Analysis of effect of sample volume and dead time on activity measurements in a gamma-ray spectrometer for radioisotope ¹³¹I

The drainage system of the high-dose ¹³¹I therapy isolation ward is connected to a delay tank for the radioactive waste to decay till the activity concentration is <0.6 µCi/l (as per AERB regulation) before discharge into the general sewage system. Gamma-ray spectrometer is a radiation measuring equipment which is employed to measure gamma emitting radioactive samples of small quantity. Sample volume and dead time have significant effect on the counting rate while counting in gamma-ray well counters. Usually, the volume of sample collected from delay tank for measurement is more than the required volume to be used in well counters. The count rate of a defined activity in a gamma counter decreases consistently with serial dilution and vice versa. However, this proportionality in count rate is lost as the volume approaches and exceeds the top of detector well. Considering this phenomenon, we aimed to analyze the effect of sample volume and dead time on activity measurements in a gamma-ray spectrometer for radioisotope ¹³¹I and further establish the upper limit of activity and volume of ¹³¹I samples for accurate measurement. (1) The dead time was calculated for the various radioactive samples ranging from 0.5 µCi to 10 µCi. The observed count of each activity was corrected for dead time to derive true count. The derived true counts were further analyzed. (2) A small sample of radioactive solution of ¹³¹I activity ranging from 0.5 µCi to 6 µCi was measured in a calibrated gamma-ray spectrometer which was further diluted progressively with normal saline up to 6 ml and the counts were recorded. (1) Least variation in dead time was observed for the activity measurement ranging from 0.5 µCi to 8 µCi and a drastic increment in dead time was observed beyond 8 µCi. (2) Least variation in counts was observed for an activity measurement in volumes ranging from 0.1 ml to 3ml and a drastic variation was observed when the volume was more than 3 ml. ¹³¹I activity of up to 8 µCi in a volume of up to 3 ml can be accurately measured in a gamma-ray spectrometer. Women in Nuclear Medicine and Miscellaneous (author)