Isotopes of dysprosium

Naturally occurring dysprosium is composed of 7 stable isotopes, ¹⁵⁶Dy, ¹⁵⁸Dy, ¹⁶⁰Dy, ¹⁶¹Dy, ¹⁶²Dy, ¹⁶³Dy and ¹⁶⁴Dy, with ¹⁶⁴Dy being the most abundant. Twenty-nine radioisotopes have been characterized, with the most stable being ¹⁵⁴Dy with a half-life of 1.4 million years, ¹⁵⁹Dy with a half-life of 144.4 days, and ¹⁶⁶Dy with a half-life of 81.6 hours. All of the remaining radioactive isotopes have half-lives that are less than 10 hours, and the majority of these have half-lives that are less than 30 seconds. This element also has 12 meta states, with the most stable being ^165mDy, ^147mDy and ^145mDy.
The primary decay mode before the most abundant stable isotope, ¹⁶⁴Dy, is electron capture to isotopes of terbium, and after beta decay to those of holmium. Dysprosium is the heaviest element to have isotopes that are theoretically stable, rather than only ones that are observationally stable and predicted to be radioactive. ¹⁶⁴Dy has a surprisingly large thermal neutron absorption and the product isotope ¹⁶⁵Dy has found medical use.

Dysprosium-165

The radioactive isotope ¹⁶⁵Dy, with a half-life of 2.332 hours, has radiopharmaceutical uses in radiation synovectomy of the knee. It had been previously performed with colloidal-sized particles containing longer-lived isotopes such as ¹⁹⁸Au and ⁹⁰Y. The major problem with the usage of those isotopes was radiation leakage out of the knee. ¹⁶⁵Dy, with its shorter half-life and thus shorter period of potential radiation leakage, is more suitable for the procedure.