The relative ionisation efficiencies of alanine dosimeters and tooth enamel differ greatly between gamma and X-ray irradiations. This can be attributed to the differences in the energy transfer mechanisms. When using Co or Cs sources, the photon energy is transferred to matter via Compton scattering, which shows little dependence on the average Z-value of the absorber. In contrast, the photon energies emitted from X-ray sources are usually much lower and energy is, at least in part, transferred to matter via the photo-electric effect, which has a cubic relationship between energy transfer and average Z -value of the absorber. The decomposition of the ESR spectra of an enamel fragment shows that the X-ray generated distribution of anisotropic View the MathML source radicals is virtually the same as in the natural sample, but quite different to gamma irradiated samples. This implies that X-rays are generally much better suited for ESR dating irradiation procedures than gamma rays. However, exact source calibration will be difficult.