Calorimetry for Absorbed-dose Measurements at BNM-LNHB
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چکیده
At BNM-LNHB, the references in terms of absorbed dose to water are based on graphite calorimeter measurements. In the cobalt-60 beam, measurements were performed directly with the graphite calorimeter in a graphite phantom. Absorbed dose to water in a water phantom was then determined using a transfer method by means of relative measurements made with ionisation chambers and Fricke dosimeters. Our medical linac (Saturne 43 from GE-Medical Systems) is being characterised in the same way for three photon beam qualities 6 MV, 12 MV and 20 MV. Special attention was given to the vacuum-gaps correction, one of the most important source of uncertainty. We carried out measurements in a graphite device similar to the calorimeter, using a very small diode embedded in a body corresponding to the calorimeter core. Measurements were made alternatively with and without vacuum gaps in the graphite phantom. The ratio of the results leads directly to the vacuum-gap correction factor. Monte-Carlo calculations were also carried out using both EGS4 and PENELOPE codes. The results are in good agreement, regarding the uncertainties. At present, a water calorimeter is being realised. The aim is to get an absorbed dose standard independent of and complementing those based on graphite calorimetry. Particular attention will be devoted to the possibilities in terms of uncertainty compared with those attainable using graphite calorimetry.
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تاریخ انتشار 2000