Mechanism of Oxygen Effect on Solid State Nuclear Track Detectors

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Solid state nuclear track detectors, especially CR-39 plastic ones, are a useful tool to detect heavy charged particles and applied to many fields, such as radiation protection measurements, nuclear fusion study, space science, etc. However, the storage of these detectors in vacuum causes severe effects on the registration sensitivity. The oxygen dissolved in a detector plays an important role in the latent track formation. The purpose of this project is to clarify the mechanism of the "Oxygen Effect" and to develop improved detectors.The oxygen concentration and the diffusion coefficient in CR-39 plastic were measured, and the relation between the oxygen concentration and the sensitivity of the detector was ascertained. Outgas rates of oxygen and some other components from CR-39 sheets commercially available were measured at room temperature with a quadrupole mass specrometer. Diffusion coefficients and oxygen concentrations dissolved at atmospheric pressure were evaluated to be(4-9)x10^<-9>cm^2/sec and(1-8)x10^<19>molecules/cm^3, respectively. The reduction rates of etch rate ratios through the storage in vacuum were given by a function of local pressures of oxygen dissolved in CR-39. These results supported the "oxygen setting model", quantitatively.The life time of the radicals formed by alpha-particle irradiation in the sufficiently outgassed CR-39 was evaluated on the basis of the examination for etch pit recovery in radius due to the air exposure after the storage in vacuum. Radicals with a life time of about 40 minutes were found to be formed as well as radicals with a life time of less than a few minutes after irradiation. It was also found that the fractions of radicals with different life times depended on the lots of CR-39, the time of storage after production and the energy of incident particles.