After a decay reaction, the nucleus is often in an “excited”
state. This means that the decay has resulted in producing a nucleus which
still has excess energy to get rid of. Rather than emitting another beta
or alpha particle, this energy is lost by emitting a pulse of electromagnetic
radiation called a gamma ray. The gamma ray is identical in nature to light
or microwaves, but of very high energy.
Like all forms of electromagnetic radiation, the gamma ray has no mass
and no charge. Gamma rays interact with material by colliding with the electrons
in the shells of atoms. They lose their energy slowly in material, being
able to travel significant distances before stopping. Depending on their
initial energy, gamma rays can travel from 1 to hundreds of meters in air
and can easily go right through people.
It is important to note that most alpha and beta emitters also emit gamma
rays as part of their decay process. However, there is no such thing as
a “pure” gamma emitter. Important gamma emitters including Technetium-99m
which is used in nuclear medicine, and Cesium-137 which is used for calibration
of nuclear instruments.