Direct measurement of the half-life of 223 Ra S.M. Collins a,n , A.K. Pearce a , K.M. Ferreira a , A.J. Fenwick a , P.H. Regan a,b , J.D. Keightley a a National Physical Laboratory, Hampton Road, Teddington, Middlesex TW110LW, United Kingdom b Department of Physics, University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom HIGHLIGHTS  Direct measurement of the 223 Ra half-life using an ionisation chamber.  New measured half-life of 11.4358 (28) days.  Result consistent with the most precise published value. article info Article history: Received 19 September 2014 Received in revised form 2 February 2015 Accepted 4 February 2015 Available online 7 February 2015 Keywords: Half-life 223 Ra NORM Nuclear medicine Nuclear data Radioactivity Ionisation chamber abstract Radioactive decay half-life measurements of 223 Ra, a member of the 235 U naturally occurring radioactive decay series, have been performed of a radiochemically pure solution with an ionisation chamber. The radioactive decay of 223 Ra was followed for 50 days, approximately 4.4 half-lives. The deduced half-life of 223 Ra was found to be 11.4358 (28) days, supporting the other published direct measurements. A detailed uncertainty budget is presented. A new evaluation of the published half-life values was performed, in- dicating significant variation across the existing published values, suggesting that further measurements of the half-life of 223 Ra are required. A new evaluated half-life has been calculated using a power moderated weighted mean of selected experimental values, with a new value of the recommended half- life for 223 Ra of 11.4354 (17) days. Crown Copyright & 2015 Published by Elsevier Ltd. All rights reserved. 1. Introduction The radioisotope of 223 Ra is a member of the naturally occur- ring decay series of 235 U (see Fig. 1), directly fed by the α- and β- decays of 227 Th and 223 Fr respectively, both being the decay pro- geny of 227 Ac. The 223 Ra nucleus decays by almost 100% α emission to a number of excited states in 219 Rn (Browne, 2001), with a small branching ratio (4.7  10 10 relative to the α branching ratio (Kutschera et al., 1985) following the rare cluster decay, whereby 223 Ra spontaneously emits a 14 C nuclear cluster (Rose and Jones, 1984), directly to 209 Pb. The 219 Rn further decays via a number of short lived α- and β-emitting decay progeny, terminating at the stable 207 Pb nucleus. As 223 Ra and decay progeny occur naturally in the environment they are capable of delivering a high radiotoxic dose within the human body, if ingested. These radionuclides are therefore of in- terest as a potential hazard associated with naturally occurring radioactive materials (NORM) and technologically enhanced NORM (TENORM) (Kathren, 1998). ‘Technological Enhancement’ refers to processes during which concentrations of the 223 Ra pre- cursors such as 227 Ac, 231 Pa or 235 U are enhanced during extraction and processing of mineral ore or crude oil. While the α emissions of the 223 Ra and decay progeny are a potential hazard they are also a potential tool to kill cancerous cells. The United States Food and Drug Administration have re- cently approved the use of 223 Ra (FDA, 2013) for clinical use, after successful clinical trials (Michalski et al., 2013; Nilsson et al., 2007; Parker et al., 2013), in targeted radiotherapy of bone metastases and bone pain palliation that occur from late-stage castration re- sistant prostate cancer. Historically, bone-targeting radio- pharmaceuticals used for such treatments have been β emitting radionuclides e.g. 89 Sr, 166 Ho and 153 Sm (Brady et al., 2013), how- ever due to the relatively long dose deposition range of β emis- sions, significant doses can be delivered to bone marrow and other healthy tissue near the tumour. As radium shares the same che- mical attributes as calcium (they are both members of the group of alkaline earth metals), radium exhibits a high level of Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/apradiso Applied Radiation and Isotopes http://dx.doi.org/10.1016/j.apradiso.2015.02.003 0969-8043/Crown Copyright & 2015 Published by Elsevier Ltd. All rights reserved. n Corresponding author. E-mail address: sean.collins@npl.co.uk (S.M. Collins). Applied Radiation and Isotopes 99 (2015) 46–53