High linear energy transfer (LET) nature of alanine radical yield by soft X-ray irradiations studied by electron spin resonance (ESR) applications

Radicals produced in alanine by irradiation of 2-7 keV X-rays were investigated by ESR applications; i.e., CWESR, pulse-ESR and ESR imaging. The relative radical yield decreased with the decreasing photon energy and the increasing dose. The 2D imaging showed that the radical concentration at the surface was extremely low even though the absorption of X-rays exponentially decreased with the penetration depth. The depth of the radical concentration peak was almost the same as the penetration depth for the X-ray irradiation with an energy lower than 5-keV. The product formed by radical recombination was observed by an LC/MS (liquid chromatographmass spectrum) analysis. For the soft X-ray irradiation, many radicals will be lost by the efficient radicalradical recombination due to the higher density of the radicals, similar to the high-LET irradiation by heavy ions. The spin-spin relaxation rate (1/T2), which correlates with the radical concentration, increased by the decreasing photon energy. The values of 1/T2 for the soft X-rays were compatible to that for the heavy ions with the high-LET. Based on these observations, it is concluded that the effects of irradiation by soft X-rays are similar to high-LET irradiation.