Correction for the effect of radionuclidic impurities in ionisation chamber measurements of 201Tl
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Date
1994-11-16
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Publisher
Australian Institute of Nuclear Science and Engineering
Abstract
The National Medical Cyclotron (NMC) in Sydney commenced commercial production of 201T1 in May I993(l). From the locally produced 202T1, Australian Radioisotopes (ARI) produces radiopharmaceuticals which are supplied to the nuclear medicine community. Under the National Measurement Act, 1960(2) the activities quoted on products supplied by ARI must be legally traceable to National standards of measurement. The Australian primary standard of measurement for radioactivity is the legal responsibility of the Radiation Standards Project at ANSTO(5). The Radiation Standards Project, in collaboration with the Quality Control section at ARI, has established an on-going calibration program to ensure that ARI's activity measuring equipment is traceable to National standards. Soon after the production of 201T1 commenced at the NMC, inconsistencies were observed in ionisation chamber measurements of 201T1 solutions. The cause for these
inconsistencies was shown to be the presence of the radionuclidic impurities, 200T1 and 202T1.
Even though the levels of these impurities were well below the recommended limits(4), their effect on ionisation chamber measurements could be relatively large. In order to correct for the effect of these impurities, calibration factors for 200T1 and 202T1 were determined using two independent methods. In the first method(5), the calibration factors were calculated from the decay scheme of the nuclides and the response curve of the ionisation chamber. However, this method can only be applied when the response curve of the ionisation chamber is reasonably well known and where there is no significant contribution due to bremsstrahlung. An alternative and more reliable method was developed which used statistical modelling techniques(6) to obtain maximum likelihood estimates of the calibration factors. This method can be used in cases where there may be significant bremsstrahlung production or where the gamma-energy response curve is unknown. By using these methods, calibration factors were determined for two TPA chambers and a Vinten IV dose calibrator. The results from the two methods were in agreement to within the measurement uncertainties. By incorporating these calibration factors into the measurement systems of both the Quality Control and Production sections at ARI, the effect of these impurities on ionisation chamber measurements was successfully eliminated. The accuracy of the Australian standard of measurement for 201T1 has now been confirmed by International intercomparison through the international reference system for gamma-emitting nuclides(7).
Description
Physical copy held by ANSTO Library at DDC 541.382/16.
Keywords
Calibration standards, Data, Ionization chambers, Radiation detectors, Radioisotopes, Thallium, Cyclotrons, Impurities
Citation
Buckman, S. M. (1994). Correction for the effect of radionuclidic impurities in ionisation chamber measurements of 201Tl. Poster presented to the AINSE Conference on Radiation Biology and Chemistry, Cuming Theatre, Chemistry Department, The University of Melbourne, Melbourne, Victoria, 16-18 November 1994, (pp. 29-30).