History of Discovery
The first gamma ray source to be discovered historically was the radioactive decay process called gamma decay. In this type of decay, an excited nucleus emits a gamma ray almost immediately upon formation. Isomeric transition, however, can produce inhibited gamma decay with a measurable and much longer half-life. Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900, while studying radiation emitted from radium. Villard's radiation was named "gamma rays" by Ernest Rutherford in 1903. Gamma rays were named in order of their penetrating power: alpha rays least, followed by beta rays, followed by gamma rays as the most penetrating.
Gamma rays were first thought to be particles like alpha and beta rays (hence the term "ray"). However, in 1910 British physicist William Henry Bragg demonstrated that gamma rays are electromagnetic radiation, not particles, and in 1914, Ernest Rutherford (who had named them gamma rays) and Edward Andrade measured their wavelengths, and found that they were similar to X-rays but with shorter wavelengths and higher frequency, and thus more energy per photon.
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