Measurement of the 225Ac half-life

The Ac-225 half-life was determined by measuring the activity of Ac-225 sources as a function of time, using various detection techniques: alpha-particle counting with a planar silicon detector at a defined small solid angle and in a nearly-2 pi geometry, 4 pi alpha + beta counting with a windowless CsI sandwich spectrometer and with a pressurised proportional counter, gamma-ray spectrometry with a HPGe detector and with a NaI(TI) well detector. Depending on the technique, the decay was followed for 59-141 d, which is about 6-14 times the Ac-225 half-life. The six measurement results were in good mutual agreement and their mean value is T-1/2(Ac-225)=9.920 (3) d. This half-life value is more precise and better documented than the currently recommended value of 10.0 d, based on two old measurements lacking uncertainty evaluations. (C) 2012 Elsevier Ltd. All rights reserved.