Search for superconductivity in neptunium and plutonium using a liquid helium-3 cryostat

SEARCH FOR SUPERCONDUCTIVITY IN N E P T U N I U M AND P L U T O N I U M USING A LIQUID HELIUM-3 CRYOSTAT G. T. MEADEN 1 and T. SHIGI ~: The Clarendon Laboratory, Oxford, U.K. Received 9 November 1963

T w o of the actinide metals have been known for many years to be superconductors. They are thorium and uranium, which have low transition temperatures of approximately 1.4 ° K 1-4 and 0.8 ° K, 4-7 respectively. We have now carried out a search for superconductivity to below 0.5 ° K in the remaining two actinides which were available to us, namely neptunium and plutonium. It was hoped to include protactinium also but we were not able to obtain it in elemental form. Previously4, 8 we have reported on experiments performed in a cryostat in which 0-75 ° K was obtained. However, in the present cryostat, which is a helium-3 one utilizing 600 cm 3 of gas at n.t.p., it was possible to reach 0-38° K and perhaps less. The experiments were complicated by the very high toxicity of both neptunium and plutonium, and precautions had to be taken to ensure that the cryostat did not become contaminated by the or-radioactivity.

helium-3 lowered to less than 0"4° K by pumping with an Edwards 2M4A mercury diffusion p u m p only. This pump is still able to operate when the backing pressure

The c r y o s t a t A schematic diagram of the lower part of the cryostat is shown in Figure 1. A liquid helium-4 container A, volume 25 cm 3, is filled from the main liquid helium bath B by opening the needle valve C. The space surrounding A is then evacuated, and A is pumped down to about 1-1°K using a rotary p u m p and an Edwards 203 oil diffusion pump. The liquid helium-3 container D is cooled to about the same temperature through gas conduction in the surrounding space. The helium-3 gas is stored in a 14 1. reservoir at room temperature at a pressure of about 32 m m Hg. This is the vapour pressure of liquid helium-3 at 1.34 ° K, so that below this temperature it is possible for the gas to liquefy in D if the storage reservoir is opened to it. I f the gas is left to condense in this simple way, then near 1.0°K 70 per cent of the available helium-3 gas has liquefied. The volume of liquid at this stage is about 0.7 cm a, which just fills the helium-3 container D. Finally, the space surrounding D is evacuated and the temperature of the liquid t Now at Centre de Recherches sur les Trb.s Basses Temp6ratures, Universit6 de Grenoble, France. On study leave from Osaka City University, Japan. 90

A. B. C. D. E. F. G. H. J.

Liquid helium-4 container Liquid helium-4 bath Needle valve Liquid helium-3 container Liquid helium-3 condensing reservoir Specimen container Gas thermometer Specimen Inner wall of metal Dewar

Figure 1. Lower part of heliura cryostat CRVOGENXCS • AVRtL .i964

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A McLeod gauge was used for liquid helium-3" vapour pressure measurements below 0-7 ° K and an oil manometer for those above 0.6 ° K.

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Figure 2. Helium-3 system

is as high as 35 mm Hg, so that, provided the pressure in the helium-3 system is not greater than this, no backing pump is required. The omission of a rotary pump simplifies the construction of the helium-3 system and reduces the possibility of leaks. The helium-3 system is so designed that the same diffusion pump can also be used to pump the helium-3 gas from the large volume reservoir into the cryostat via a cold trap during the initial cooling from 1.34 ° K (see flow diagram in Figure 2). This procedure speeds up considerably the rate at which the gas condenses. Generally, as the helium-3 gas evaporates, it is pumped straight back into the large volume reservoir at room temperature. However, on those occasions when it is desired to maintain a steady temperature below I ° K for a long period of time, the gas, on leaving the diffusion pump, is passed instead into the helium-3 condensing reservoir E where it liquefies. This little reservoir is at the temperature of the helium-4 container A (about 1° K) and, until it is filled, this ensures that the pumping speed remains constant. Otherwise, if the gas goes to the 14 1. room temperature reservoir, the pumping speed gradually decreases as the backing pressure rises, and continual adjustment of the main control valve in the helium-3 line before the pump is necessary. It was experimentally established that, provided the liquid helium level in B was maintained and there was no heat input into D, the liquid helium-3 itself could last for 10 hr at 0.4 ° K. A metal Dewar having an outer tail diameter of 4.5 cm was employed to hold the main liquid helium bath B. CRYOGENICS.

