The effect of x irradiation on the spin-lattice relaxation times of transitions between the lowest spin levels of Cr 3+ in α-Al 2 O 3 has been examined. The values for the first-order transitions at 4 2 °K are reduced by a factor of ten after x irradiation. Comparison of the properties of orange rubies grown from the vapour phase and produced by x irradiation of pink ruby indicates that this is due to the conversion of some of the Cr 3+ into Cr 4+ and Cr 2+ .
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Mason et al. (1966) studied this question.