Rare-earth regenerator material particles, and group of rare-earth regenerator material particles, refrigerator and measuring apparatus using the same, and method for manufacturing the same

1. A refrigerator for accumulating an extremely low temperature cold, comprising:

a vacuum chamber;

an external cylinder disposed in the vacuum chamber, the external cylinder enclosing an inner space;

a first regenerator container for accumulating cold and a second regenerator container for accumulating cold which are arranged at the inner space inside of the external cylinder, the first regenerator container for accumulating a first cold generated at a first cooling stage, the second regenerator container for accumulating a second cold generated by further cooling the first cold at a second cooling stage,

a mesh-like member packed in the first regenerator container for accumulating cold, and

regenerator material particles packed in the second regenerator container, the regenerator material particles comprising a rare-earth metal and having an average particle size of 0.045 to 3 mm,

wherein a proportion of particles having a ratio of a long diameter to a short diameter of 2 or less is 90% or more by number,

a proportion of particles having a depressed portion having a length of 1/10 to ½ of a particle circumferential length of a particle on a particle surface is 30% or more by number and wherein the depressed portion has depth of 1/10 or less of a particle diameter, and the depressed portion has channel-shape or a hole-shape.

2. The refrigerator according to claim 1 , wherein the extremely low temperature is lower than 10K.

3. The refrigerator according to claim 1 , wherein the second group of regenerator material particles are arranged in one packing layer formed between mesh-like members which are disposed respectively at inner end portions of the second regenerator container.

4. The refrigerator according to claim 1 , wherein the second group of regenerator material particles are arranged in a plurality of packing layers formed between a plurality of mesh-like members which are disposed within a space in the second regenerator container.

5. The refrigerator according to claim 1 , wherein the second group of regenerator material particles comprising at least one element selected form a group consisting of yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), and ytterbium (Yb).

6. The refrigerator according to claim 1 , wherein the second group of regenerator material particles comprising at least one selected from the group composed of a rare-earth elemental of single substance, and alloy with Cu (including an intermetallic compound), a rare-earth with Ni (including an intermetallic compound), a rare-earth oxide (including a rare-earth complex oxide), and a rare-earth sulfide.

7. The refrigerator according to claim 1 , wherein the second group of regenerator material particles comprising at least one selected from the group consisting of Nd, Er 3 Ni, ErNi, HoCu 2 , Gd 2 O 3 , and Gd 2 O 2 S.

8. A measuring apparatus, comprising a superconducting magnet comprising the refrigerator of claim 1 .

9. The measuring apparatus of claim 8 , which is at least one apparatus selected from the group consisting of an MRI and an NMR.

10. A cryopump, comprising a superconducting magnet comprising the refrigerator of claim 1 .