Various radionuclides emit beta particles, high-speed electrons or positrons, through radioactive decay of their atomic nucleus.
Strontium-90 has a shorter half-life, produces less power, and requires more shielding than plutonium-238, but is cheaper as it is a fission product and is present in a high concentration in nuclear waste and can be relatively easily chemically extracted.
[2] Strontium-89 is a short-lived beta emitter which has been used as a treatment for bone tumors, this is used in palliative care in terminal cancer cases.
Tritium is a low-energy beta emitter commonly used as a radiotracer in research and in traser[check spelling] self-powered lightings.
Tritium can also be found in metal work in the form of a tritiated rust, this can be treated by heating the steel in a furnace to drive off the tritium-containing water.
Also a large amount was generated by the neutrons from the air bursts during nuclear weapons testing conducted in the 20th century.