text: Yttrium in the Solar System was created through stellar nucleosynthesis, mostly by the s-process (≈72%), but also by the r-process (≈28%). The r-process consists of rapid neutron capture by lighter elements during supernova explosions. The s-process is a slow neutron capture of lighter elements inside pulsating red giant stars.
  Yttrium isotopes are among the most common products of the nuclear fission of uranium in nuclear explosions and nuclear reactors. In the context of nuclear waste management, the most important isotopes of yttrium are Y and Y, with half-lives of 58.51 days and 64 hours, respectively. Though Y has a short half-life, it exists in secular equilibrium with its long-lived parent isotope, strontium-90 (Sr) with a half-life of 29 years.
  All group 3 elements have an odd atomic number, and therefore few stable isotopes. Scandium has one stable isotope, and yttrium itself has only one stable isotope, Y, which is also the only isotope that occurs naturally. However, the lanthanide rare earths contain elements of even atomic number and many stable isotopes. Yttrium-89 is thought to be more abundant than it otherwise would be, due in part to the s-process, which allows enough time for isotopes created by other processes to decay by electron emission (neutron → proton). Such a slow process tends to favor isotopes with atomic mass numbers (A = protons + neutrons) around 90, 138 and 208, which have unusually stable atomic nuclei with 50, 82, and 126 neutrons, respectively. This stability is thought to result from their very low neutron-capture cross-section. Electron emission of isotopes with those mass numbers is simply less prevalent due to this stability, resulting in them having a higher abundance. Y has a mass number close to 90 and has 50 neutrons in its nucleus.
  At least 32 synthetic isotopes of yttrium have been observed, and these range in atomic mass number from 76 to 108. The least stable of these is Y with a half-life of >150 ns (Y has a half-life of >200 ns) and the most stable is Y with a half-life of 106.626 days. Apart from the isotopes Y, Y, and Y, with half-lives of 58.51 days, 79.8 hours, and 64 hours, respectively, all the other isotopes have half-lives of less than a day and most of less than an hour.
  Yttrium isotopes with mass numbers at or below 88 decay primarily by positron emission (proton → neutron) to form strontium (Z = 38) isotopes. Yttrium isotopes with mass numbers at or above 90 decay primarily by electron emission (neutron → proton) to form zirconium (Z = 40) isotopes. Isotopes with mass numbers at or above 97 are also known to have minor decay paths of β delayed neutron emission.
  Yttrium has at least 20 metastable ("excited") isomers ranging in mass number from 78 to 102. Multiple excitation states have been observed for Y and Y. While most of yttrium's isomers are expected to be less stable than their ground state, Y, Y, Y, Y, Y, Y, and Y have longer half-lives than their ground states, as these isomers decay by beta decay rather than isomeric transition.