Determination of the activity and half-life of thorium-227

In nuclear medicine, alpha–emitting radionuclides offer interesting possibilities for cancer therapy. In spite of its relatively high energy, alpha radiation has a short particle range. If an alpha emitter is successfully introduced into the tumor tissue, the alpha radiation essentially destroys tumor cells whereas severe damage to the healthy surrounding tissue is kept low. An alpha emitter may be introduced into the tumor tissue, e.g., by radioimmunotherapy, whereby the corresponding radionuclide is coupled with antibodies which preferably target the diseased tissue. One of the radionuclides investigated for such a therapeutic approach is thorium–227. This method is particularly efficient since the alpha emitter thorium–227 has several radioactive daughter nuclides (see Fig. 1) which, in turn, emit alpha radiation. In applications of nuclear medicine, the progenies are separated from thorium–227 before coupling to the antibodies takes place.

Fig. 1: Decay chain of thorium–227.

At PTB, new methods have now been developed to determine the activity of thorium–227 accurately. A particularity of thorium–227 is that it is not in a radioactive equilibrium with its progenies. The activity of its progenies relative to the activity of thorium–227 changes over time, which causes many measuring instruments to indicate measurement results (count rate or ionization current) that first increase and only decrease later.

The activity was determined by means of liquid scintillation counting of weighed aliquots of a thorium–227 solution. In this procedure, the detection probability of the individual radionuclides of the decay chain is determined by means of an arithmetic method that was previously developed to determine the activity of actinium–227 and radium–223 [1]. In the case of thorium–227, however, it must additionally be kept in mind that the activity situation changes over time because there is no radioactive equilibrium. A procedure has been developed for this purpose: by means of a numerical minimization algorithm, this procedure allows the activity concentration to be determined as well as the point in time when the chemical separation of the progenies takes place. For the activity concentration, a relative uncertainty of only 0.25 % has been achieved. A detailed description of this new method has now been published [2].

A count rate is determined as follows: Events that have been registered by the detector are counted over a defined period of time and then divided by the measurement duration. Since the activity of the sample changes during the measurement period, a correction for decays during the measurement period must be carried out. For radioactive samples with an exponential decay, such a correction is easy to determine. It merely depends on the half–life of the radionuclide considered and on the duration of the measurement. In the case of thorium–227, however, the count rate first increases. For such cases, a new computation method was therefore developed and applied for the correction for decays during the measurement period. In the case of thorium–227, the corresponding correction now also depends on when the measurement takes place [2].

The measurement data obtained by means of liquid scintillation counting have furthermore been used to determine the half–life of thorium–227. The result depends on the half–life of radium–223, which is its longest–lived progeny. The dependence on the half–life of radium–223 is strongest directly after the daughter products of thorium have separated. It diminishes if the measurement period is particularly long¹. The measurements at PTB took place over a period of approx. 150 days in total, which is a significant advantage compared to other experiments which have considerably shorter measurement periods.

At PTB, measurements in an ionization chamber were even performed over a period of approx. 180 days, which provided a second independent determination of the half–life. The results of both procedures are in good agreement, and by combining them, a half–life of T_1/2 = 18.681(9) days is obtained.

The novel methods can also be adapted to other radionuclides such as radium–224.

Literature

[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, DOI: doi.org/10.1016/j.apradiso.2018.12.010.

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¹ Die Abhängigkeit verschwindet, wenn alle Messungen im radioaktiven Gleichgewicht erfolgen.