Polycrystalline samples of amino acids and proteins were exposed to electrons and beams of helium, carbon, neon, and argon ions from the Berkeley heavy-ion linear accelerator. Both the irradiations and the electron-spin resonance observations were carried out at 77°K. The results showed that the same types of radicals were formed independent of the stopping power of the bombarding particle. The results for the amino acid DL-valine have been interpreted in relation to the "thermal spike model." The irradiation of DL-valine at 77°K results in the formation of a radical which converts upon heat treatment into a secondary radical. On the basis of the thermal spike model and the kinetics for the radical conversion, it is possible to calculate the expected amount of secondary radicals formed at 77°K along the tracks of the heavy ions. The observation that no spectral changes take place with increasing stopping power is clearly in contrast to the predictions from the thermal spike model. Therefore, the model in its present form does not apply to radical formation or secondary radical reactions.

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