Effect of annealing on track density in unirradiated and gamma irradiated CR-39 when used for fast neutron detection

Abstract

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 × 12.7 × 0.635 mm) was irradiated with 1.3 kGy of 60 Co 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 (20) to 150°C for 30 min. The detectors were then overlaid with 0.77 mm of polyethylene and exposed to 252 Cf 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 h. 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 and 120°C then sharply decreased between 120 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.