Measurement of photon mass-energy absorption coefficients of paraffin wax and gypsum at 662 keV

One of the most important gamma radiation quantities with respect to radiation dosimetry and health physics applications is mass-energy absorption. Direct measurements of the coefficient of mass-energy absorption, μen/ρ, are difficult and typically recourse is taken to theoretical computations. In this study, we have determined μen/ρ experimentally using a simple and direct method based on paraxial sphere transmission, using a proportional response gamma detector. The effects of nonparaxiality and of finite sample thickness have been accounted for, using extrapolation procedures. The deviation from nonproportionality and other corrections have been shown to be small. For 662 keV photons, the measured value of μen/ρ for paraffin wax has been determined using the above method as (3.37±0.05)x10-3 m2/kg. This compares favourably with the theoretically computed value of 3.35x10-3 m2/kg given by Hubbell and Seltzer (1997). The measured value of μen/ρ for gypsum has been determined as (2.96±0.01)x10-3 m2/kg, which also compares favourably with the theoretically computed value of 2.99x10-3 m2/kg given by Hubbell and Seltzer (1997). Comparison of measurements made using spherical shells and cylindrical shells has been provided with the intent of determining the equivalence of these two phantom shapes. In addition to the question of sensitivity of measurements to shell thickness, the investigation also examined the dependency of measured values on source-to-detector separation. Results obtained for the two shapes show close agreement with regard to the values of μen/ρ obtained using either shape