Electrical Conductivity of γ-Irradiated 3-MP Glass and of Photoionized 3-MP—TMPD during Warming from 4° and 77°K

Abstract

The electrical conductivity of γ-irradiated 3-methylpentane (3-MP) glass has been studied as a function of γ dose, temperature, applied voltage, post γ illumination, and the nature and concentration of additives. Three distinct populations of charge carriers are found in samples which have been γ irradiated at 77°K. The first, attributed to the combined mobility of weakly trapped electrons and positive charges, decays almost completely in 30 min in the dark at 77°K. It is reduced by 0.01 mole % of the electron scavenger CCl4, but is not affected by 1 mole % of the positive hole trap 2-methylpentene−1 (2-MP−1). The second, attributed to trapped electrons, is revealed by current flow between 83° and 88°K during warmup. It is eliminated by infrared illumination at 77°K before warm up (which causes a burst of current), or by alkyl halides, and is increased by 2-MP−1. The 85°K peak is absent on warmup of γ-irradiated 3-MP containing biphenyl, but appears if electrons are liberated from the biphenylide ion by illumination with 4100-Å light prior to warmup. The third population of charge carriers becomes mobile above about 92°K, giving maximum current at about 102°K. It is not affected by alkyl halide additives, and must be ascribed to migration of positive and negative ions. Additives capable of forming larger ions of lower mobility lower this current peak and produce new peaks. The areas of the peaks at 85° and 102°K are proportional to the applied voltage over the range tested (4×102 to 8×104 V cm−1). They increase linearly with dose below 1×1018 eV g−1 but more slowly at higher doses. Blocking electrodes reduce these peaks. Between about 88° and 92°K the warmup current of γ-irradiated 3-MP becomes negative, suggesting randomization by thermal agitation of radiation-produced charges which have been oriented by the applied electric field at 77°K. The warmup current spectrum of 3-MP following γ irradiation at 4°K shows a peak at about 60°K in addition to the peaks observed for warmup from 77°K. When tetramethyl-p-phenylenediamine (TMPD) in 3-MP at 77°K is photoionized, the warmup current shows a peak centered at 108°K, which differs from the similar peak produced by γ irradiation in that it is reduced by the presence of alkyl halides. The effect is attributed to complexing of TMPD by the alkyl halide with resultant trapping of the photoionized electrons in close proximity to the positive ion.