The effect of water content on the dark and radiation induced microwave conductivity of frozen gelatin gels

Abstract

The migration of charge carriers created in gelatin gels during nanosecond pulses of 3 MeV electrons has been studied using a time resolved microwave conductivity technique for temperatures from -20 to -150°C. Both the initial height and the decay kinetics of the measured conductivity transients were found to be dependent on the water content of the gels. Below a critical water content (approximately 32% water by weight) no induced conductivity was observed. Above this threshold the height of the signal was found to increase steadily with water content, tending to a conductivity value of approximately 1 3 of that found for pure ice at 100% water. At 138 K, the decay kinetics were non-exponential and the transient conductivity was found to decay over a timescale of tens of microseconds for the lowest water content gels. The lifetime of the carriers decreased with increasing water content, to submicrosecond timescales at the highest water concentrations studied, similar to the decay found in pure ice. For temperatures above-80°C the decay kinetics were exponential and the lifetime almost independent of water content. It is concluded that in the gels at low temperatures the water is present in different structures. The water most closely associated with the gelatin cannot sustain charge migration and was shown, by dielectric loss measurements, to correspond closely to the so called “non-freezing” water. Above this primary hydration layer, ice-like regions are present, some of which support charge migration similar to that in bulk ice, but some of which appear to form non-conducting ice regions. The presence of the latter regions is ascribed to the random coil configurations of the gelatin.