Measurement of the 225Ac half-life

The 225Ac half-life was determined by measuring the activity of 225Ac sources as a function of time, using various detection techniques: α-particle counting with a planar silicon detector at a defined small solid angle and in a nearly-2π geometry, 4πα+β counting with a windowless CsI sandwich spectrometer and with a pressurised proportional counter, gamma-ray spectrometry with a HPGe detector and with a NaI(Tl) well detector. Depending on the technique, the decay was followed for 59–141d, which is about 6–14 times the 225Ac half-life. The six measurement results were in good mutual agreement and their mean value is T1/2(225Ac)=9.920 (3)d. This half-life value is more precise and better documented than the currently recommended value of 10.0d, based on two old measurements lacking uncertainty evaluations.