Standardisation of Pb-210

The standardisation of Pb-210 is complicated by the presence of the daughte rs, Bi-210 and Po-210. In addition, the low energies of the beta emissions from Pb-210 make it difficult to obtain high detection efficiencies in an a tmospheric proportional counter and hence produce the need for large extrap olations with consequential large uncertainties when extrapolating to unit efficiency with the conventional 4 pi(PC)-gamma-coincidence technique. In o rder to produce a reliable standardisation, it is necessary to remove the d aughter products. A solution of Pb-210 was therefore chemically separated f rom its daughters and then standardised using the conventional 4 pi(LS)-gam ma-coincidence technique. The low energy (46 keV) and low emission probabil ity (4%) of the associated photon emissions effectively rules out the possi bility of using ionisation chambers as secondary standard transfer instrume nts for this nuclide. A germanium spectrometer therefore was calibrated for this purpose using Am-241 as a normalising agent. The results of this work are presented together with an analysis of the sta ndardisation uncertainties that can be achieved in practice. Crown Copyrigh t (C) 2000 Published by Elsevier Science Ltd. All rights reserved.