Refrigerator including rare earth cold accumulating material particles, and superconducting magnet, inspection device and cryopump using same

1. A refrigerator for accumulating an ultralow temperature of 20 K or lower, comprising:

at least one cold accumulating vessel,

wherein rare earth cold accumulating material particles are filled in the cold accumulating vessel,

wherein the rare earth cold accumulating material particles consist essentially of a rare earth oxide or a rare earth oxysulfide,

wherein the rare earth cold accumulating material particles define a sintered body,

wherein an average crystal grain size of the sintered body is 0.5 to 5 μm,

wherein a porosity of the sintered body is 10 to 50 vol %,

wherein an average pore size of the sintered body is 0.3 to 3 μm, and

wherein in an arbitrary cross section of the rare earth cold accumulating material particles, a number of pores per a unit area of 10 μm×10 μm is 20 to 70.

2. The refrigerator according to claim 1 , wherein a group of the rare earth cold accumulating material particles packed in the cold accumulating vessel includes the rare earth cold accumulating material particles according to claim 1 in a content of 50% by mass or more and 100% by mass or less.

3. The refrigerator according to claim 1 , wherein the refrigerator is a GM refrigerator.

4. The refrigerator according to claim 1 , wherein the refrigerator is a Stirling refrigerator.

5. The refrigerator according to claim 1 , wherein the refrigerator is a pulse tube refrigerator.

6. The refrigerator according to claim 1 , wherein two or more stages of cold accumulating material-filled zones are formed, which are divided by a metal mesh material in the cold accumulating vessel, and

wherein the rare earth cold accumulating material particles are packed in a stage of regenerator material-filled zones.

7. The refrigerator according to claim 6 , wherein the metal mesh material comprises a metal mesh member composed of copper or a copper alloy.

8. The refrigerator according to claim 7 , wherein the mesh material comprising a metal mesh member composed of copper or a copper alloy is packed.

9. The refrigerator according to claim 6 , wherein an inside of the cold accumulating vessel is divided into two stages,

wherein a group of HoCu 2 particles is filled as a first group of cold accumulating material particles, and

wherein a group of rare earth cold accumulating material particles composed of rare earth oxides or rare earth acid sulfides is filled as a second group of cold accumulating material particles.

10. The refrigerator according to claim 6 , wherein an inside of the cold accumulating vessel is divided into three stages,

wherein a group of lead cold accumulating material particles is filled as a first group of cold accumulating material particles,

wherein a group of HoCu 2 particles is filled as a second cold accumulating material particles, and

wherein a group of rare earth cold accumulating material particles composed of rare earth oxides or rare earth acid sulfides is filled as a third group of cold accumulating material particles.

11. The refrigerator according to claim 1 , wherein the refrigerator accumulates an ultralow temperature of 10 K or lower.

12. The refrigerator according to claim 1 , wherein the rare earth cold accumulating particles have an average particle size of 100 to 500 μm.

13. The refrigerator according to claim 1 , wherein a filling factor of the cold accumulating material particles in the cooling stage of a refrigerator falls within a range from 55 to 70%.

14. The refrigerator according to claim 1 , wherein the rare earth cold accumulating material particles have an aspect ratio of 2 or less.

15. The refrigerator according to claim 1 , wherein when L represents a perimeter length of a projection image of each of the cold accumulating material particles constituting a group of rare earth cold accumulating material particles, and A represents the actual area of the projection image, in the group of rare earth cold accumulating material particles, a proportion of the rare earth cold accumulating material particles having a shape factor R being represented by L 2 /4πA and exceeding 1.5 is 5% or less.

16. The refrigerator according to claim 1 , wherein the rare earth cold accumulating material particles comprise gadolinium aluminum oxide (GdAlO 3 ) or gadolinium oxysulfide (Gd 2 O 2 S).

17. A pulse tube refrigerator for accumulating an ultralow temperature of 20 K or lower, comprising:

a first cooling stage; and

a second cooling stage,

wherein a Cu mesh cold accumulating material is filled in the first cooling stage,

wherein a filling space of the second cooling stage is partitioned into first, second and third filling spaces,

wherein a group of lead cold accumulating material particles is filled into the partitioned first filling space,

wherein a group of HoCu 2 cold accumulating material particles is filled into the partitioned second filling space,

wherein a group of rare earth cold accumulating material particles is filled into the partitioned third filling space,

wherein the rare earth cold accumulating material particles consist essentially of a rare earth oxide or a rare earth oxysulfide,

wherein the rare earth cold accumulating material particles define a sintered body,

wherein an average crystal grain size of the sintered body is 0.5 to 5 μm,

wherein a porosity of the sintered body is 10 to 50 vol %,

wherein an average pore size of the sintered body is 0.3 to 3 μm, and

wherein in an arbitrary cross section of the rare earth cold accumulating material particles, a number of pores per a unit area of 10 μm×10 μm is 20 to 70.

18. The pulse tube refrigerator according to claim 17 , wherein the pulse tube refrigerator is a 4 K pulse tube refrigerator.

19. A superconducting magnet comprising the refrigerator according to claim 1 .

20. A MRI comprising the refrigerator according to claim 1 .

21. A NMR comprising the refrigerator according to claim 1 .

22. A cryopump comprising the refrigerator according to claim 1 .