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

A G-M type refrigerator, comprising:

• a regenerator container;
• a first regenerator material particle group filled in the regenerator container;
• and
• a second regenerator material particle group filled in the regenerator container and divided with the first regenerator material particle group by a metal mesh material, and having a specific heat peak temperature lower than that of the first regenerator material particle group,
• wherein a regenerator material particle constituting at least one of the first and second regenerator material particle groups is a rare earth generator material particle comprising a rare earth element as a constituent component,
• 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 rare earth generator material particle by an X-ray photoelectron spectroscopy analysis,
• an entire content of carbon in the rare earth generator material particle including the carbon component is 5 mass ppm or more and 100 mass ppm or less, and
• the carbon component detected in the surface region of the rare earth generator material 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.