THE DISTRIBUTION OF ABSORBED ENERGY FROM A POINT BETA SOURCE

Abstract

The distribution of absorbed energy/g (dose) around a point source in a homogeneous medium has been measured for a variety of beta emitters in air, argon, ethane, and expanded polystyrene. At short distances the dose falls off faster than 1/r², contrary to the results of measurements in solids but in agreement with previous measurements in air for maximum beta energies below 1.6 MeV. Except at short distances, distributions in different media differ only by a constant scaling factor on distance (mg/cm²) that is the same within ± 1% for all beta energies considered, but that is not proportional to electron density. Except close to the end of the range, the absolute doses calculated from Spencer's numerical solutions of the transport equation agree with the measured values within about ± 4%, for maximum beta energies from 0.16 to 3.58 MeV and for a wide variety of spectral shapes. The calculations predict correctly the observed variation of attenuation with the atomic number of the medium.