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

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 gamma-spectroscopy on large masses of the two types of lead, show the absence in both samples of radioactive contamination from Bi-214 and Th-232, 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 Pb-210 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 Pb-210 of 4 and 7 mBq kg(-1), the lowest ever determined for any type of lead. (C) 1998 Published by Elsevier Science B.V. All rights reserved.