Thermal Expansion of Dilute Solid Solutions of Silver in Aluminium at Low Temperatures BY V.P. POPOV andV.A. PERVAKOV It i s known that resonance modes caused by heavy substitutional impurities lead t o a substantial increase of the specific heat of crystals at low temperatures (1 to 6).

It has been shown by us in (1) that a t quenching of thin molybdenum wires (d = = 80 pm) in liquid helium-I1 the completeness of fixation of high-temperature thermodynamic equilibrium defects substantially depends on grain size. Full fixation was attained only in the case when samples had a bamboo structure. This points to the fact that the disappearance of defects at supersaturation in a high-temperature region takes place essentially by means of their migration to grain boundaries and to the sample surface. In accordance with the data given in (1) it is believed that the investigated defects were single vacancies. As the nature of high-temperature vacancy sinks of metals with b.c.c. lattices was not studied earlier, we have per-'formed the investigation of this problem with molybdenum of higher frequency and tungsten. IThe samples were thin wires of various diameters: molybdenum wires with 80, 40, 20 p, tungsten ones with 60 pm. By means of high-temperature annealing various grain structures of samples were produced. The purity was estimated by the resistance ratios at 293 and 4.2 K. The 80 p m molybdenum wires had e 29 3/e 4.2 = = 1800 to 2000, the tungsten ones e293/q4.2 =400 to 500.The quenching was produced in liquid helium-I1 (T w 1.5 K). The samples were heated by a current above the liquid helium surface and then they were immersed in helium with simultaneous current shut off. In this case the cooling rate for more massive samples on an initial section (the first 1000°) reached (1 to 2)%10 deg/s and the mean rate cooling nearly to room temperature was (0.6 to 0.8)xlO deg/s.Molybdenum samples of smaller diameters were cooled faster, however by means of capacitors connected with them in parallel the cooling rate was lowered to the above-mentioned values. It enabled us to correlate the completeness of que-ing on samples of various diameters at comparable cooling rates. The employed appa-

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