IN SITU GAMMA-RAY DOSE RATE MEASUREMENT FROM PLANAR SOURCE USING HIGH PURITY GERMANIUM DETECT0R

Abstract

This research proposes a simple method for dose rate measurement from planar source using a high purity germanium (HPGe) detector by measurement of gamma-ray spectrum. Net peak intensities of all energies from the measured spectrum were then used to obtain the total dose rate. First of all, 4 ceiling paint samples containing uranium and thorium were purchased and determined uranium and thorium contents by using high resolution gamma-ray spectrometry equipped with an HPGe detector of 30% relative efficiency. Radioactive isotopes including uranium daughter radionuclides such as 214Pb and 214Bi as well as thorium daughter radionuclides such as 208Tl and 228Ac were found in all samples. The specific activities of 238U in the range of 144.61±3.28 to 299.76±4.87 Bq/kg and 232Th in the range of 1773.59±23.66 to 2121±26.52 Bq/kg respectively. After that 8 test specimens were prepared from the four paint samples by painting on 30 cm x 30 cm plywood, two pieces per paint sample. The specific activities of 238U and 232Th on the plywood samples were in the range of 0.018 – 0.071 Bq/cm2 and 0.22 – 0.49 Bq/cm2 respectively. The other two radioactive paint samples were prepared by mixing non-radioactive paint sample with standard uranium and standard thorium ores of different proportions to make the specific activities of 238U and 232Th on the plywood of 0.027 and 0.030 Bq/cm2 and 0.043 and 0.056 Bq/cm2 respectively. A portable gamma-ray spectrometer equipped with 10% relative efficiency HPGe was then used to measure radioactivity from the prepared ten plywood samples at 2 cm distance at right angle from the middle of the samples. To limit the detector detecting area to 10 cm x 10 cm, the detector sides were surrounded by lead blocks. Unfortunately, the radionuclides after 222Rn in 238U series and after 220Rn in 232Th series were not in radioactive equilibrium with the parent radionuclides which made the measured dose rates lower than the calculated values. However, the dose rate of 228Ac was in good agreement with the calculated value because it was the only radioisotope that was in radioactive equilibrium with the parents 232Th. To overcome the problem, a new technique was then introduced by calibrating the detector with point gamma-ray standard sources at different position to obtain geometrical correction factors. The correction factor was found to be 0.74 for all energies. The results of dose rate measurements from the ten test specimens using the proposed technique were very satisfactory. In addition, a worksheet with necessary formulae was prepared for convenience in calibration and calculation of the dose rate.