A large area detector for thermal neutron flux measurements at the KamLAND site

TL;DR

This paper presents the development and testing of a large-area thermal neutron detector using $^{6}$LiF:ZnS(Ag) scintillator sheets, capable of measuring low flux neutron variations in underground environments, with demonstrated application at KamLAND.

Contribution

The paper introduces a novel large-area neutron detector with digital pulse shape discrimination for background reduction, and provides the first measurement of thermal neutron flux at the KamLAND site.

Findings

Neutron detection efficiency of 0.368±0.018 before event selection.

Measured neutron flux at KamLAND: (6.43±0.50)×10^{-6} n/cm^2/sec.

Identified noise source in photomultiplier tubes affecting measurements.

Abstract

A large area (0.5~m $\times$ 0.5~m) thermal neutron detector was developed based on 0.32~mm-thick $^{6}$LiF:ZnS(Ag) scintillator sheets. The detector was constructed for measurements of seasonal variations in low intensity thermal neutron flux at deep underground sites. The detector response to neutrons was studied using a low intensity $^{241}$Am/$^{9}$Be($\alpha$,n) neutron source. The digital pulse shape discrimination technique was applied to separate neutron capture events from the background caused by traces of $\alpha$-emitters from Uranium and Thorium decay chains in scintillator components. The thermal neutron detection efficiency was 0.368$\pm$0.018 (0.155$\pm$0.009) before (after) event selection based on the digital pulse shape discrimination method. The measured $\alpha$-particle background event rate in the $^{6}$LiF:ZnS(Ag) scintillator was (4.88$\pm$0.06(stat))$\times$10$^{-6}$ $\alpha$ cm$^{-2}$sec$^{-1}$. As a test of detector performance we measured the thermal neutron flux at the KamLAND area of the Kamioka neutrino observatory and compared it with result of the neutron flux measurement carried out at the Super-Kamiokande site. The thermal neutron flux at KamLAND was (6.43$\pm$0.50)$\times$10$^{-6}$ n cm$^{-2}$sec$^{-1}$ that included $\sim$10~\% contribution from neutrons with energies above 1~eV. During the detector construction we identified a source of high amplitude noise pulses generated by 5-inch R1250 photomultipliers manufactured by Hamamatsu Photonics K. K..