1900: Gamma ray - The book of science

Gamma ray
Gamma rays
Gamma ray guns
Commentary

Gamma ray

Ernest Rutherford identified two types of radioactive decay.
Alpha decay created rays that a thin aluminum sheet blocked.
Beta decay created rays that penetrated aluminum.
Paul Ulrich Villard studied radiation from radium
and identified a third type of ray that,
like beta rays, penetrated a lead shield,
but, unlike beta rays,
were not deflected by a magnetic field.
Rutherford named this third type of decay
with the third letter of the Greek alphabet, gamma.

Gamma rays

Gamma rays are energetic photons
emitted from excited atomic nuclei
because the atom’s been struck by lightning
because it’s inherently unstable
because it’s been created in a nuclear reaction
because it’s energized by a pulsar, quasar, or blazar.

Gamma ray guns

Gamma rays kill bacteria
to sterilize your peaches.
Gamma rays flow from magnetars
deep in the Andromeda galaxy.
Gamma rays stream from guns
wielded by beasts from planet Φar.

At the time, they didn’t know what these rays were. They didn’t know that alpha rays are helium-4 nuclei; they didn’t know that beta rays are electrons; they didn’t know that gamma rays are, like X-rays, photons. X-rays are emitted by excited electrons; gamma rays were emitted by the decay of nuclei or other particles.