Calibration of ionization chamber and GM counter survey meters, (2)

Five types of ionization chamber survey meters and a type of GM counter survey meter were calibrated for measuring the β-ray absorbed dose rate due to a skin contamination, using natural uranium, 198Au and 204Tl circular sources. To estimate the β-ray absorbed dose rate of a contaminated skin, measurements were done twice by a survey meter without and with a filter, keeping the distance from the contaminated skin surface to the survey meter at 10 mm. The absorbed dose rate was obtained by multiplying a net reading (a difference between a reading of survey meter's indicator measured without the filter and a reading measured with the filter) by a multiplying factor. Multiplying factors varied approximately in proportional to the inverse square of source diameter when the source diameter was smaller than the detector's diameter, and approached gradually to a constant value as the value of source diameter increased over the detector's diameter. The multiplying factors of each ionization chamber survey meter were equal to one another within +- 30% independently of type (manufacturer) and the maximum energies of sources. The multiplying factors of GM counter survey meter to 204Tl and 198Au sources were three times as large as those to natural uranium sources. The change in the multiplying factor was less than +- 20%, when the measuring distance was 10 +- 2 mm. (author)