Establishment of known radiation fields for calibration of survey meter

Abstract

       The reference radiation fields for calibration of survey meter of the Secondary Standard Dosimetry Laboratory (SSDL) is gamma ray from Cesium-137 radioactive source with an ambient dose equivalent rate (H*(10)) of 1849.82 μSv/h. It is necessary to establish other dose rate to cover measurable ranges of survey meters. These measurable ranges are as follow, 3000, 2000, 1000, 500, 300, 200, 100, 50, 30, 20, 10 and 5 μSv/h. A new shielding box made of lead was designed with a special window. The window could be shielded by different lead thickness. Thus, dose from the window could be controlled. The appropriate dose measured by survey meter is said to be about 30% to 70 % of full scale. This approximation gave rise to thicknesses of lead sheet in a series as follow, 2, 6, 12, 18, 22, 26, 32, 40, 44, 48, 54 and 60 millimeter. The doses were measured by working standard instrument. The results of H*(10) were found to be 1435.89, 947.21, 496.96, 239.11, 166.71, 99.93, 53.32, 22.99, 14.40, 10.64, 5.43 and 3.44 μSv/h respectively. A measurement uncertainty for calibration was estimated to be 2.0% at confident level of 95%. It is found that gamma radiation fields are appropriate except the 5 μSv/h range.