Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting ionizing particles and radiation.
The half life T of 5.27 year corresponds to the activity A = N [ ln(2) / T ], where N is the number of atoms per mol, and T is the half-life.
This table gives an indication why - despite its enormous cost - 238Pu with its roughly eighty year half life and low gamma emissions has become the RTG nuclide of choice.
90Sr performs worse than 238Pu on almost all measures, being shorter lived, a beta emitter rather than an easily shielded alpha emitter and releasing significant gamma radiation when its daughter nuclide 90Y decays, but as it is a high yield product of nuclear fission and easy to chemically extract from other fission products, Strontium titanate based RTGs were in widespread use for remote locations during much of the 20th century.
Cobalt-60 while widely used for purposes such as food irradiation is not a practicable RTG isotope as most of its decay energy is released by gamma rays, requiring substantial shielding.