Dose Enhancement Due to Backscattered Secondary Electrons at the Interface of Two Media

When X or gamma rays pass through an interface of materials with low and high atomic numbers, a significant fraction of the secondary electrons are backscattered toward the interface by the material with a high atomic number. This considerably increases the dose at the interface. The aim of the present work was to measure the dose at the interface (D) of Perspex and materials of various atomic numbers and compare it with the dose in a homogeneous medium of Perspex (<tex-math>$D_{{\rm eq}}$</tex-math>). Thin samples (<tex-math>$3\ {\rm mg}/{\rm cm}^{2}$</tex-math>) of <tex-math>${\rm Li}_{2}{\rm B}_{4}{\rm O}_{7}\colon {\rm Mn}$</tex-math> thermoluminescent material were used for this study. Cobalt-60 gamma rays and X rays of different energies were used as primary radiations. This investigation has shown that: (i) The dose-enhancement ratio (<tex-math>$D/D_{{\rm eq}}$</tex-math>) at the interface increased with the square root of the atomic number of the backscatterer for both X and gamma rays; for X rays the increase was found to be nonlinear in the region of low atomic numbers. (ii) For a given backscatterer (silver) the <tex-math>$D/D_{{\rm eq}}$</tex-math> ratio varied in a complex way with the effective photon energy. (iii) The contribution by the backscattered secondary electrons to the dose at the Perspex-Perspex interface decreased rapidly with increasing air gap at the interface and was negligible for gap distances of more than 10 mm. (iv) For the Perspex-silver interface the electrons backscattered from silver had a wide range of energies. The maximum energy was about 600 keV for irradiation by⁶⁰ Co gamma rays and was about 100 keV for irradiation by 250 kV X rays. (v) The dose-enhancement ratio is dependent on the grain size of the thermoluminescent material. Larger grain sizes underestimated this ratio.