The spatial variation of dose-rate from 90Sr/ 90Y beta sources for use in luminescence dating

Abstract

The spatial variation of dose-rate from a 1850 MBq (50 mCi) 90Sr/ 90Y Amersham International SIF type ceramic-substrate beta particle source (5 mm diameter active area) was measured and compared with that from a 1480 MBq (40 mCi) 90Sr/ 90Y Amersham International SIP type silver plaque type beta particle source (12 mm diameter active area), for a variety of source–target spacings. The radiation field intensities were mapped using the intensity of radioluminescence induced in a probe comprising an ≅1 mm diameter calcium fluoride grain attached to the end of a glass fibre optic cable, and detected by a photomultiplier. Both sources were found to have radially symmetric dose distributions with no significant local irregularities, and dose-rate radial profiles indistinguishable at the closest source–probe spacing used (15 mm). At the largest source–probe spacing, 25 mm, the ceramic-substrate source exhibited slightly lower centre-periphery dose-rate decrease. Further measurements, using in addition a 3700 MBq (100 mCi) ceramic-substrate source, found an apparently lower dose-rate/MBq from the ceramic-substrate sources than from the silver plaque. This somewhat negates the practical benefits arising from the increased source activity permissible on radiation safety grounds for ceramic-substrate sources for a given shielding configuration, as although a given shielding configuration can house a ceramic-substrate source delivering a greater dose-rate than can safely be achieved from a silver plaque source, a considerably higher source activity is required to do so. It was concluded that the overall suitability of both types of sources for luminescence applications differs little in that both are suitable, and the choice between types is best governed by case-to-case considerations, such as practical constraints on shielding.