Yttrium-90 is a medically significant isotope

Isotope

Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

 of yttrium

Yttrium

Yttrium is a chemical element with symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and it has often been classified as a "rare earth element". Yttrium is almost always found combined with the lanthanides in rare earth minerals and is...

.

It emits beta rays of 2.3 MeV.
Yttrium-90 is a decay product of Strontium-90 which makes up about 5% of the Nuclear daughter isotopes when Uranium

Uranium

Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...

 is fissioned.
http://www.epa.gov/radiation/radionuclides/strontium.html
Yttrium-90 is produced by chemical high-purity separation from strontium-90, a fission product of Uranium

Uranium

Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...

in Nuclear Reactors.
http://radioisotopes.pnnl.gov/yttrium-90.stm