Measurement of ${\hat{I}}^3$ rays from ${}^6$LiF tile as an inner wall of a neutron-decay detector

Abstract

A neutron lifetime measurement conducted at the Japan Proton Accelerator Research Complex (J-PARC) is counting the number of electrons from neutron decays with a time projection chamber (TPC). The ${\hat{I}}^3$ rays produced in the TPC cause irreducible background events. To achieve the precise measurement, the inner walls of the TPC consist of ${}^6$Li-enriched lithium-fluoride (${}^6$LiF) tiles to suppress the amount of ${\hat{I}}^3$ rays. In order to estimate the amount of ${\hat{I}}^3$ rays from the ${}^6\mathrm{LiF}$ tile, prompt gamma ray analysis (PGA) measurements were performed using germanium detectors. We reconstructed the measured ${\hat{I}}^3$-ray energy spectrum using a Monte Carlo simulation with the stripping method. Comparing the measured spectrum with a simulated one, the number of ${\hat{I}}^3$ rays emitted from the${}^6\mathrm{LiF}$ tile was $(2.3_{-0.3}^{+0.7})\times 10^{-4}$ per incident neutron. This is $1.4_{-0.2}^{+0.5}$ times the value assumed for a mole fraction of the ${}^6\mathrm{LiF}$ tile. We concluded that the amount of ${\hat{I}}^3$ rays produced from the ${}^6\mathrm{LiF}$ tile is not more twice the originally assumed value.

14 pages, 11 figures, will be submitted to JINST