Cold accumulating material and method of manufacturing the same

A refrigerator for accumulating an extremely low temperature cold, comprising:

• a vacuum vessel;
• an outer shell disposed in the vacuum vessel, the outer shell enclosing an inner space;
• a first cold accumulating means for accumulating cold and a second cold accumulating means for accumulating cold which are arranged at the inner space inside of the outer shell, the first cold accumulating means accumulating a first cold generated at a first cooling stage, the second cold accumulating means accumulating a second cold generated by further cooling the first cold at a second cooling stage,
• a mesh-like member packed int he first cold accumulating means for accumulating cold, and
• a second cold accumulating material packed int eh second cold accumulating means, the second cold accumulating material comprising a set of particles having a total weight, wherein each particle comprises at least one element selected form a group consisting of yttrium (Y), lanthanum (La), and rare earth elements cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), and ytterbium (Yb), and wherein at least 70% of said total weight is comprised of particles which have a particle size of 0.01 to 3 mm and at least 70% of said total weight is comprised of particles which each have a major diameter to a minor diameter ratio that is not greater than 5.