Rare earth regenerator material particle, rare earth regenerator material particle group, and cold head, superconducting magnet, examination apparatus, and cryopump using the same

1. A rare earth regenerator material particle, comprising a rare earth element as a constituent component,

wherein a peak indicating a carbon component and a peak indicating a compound containing the rare earth element and oxygen are detected in a surface region of the particle by an X-ray photoelectron spectroscopy analysis,

an entire content of carbon in the particle including the carbon component is 5 mass ppm or more and 50 mass ppm or less, and

the carbon component detected in the surface region of the particle has at least one selected from the group consisting of a C—C bond, a C—H bond, a C—O bond, a C═O bond, and an O—C═O bond.

2. The particle according to claim 1 , comprising an intermetallic compound represented by a composition formula of RMa, wherein R is at least one element selected from the group consisting of rare earth elements, M is at least one element selected from the group consisting of Cu, Ni, and Co, and a is a number (atomic ratio) satisfying 0.1≤a≤4.0.

3. The particle according to claim 2 , wherein a peak indicating a compound of the rare earth element and oxygen and a peak indicating a compound of the M element and oxygen are detected in the surface region by the X-ray photoelectron spectroscopy analysis.

4. The particle according to claim 2 , wherein a ratio (A/B) of an atomicity A of the rare earth element in a compound of the rare earth element and oxygen to an atomicity B of the M element in a compound of the M element and oxygen exceeds the value of the a.

5. The particle according to claim 2 , wherein the intermetallic compound is at least one selected from the group consisting of HoCu 2 , HoCu, Er 3 Ni, ErNi, and Er 3 Co.

6. The particle according to claim 2 , wherein the intermetallic compound is HoCu 2 , and a reflectance of light with a wavelength of 600 nm in the particle is 20% or more.

7. A rare earth regenerator material particle group, comprising a plurality of the rare earth regenerator material particles according to claim 1 .

8. The particle group according to claim 7 , wherein a ratio of the particle in which a form factor R represented by L 2 /4πA exceeds 1.5 is 5% or less, wherein L is a circumferential length of a projected image of the particle, and A is an actual area of the projected image.

9. A cold head, comprising the rare earth regenerator material particle group according to claim 7 .

10. A cold head, comprising:

a first stage having a regenerator material; and

a second stage having a plurality of regenerator material-filled layers in which different compounds of regenerator material particle groups are respectively filled,

wherein at least one of the regenerator material-filled layers is provided with the rare earth regenerator material particle group according to claim 7 .

11. A superconducting magnet, comprising the cold head according to claim 9 .

12. An examination apparatus, comprising the cold head according to claim 9 .

13. A cryopump, comprising the cold head according to claim 9 .