Speaker
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Authors and affiliations: R. Broda 1, T. Ziemek 1, J. Marganiec-Gałązka 1, M. Czudek 1, K. Kossert 2, A. Listkowska 1, E. Lech 1, Z. Tymiński 1
1 NCBJ Radioisotope Centre POLATOM, A. Sołtana 7, 05-400 Otwock, Poland
2 Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116, Braunschweig, Germany
The applications of an alpha emitter 225Ac in targeted cancer therapy are being investigated at the NCBJ RC POLATOM. Thus, a method for absolute measurements of 225Ac activity in equilibrium with its progeny was developed. Measurements were performed using the triple-to-double coincidence ratio (TDCR) method in two different TDCR counters. The parameter TDCR of each measured source was determined. The counting efficiency of α particles with energy above 5 MeV in a liquid scintillator was assumed to be 100%. The theoretical counting efficiency of the beta branches was calculated using the free parameter model (Broda et al., Metrologia 44, 2007, S36-S52). The shape of the beta spectra was taken from calculations with the BetaShape program (Mougeot, Phys. Rev. C91, 2015, 055504). The total efficiencies of 225Ac in equilibrium with its progeny in T and D coincidence channels were calculated by the MetroActivityLSC program for various values of the free parameter and presented as functions of the TDCR parameter. The counting efficiency for individual sources was determined by interpolation from the above functions. The final massic activity of 225Ac solution was determined as the arithmetic mean of activity of all measured sources. In calculation of counting efficiency by Kossert et al. (ARI 156, 2020, 109020) with the MICELLE2 program, gamma transitions that occur directly after beta decays of 213Bi, 209Tl and 209Pb were taken into account. The massic activity determined in this case differed by 0.06%. The measurement result and an 225Ac solution were sent to the international reference system SIR in BIPM for a comparison. The decay half-life of 225Ac was also determined with high precision. One 225Ac source in an Ultima Gold scintillator was measured in one TDCR counter for a total of 9.9 hours over 43 days. The arithmetic mean of the measurements in channel T and D of 9.9139(63) days was taken as the final result, which was consistent with the values of 9.920(3) days and 9.9179(30) days reported by Pommé et al. (ARI 70, 2012, 2608-2614) and Kossert et al. (ARI 156, 2020, 109020), respectively. For validation, the 225Ac half-life was also determined in a scintillation counter with NaI(Tl) crystal. One source was measured for a total of 16.2 hours over 85 days and the result of 9.942(26) days was obtained.
