Gamma-Ray and Neutron-Induced Conductivity in Insulating Materials

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Abstract
Excess conductivities induced by steady-state sources of gamma rays and by pulsed sources of neutrons and gamma rays in polyethylene, polystyrene, polypropylene, Nylon, polyisobutylene (impregnated paper), mylar, Teflon, diallylphthalate, H-film, cellulose acetate, reconstituded mica, tantalum oxide, and an epoxy formulation have been measured. The measurements were made at steady-state gamma-ray dose rates within the interval from 1.0 × 10-3 rads(H2O)/sec to 1.0 × 104 rads(H2O)/sec, and at combined pulsedneutron and gamma-ray dose rates less than 2.0 × 108 rads(H2O)/sec. All measurements were made at controlled temperatures between 25°C and 71°C. With steady-state gamma-ray irradiation, an excess conductivity is induced which has distinct features in three time intervals denoted as A, B, and C. In interval A, induced conductivity (σ - σo) is responding to a step increase in gamma-ray dose rate. The conductivity response is exponential (σ - σo) = A(1 - e-t/τo), with the time constant (τo) decreasing with increased gamma-ray dose rare (γ). The change in time constant as a function of gamma-ray dose rate at a fixed temperature is approximated by τo = koγ-μ where ko and δ are empirical constants. In interval B the induced conductivity has arrived at an equilibrium value whose magnitude as a function of gamma-ray dose rate at a fixed temperature is characterized to a good approximation by (σ - σo) = Aγγδ, where Aγ and δ are empirical constants. In interval C the conductivity is recovering upon removal of the sample from the radiation environment.

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