Measurements of internal radioactive contamination in samples of Roman lead to be used in experiments on rare events

Abstract

Improved results of a series of measurements carried out on two different types of Roman lead to be used in shields for experiments on rare events are reported. The chemical impurities in both samples have been determined by neutron activation. Underground measurements based on γ-spectroscopy on large masses of the two types of lead, show the absence in both samples of radioactive contamination from 214Bi and 232Th, in secular equilibrium, with upper limits of a few tenths of a mBq kg −1. The contamination from 40K is less than a few mBq kg −1. Much care has been addressed to the contamination due to 210Pb which breaks secular equilibrium and which contributes to most of the background in experiments searching low energy events like direct interactions of Weakly Interacting Massive Particles (WIMPS). We have applied to this problem the technique of cryogenic detection and found for the two samples upper limits for contamination of 210Pb of 4 and 7 mBq kg −1, the lowest ever determined for any type of lead.