title: The half-life of ²²⁷Th by direct and indirect measurements.
abstract: Utilising a chemically purified solution the radioactive half-life of (227)Th has been determined indirectly by observation of the ingrowth of (223)Ra 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 (227)Th as determined by ingrowth can be modified if a different (223)Ra half-life has been determined, evaluated and adopted.
authors.affiliation: National Physical Laboratory, Hampton Road, Teddington, Middlesex TW11 0LW, UK. Electronic address: sean.collins@npl.co.uk.
authors.forename: S M
authors.initials: SM
authors.lastname: Collins
authors.affiliation: European Commission, Joint Research Centre, Institute for Reference Materials and Measurements, Retieseweg 111, B-2440 Geel, Belgium.
authors.forename: S
authors.initials: S
authors.lastname: Pommé
authors.affiliation: National Physical Laboratory, Hampton Road, Teddington, Middlesex TW11 0LW, UK.
authors.forename: S M
authors.initials: SM
authors.lastname: Jerome
authors.affiliation: National Physical Laboratory, Hampton Road, Teddington, Middlesex TW11 0LW, UK.
authors.forename: K M
authors.initials: KM
authors.lastname: Ferreira
authors.affiliation: National Physical Laboratory, Hampton Road, Teddington, Middlesex TW11 0LW, UK; Department of Physics, University of Surrey, Guildford, Surrey GU2 7XH, UK.
authors.forename: P H
authors.initials: PH
authors.lastname: Regan
authors.affiliation: National Physical Laboratory, Hampton Road, Teddington, Middlesex TW11 0LW, UK.
authors.forename: A K
authors.initials: AK
authors.lastname: Pearce
keywords: (227)Th; Half-Life; Ionisation chamber; NORM; Nuclear data; Nuclear medicine; Radioactivity; TENORM; γ-ray spectrometry