A Comparison of Measurements and Calculations of the Effects of Scattered Radiation on Dosimeter Calibration in a Calibration Range

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The method presented provides an alternative to the shadow shield method for experimentally measuring the contribution due to scattering radiation in a calibration range. Scattering of 0.6616 MeV photons from a 137Cs irradiator in a calibration range does not only occur due to the walls and floor of the range. It also occurs due to the source material itself, its encapsulation, the brass cup that holds the source, the two stainless tubes that surround the source capsule and the brass cup, and the irradiator structure that surrounds the aforementioned objects. The shadow shield method underestimates this scattering radiation that originates in the irradiator. By measuring the uncollided effective activity using a CZT detector and a total effective activity using a survey meter, the contribution of scattered radiation (including all of the abovementioned sources of scatter) to the effective source activity is able to be measured. In this paper, the measured mean effective source activities for a 137Cs irradiator in a calibration range are reported for a CZT detector and a survey meter. The measured activities are compared among themselves and with the results of MCNP calculations. From these comparisons, the new alternative method for measuring the scatter contribution was validated by agreement in both the MCNP calculations and experimental measurements that scattering contributed about 28% to the overall effective activity of the range.

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