Growth of color centres and hardening of CaF2 by heavy dose of γ-irradiation

Abstract

The growth of color centers and the accompanying changes in the mechanical properties (flow stress, Vicker's microhardness and dislocation mobilities) of CaF, single crystals due to y-ray irradiation has been studied in the dosage range of lo4 to 5 x lo8 R at room temperature. Of the four absorption band spectra with peaks a t 225, 335,400, and 580 nm observed, the 225, 400, and 580 nm bands were seen to saturate at about 105 R and remain saturated in the entire dosage range studied. On the other hand the 335 nm band, after initial saturation a t N lo5 R was observed t o start growing again a t about 2 x 107 R.The observed changes in the mechanical properties are seen to correlate well with the growth of the 335 nm band. A tentative model is suggested for the color center responsible for the 335 nm band and the mechanical property changes. Es wurde das Wachstum vonFarbzentren unter y-Bestrahlung bei Raumtemperatur und damit verbnndene Bnderungen der mechanischen Eigenschaften (FlieBgrenze, Vickers Mikroharte und Versetzungsbeweglichkeiten) in CaF, Einkristallen im Dosisbereich von 104 bis 5 x 108R untersucht. Von den vier beobachteten Absorptionsbanden bei 225, 335, 400 und 580 nm erreichten die Banden bei 225. 400 und 580 nm die Sattigung bei etwa 105 R und bIieben iiber den gesamten untersuchten Dosisbereich gesattigt. Die 335 nm-Bande dagegen erreichte eine vorlaufige Sattigung bei ungefahr 105 R und zeigte einen weiteren Anstieg bei etwa 2 x 10' R. Die beobachteten VerLnderungen der mechanischen Eigenschaften konnten mit dem Wachstum der 335 nm-Bande gut in Beziehung gebracht werden. Ein Modell fur das Farbzentrum, das AnlaD fur die 335 nm-Bande und die Anderungen der mechanischen Eigenschaften gibt, wird vorgeschlagen.

Coloring of synthetic fluorite

“…The absorption bands obtained from this study are not the same as those obtained by Sashital and Vedam [18], although in each case the same radiation doses and thicknesses were maintained. Neither the number of absorption bands nor the position of the absorption bands are the sanie, although the fluorite samples of the Harshaw Chemical Company were used in both experiments.…”

“…Neither the number of absorption bands nor the position of the absorption bands are the sanie, although the fluorite samples of the Harshaw Chemical Company were used in both experiments. However, the resulting absorption spectra were expected to be different from Sashital and Vedam's [18] work as a result ofe the different impurities present.…”

“…Chemical analyses of the synthetic fluorite show that even the purest available synthetic fluorite is not completely free of chemical impurities. I n a similar study, Harshaw Chemical Company fluorite, that was analyzed by Sashital and Vedam [18] was found to contain 20.0 ppm sodium and 4.5 pprn yttrium. The presence of these various impurities, even in fluorite samples of t,he same company, explains why many discrepancies have arisen between researchers as to the causes of the coloration of fluorite.…”

“…The cause of coloration for synthetic flnorit,es has been mainly attributed to electrons trapped a t the lattice defects [l], electrons trapped in a fluorine vacancy (F-centre) [5, 10 t o 131, aggregation of F-centres [lo, 13 t,o 171, self-trapped hole F, [18] and impurity-related complex centres [ 191. Many of these studies used differently originated synthetic fluorites which were considered t,o be pure, but in fact contained minor and variable amounts of chemical impurities.…”