The high energy dosimetry system Goettingen. Twelve years controlled accuracy and stability

The high energy dosimetry system Goettingen consists of a graphite double extrapolation chamber as the primary standard, a small parallel-plate chamber as a secondary standard and field dosimeter chamber, and a ferrosulphate dosimeter as a transfer dosimeter. The estimate of the Bragg-Gray dose of air, Da, in the graphite chamber (phantom) is 'absolute'. The dose to water, Dw, at the same point is Dw=sw,a·Da. The accuracy of the system is given - apart from the fact of direct instead of 60Co or 2 MV calibration with a number of semi-empirical correction factors - by that of all magnitudes needed and correction factors gained experimentally by the system itself and reduced to general physical and dosimetric constants. The relative mass stopping power, sw,a, is experimentally evaluated combining ionization and ferrosulphate dosimetry, and essentially given by the product of the dosimetric constants W·G·ε. Except for air density and saturation losses, corrections are included in the calibration factor, also for field perturbation. The simplicity of the system is first given by the choice of the reference energies: Ep,0, the most probable energy at the surface, and Er the residual energy at depth z. sw,a if referred to Er depends only on this one parameter for all initial energies and usual spectra between Er=2 and 42 MeV and depths z ≤ 0.8 Rp. sw,a (Er) is simple to calculate by an interpolation formula to better than ±0.3%. Another simplifying factor is the energy independence of the air dose calibration factor, and a pertinent small polarity effect of <0.5%. The latter is valid also for high energy photon beams, at least between 8 and 42 MeV. An analogous procedure as for electrons can be applied. A comparison of the long term stability of the primary/secondary and the primary/ferrosulphate standards shows random deviations of <0.1 and <0.2, respectively. 12 refs, 12 figs