the same esr characteristics as the dosimeter substance i e for example have a comparable line width i e modulation saturation behavior a comparable microwave saturation behavior a comparable temperature coefficient an isotropic behavior etc according to the present invention a chromium doped magnesium oxide cr mgo is utilized as a reference sample the amount of dotation is preferably 0 1 the chromium cation cr 3 may be utilized in natural or in isotope clean abundance as will be explained below fig3 illustrates an esr spectrum of this sample material for a measurement in x band i e at a microwave frequency of about 9 8 ghz as one may clearly see from fig3 the esr spectrum of cr mgo has a primary line 82 as well as a smaller secondary line 84 both lines are sufficiently at a distance from the gyromagnetic ratio g 2 i e the resonance position of a free electron the amplitude a 1 of primary line 82 measured from tip to tip is about 38 u00d7 10 3 arbitrary units within the scope of the present invention one has now found that a considerable improvement may be achieved insofar when instead of cr mgo a reference sample is utilized in which the chromium ions appear as the isotope 52 cr in that case the chromium cation will be used in its isotope clean abundance gt 95 52 cr 3 fig4 in an analog illustration as compared to fig3 the esr spectrum 86 of 52 cr mgo is shown one can see that the spectrum only comprises a primary line 88 but no secondary line moreoever the signal to noise.