Absolute ionization dosimetry of high enrgy electrons with an energy of more than 15 MeV

By means of a graphite double extrapolation chamber, the self consistent dosimetry system already used for an electron energy until 15 MeV also allows an absolute determination of the cavity ion dose until 42 MeV which can be performed under practically ideal Bragg-Gray conditions with the precision of a primary standard. For the conversion into energy-dose, the relative mass collision stopping power Ssub(w,l) reduced to the universal dosimetric constants W/e, G and epsilon is used which is determined by the system itself and corresponds to the theoretic relative unrestricted mass collision stopping power. A clear function for Ssub(w,l) only depending on anti Esub(r) is achieved by a co-ordination with the mean rest energy of electrons anti Esub(r), and that in any measuring depth. This function also harmonizes with the theoretical values of the relative unrestricted mass collision stopping power in the enlarged energy range. The performance specifications and influence quantities of the small parallel-plate chamber ('Electron Chamber') which also has been already used until 15 MeV as a secondary standard dosemeter and field instrument in the water, M3 or plexiglas phantom keep the same values and ranges of use. The calibration factor of the cavity ion dose is independent of the energy and remains constant also for the enlarged energy range. The measuring accuracy for the cavity ion dose and the energy-dose are unchanged. (orig.)