Effect of Gamma Radiation on Silica Gel

Silica gel degassed at elevated temperature (>400°C) forms color centers absorbing light at 320 and 530 μ upon γirradiation. Moreover, these gels adsorb H 2 irreversibly. The color centers are readily bleached by H 2 . From the observed dependence on radiation dose and degassing temperature shown by the irradiated gel in respect of color center formation and H 2 adsorption, the color centers-which have hitherto been tentatively attributed to positive holes-are now ascribed to the sites of the H 2 adsorption. Assuming that one H 2 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 530 mμ seen with increasing H 2 adsorption. Inference from chemical behavior indicates the presence of trapped electrons on irradiated gel degassed above 400°C. When present during irradiation, electron scavengers such as N 2 O or SF 6 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 e V. A mechanism is presented to explain some of the foregoing findings.