IRMM participated in a recent international comparison for the standardisat
ion of a Tl-204 solution organised by BIPM. The activity concentration of t
he Tl-204 solution was measured at IRMM using 3 independent methods; Liquid
Scintillation Counting using the CIEMAT/NIST method, 4 pi-CsI counting and
4 pi beta counting using a large pressurised proportional counter. This ar
ticle describes the measurements in detail and discusses potential problems
in the standardisation of Tl-204. (C) 2000 Elsevier Science Ltd. All right
s reserved.