dysprosium (Dy) /disprō″sē·əm/ [Gk, dys + prositos, to approach] , a rare-earth metallic element. Its atomic number is 66, and its atomic mass is 162.50. Dysprosium, in conjunction with other elements, is used in making laser materials, commercial lighting, neutron-absorbing control rods in nuclear reactors, and data-storage devices such as hard disks. It is employed as a source of infrared radiation. It is used in dosimeters for measuring ionizing radiation. Radioactive isotopes of dysprosium are used in radioisotope scanning, particularly in studies of the bones and joints.
