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 differ ent 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 g amma-spectroscopy on large masses of the two types of lead, show the a bsence in both samples of radioactive contamination from Bi-214 and Th -232, in secular equilibrium, with upper limits of a few tenths of a m Bq kg(-1). The contamination from 40K is less than a few mBq kg(-1). M uch care has been addressed to the contamination due to Pb-210 which b reaks secular equilibrium and which contributes to most of the backgro und in experiments searching low energy events like direct interaction s of Weakly Interacting Massive Particles (WIMPS). We have applied to this problem the technique of cryogenic detection and found for the tw o 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 Publishe d by Elsevier Science B.V. All rights reserved.