Rare earth cold accumulating material particles, and refrigerator, superconducting magnet, inspection device and cryopump using same

A method of manufacturing a pulse tube refrigerator for accumulating an ultralow temperature of 20 K or lower, the method comprising the steps of:

• filling a Cu mesh cold accumulating material in a first cooling stage;
• partitioning a filling space of a second cooling stage into a first to a third filling spaces;
• filling a group of lead cold accumulating material particles into the partitioned first filling space;
• filling a group of HoCu2 cold accumulating material particles into the partitioned second filling space;
• and
• filling a group of cold accumulating material particles into the partitioned third filling space wherein the rare earth cold accumulating material particle consists essentially of a rare earth oxide or a rare earth oxysulfide;
• wherein the rare earth cold accumulating material particle is 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 particle, a number of pores per a unit area of 10 μm×10 μm is 20 to 70.