Generation Of Color Centers Research Articles

Optical absorption in fused silica during irradiation: Radiation annealing of the C-BAND

The results of extensive measurements of the optical transmission of Corning 7940 u.v. grade fused silica made during ambient and elevated temperature irradiations are described. The irradiations include high dose rate pulsed reactor irradiation, moderate dose rate steady state reactor irradiation and a selectable dose rate steady state 1·5 MeV electron irradiation. The growth of the C-band absorption at 2150 Å during ambient temperature irradiation indicates that this absorption is due to the trapping of free electrons at defects; the free electrons and defects both being generated by ionization process. During prolonged elevated temperature irradiation, the C-band absorption reaches a steady-state value due to the combined effects of the generation and annealing of color centers. At a given dose rate, these steady state values decrease with increasing temperature due to the corresponding increase in the annealing rate. Above a dose rate of 0·4 Mrad/sec the steady-state absorption coefficient decreases with increasing dose rate due to radiation annealing. A mathematical model describing the temporal behavior of the C-band absorption during irradiation is also presented.

X-Ray Generation of Color Centers in Zone-Refined KCl and KBr

The techniques of zone purification of KCl and KBr crystals are described, and the success of this technique in removing impurities such as hydroxyl ions and divalent cations is demonstrated. A comparison of the defects introduced into zone-purified salts and commercial salts by x irradiation at room temperature and at liquid-helium temperature is given. Studies of the optical bleaching of $F$, $H$, and ${V}_{K}$ centers by $F$-band light at liquid-helium temperature gives evidence of the spatial proximity of these centers.

Structure Sensitivity of the X-Ray Coloration of NaCl Crystals

Room temperature measurements of the rate of coloring of NaCl crystals by x-rays at different depths below the irradiated surface and for different states of deformation and heat treatment are reported. From the results it is concluded that two mechanisms of coloring operate in these crystals. The first, or "rapidtype" coloring, approaches a saturation density of $F$-centers of the order of ${10}^{17}$/${\mathrm{cm}}^{3}$, and appears to result from the generation of color centers from vacancies already present in the unirradiated crystal. The second, or "slow-type" coloring, takes place at a constant rate until $F$-centers in excess of ${10}^{18}$/${\mathrm{cm}}^{3}$ are formed. This type of coloring, which is usually observed only near the irradiated surface, is due to the generation of $F$-centers at dislocations, and is responsible for the hardness and density changes produced by the x-rays. Rapid-type coloring is found to occur at essentially the same rate in deformed crystals and in carefully annealed crystals; recovery at low temperatures after deformation, however, decreases the colorability. These results indicate that the principal effects of deformation and heat treatment on colorability may be related to the state of dispersion of impurities.