Radioactivity Gamma (γ)
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’.
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.
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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