Radical formation in the radiolysis of solid alanine by protons and helium ions

Radical formation in solid α-alanine irradiated with 0.5-, 1-, 2-, and 3-MeV H ⁺, 0.5-, 1-, and 3-MeV He ⁺ ions were studied by means of electron spin resonance. The radical yield (number of radicals per incident ion) is constant below the critical fluence of about 10 ¹² ions cm ^-2. Above the critical fluence, the yield decreases with increasing ion fluence. G-value of the radical formation was obtained from the constant yield at the low fluences. Assuming a simple cylindrical shape of the ion tracks, the average radii of the ion tracks were estimated from the G-values. The estimated value of the radius is from 2 nm to 5 nm. The dependence of the radical yield on the ion fluence was simulated with the cylindrical model of ion tracks and dose-yield relationship for γ-irradiation. The observed yield decreases more than the simulated one above the critical fluence. This difference is ascribed to higher damage on the solid alanine by the ion irradiation than by γ-irradiation.