MEASUREMENTS ON RADIOACTIVITY OF ANCIENT-ROMAN LEAD TO BE USED AS SHIELD IN SEARCHES FOR RARE EVENTS

An ingot from the load of lead carried by a Roman ship sunken near Sardinia has been analysed for its composition and purity from radioactive contaminants in view of its possible use to shield low level counting experiments. Measurements carried out with X-ray fluorescence, X-ray diffraction and neutron activation show that the lead was desilvered, a beneficial procedure as far as the elimination of radioactive contaminants is concerned. Measurements of gamma and X-rays and of alpha particles have been carried out on a substantial part of the ingot and, for comparison, on samples of modern electrolytic and of specially prepared lead and of lead from a presumably 500 year old Dutch ship. Contaminations yielding high energy gamma rays have been found to be negligible in all samples. The analysis of gamma rays, X-rays, alpha particles and bremsstrahlung spectra on the contrary have consistently shown that the modern lead contains a considerable amount of Pb-210, which is totally absent in Roman lead. This metal, together with the special low radioactivity lead, is by far the best shielding material among the samples investigated in this study.