Variation of the dose rate due to an external CO-60 gamma beam by lead absorbers for calibration of dosemeters in the radiation protection field

Measurements carried out in external photon beams for radiation protection purposes are within a large dose rates range. Consequently, radiation protection dosemeters used for such measurements require to be calibrated within a large dose rates range to cover the whole range of instrument scale sensitivities (such as from about 10 n Sv/h to about 10 Sv/h, in terms of ambient dose equivalent rate). An appropriate experimental arrangement is then required to offer a complete service for calibration and examination of radiation protection dosemeters. Generally, the calibration laboratory obtains the required dose rates range by using several radioactive source of different activity and varying the source-detector distance. This demands multi-sources irradiators with accurate mechanical system to exchange the sources. Moreover it is necessary to accurately investigate the energy spectral component due to photons scattered by the walls of the irradiation room at the different measuring distances and the effects of this variable component on the dosemeter calibration coefficient. The present paper refers to a try of a different approach in which the dose rates variation is obtained by a high activity Co-60 source and a set of lead absorbers. At the Italian Metrological Institute in ionising radiation field, the 'Instituto Nazionale di Metrologia delle Radiazioni Ionizzanti' of the ENEA (ENEA-I.N.M.R.I.), some attenuated Co-60 beam qualities were realized by lead absorbers with thickness from 3.5 cm to 14.0 cm. This allows varying the Co- 60 beam intensity of a factor from 1 x 10-1 to 1 x 10-4 keeping constant the source-detector distance. The attenuated beams were characterized by accurate Monte Carlo determination of the spectral distributions to quantify the variation of the spectra mean energy and the increase of scattered radiation percentage with the absorbers thickness. Absolute air kerma measurements were carried out with the ENEA-I.N.M.R.I. standard cavity chamber, at a fixed distance of 200 cm in each beam quality. At this end the some correction factors of the ENEA-I.N.M.R.I. standard chamber were redetermined, with particular reference to the wall effect factor calculated by Monte Carlo simulation using for each beam quality the effective energy spectral distribution. Moreover, the variation of the response with the absorber thickness was investigated for a number of radiation protection dosemeters on the aim to estimate the uncertainty component due to the difference in beam spectral distributions in calibration and in field measurements, respectively. It must be pointed out that radiation protection measurements are often carried out with shielded radiation more similar to the attenuated than to the un-attenuated beam and that keeping the calibration distance constant represents an undoubted advantage. Results will be presented and discussed in the paper. (authors)