Use of lead for the reduction of background gamma rays in underground experiments

Braoudakis, G., Peak, L.S. and Soler, F.J.P. (1998) Use of lead for the reduction of background gamma rays in underground experiments. Nuclear Instruments and Methods in Physics Research. Section A: Accelerators, Spectrometers, Detectors, and Associated Equipment, 403(2-3), pp. 499-512. (doi: 10.1016/S0168-9002(97)01126-1)

Abstract

In order to determine the effectiveness of lead for the reduction of gamma-rays at a site 1230 m underground, a sodium iodide crystal detector has been used to measure the gamma-ray spectrum up to 9 MeV emanating from a shield of lead that totally surrounded the detector. Below 3 MeV, the contribution to the gamma-ray spectrum was from long-lived radioactive trace elements in the shield itself. From these measurements it was possible to estimate the concentration of ²³⁸U and ²³²Th in the lead to be 1.9 × 10⁻⁷ and 3.4 × 10⁻⁸ gg⁻¹, respectively, more than one order of magnitude less than what is observed in the environment. From 4 to 6 MeV the activity observed had to be internal to the detection system. The most probable source of these events is due to a combination of alpha and beta decays from daughter products of ²³²Th. Above 6 MeV, the activity observed is predominantly due to (n,γ) interactions in the NaI crystal, in the lead shield, in the stainless-steel housing of the detector and in the materials that make up the photomultiplier tube, with a negligible contribution from ²³⁸U fissions. In consequence, for most applications, a solitary shield of lead would not be sufficient to achieve a background-free environment, so it would be recommended that an additional neutron shield external to the lead would be needed to reduce the higher-energy gamma rays that originate from these (n,γ) reactions.

• Item Type: Articles
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Soler, Professor Paul
• Authors: Braoudakis, G., Peak, L.S., and Soler, F.J.P.
• College/School: College of Science and Engineering > School of Physics and Astronomy
• Journal Name: Nuclear Instruments and Methods in Physics Research. Section A: Accelerators, Spectrometers, Detectors, and Associated Equipment
• Publisher: Elsevier
• ISSN: 0168-9002
• ISSN (Online): 1872-9576