Variations in the shape and the power saturation of EPR spectra of L-alanine irradiated with photons, electrons, neutrons and protons are reported. It is shown that the ratio of the intensities of the satellite lines attributable to "spin flips" and the central line depend on the EPR microwave power, and it is proposed as a quantitative measure of the signal saturation effect. Dependence of this ratio on the microwave power is effected by the radiation quality and for doses in excess of 10 kGy by the absorbed dose. At high doses of low-LET radiation these changes are attributed to a high local density of free radicals, while for low doses of high-LET radiation these are due to changes induced in the crystal lattice. Consequently, the conventional peak-to-peak amplitude measurement of the EPR signal intensity is inaccurate when used for high doses and for comparison between radiations with different beam quality. The possibility to determine radiation quality from an EPR measurement is discussed.

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