Determination of the width of gamma radiation field of the OB-85 (137Cs source) at calibration facility of PTKMR BATAN

The OB-85 is one of the main equipments for calibration of Gamma Radiation Measuring Instrument’s (RMIs, such as Surveymeter, TLD, pocket dosimeter) at Calibration Facility of PTKMR-BATAN, however, without standard instrument and other supporting tools, the OB-85 will not function properly. To get an accurate calibration result, the width of radiation field of the OB-85, the Air Kerma rate, Ka [µGy/h], the Ambient Equivalent Dose rate, H*(10) [µSv/h], the Personal Equivalent Dose rate, Hp (10) [µSv/h], the Exposure rate, X [mR/h], and the expanded uncertainty, uexpand of these quantities must be measured and determined as they are indispensable for calibration or irradiation of the RMIs . The purpose of determining of the width of radiation field of the OB-85 is to fulfil quality audit of the Calibration Facility, for ascertaining what the
actual width of radiation field of the OB-85, belong to the PTKMR-BATAN. At the time of calibration, surveymeter or pocket dosimeter must be placed at the center of radiation field and in the width of radiation field because if it was partially outside of the radiation field then the
calibration result was inaccurate. This paper presents the determination of the width of radiation field of the OB-85 by three types of ionization chamber detectors. with different shapes and volumes, with two settings of SDD (Source Detector Distance). The width of radiation field of the OB-85 measured by the IC/SN #M23332, IC/TK-30 #SN107, and IC Exradin/A4 were 18 cm for SDD = 50 cm, and 36 cm for SDD = 100 cm. From this experiment, the information was obtained that the width of radiation field of the OB-85 does not depends on the type of the detectors used to measure but it depends on the calibration facility settings (the shape or geometry of the source, SDD, and diameter of collimator).