Measurement of the Spectral Distribution of Scattered 400 kVp X Rays in a Water Phantom

A scintillation counter employing a large sodium iodide crystal has been used to measure the spectral distribution of the scattered radiation in a water phantom irradiated with 400 kVp X rays. The spectrum was determined at various points along the central axis of the X-ray beam for various scattering angles. The complete energy spectrum at each point was found by integrating over all angles. The shape of the spectral distribution of the scattered radiation was found to be nearly independent of the depth below the surface of the phantom. The variation of spectral distribution with field size was also investigated and in this case also the shape of the distribution was very nearly constant but was somewhat shifted to lower energies as the area of the field was increased. The scattered spectra were also plotted as distributions of dose and then combined with the dose distributions of primary radiation to give the total dose distribution. As an illustration of the use of radiation spectra, these dose distributions were used to calculate the effective half-value layer in copper and the average linear energy transfer of the radiation at various depths and for various field sizes. In addition, the absorbed dose in bone compared to the dose in muscle was calculated from the spectral distributions.