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Determination of the activity and half-life of actinium-225


As alpha–emitting radionuclides are particularly interesting for nuclear medicine, procedures to determine activity accurately have been developed at PTB over the past few years [1,2]. Now, the activity concentration of an actinium–225 solution has also been successfully determined with a very small uncertainty. For this purpose, liquid scintillation counting was used again. A triple–to–double coincidence ratio (TDCR) analysis [3], which had previously been developed for thorium–229, was successfully adapted to actinium–225. A particularity of the decay chain of actinium–225 is the short–lived progeny polonium–213 (see figure). If the prior decay of bismuth–213 is detected, then the decay of polonium–213 may occur within the detector's dead time. To determine activity, corrections must therefore be applied. They are described in detail in the article that has now been published [4]. For the activity concentration, a relative standard measurement uncertainty of only 0.16 % was achieved.


Fig. 1: Decay chain of actinium-225.

The measurement data obtained by means of liquid scintillation counting have also been used to determine the half–life of actinium–225. The measurements were carried out over a period of more than 110 days, which has been the longest uninterrupted half–life measurement of this radionuclide so far. Since the measurements were performed with a TDCR liquid scintillation counter, it was possible to simultaneously check the stability of the sample and of the detector – which distinguishes this method from numerous other measurement procedures. The half-life was additionally determined by means of long–term measurements with ionization chambers. By combining these two methods, a half–life of T1/2 = (9.9179 ± 0.0030) days is obtained. PTB's result is in very good agreement with a value that has recently been determined at JRC–IRMM in Geel [5]. The uncertainties of the new half–life determinations of the two institutes are smaller by a factor of 33 than the uncertainty of the value provided in the literature to date.

Moreover, it was also possible to measure the half–life of polonium–213, which is very short–lived. This was achieved by means of another TDCR device that contains an additional CeBr3 detector to detect photon radiation as well as fully digital measurement data acquisition. The experiment and the subsequent data evaluation were used to determine the time difference between the beta decay of bismuth–213 (start signal) and the alpha decay of polonium–213 (stop signal) for a number of decays. Bismuth decay is identified by additionally detecting the 440 keV photons from the gamma transition which follows beta decay. From the time difference spectrum thus obtained, a half–life of (3.709 ± 0.012) μs was determined for polonium–213. In this case too, the result is in very good agreement with a value determined at JRC–IRMM [6]. Again, these two values represent a tremendous improvement compared to previous measurement results. These new measurement results are also particularly important for future evaluations of radionuclide data such as those performed within the scope of the Decay Data Evaluation Project [7].


[1]    Kossert, K., Bokeloh, K., Dersch, R., Nähle. O.J.: Activity determination of 227Ac and 223Ra by means of liquid scintillation counting and determination of nuclear decay data. Applied Radiation and Isotopes 95 (2015) 143-152.

[2]    Kossert, K., Nähle. O.: Determination of the activity and half-life of 227Th. Applied Radiation and Isotopes 145 (2019) 12-18.

[3]    Kossert, K., Nähle, O.J., Janßen, H.: Activity determination of 229Th by means of LS counting. Applied Radiation and Isotopes 87 (2014) 274-281.

[4]    Kossert, K., Takács, M.P., Nähle, O.: Determination of the activity of 225Ac and the half-lives of 213Po and 225Ac. Applied Radiation and Isotopes 156 (2020) 109020.

[5]    Pommé, S., Marouli, M., Suliman, G., Dikmen, H., Van Ammel, R., Jobbágy, V., Dirican, A., Stroh, H., Paepen, J., Bruchertseifer, F., Apostolidis, C., Morgenstern, A.: Measurement of the 225Ac half-life. Applied Radiation and Isotopes 70 (2012) 2608–2614.

[6]    Suliman, G., Pommé, S., Marouli, M., Van Ammel, R., Stroh, H., Jobbágy V., Paepen, J., Dirican, A., Bruchertseifer, F., Apostolidis, C., Morgenstern, A.: Half-lives of 221Fr, 217At, 213Bi, 213Po and 209Pb from the 225Ac decay series. Applied Radiation and Isotopes 77 (2013) 32-37.

[7]    www.lnhb.fr/donnees-nucleaires/donnees-nucleaires-tableau/