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

A pulse type refrigerator, comprising:

• a regenerator container;
• and
• a plurality of regenerator material particle groups which are different in kind, and are filled in the regenerator container while being divided from each other,
• wherein at least one of the plurality of regenerator material particle groups comprises a rare earth generator material particle containing a rare earth element,
• 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.