40 PULSE HEIGHTFIG. 2, Coincidence counting rate in a small pulse-height interval as a function of pulse height. Pulse-height interval was 2.5 pulse-height divisions.

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this case because the hyperfine, quadrupole, and even nuclear magnetic energies are all of the same order of magnitude. The above model accounts for most of the features of the spectrum and is quite similar to the model proposed by Griffiths et al. 1 and by O'Brien and Pryce 2 to explain the resonances observed in irradiated (smoky) quartz where the effects were ultimately due to the presence of naturally occurring aluminum impurities.The assumption in the above model that each hole is oriented along a definite tetrahedral bond direction may imply that in the diamond-Al system the spin precession frequency in a field of approximately 3 kilogauss is large compared to a molecular resonance frequency governing the rate at which the hole flips from one tetrahedral bond direction into another. A more detailed analysis of the observed resonances is in progress.

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