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

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.