Dense radical formation in L-alanine-3,3,3-d3 and L-alanine-d4 by 1.5 keV soft X-ray irradiation

Radicals produced in crystalline L-alanine-3,3,3-d3 and L-alanine-d4 were observed by the electron spin resonance (ESR) technique during 1.5 keV soft X-ray irradiation. The spectral change from •CHCD3COOH to •CDCD3COOH by a hydrogen exchange reaction was directly observed in L-alanine-3,3,3-d3. The line width of the ESR spectra obtained by the soft X-ray irradiation was 1.5 times wider than that of hard X-rays from a previous report, meaning a higher density of radicals. The efficiency of the radical yield by the soft X-ray irradiation was 10−6 relative to that by γ-irradiation. The product formed by the radical recombination was observed by an LC/MS analysis. For the soft X-ray irradiation, many radicals will be lost by the efficient radical-radical recombination due to the higher density of the radicals, just as the high-LET irradiation by heavy ions. The result was the same as the yield of hydroxyl radicals in an aqueous solution that decreased with the decreasing photon energy from 1.25 MeV to 1.5 keV. We concluded that the high LET nature of the lower energy photons leads to the dense radical formation in the crystalline alanine powder.