Page "Holmium" Paragraph 19

Natural holmium contains one stable isotope, holmium-165.

Some synthetic radioactive isotopes are known ; the most stable one is holmium-163, with a half life of 4570 years.

All other radioisotopes have ground-state half lives not greater than 1. 117 days, and most have half lives under 3 hours.

However, the metastable < sup > 166m1 </ sup > Ho has a half life of around 1200 years because of its high spin.

This fact, combined with a high excitation energy resulting in a particularly rich spectrum of decay gamma rays produced when the metastable state de-excites, makes this isotope useful in nuclear physics experiments as a means for calibrating energy responses and intrinsic efficiencies of gamma ray spectrometers.