This element also has 19 meta states, of which the most stable is 58m1Co with a half-life of 8.853 h. The isotopes of cobalt range in atomic weight from 50Co to 78Co.
The 60Co source is about 2 cm in diameter and as a result produces a geometric penumbra, making the edge of the radiation field fuzzy.
The metal has the unfortunate habit of producing fine dust, causing problems with radiation protection.
The 60Co source is useful for about 5 years but even after this point is still very radioactive, and so cobalt machines have fallen from favor in the Western world where Linacs are common.
Cobalt-60 (60Co) is useful as a gamma ray source because it can be produced in predictable quantities, and for its high radioactivity simply by exposing natural cobalt to neutrons in a reactor.