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Since neptunium and plutonium are very toxic, both metals had to be sealed inside small containers made of copper before they could be used with safety. This eliminates radiation hazards provided the containers do not actually break or leak, as the radiation concerned is due only to short rangg 0t-particles. With plutonium there is the additional difficulty that the spontaneous self-heating is very high. The actual rate depends on the particular isotopic constitution which was not known for the specimen we eventually used, but data for similar material suggest that the self-heating was probably in the region of 2.2 mW/g. Such a rate from a 1 g specimen would boil away all the liquid helium-3 (0.7 cm 3~,in less than 5 min. As the experiment would be quite impossible under such conditions, this meant that we had to obtain and use specimens which were at least an order of magnitude smaller ( < I00 mg). The density of a-plutonium is very high at about 19.6 g/cm 3, so the volume required was to be less than 0.005 cm 3. Calculations based on this small volume, remembering that the specimen had to be encased in copper, showed at once that a ballistic mutual inductance method for detecting superconductivity would not be sensitive enough. An electrical resistance method had therefore to be used.. Nevertheless, it was still hoped to use a magnetic method with neptunium, especially since there was no self-heating problem with this metal. Accordingly, tests were made on specimens of superconducting indium, thorium, and uranium of a similar shape and volume to that of the available neptunium sample (volume about 0.02 cm3). Two secondary coils were wound in opposition on a copper former which was anchored to the helium-3 container D and which was vertically beneath it. Each coil consisted of 5,750 turns of 46 s.w.g, double cotton covered enamelled copper wire. The sheathed specimen, also anchored to D, was positioned within an axial hole in the former so that it was within the lower coil. The method then consisted of reversing a small field due to a coil external to the cryostat which was within a steady larger magnetic field, and observing the e.m.f, induced in the secondaries. This method worked well enough with indium above 1.2 ° K while there was still exchange gas in the inner vacuum jacket, but failed completely at lower temperatures, when the jacket was evacuated. This was attributed to eddy current heating in the secondary coils and poor heat exchange between the coil wire and former when there was no exchange gas. Varied attempts at improving this situation and increasing the sensitivity did not meet with success. Finally it was decided to employ a 91

resistance method with neptunium also and possibly to return to a magnetic method if superconductivity was discovered.

Experimental details and results Each specimen was enclosed inside a small hollow cylindrical container of copper which was provided with a platinum-glass seal at one end (F in Figure 1). Four wires passed through the seal for the specimen to be used as a four-terminal resistor. Contacts to the specimen were made by spot welding. The previously evacuated container was filled with 10-30 per cent helium-3 gas, the remainder being helium-4 gas, immediately prior to Wood's-metalling the seal in place. Apart from a much shorter overall length the containers were very similar to those described previously. 9 However, the new ones were terminated at one end by a 5 mm long 6 B.A. threaded section which could be screwed tightly into the base of the liquid helium-3 container D. A liberal coating of Apiezon L-grease aided the thermal contact between D and F. Neptunium. The source of our neptunium has been described in a previous publication: About a year ago all the metal was processed once again, and later some of it extruded as a 0.8 mm diameter wire. The present sample is a piece 19.5 mm in length with a mass of 0-I 69 g. It was found to have a resistivity at 4.2 ° K o f 27.0 laf~ cm, which is 22 per cent of the room temperature value (121.5 I~f~cm at 295°K). The latter value is almost identical to that obtained before, 4:° but the helium value is over seven times higher. The electrical resistances were measured using a thermoelectric-free Diesselhorst potentiometer, and the sensitivity was such that a change in resistance of I part in 200 would have been readily discernible. In fact, no change at all could be found between 1.2 and 0-414 ° K, the lowest temperature reached. Furthermore, this lowest temperature, once attained, remained at a constant value for 40 min before we started raising it again, so that it was certain that thermal equilibrium had become established between the liquid helium-3 container and the specimen and its container. We may therefore say that our neptunium sample showed no sign of superconductivity down to a temperature of 0.414 ° K. Previously,10,11 we had suggested there was a possibility of superconductivity below 0.75 ° K. Plutonium. The a-plutonium sample was in the form of a short strip made from good purity rolled sheet. Its dimensions were 20 x 1 x 0.175 mm and its mass 0.072 g. The self-heating contribution to the general heat leak was therefore about 0-16 mW. In the experiment the liquid helium-3, whose starting volume was about 0-6 cm a, lasted for 45 min a~d calculation 92

