Dose distribution measurements in irradiation volume of a gamma chamber-900 -its importance in calibration of dosimeters

Full text: Cobalt-60 Gamma chamber-900 is an irradiation unit designed and developed by BRIT. It is self-shielded, compact, dry storage irradiation unit offering an irradiation volume of few hundred cubic centimeters. It is an ideal irradiation facility for R and D establishments, for calibration of dosimeters maintained by Radiation Standards laboratories and also for Radiation Processing facilities. The source cage of Gamma chamber holds the radiation sources in an annular cylinder. During irradiation, the sample chamber rests at the center of the annular cylinder, thus providing a typical dose distribution within the irradiation volume. Generally, manufacturer supplies, along with Gamma chamber, a generic dose distribution profile within the irradiation volume and the dose rate at its central position. In the present paper, dose distribution profile along the central vertical plane of the irradiation volume of Gamma chamber-900 was measured along its central vertical plane. A specially designed polystyrene jig with a 35-point grid was used for placement of the dosimeters. Alanine (spectrophotometric readout) dosimeter and Glutamine (spectrophotometric readout) dosimeter - a transfer standard; maintained by Chemical dosimetry group were used for this purpose. The results were further confirmed using Gafchromic film dosimeter. The measured dose distribution profile differed from the standard pattern. An asymmetric dose distribution pattern revealed that the source-chamber overlap alignment of the Gamma chamber has changed. The importance of this dose distribution measurement, in calibration of dosimeters as well as irradiation of samples for R and D activities has been highlighted