FACTORS AFFECTING TRACK REGISTRATION CHARACTERISTICS OF CR-39 POLYMER WHEN USED AS A FAST NEUTRON DETECTOR (GAMMA, TEMPERATURE, ULTRA VIOLET ABSORBANCE)
Abstract
Certain factors affecting track registration characteristics of CR-39 were investigated for Cf-252 fission neutrons. The etching conditions used for all experiments were 6.25N NaOH at 70(DEGREES)C for 4 hours. Track densities were determined with an optical microscope. To determine the effect of polyethylene thickness on track density, groups of CR-39 detectors (1 x .05 x 0.025 in.) covered with different thicknesses of polyethylene (0-5.2 mm) were exposed to a known neutron fluence and etched. The net track density increased with polyethylene thickness up to 4.45 mm. The neutron fluence response was determined by exposing groups of detectors overlaid with 0.77 mm of polyethylene to different neutron fluences (2.87 x 10('7) to 2.87 x 10('8) n/cm('2)). Net track density increased linearly with neutron fluence with a sensitivity of 5.5 x 10('-4) tracks/n. The effect of pre-gamma irradiation on track density and bulk etch rate was determined by irradiating groups of polyethylene covered detectors with different gamma doses (0-24 Mrads) and exposing them to a known neutron fluence. Net track density decreased and bulk etch rate increased non-linearly with gamma dose. The effect of pre-gamma irradiation and pre-annealing on track density and bulk etch rate was determined by exposing groups of polyethylene covered detectors to a fixed gamma dose, annealing them at various temperatures (20(DEGREES)C-170(DEGREES)C) for 30 minutes and irradiating them with a fixed neutron fluence. Net track density and bulk etch rate increased non-linearly with temperature. To determine the effect of pre-annealing on track density and bulk etch rate groups of detectors were annealed at different temperatures (20(DEGREES)C-170(DEGREES)C) for 30 minutes and then exposed to a known neutron fluence. The net track density initially decreased with increasing temperature up to 60(DEGREES)C, increased between 60(DEGREES)C and 120(DEGREES)C, then sharply decreased between 120(DEGREES)C and 130(DEGREES)C, remaining constant thereafter. The bulk etch rate initially decreased with increasing temperature to 60(DEGREES)C, increased between 60(DEGREES)C and 80(DEGREES)C, remained constant to 130(DEGREES)C after which it sharply increased. UV spectrophotometry measurements showed significant changes in light absorbance for gamma irradiated and for annealed CR-39.
Degree
Ph.D.
Subject Area
Radiation
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