Absolute calibration of low energy, thick target bremsstrahlung

The applicability of the Monte Carlo program GEANT3 for the calculation of low energy, thick target bremsstrahlung is investigated. The results of GEANT3, version 15, show very good correspondence to those of the well established EGS4 code. In contrast it is shown that GEANT3, version 11, leads to erroneous angular distributions, total intensities and even spectral shapes for angles far from the critical angle. The use of the 115In(γ,γ')115Inm reaction for an absolute calibration of bremsstrahlung spectra is also demonstrated. This method relies on the reconstruction of the isomer yield excitation function from a set of 115In levels with well characterized properties. It can be utilized for continuous photon spectra up to about 3 MeV. (orig.)