Ionic Processes in γ-Irradiated Organic Solids: Electrons in 3-Methyl Pentane

Abstract

γ-irradiated 3-methylpentane (3MP) and solutions of biphenyl (φ) have been examined at −196° at doses of 1 to 26×1018 eV ml−1. The half-life for decay of the trapped electron (e−) in 3MP is independent of dose for t<1 h and the rate is approximately second order for t>1 h. This behavior, as well as very efficient formation of φ− at small % φ, requires electron track lengths of ∼10−4 cm. The 100 eV yield of (e−) shortly after γ irradiation is ∼0.9, in terms of the measured extinction coefficient ε(e−)≅3.0×104 liter mole−1·cm−1 at 2.3×1018 eV ml−1, but only 60% as great at 10-fold greater dose. At 0.04% φ, the rate of (e−) decay is much faster and approximates first order. The ratio of reaction cross sections of (e−) with positive holes and φ is ∼120. Addition of 2% 2-methylpentene−1 as positive hole trap increases the yield of (e−) and decreases the rate of decay, which becomes second order throughout. Recombination luminescence intensity from TMPD at t>1 h decays at the same rate following uv and γ excitation. The evidence for randomization of relatively large yields of ion pairs and very long track lengths, as well as trapping and escape, may require a new theoretical framework.