Effect of Annealing on Track Density in Unirradiated and Gamma Irradiated Cr-39 When Used for Fast Neutron Detection*

The effect of annealing on track density in unirradiated CR-39 and gamma irradiated CR-39 neutron detectors was investigated. One group of CR-39 detectors (25.4 x 12.7 x 0.635 mm) was irradiated with 1.3 kGy of 6oCo gamma rays. The other group of detectors was not irradiated with gamma radiation. The two sets of detectors were then annealed at temperatures ranging from room temperature (2OOC) to 150°C for 30 minutes. The detectors were then overlaid with 0.77 mm of polyethylene and exposed to 252Cf spontaneous fission neutrons. Eight detectors were used as controls for the determination of background track density. All detectors were then etched in 6.25 N sodium hydroxide solution at 70°C for 4 hours. The track densities were determined with an optical microscope. In the case of gamma irradiated detectors the net track density increased non-linearly with temperature until 150°C. For the unirradiated detectors the net-track density initially decreased with increasing temperature up to 60°C, increased between 60°C and 120°C then sharply decreased between 120°C and 130°C, remaining constant thereafter. Ultraviolet spectrophotometry measurements showed significant changes in light absorbance indicating heat induced structural changes in the detector for both annealed CR-39 detectors and gamma irradiated annealed CR-39 detectors. INTRODUCTION CR-39 is a polymeric nuclear track detector which is being increasingly used for fast neutron dosimetry. An ideal dosimeter should retain most of the dose information under various environmental conditions. If the environmental effects on the dosimeter are unknown then the estimate of the dose could lead to erroneous results. During use track detectors can be exposed to temperatures higher than ambient levels. Elevated temperatures can induce changes in the bulk detector material and in the latent damage tracks. The effect of temperature on the material and on track registration depends on whether the material has been subjected to heating prior to or after the formation of radiation induced latent damage tracks. Thus there are two temperature effects, the so called preand post-irradiation annealing effects (Ogura and Tamai (1982)). Pre-irradiation annealing alters the property of the bulk material and post-irradiation annealing affects both the bulk material as well as the latent damage tracks. Some aspects of the effect of temperature on the registration properties of CR-39 have been reported (Al Najjar et al. (1982); Gruhn et al. (1980); O’Sullivan et al. (1981); Jamil et al. (1981); O’Sullivan et al. (1984); Hamasaki et al. (1984); and Wong and Field (1984)). Ultraviolet absorbance studies are powerful techniques for the indication of structural changes within a material. Hence the effect of annealing on ultraviolet light absorbance in unirradiated and gamma irradiated CR-39 was also investigated.