Experimental study of quasi-monoenergetic 200 keV photon field using a radioactive source with backscatter layout

At present, Cs-137 is used as a standard in the calibration of radiation detectors or dosimeters because it is a monoenergetic source (662 keV) and has a long half-life of 30.07 years. When radiation detector is calibrated, it is desireable to calibrate for wide energies. For example 200 keV energy region is important because some detector response is too high and scatter photon energy tend to be that energy even if incident energy is 662 keV or 1.25 MeV. While many available stable calibration sources have high energies (above 300 keV), a stable source with energy of ~200 keV is desirable. However, there are currently no stable sources that fulfill these energy requirements. With Cs-137, a backscattered source energy of ~200 keV can be obtained by using the same source. We aimed to use this backscattered radiation as a source to calibrate our detectors. Lead block of 10 cm was used to stop the full energy peak of Cs-137. The backscatter layout variables were optimized experimentally to minimize Compton scattering. Considering the backscatter radiation field uniformity both experimentally and theoretically, we obtained a backscatter peak at 189 keV with full width at half maximum of 4.3% (8.1 keV, 13.6 ch) uniformly over 10 cm × 10 cm × 10 cm volume. CdZnTe detector of 1 cm³ sensitive volume was used for photon energy measurement. We obtained a suitable dose rate of 4.06 μSv/hr for calibration applications with a Cs-137 source intensity of 207.8 MBq.