Mechanism of Oxygen Effect on Solid State Nuclear Track Detectors

固体核径迹探测器，特别是CR-39塑料核径迹探测器，是探测重带电粒子的重要工具，在辐射防护测量、核聚变研究、空间科学等领域有着广泛的应用。然而，这些探测器在真空中储存会对配准灵敏度产生严重影响。探测器中溶解的氧对潜迹的形成起着重要的作用。该项目的目的是阐明“氧效应”的机制，并开发改进的探测器。测定了CR-39塑料中的氧浓度和扩散系数，确定了氧浓度与探测器灵敏度的关系。用四极杆质谱仪在室温下测量了市售CR-39片材的氧和其他组分的出气率。在大气压下的扩散系数和溶解氧浓度分别为（4-9）x10^<-9>cm^2/秒和（1-8）x10^<19>分子/cm^3。真空储存过程中蚀刻速率比的降低率由CR-39中溶解氧局部压力的函数给出。这些结果在定量上支持了“氧定形模型”。在充分放气的CR-39中，通过检测真空贮存后暴露在空气中的蚀坑恢复半径，评价了α粒子辐照形成的自由基的寿命。发现辐照后形成了寿命约为40分钟的自由基和寿命小于几分钟的自由基。具有不同寿命的自由基的数量与CR-39的数量、产生后的储存时间和入射粒子的能量有关。

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.