Measurement of $\gamma$ rays from $^6$LiF tile as an inner wall of a neutron-decay detector

TL;DR

This study measured gamma-ray emissions from $^6$LiF tiles used in a neutron decay detector to better estimate background noise, employing prompt gamma-ray analysis and Monte Carlo simulations for accurate quantification.

Contribution

The paper provides the first detailed measurement of gamma-ray emissions from $^6$LiF tiles in a neutron decay detector, refining background estimates for neutron lifetime experiments.

Findings

Gamma-ray emission from $^6$LiF tiles is about 1.4 times higher than initially assumed.

The gamma-ray production is not more than twice the originally estimated value.

Quantitative data on gamma-ray emissions aids in improving neutron decay measurements.

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 $\gamma$ 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 $\gamma$ rays. In order to estimate the amount of $\gamma$ rays from the $^{6}{\rm LiF}$ tile, prompt gamma ray analysis (PGA) measurements were performed using germanium detectors. We reconstructed the measured $\gamma$-ray energy spectrum using a Monte Carlo simulation with the stripping method. Comparing the measured spectrum with a simulated one, the number of $\gamma$ rays emitted from the$^{6}{\rm LiF}$ tile was $(2.3^{+0.7}_{-0.3}) \times 10^{-4}$ per incident neutron. This is $1.4^{+0.5}_{-0.2}$ times the value assumed for a mole fraction of the $^{6}{\rm LiF}$ tile. We concluded that the amount of $\gamma$ rays produced from the $^{6}{\rm LiF}$ tile is not more twice the originally assumed value.