background neutron detector array was developed to measure the cross section of the ${}^{13}$C($\hat{I}\pm$,n)${}^{16}$O reaction, which is the neutron source for the $s$-process in AGB stars, in the Gamow window ($E_{c.m.}$ = 190 $\pm$ 40 keV) at the China Jinping Underground Laboratory (CJPL). The detector array consists of 24 ${}^3$He proportional counters embedded in a polyethylene cube. Due to the deep underground location and a borated polyethylene shield around the detector array, a low background of 4.5(2)/hour was achieved. The ${}^{51}$V(p, n)${}^{51}$Cr reaction was used to determine the neutron detection efficiency of the array for neutrons with energy $E_n$ $<$ 1 MeV. Geant4 simulations, which were shown to well reproduce experimental results, were used to extrapolate the detection efficiency to higher energies for neutrons emitted in the ${}^{13}$C($\hat{I}\pm$,n) ${}^{16}$O reaction. The theoretical angular distributions of the ${}^{13}$C($\hat{I}\pm$,n)${}^{16}$O reaction were shown to be important in estimating the uncertainties of the detection efficiency.