Spatial distribution of radicals produced by fast neutron irradiation

Abstract

The linear energy transfer (LET) of ∼0.5 MeV fast neutron in organic compounds is known to be more than 100 times larger than that of 60Co γ-ray. The local concentration of radicals produced by irradiation with two different kinds of radiation were evaluated. Microwave power saturation of electron spin resonance (ESR) spectra of radicals formed by fast neutron irradiation in a fast neutron source reactor “YAYOI” at 77K has been measured. Although ESR spectra of irradiated cyclohexane, polymethylmethacrylate and polyethylene show no large difference between neutron and γ-irradiations, power saturations of the ESR spectra for the two irradiations are very different. From the saturation characteristics, the spin-spin relaxation time in fast neutron irradiated samples was evaluated to be 3∼7 times smaller than that of γ-irradiation. In other words, the local concentration of radicals formed by fast-neutron irradiation is 3∼7 times higher than that formed by γ-irradiation. The spatial distributions are discussed in connection with LET of the radiations and structures of spurs in the matrix.