showed that 90 per cent of it was lost due to the selfheating. The resistivity measured at 1.24 ° K was found to be 19 per cent of the room temperature value (295 ° K). This is quite a low figure for plutonium and indicates that the specimen was relatively strain-free and of good purity. 4 No change in resistivity was observed to within I part in 200 between 1.2 ° K and the lowest temperature attained. This temperature was held very constant indeed for 15 min indicating that thermal equilibrium had become established, i.e. the plutonium and its container had reached a steady temperature that was the lowest possible. The corrected vapour pressure reading of the liquid helium-3 was 0.454 ° K. The specimen was certainly slightly warmer than this because of the self-heat, but an estimate of the rate of heat removal showed that the temperature of the specimen was unlikely to have been higher than 0.50 ° K.

Conclusion Unlike thorium and uranium, neptunium and plutonium have not proved to be superconductors. Neptunium was cooled to 0.41 ° K and plutonium to 0.50°K. We have "discussed in previous publications4: °-12 the possibility of plutonium being antiferromagfietic, so its non-superconductivity was not unexpected. In a similar way actinium and protactinium, both very rare radioactive metals, should also b~ investigated. Their positions in the Periodic Table in relation to lanthanum, thorium, and uranium suggest that they may~ be superconductors. This work was carried out at the Clarendon Laboratory in collaboration with A.E.R.E., Harwell, under a research grant to one of us (G.T.M.). We are grateful to Dr. K. Mendelssohn, F.R.S., for his interest and discussions and to Dr. J. A. Lee, A.E.R.E., for much help in providing the specimens.

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MEISSNER,W. Naturwissenschaften 17, 390 (1929) SHOENBERG,D. Proc. Camb. phil. Sac. 36, 84 0940) WOLCOTT,N. M., and HEn~, R. A. Phil. Mag. 3, 591 0958) MEADEN,G. T. Proc. roy. Soc. A276, 553 (1963) GOODMAN,B. B., and SHOENnEgG, D. Nature, Lond. 165, 441 (1950) KILPATRICK,J . E., HAMMEL, E. F., and MAPOTHER, D. Phys. Rev. 97, 1634 (1955) HEIN, R. A., HENRY, W. E., and WOLCOTT, N. M. Phys. Rev. 107, 1517 (1957) LEE, J. A., MEADEN, G. T., and MENDELSSOHN,K. Proc. phys. Soc. Lond. 74, 671 (1959) MEADEN,G. T., and LEE, J. A. Cryogenics 1, 33 (1960) LEE,J. A., MEADEN,G. T., and MENDE~SOHN,K. Cryogenics 1, 60 (1960) MEADEN,G. T. Thesis (Oxford, 1961) LEE, J. A., MENDELSSOHN,K., and WIC;LEV, D. A. Physics Lett. 1, 325 (1962) C R Y O G E N I C S . APRIL

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