Effect of gamma radiation on silica gel

Silica gel degassed at elevated temperature (>4000C) forms color centers absorbing light at 320 and 530 mμ upon γ-irradiation. Moreover, these gels adsorb H2 irreversibly. The color centers are readily bleached by H2. From the observed dependence on radiation dose and degassing temperature shown by the irradiated gel in respect of color center formation and H2 adsorption, the color centers - which have hitherto been tentatively attributed to positive holes - are now ascribed to the sites of the H2 adsorption. Assuming that one H2 molecule reacts with one positive hole, a G-value (positive hole) of about 0.3 is derived from the linear decrease of the color center at 530mμ seen with increasing H2 adsorption. Inference from chemical behavior indicates the presence of trapped electrons on irradiated gel degassed above 4000C. When present during irradiation, electron scavengers such as N2O or SF6 promote the formation of the color center. A visual verification was conducted on the presence or absence of coloration undergone by irradiated gel upon exposure to each of sixteen different gaseous reagents. The results of this survey, referred to the known values of the ionization potential of these reagents, indicated the effective electron affinity of the positive hole to be in the vicinity of 11 eV. A mechanism is presented to explain some of the foregoing findings. (auth.)