An investigation is made of the absorption spectra, the spectral dependence of the luminescence quantum output, glow curves, and the radiation spectra of the glow curves of ruby crystals. These crystals are coloured by intense light flashes or by γ-rays from Co60. It is found that the absorption is greater in coloured crystals while the luminescence quantum output is smaller. The thermal decoloration of crystals is accompanied by a luminescence in the R-line region. A decrease in the Cr3+ ion concentration is detected in coloured crystals by ESR measurements. The similarity of all the characteristics obtained allows a conclusion about the identity of the colour centres arising under optical and γ-irradiation of the crystals. The results obtained here and published data suggest that Cr2+ and Cr4+ act as colour centres, and that their formation under optical coloration is duo to Cr3+ ion ionization via a two-step process. It is shown that the gradual increase of the ruby generation threshold observed experimentally, which gives laser operation, is due to the formation of colour centres which cause a parasitic absorption of the pumping light.



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Colour Centres in Ruby Crystals

Lasers offer many potential advantages for space communications and space instrumentation. Their performance is, however, affected by the high energy radiation found in the space environment, e.g., trapped proton and electrons, solar flare protons, and possibly gamma rays and neutrons from on-board nuclear power, auxiliary or propulsive. We report here on some studies of the effects observed on lasers and laser components exposed to irradiation, and in particular on the mechanisms of these effects.

Mechanisms of Radiation Effects on Lasers

An investigation is made of the spectral dependence of the luminescence quantum yield ratio (η/η0) of coloured and non-coloured ruby and of the yield (ηadd) calculated for additional absorption. It is shown that in the additional absorption (Δk) are bands of different nature. Those with ηadd = 0 are called “active”, the others, with ηadd = 0, are called “inactive”. The active absorption connected with an electron transition to the conduction band and a following recombination with R-line radiation, is attributed to Cr2+ ions, the inactive to Cr4+ ions. Experiments of monochromatic light action on coloured ruby have confirmed this conclusion; light absorbed in the region of active bands bleaches the crystal, whereas bleaching does not occur in the region of inactive bands. The results obtained allow to suggest a level scheme of corundum with a Cr2+ centre.



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On the Nature of the Colour Centres of Ruby

An increase in the absorption cross section responsible for optical pumping of a ruby laser occurs as a result of γ-irradiation. As single crystal ruby is irradiated with Co60γ-rays, the usual absorption bands which exist between 200 and 550 mμ change from four distinct bands to one continuous absorption band. Similar work on sapphire, the host crystal for ruby, indicates that the majority of this increased absorption is experienced by the Cr+++ion in ruby. Thermoluminescence data are presented showing that electron traps with activation energies of about 0.64 and 0.78 eV are formed in ruby as a result of γ-irradiation. These traps remain in the crystal until it is annealed at about 800°C. It is found that the threshold energy for laser action increases in relation to the amount of irradiation. This effect is also manifested as a decrease in the luminescence of the R 1 line after γ-irradiation. A model has been devised to explain the changes in Cr+++ions due to γ-irradiation and to show how this affects laser action. Theoretical curves are presented indicating how the energy output vs. energy input varies with the amount of γ-irradiation. These data show a definite increase in threshold energy, and at higher pump energies they suggest the possibility of higher efficiencies than possible before irradiation. These theoretical results are supported by experimental data.

M. K. Moss

Papers

Effects of Gamma Irradiation on the Energy Output of Ruby Laser Crystals