A Calibration Factor of Semiconductor Field Detector as a Function of 60Co Gamma Dose Rate

Full text: The dependence of the calibration factor of Si n-type semiconductor detector exposed to different dose rates was investigated. The special detector with increased sensitivity (1200 nC/Gy, Scanditronix prototype) served in radiation field analysis of total body (TBI) 60Co gamma irradiation. The sensitive Si crystal layer, situated very close to the detector surface, enabled precisely radiation measurements in a build up region. The absorbed dose was determined with an ionisation chamber (RK Scanditronix type) by IAEA protocol. The electrometers DPD-5 and RDM-1F, connected to Si detector and RK ionisation chamber, respectively, were used. We defined the calibration factor as: Ncal=D/M, (D - absorbed dose by ionisation chamber, M - monitor reading by semiconductor detector). The source to detector distances (SDD) were 80, 160, 267 and 400 cm and collimator openings formed the following field dimensions (in cm2) on the front surface of detector's tray slab: 20x20 (all SDDs), 50*50 (SDD=160, 267, 400 cm), 100x100 (SDD=267, 400 cm) and 150x150 (SDD=400cm, as in TBI). It was found an 11% (field 20x20 cm2) decrease in relative Ncal (normalised to TBI conditions) as the dose rate decreases from 46.17 cGy/min (SDD = 80 cm) to 1.59 cGy/min (SDD=400 cm). We obtained the similar results for other fields. Also, the relative Ncal changed up to 2.0 % at fixed SSD=400 cm, by changing the fields from 150*150 to 20x20 cm2. In conclusion, the calibration factor of n-type semiconductor detector, which was constructed specially for the purpose of radiation field analysis at extended distances i.e. at low dose rate conditions, had a pronounced dose rate dependence in 60Co beam. Therefore, it is always necessary to determine it in actual irradiation conditions. (author)