Development of a novel relative measurement procedure for activity determination with the aid of liquid scintillation counters

At PTB, a new relative measuring procedure for activity determination with the aid of 4π liquid scintillation counting was developed and tested. In the procedure, the detection probability of a radionuclide is traced back to a ³H reference solution which makes the method very resistant to changes of the measuring device. The procedure is also suitable for small activities and shall be further developed for use in interna-tional comparison measurements.


At PTB, a new relative measuring procedure for activity determination with the aid of 4π liquid scintillation counting (LSC) was developed and tested. In the procedure, two measurements are first carried out with solutions of known specific activity. One of the measurements is performed with a ³H activity standard, the other with an activity standard of another radionuclide. As the specific activities of both solutions are known, the detection probability can be determined by experiment. In addition, the so-called quench indicator is determined for each measurement with the aid of external sources of ¹⁵²Eu, ¹³³Ba or ²²⁶Ra. The quench indicator is a measure of the detection probability and, under requirements easily to be met, independent of the radionuclide in the respective sample. This allows the detection probabilities of different nuclides to be correlated and, consequently, an efficiency curve to be determined.

If one wants to determine the activity of a solution with the same nuclide later on, two additional measurements are required. First, the respective detection probability is again determined with a ³H activity standard as a function of the quench indicator. Additionally, a measurement is performed with the nuclide to be investigated. This measurement also includes the determination of quench indicator and net counting rate. With the aid of the quench indicator, a ³H detection probability can be assigned to each sample, and the efficiency curve determined before also allows the detection probability of the nuclide valid for the sample to be detected so that the activity can finally be determined.

As the detection probability is traced back to a ³H reference solution, the procedure is largely independent of changes of the measuring device parameters. The efficiency curve can be preserved over decades and is, under certain conditions, even transferable to other LSC equipment. If aliquots of the same ³H reference solution are used for all measurements, the procedure can even be applied without exact knowledge of the ³H activity. Due to these properties, the method also seems suitable for international comparison measurements. In September 2005, a project was proposed on the occasion of the International Conference on Radionuclide Metrology and its Applications (ICRM) in Oxford, by which applicability of the method to comparison measurements shall be tested in cooperation with the metrology institutes of other countries.

The new procedure could, for example, supplement measurements with 4π ionization chambers which are also used as secondary standard measuring devices and for comparison measurements. Compared to ionization chambers, a special advantage of liquid scintillation counting are high detection probabilities even for low-energy ß-ray emitting radionuclides and radionuclides with small atomic number which decay by electron capture. In addition, the method can also be used for small activities down to a few Becquerel and is therefore, among other things, used for measurements within the scope of radiation protection or environmental radioactivity.