ESR-aided evaluation of lyoluminescence processes in ‘Tris’

ESR spectral analysis of γ-ray irradiated lyoluminescence (LL) phosphor powder, Tris (hydroxymethyl) aminomethane—referred to as ‘Tris’ showed the presence of two free radical species. It was observed that at higher exposure levels radical II with a singlet spectrum shows a higher growth compared to radical I with a three component spectrum. Radical I had been identified as RĊHOH. The ESR-LL correlation studies indicate that the RĊHOH radical formed in a low dose range is responsible for LL emission and the radical II either inhibits or does not have any role in the LL process. The LL yield of Tris measured under deoxygenated conditions showed a 40% reduction in the yield in luminol solution and 87% in distilled water. The range of linearity of the γ-ray response extends further from 200 to 2000 Gy when the LL is measured under oxygen equilibrated condition.

Based on the results observed, two models for the mechanism of LL in Tris have been proposed. The first LL model of Tris envisages the production of a dinegative luminol molecule and a restored Tris molecule when the LL enhancer luminol is used as the solvent. This is based on the hydrogen abstraction reaction of the RĊHOH radical. The luminol dianion reacts with O₂ to produce the excited singlet state of the aminophthalate ion which gives the LL emission. The second model proposes that the oxidation of RĊHOH radicals will yield organic peroxy radicals which in turn produces hydroperoxy (HO₂) radical and a superoxide anion. The HO₂ disproportionates, or reacts with the superoxide anion to give a singlet oxygen. The LL process in distilled water is probably due to the liberation of these singlet oxygen molecular pairs which are chemiluminescnt. H₂O₂ which is also produced on the disproportionation of HO₂, or one of the interconversion products OH, can also oxidize the luminol molecule to give the emission. The LL saturation, and subsequent reduction at the increasing free radical concentration, are also discussed.