Fig. 1-12 Concept of liquid-He-free dilution refrigerator for neutron scattering The sample is fixed at the bottom of the mixing chamber where the temperature is lowest, and scatters the incident neutron beam. In a steady state at an ultra low temperature, liquid mixtures of He-3 and He-4 separate into two phases in the mixing chamber, one being rich in He-3 and the other rich in He-4 (i.e., dilute in He-3), and because of its lower density the He-3 concentrated phase floats on top of the He-3 dilute phase. By reducing the pressure of the still using a turbomolecular pump, He-3 is selectively pumped out from the still to the outside. Then, He-3 is returned to the inside using the compressor, cooled and liquefied. With the circulation of He-3, the cooling occurs in the mixing chamber where He-3 is continuously dissolved from the He-3 concentrated phase into the dilute phase and is diluted. The hybrid cryocooler consists of the GM stage and the pulse tube stage, and produces the low temperature atmosphere required for the cooling and liquefaction of He-3.

Fig. 1-13 Cooling characteristics of using only the hybrid cryocooler developed. The abscissa is the elapsed time from the start of the hybrid cryocooler. By following the precooling operation as shown in the inset, each part reaches its steady state at low temperatures after about one day. Then, the production of ultra low temperatures by the dilution refrigeration is continuously made by circulating He-3.

For the neutron scattering experiments at JRR-3M, we often needed to cool the sample to ultra low temperatures below 0.1 K. The ultra low temperatures can be realized by the principle of He-3/He-4 dilution refrigeration. The conventional dilution refrigerator, however, is a big machine such that we have to produce a low temperature atmosphere on a sufficiently large scale to circulate the operating gas of He-3/He-4. In particular, the commercial refrigerators consume a large quantity of liquid helium coolant (He-4 liquid) to produce the low temperature atmosphere, which makes the neutron scattering experiments very difficult to access at ultra low temperatures. Instead of this conventional method of using a large quantity of liquid helium coolant, we have developed a new method of using a hybrid cryocooler which combines a Gifford-McMahon (GM) cryocooler with a pulse tube cryocooler. As the result, we can now easily reach a temperature of 50 mK using the new method. This liquid-He-free dilution refrigerator is named "mK Cryocooler" for the neutron scattering research. The mK Cryocooler can produce any ultra low temperature just by pushing a start button and can be safely operated without need of any maintenance during the long-term neutron scattering experiments. In addition, the mK Cryocooler being compact, it is very easy to rotate or tilt the sample held at ultra low temperatures together with the mK Cryocooler in the neutron scattering experiments.

Reference Y. Koike et al., A Dilution Refrigerator Using the Pulse Tube and GM Hybrid Cryocooler for Neutron Scattering, Cryogenics, 39, 570 (1999).

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