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Determination of the activity and photon emission probabilities of antimony-124


The radionuclide antimony-124 (124Sb) is known from the following applications: as tracer in medicine and technology, as part of neutron sources and as calibration source for high-energy photon radiation in research. Especially for the last-mentioned application, it is of utmost importance to determine the activity of 124Sb sources with best possible precision and to know the photon emission probabilities with low measurement uncertainty.

In 2007, the "Unit of Activity" Working Group of the Physikalisch-Technische Bundesanstalt (PTB) was one of the eight partners of the Project 907 of the European Collaboration in Measurement Standards (EUROMET). It was the aim of this project to clarify discrepancies which have so far existed in the field of the activity measurement of 124Sb and to redetermine the photon emission probabilities.

At the outset, suitable radionuclide sources had to be prepared from a 124Sb-containing aqueous antimony chloride solution for the planned measurement procedures. Standard handling of this solution for the preparation of radionuclide solid sources was out of the question, as the binary antimony chlorides - like many other compounds of antimony - are volatile. To achieve an optimal chemical fixing of 124Sb, test series were performed with different reagents and under varying conditions of preparation. A procedure making use of reference surfaces for the semi-quantitative detection of possibly occurring evaporation losses of 124Sb during essential periods of the preparation was developed; the measurement was carried out with an autoradiographic method.

The activity was determined with two procedures. In the case of the classical 4πβ-γ coincidence method, the β-counting rate, the γ-counting rate and the counting rate of β-γ-events (which are occurring at the same time) were measured. These measurands allow the activity to be determined directly by extrapolation, without knowledge of the detection probabilities. With 4π-liquid scintillation counting (LSC), the activity of the solution was determined in accordance with the CIEMAT/NIST procedure. A difficulty is the great number of different β--transitions which may occur in coincidence with several γ-transitions. A programme developed at PTB allowed the complex calculations to be carried out.

As activity, the mean value of the results from the 4πβ-γ coincidence method and the LSC measurements was determined with a relative standard measurement uncertainty of 0.60 %. Preliminary results of the comparison show very good agreement of the PTB result with the mean value of the results of all participants in the exercise.

The second part of the project - the determination of the photon emission probabilities - was performed with the aid of a germanium detector system. The energy-specific photon detection probabilities of the system were calibrated, using PTB activity standards. As in the case of all measurement methods, the very complex decay scheme of the 124Sb represented a challenge. The decay scheme determines with which probability several photons from the decay of a 124Sb atom are detected at the same time. For this complex case, the KORSUM software, which takes these coincidences into account, had to be extended; this occasion was used for a fundamental modernization so that KORSUM II now uses - among other things - the data bank of the Decay Data Evaluation Project (DDEP) - in the establishment of which PTB had participated - as a basis for the required nuclide data.

For 41 γ-transitions, absolute photon emission probabilities with values between 97.6 and 0.0024 emissions per 100 decays of 124Sb were determined with relative standard measurement uncertainties between 0.68 % and 29 %. Moreover, upper limits were determined for seven additional photon emission probabilities.