Absolute Determination of the Activity of Th-228

²²⁸Th is a radionuclide of the natural thorium decay chain. ²²⁸Th and its progenies decay by alpha and beta transitions, as a result of which a high detection probability in liquid scintillation counting is expected.

At PTB, absolute measurements for the determination of the activity of ²²⁸Th solutions have now been carried out for the first time. For that purpose, liquid scintillation sources with weighed aliquotes of ²²⁸Th solutions were measured in systems with two photomultiplier tubes. The detection probability, which is at first unknown, was determined by an efficiency tracing procedure [1]. The procedure requires measurements with ³H sources under identical experimental conditions, and a calculation of the detection probabilities as a function of a free parameter. As the solubility of the noble gas isotope thoron (²²⁰Rn) - which plays, as a progeny, a role in the thorium decay chain - is very high in organic liquids, the measurements can be performed without any problems.

The ephemeral ²¹²Po represents, however, a challenge in the decay chain of ²²⁸Th. With a half-life of only approximately 300 nanoseconds, a great number of decay processes of this isotope occur within the dead time of the detector, which was released as a result of the detection of the preceding ²¹²Bi decay. It has, however, been possible to take this effect into account as well as the equilibrium factors.

In addition, the first measurements worldwide have been carried out in a liquid scintillation system with three photomultiplier tubes. When measurements are performed in such a system, tracer measurements with ³H need not be performed. Instead, the respective free parameter and, thus, the detection probability searched for, are determined by the ratio of the calculated probabilities of triple- to double-coincidences [1]. This ratio - referred to as TDCR - must agree with the experimentally determined ratio of the respective net counting rates.

The results of the two methods agree very well. In addition, PTB has - as the first national metrology institute - sent a flame-sealed glass ampoule with a ²²⁸Th solution of known activity to the Bureau International des Poids et Mesures (BIPM) in Paris, to participate in a comparison within the scope of the International Reference System (SIR). Up to now it has, however, not been possible to compare the value - which has been determined with a relative standard measurement uncertainty of only 0.31% - with other values, as the results of other metrology institutes are not yet available. This emphasises PTB’s pioneer work.

The new procedure, which will soon be presented to the public on the occasion of a conference in Paris [2], allows activities to be determined with very small uncertainties. Furthermore, a simple determination of the activity concentration of ²²⁸Th solutions is now possible. Although the activity can also be determined by measurements with an alpha spectrometer with a known solid angle of the source detector system, the uncertainties achieved so far are clearly larger. This can, among other things, be attributed to the source properties, provided that they have been prepared quantitatively - i.e. by the dropping on of a weighed solution quantity. Measurements with ionisation chambers do not represent any competition, as in these measurements - in contrast to liquid scintillation counting - a large dependence on the solution composition (as, for example, the acid concentration and the ampoule type used in which the solution is contained) has been found.

The new method can also easily be adapted to activity determinations of ²²⁴Ra which is used for treatments of the Morbus Bechterew disease (also: spondylitis ankylosans).

In a recently published work, Cardone and colleagues report on a decrease in the activity of ²²⁸Th solutions, when these are exposed to cavitation caused by ultrasound [3]. According to the authors’ opinion, the decrease is larger by a factor of 10⁴ than the decrease due to the natural radioactive decay. The experimental findings are, however, very controversial, and the results have so far not been confirmed by other groups of researchers.

At PTB, some of the liquid scintillation sources with ²²⁸Th were exposed for approx. 130 minutes to an ultrasound bath with 320 watt and a frequency of 35 kHz. The activities of the samples were determined before and after the ultrasound bath, and the corresponding relative deviation of the results amounted on average to only 0.04% and is not significant. This is why this experiment does not indicate that the activity of the ²²⁸Th is influenced by ultrasound.

Literature

1. Broda, R., Cassette, Ph., Kossert, K.:
   Radionuclide Metrology using Liquid Scintillation Counting.
   Metrologia 44 (2007) S36-S52.
2. Kossert, K., Nähle, O. J.:
   Activity determination of ²²⁸Th by means of liquid scintillation counting.
   Advances in Liquid Scintillation Spectrometry (LSC 2010), 6.-10. September 2010, Conference Proceedings in Radiocarbon, in Vorbereitung.
3. Cardone, F., Mignani, R., Petrucci, A.:
   Piezonuclear decay of thorium.
   Physics Letters A 373 (2009) 1956-1958.