A simple function describing the absorption in platinum for dose-rate calculations around radium applicators.

The diminution of exposure rate from radium gamma radiation by filtration through platinum can be described very accurately by gamma 0/(1 + mu0d) for wall thickness d from 0.2 mm to 4.0 mm (gamma 0 = 6.356 x 10(-5) A m2/kg2 or 8.869 R cm2 h-1 mg-1; mu0 = 1.501 cm-1). This function fits generally accepted data to within experimental error, to an accuracy not achieved by any single exponential factor. The expression is derived theoretically from a simple approximation to the spectral distribution of radium gamma radiation. With this type of transmission function, the expression for the exposure rate around linear radium sources in platinum tubes (generalized Sievert integral) is readily integrated in terms of elementary functions. Used in place of the Sievert integral in calculations of dose distributions, this new fomulation makes possible straightforward numerical evaluation and avoids the storage of tabulated values in computer programmes.