Energy Calibration of Dosimeters for Gamma and X-Rays

The evaluation of energy dependence of health physics instrumentation, especially for low-energy radiation, is of great interest. If the exact knowledge of the response of an instrument as a function of photon energy is needed, sources of monoenergetic radiation are required. Since, in the energy range considered, a suitable set of nuclides is not available and beams of fluorescence X-rays, limited in energy to about 100 keV, are difficult to realize in practice at a sufficient dose rate, the only method is to modify the heterogeneous output of an X-ray machine by filtration. The result of a calibration of an instrument strongly depends on the spectral distribution of the beam, and this makes it necessary to use well-defined radiation for this purpose. Defining an 'effective energy' (Eeff) is not sufficient to characterize the beam. This led to the establishment, for the energy range 40 to 300 keV, of a set of nine radiation qualities, which have the advantage of being reproducible and having a similar spectral dose distribution, making detailed description of the properties of the calibration beam possible. To illustrate the Influence of the spectral distribution, the response of several instruments has been compared for radiations having the same mean energy. From the results it can be seen that only a knowledge of the photon spectra gives enough information to interpret the deviations in the various response functions. (author)