Isotopes of germanium
has five naturally occurring isotopes, 70Ge, 72Ge, 73Ge, 74Ge, and 76Ge. Of these, 76Ge is very slightly radioactive, undergoing double beta decay with a half-life of 2.02 × 1021 years.
Stable 74Ge is the most common isotope, having a natural abundance of 36.52%; 76Ge is the least common with a natural abundance of 7.75%.
At least 27 additional radioisotopes have also been synthesized ranging in atomic mass from 58 to 89. The most stable of these is 68Ge, decaying by electron capture with a half-life of 271.05 days, whose daughter is the medically useful positron-emitting isotope 68Ga. The next after that is 71Ge, also decaying by electron capture with half-life 11.468 days, and the rest are all less than two days, most under two hours.
While most of germanium's radioisotopes decay by beta decay - β+ for isotopes lighter than 74-76, and β- for those heavier - isotopes as heavy as 65Ge can also decay by β+-delayed proton emission, and those as light as 84Ge by β−-delayed neutron emission.
76Ge is used in experiments on the nature of neutrinos, by searching for Double beta decay#Neutrinoless [double beta decay|neutrinoless double beta decay].