Two-stage heat regenerating cryogenic refrigerator

A two-stage heat regenerating cryogenic refrigerator comprising:

• a vacuum vessel;
• a first cylinder disposed in the vessel;
• a second cylinder disposed in the vessel, the second cylinder coaxially connected to the first cylinder;
• a first regenerator disposed in the first cylinder, the first regenerator accommodating a first heat regenerating material;
• and
• a second regenerator disposed in the second cylinder, the second regenerator accommodating a second heat regenerating material, the second heat regenerating material including a plurality of heat regenerating material particles, each of the heat regenerating material particles including a heat regenerating substance having a maximum value of specific heat at a temperature of 20 K or less of 0.3 J/cm3·K or more and one metal element selected from the group consisting of calcium (Ca), magnesium (Mg), beryllium (Be), strontium (Sr), aluminum (Al), iron (Fe), copper (Cu), nickel (Ni), and cobalt (Co);
• wherein each of the heat regenerating material particles includes a first region and a second region, the second region is closer to an outer edge of each of the heat regenerating material particles than the first region, and the second region has a higher concentration of the metal element than the first region,
• the first region and the second region contain the heat regenerating substance, and
• the heat regenerating substance contains a component in a form of an oxide or an oxysulfide.