Differential Dose-Rate Measurements of Backscattered Gamma Rays from Concrete, Aluminum and Steel

The angular distribution of backscattered dose-rate ratio was measured for ‘semi-infinite’ concrete, aluminum and steel slabs irradiated by plane-parallel beams of Co⁶⁰ and Cs¹³⁷ gamma photons. The photons were incident on the slabs at angles, measured from the normal to the slab, of arccos 1.00, 0.75 and 0.50. For the necessary sensitivity, the backscattered dose rate was measured by a digital dosimetry system using a plastic scintillator as the detector. An empirical formula for differential dose-rate ratio was derived from the experimental data. Comparisons are made with two other experiments and a semi-empirical formula fitted to a Monte Carlo calculation. The experiments generally agree to within 20%. The values based upon the Monte Carlo calculation are generally 20% to 35% lower than the experimental values.