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Using an ultrasonic pulse echo technique operating at frequencies ,....,10 Mc/sec we have obtained the following elastic constants for cubic PbF2: CI1=8.88X1O lI dyn/cm 2 , C12=4:72X1O lI dyn/cm 2 , C'4 =2.454X1O lI dyn/cm 2 at 27°C. From these data a Debye 0 of 221.2°K was calculated.

An anisotropic relaxation peak has been observed in the internal friction of crystalline quartz. The height of the loss peak increases by a factor of 2.1 between a sample stressed in the Y direction and one stressed in a direction -18.5° from the Y axis. The shape of the observed relaxation peaks can be fitted by a theory of Zener for a relaxation of a preferred distribution of impurities induced by stress, using an activation energy of 24 kcal/mole and a relaxation time of 0.6X10 -14 sec. The possibility is discussed that this relaxation peak and one observed by Cook and Breckenridge are due to lithium.

SummaryThe materials studied were (1) Schott glass absorptive filters, (2) General Electric's (GE) polycarbonate thermoplastic Lexan, (3) single and polycrystalline Ge, and (4) Eastman Kodak Irtran 1, sintered MgF2. The response to thermal radiation of the optical materials investigated ranged from no discernible damage for Irtran 1 to catastrophic breakage at fairly short irradiance times (tma! of 0.2 to 0.45 s) in the lower intensity (40-cal/cm s) focus for the Schott glass absorptive filters. The observed damage levels and their variation with sample size was satisfactorily explained by our thermal stress model. The lack of damage in Irtran 1 was also explained by the same model. It was probably differential thermal expansion between the anti-reflection layers and the polycrystalline Ge substrate that caused the plastic flow that occurred on this sample at irradiation time of tmax = 1.00 s in the 300-cal/cm2s focus. The optical transmission was not affected, but the surface polish was destroyed. An uncoated sample of single crystal germanium irradiated under similiar conditions showed evidence of surface melting, but no plastic flow. Lexan showed a continuum of damage beginning with the generation of small bubbles beneath the surface and ending with charring and complete destruction of the surface.

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