In this paper the suitability of a PTW natural diamond detector (DD) for relative and reference dosimetry of photon and electron beams, with dose per pulse between 0.068 mGy and 0.472 mGy, was studied and the results were compared with those obtained by a stereotactic silicon detector (SFD). The results show that, in the range of the examined dose per pulse the DD sensitivity changes up to 1.8% while the SFD sensitivity changes up to 4.5%. The fitting parameter, D, used to correct the dose per pulse dependence of solid state detectors, was D = 0.993±0.002 and D = 1.025±0.002 for the diamond detector and for the silicon diode respectively. The D values resulted independent of particle type of two conventional beams (a 10 MV x-ray beam and a 21 MeV electron beam). So if D is determined for a radiotherapy beam, it can be used to correct relative dosimetry for other conventional radiotherapy beams. Moreover the diamond detector shows a calibration factor independent of beam quality and particle type, so an empirical dosimetric formalism is proposed here to obtain the reference dosimetry. This formalism is based on a dose-to-water calibration factor and on an empirical coefficient, that takes into account the reading-dependence on the dose per pulse. This procedure is applicable also to silicon diode determining its calibration factor for each energy or quality of the beam.