We have observed the propagation of sound in liquid He 3 at 0.32 atm and at frequencies of 15.4 and 45.5 MHz down to a temperature T* of 2 mdeg on the magnetic temperature scale valid for powdered cerium magnesium nitrate (CMN) in the form of a right circular cylinder with diameter equal to height. As the temperature rises the sound attenuation increases, reaches a maximum, and then decreases. At low temperatures the attenuation is proportional to T* 2 and is independent of frequency. At high temperatures the attenuation is proportional to co 2 /T* 2 , where co is the angular frequency. The sound propagation velocity is relatively temperature independent at both high and low temperatures but near the attenuation maximum the velocity changes.In 1957 Landau 1 predicted that at sufficiently low temperatures a new type of sound, which he called zero sound, could be propagated in liquid He 3 . Based on Landau's idea, a more detailed theory of the velocity and attenuation of sound in both the hydrodynamic (first sound) and zero-sound regions was worked out by Khalatnikov and Abrikosov. 2 At temperatures sufficiently high that quantum effects are unimportant, it is predicted that the attenuation of zero sound be proportional to T 2 and independent of frequency. In the first-sound region it is predicted that the attenuation is proportional to OJ 2 /T 2 , corresponding to classical viscous attenuation with viscosity proportional to T~2. Both of these temperature and frequency dependences are observed in the present experiments. In what follows we shall show that there is quantitative agreement with theory on veloc-

Following a b r ie f introduction in Section T, the properties of pure He3 and of d ilu te solutions of He3 in superfluid He4 which are relevant to the design and understanding of d ilu tio n refrig erato rs are discussed in Section I I. In Section I I I the basic model fo r discussing the thermodynamics of the d ilu tio n process is developed and applied to both a continuously operating re frig e ra to r and to singlecycle refrig e ra to rs using both He3 flow and super f l u id He4 flow. The re la tiv e merits of He3 flow and super leak operated devices are also mentioned. In Section IV the thermodynamical properties of He* in the d ilu te phase of the re frig e ra to r are calculated, including the dependence of concentration on temperature and the e ffe ctiv e enthalpy function. The heat exchange problem is discussed in Section V, f i r s t by introducing new measurements of the Kapitza resistance fo r saturated d ilu te solutions and then by re la tin g these to the concept of a He*-phonon resistance in the d ilu te s o lu tio n s. F in a lly specific design estimates are made fo r the heat exchangers. In Section V I the physics of the He3 c irc u la tio n problem is discussed. The effects of circulated He4 and of viscous heating due to He3 flow are also emphasized. Both the effects of thermal conduction and viscosity lim it the low temperature attainable in a He3 flow type of d ilu tio n r e fr ig e r a to r. There is a lower lim it to 'the temperature attainable as a resu lt of these in trin s ic factors which is calculated in Section V II. The general characteristics of the re frig e ra tio n device i t s e lf are reviewed in Section V I I I. Section IX contains a d etailed description of two systems b u ilt and tested by us. Parts of the apparatus discussed are the s t i l l , the heat exchangers, the mixing chamber, the condenser and main impedance, interconnections, and mechanical support. The a u x ilia ry cryogenic and pumping systems are also described. In Section X the operational characteristics of two d ilu tio n refrig erato rs are d iscussed, both in regard to s ta rtin g them up and to th e ir steady state operating c h a ra c te ris tic s. The e ffec t of a heat load, of s t i l l power, and of the number of exchangers is given. Comparison with the thermodynamical treatment is made. Under suitable conditions a temperature of 10 m°K can be maintained continuously and a temperature of 4.5 m°K fo r short periods of time.

The heat capacity of pure He 3 has been measured using a difference method from 6 to 50 m°K at 0.28 atm and from 4 to 30 m°K at 27.0 atm. In neither case is the ratio of heat capacity to temperature constant over the range of temperature of the measurements. Moreover, the high-and low-pressure heat capacities seem to have qualitatively different temperature dependences. The raw heat-capacity data down to a temperature [on the magnetic temperature scale valid for powdered cerium magnesium nitrate (CMN) in the form of a right circular cylinder with diameter equal to height] of 2 m°K at 0.28 atm and 4 m°K at 27.0 atm show no evidence for anomalous behavior. As a by-product of the measurements the heat capacity of the CMN cooling salt was also obtained.

The thermal conductivity K of pure He 3 and of two dilute solutions of He 3 in He 4 has been measured from 30 down to 5 mdeg K or below. For pure He 3 , KT increases with increasing temperature. For the dilute solutions at low enough temperatures, K is consistent with the T~i temperature dependence of a normal Fermi liquid, but the magnitudes of KT do not agree with values computed from an effective potential based on spin-diffusion coefficient measurements.We have measured at saturated vapor pressure the thermal conductivity both of pure liquid He 3 and of the same two dilute solutions of He 3 in He 4 for which measurements of specific heat, spin-diffusion coefficient, and magnetic susceptibility have already been reported. 1 The results relate to the question of the anomalous behavior of pure He 3 at low temperatures 2 ' 3 and to the effective interactions between He 3 quasiparticles in the dilute solutions. 4

Impact of the metal cathode and CsF buffer layer on the performance of organic light-emitting devices