High energy gamma-ray energy deposition within shielding materials

Arrays of thermoluminescent dosimeters were used to determine spatial dose distributions from a 0.375 inch diameter beam of essentially monoenergetic high energy gamma rays normally impingent on water, aluminum, and lead phantoms. Photon energies considered were 1.25, 6, and 8 MeV. A conventional Monte Carlo program was used to predict the spatial- and energy- dependent gamma-ray flux in the phantom, and thus to provide a basis for determining average dosimeter response functions at various locations in the phantoms. The experimentally determined dose distributions were compared to the Monte Carlo results and discrepancies were apparent in several cases. At high energies in water and aluminum, considerably more energy was removed from the beam volume than was predicted. This is attributed to the finite range of the energetic secondary electrons which was not considered in the Monte Carlo code. At 6 and 8 MeV, the energy deposition in lead was found to be considerably greater than that predicted. In this case measured off-beam absorption was up to an order of magnitude greater than the Monte Carlo predictions. This behavior is considered to be a result of reradiation and subsequent reabsorption further removed from the initial beam. These results reaffirm the necessity for considering secondary electron transport mechanisms, particularly in the high-Z, high photon energy case. Failure to include these effects in complex source-shield-receptor geometries could lead to serious discrepancies between calculated and measured energy absorption rates