How nuclei get rid of excess energy

It was in 1900 that the French physicist Paul Villard first found evidence for gamma radiation. The fact that this radiation, unlike both alpha and beta rays, was not deflected by either electric or magnetic fields led to the conclusion that gamma radiation was carried by electrically neutral particles later identified as ‘photons’.

Example of gamma radioactivity
Gamma radioactivity takes place when a decay, or an event like a neutron capture, has left the nucleus with an energy surplus. The ‘excited’ nucleus generally returns to its ground state very quickly. The figure shows a deformed nucleus rotating around an axis before returning to its spherical shape and losing its rotation through the emission of a gamma ray. The gamma rays are of the same nature as photons that form the light emitted by atoms, but have energies several hundred thousands of times greater.
© IN2P3

These ‘gamma ‘ (γ) rays are of the same nature as X-rays or even the light emitted by atoms. The energy they carry away, however, is far larger; anywhere between 100,000 and a million electronvolts (MeV).

The term of gamma radioactivity is slightly misleadiong, as it is a desexcitation phenomenon similar to atomic desexcitations.

Cascade of gamma
Gamma radioactivity generally accompanies alpha or beta decay, as this example of cobalt 60 shows. The nucleus of Cobalt-60 (a radioisotope with a half-life of 5,271 years) decays by undergoing beta radioactivity and forms a stable nucleus of nickel 60. The transformation, accompanied by the emission of an electron and an antineutrino, results almost always in an excited nickel 60 nucleus. The nucleus loses the 2,158.80 keV of its excess energy by emitting a first gamma photon, followed by a second. The emission of the two photons follows that of the electron and the antineutrino.
© IN2P3

The emission of a gamma ray often follows rarely the emission of an alpha particle, but frequently a beta decay, or a neutron capture by a nucleus. These events leave the nucleus in an excited state, possessing more energy than its ‘ground’ state, which causes it to release the extra energy through the emission of one or more gamma photons, the ‘grains of electromagnetic energy’.

The gamma emission is almost always instantaneous, though it can occasionally take place with a delay. This is the case with technetium in its excited state; a state which can last for several hours and thus allows technetium to be used as a pure gamma source in hospital scans.

In an interesting parallel with the atom, nuclei have well-defined energy states. The jump from one energy level to another is accompanied by the emission of a gamma ray with a specific energy value, characteristic both of the specific energy transition and of the nucleus involved. Measuring the energy of the gamma rays emitted therefore allows for positive identification of the emitter nucleus.

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