First measurement of low intensity fast neutron background from rock at the Boulby Underground Laboratory

Abstract

A technique to measure low intensity fast neutron flux has been developed. The design, calibrations, procedure for data analysis and interpretation of the results are discussed in detail. The technique has been applied to measure the neutron background from rock at the Boulby Underground Laboratory, a site used for dark matter and other experiments, requiring shielding from cosmic ray muons. The experiment was performed using a liquid scintillation detector. A 6.1 litre volume stainless steel cell was filled with an in-house made liquid scintillator loaded with Gd to enhance neutron capture. A two-pulse signature (proton recoils followed by gammas from neutron capture) was used to identify the neutron events from much larger gamma background from PMTs. Suppression of gammas from the rock was achieved by surrounding the detector with high-purity lead and copper. Calibrations of the detector were performed with various gamma and neutron sources. Special care was taken to eliminate PMT afterpulses and correlated background events from the delayed coincidences of two pulses in the Bi-Po decay chain. A four month run revealed a neutron-induced event rate of 1.84 +- 0.65 (stat.) events/day. Monte Carlo simulations based on the GEANT4 toolkit were carried out to estimate the efficiency of the detector and the energy spectra of the expected proton recoils. From comparison of the measured rate with Monte Carlo simulations the flux of fast neutrons from rock was estimated as (1.72 +- 0.61 (stat.) +- 0.38 (syst.))*10^(-6) cm^(-2) s^(-1) above 0.5 MeV.

37 pages, 24 figures, to be published in Astroparticle Physics

References in corpus (5)

• Neutron Flux at the Gran Sasso Underground LaboratoryRevisited
• Limits on WIMP cross-sections from the NAIAD experiment at the Boulby Underground Laboratory
• Neutron background in large-scale xenon detectors for dark matter searches
• Low energy neutron propagation in MCNPX and GEANT4
• Measurements of muon flux at 1070 metres vertical depth in the Boulby underground laboratory

Cited by in corpus (14)

• The case for a directional dark matter detector and the status of current experimental efforts
• Low energy neutron background in deep underground laboratories
• Low Threshold Results and Limits from the DRIFT Directional Dark Matter Detector
• First Results from the DRIFT-IIa Dark Matter Detector
• Neutron- and muon-induced background in underground physics experiments
• Measurements of neutrons produced by high-energy muons at the Boulby Underground Laboratory
• Simulation of neutrons produced by high-energy muons underground
• Radioactivity Backgrounds in ZEPLIN-III
• Direct Dark Matter Searches
• Neutron background measurement for rare event search experiments in the YangYang Underground Laboratory
• Neutron background measurements at China Jinping underground laboratory with a Bonner Multi-sphere Spectrometer
• Measurement of the fast neutron background at the China Jinping Underground Laboratory
• Direct measurement of neutrons induced in lead by cosmic muons at a shallow underground site
• Response of gadolinium doped liquid scintillator to charged particles: measurement based on intrinsic U/Th contamination