Dosimetry and dose distribution in cesium-137 irradiation unit used at the Eastern Regional Research Center

Abstract

A self-contained, cesium-137 Research Irradiator with the strength of approximately 147, 000 Ci is located at the Eastern Regional Research Center (ERRC). When the irradiator was installed, the absorbed dose in a reference position of the irradiation field was calibrated and the absorbed doses in other locations of the irradiation field relative to the calibrated positions were measured to map its irradiation field. Self-read pocket dosimeters for gamma-ray were calibrated with a 120 M Ci Calibration Cs-137 source which delivered about 50 mR exposure during a 20-min irradiation at 50 cm from the source. A ferrous sulfate/cupric sulfate solution was used to calibrate the dosimetry of the irradiation chamber which holds three number 10 cans (16 cm inside diameter X 17.5 cm height each). At 20-min exposure, Dmax:Dmin = 1.35, except at the top and bottom $\text{1}\text{3}$ portions of the cans, all positions received a more or less uniform dose (about 0.13 kGy/min) during irradiation. A straight line of total dose (kGy) vs time (min) was constructed following the equation y = 0.127 + 0.137 with r = 0.99, where y is the total dose, and x is time. Radiochromic films and PVC strips were also used to monitor the irradiation operation after comparing results obtained from the chemical dosimeter. Two equations were constructed, respectively, as follows: y = 0.0091x + 0.0015 with r = 0.99, where y = absorbance and x = time (min), and y' = 0.0041x' + 0.0185 with r = 0.99, where y' = absorbance and x' = time (min). Controlled environment is important when such chemical and solid state dosimeters are used during irradiation at subfreezing, refrigerated, or ambient temperature conditions; the equipment has this capability.