The half-life of 227Th by direct and indirect measurements

• First direct measurement of 227Th half-life by HPGe γ-ray spectrometry.
• Precision half-life measurement by ionisation chamber.
• New half-life of 18.695 (4) days determined.
• Critical evaluation of published half-lives and recommended value determined.

Utilising a chemically purified solution the radioactive half-life of 227Th has been determined indirectly by observation of the ingrowth of 223Ra using an ionisation chamber (IC) and for the first time by direct observation of the change in activity with time using a high-purity germanium (HPGe) γ-ray spectrometer. The radioactive decay was observed for ~104 days (~5.6 half-lives) by γ-ray spectrometry and approximately 63 days and 72 days (~3.4 and ~3.9 half-lives) using an ionisation chamber (IC). The resulting half-life values – 18.695 (4) days (IC) and 18.683 (20) days (HPGe) – are consistent and detailed uncertainty budgets are presented for the two measurement techniques. A weighted mean of our results of 18.695 (4) days is inconsistent with the most precise published half-life value of 18.7176 (52) days (Jordan and Blanke, 1967). A critical evaluation of literature data has been performed, indicating a paucity of reliable and independent measurements. Selected independent published values have been used to determine a recommended half-life of 18.697 (7) days. A method has been introduced in the course of this work so that the recommended half-life of 227Th as determined by ingrowth can be modified if a different 223Ra half-life has been determined, evaluated and adopted.