Study of Neutron Induced Modification on Optical Band Gap of CR-39 Polymeric Detector

Abstract

Neutron- irradiation effects on optical absorption of solid-state nuclear track detector, CR-39 was studied for the various fluences ranging from 2.38x106 to 2.7x108 n/cm² by ultraviolet-visible (UV-Vis) spectroscopy. The optical absorption spectra in the wavelength range of 200-700nm were recorded for the pristine and the neutron irradiated samples in the above fluence range. The UV-visible spectra shows a shift in the absorbance edge towards the higher wavelength, which can be correlated to the transition involved in the CR-39 polymer and variation in the band gaps (Eg) using Tauc’s expression. Both the indirect and direct band gap in pristine and neutron irradiated CR-39 have been studied, it is interesting to note that the values of indirect band gap found to be lower then the corresponding values of the direct band gap. Indirect and direct bandgap of pristine and exposed CR-39 was found increasing with increasing fluences. However values of the indirect band gap found lower than that of direct band gap.  A decrease in the optical energy gap with increasing fluence can be discussed on the basis of neutron-irradiation defects in CR-39 and the numbers of carbon atoms in a cluster were determined from UV-visible spectra of the neutron irradiated samples, using Tauc’s expression.