Measurements of the attenuation of sound in liquid 4 He down to 0.1 K have been performed at 12, 30, 36, 60, 84, 90, 108, 132, 150, and 208 MHz. Measurements of the temperature dependence of the velocity of sound were made at 12, 36, 60, and 84 MHz. These data are compared with recent theoretical work, particularly that of Khalatnikov and Chernikova. The attenuation data agree well with theory in the vicinity of the peak in the attenuation near 1 K but do not agree elsewhere, the observed attenuation being greater than that predicted by theory. The temperature dependence of the velocity at low temperatures is found to be less than that predicted by theory, while the frequency dependence of the velocity (at finite temperature) is opposite to that predicted by theory.
Using the ultrasonic pulse technique at a carrier frequency of 10 Mc/sec, we have measured the longitudinal velocity of sound in liquid and solid He 8 , He 4 , and He 3 -He 4 mixtures (0.0, 5.03, 25.0, 74.9, 98.00, 99.84% He 3 ) in the temperature range from 1 to 4.2°K and the pressure range from 1 to 150 atm. An upper limit to the attenuation of sound in the region of the solid investigated is on the order of 0.3 to 0.7 cm -1 . Values of the adiabatic compressibility of the liquid along the melting curve of He 4 are given. The discontinuity in the propagation velocity at a first-order phase boundary was utilized to locate the solidification curves of these samples and also to investigate the bcc-hcp crystallographic transition in solid samples, including pure He 4 , similar to that observed in pure He 3 by Grilly and Mills. The triple points for this transition in He 4 are given by T t = 1.449±0.003°K, P* = 26.18=fc0.05 atm; T u = 1.778db0.003°K, P"=30.28±0.05 atm. The upper triple points for the mixtures lie on a straight line connecting He 3 and He 4 in temperaturepressure space. In addition, by observing the peak in the attenuation of sound, it was possible to measure the X line in He 4 and in the 5.0% and 25.0% He 8 liquid mixtures. The upper X point for pure He 4 is given by T a = 1.765db0.003°K and P a = 29.90:fc0.05 atm. Comparison is made with existing data wherever possible.
Recent measurements of the longitudinal velocity of first sound in solid He 4 have revealed a first order phase transition, and hence a new solid phase (designated the y phase), between 1.45°K and 1.78°K in a narrow range of pressure adjoining the melting curve. The transition between the previously established hexagonal closepacked a phase 1 ' 3 and the inferred y phase was detected by observing the discontinuity in the velocity of sound at the phase boundary. Subsequent measurements of a change in molar volume across the phase boundary and a discontinuity in the slope of the melting curve at the lower liquidsolid a-solidy triple point have substantiated that this is indeed a first order phase transition.The standard ultrasonic pulse technique at a carrier frequency of 10 Mc/sec has been employed to measure the longitudinal velocity of sound in solid as well as liquid He 4 . First order phase transitions can also be explored since there is a discontinuous change in velocity on crossing the transition. The melting curve of He 4 has been investigated in this manner and is shown in Fig. 1 (solid circles). Investigation of the velocity of sound in the solid region near the melting curve indicated the existence of another first order phase transition. The locus of the discontinuity in velocity observed in this region is shown in Fig. 1 (open circles). This discontinuity was found to be reversible in both temperature and pressure. The locations of the liquid-solid asolid y triple points are given by T u = 1.778 ±0.003°K, P u = 30.28±0.05 atm and T z = 1.449 ±0.003°K, P z = 26.18±0.05 atm. The velocity of sound in the solid a region, both along the melting curve from 1°K to the lower triple point and along the a-y phase boundary, was found to be essentially constant and equal to 478 m/sec. Throughout the solid y region the velocity has values ranging from 520 m/sec to 545 m/sec, these two extremes being the predominant values. We attribute this variation to different orientations of a single crystal or a few large crystals. This tendency for helium to form large crystals is consistent with previous observations. 1 " 3 For reference the velocity of sound in the liquid at the melting curve is 365 m/sec from 1 C K to the \ point and then climbs to 380 m/sec at 1.8°K. The lambda line shown in Fig. 1 (solid triangles) 32 30 6 2 28 I-26 24 rr h i -r~ [-
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