History of discovery

File:NASA's Fermi Explores the Early Universe.ogv


The first gamma ray source to be discovered was the radioactive decay process called gamma decay. In this type of decay, an excited nucleus emits a gamma ray almost immediately upon formation.note Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900, while studying radiation emitted from radium. Villard knew that his described radiation was more powerful than previously described types of rays from radium, which included beta rays, first noted as "radioactivity" by Henri Becquerel in 1896, and alpha rays, discovered as a less penetrating form of radiation by Rutherford, in 1899. However, Villard did not consider naming them as a different fundamental type. Later, in 1903, Villard's radiation was recognized as being of a type fundamentally different from previously named rays by Ernest Rutherford, who named Villard's rays "gamma rays" by analogy with the beta and alpha rays that Rutherford had differentiated in 1899. The "rays" emitted by radioactive elements were named in order of their power to penetrate various materials, using the first three letters of the Greek alphabet: alpha rays as the least penetrating, followed by beta rays, followed by gamma rays as the most penetrating. Rutherford also noted that gamma rays were not deflected (or at least, not easily deflected) by a magnetic field, another property making them unlike alpha and beta rays.

Gamma rays were first thought to be particles with mass, like alpha and beta rays. Rutherford initially believed that they might be extremely fast beta particles, but their failure to be deflected by a magnetic field indicated that they had no charge. In 1914, gamma rays were observed to be reflected from crystal surfaces, proving that they were electromagnetic radiation. Rutherford and his co-worker Edward Andrade measured the wavelengths of gamma rays from radium, and found that they were similar to X-rays, but with shorter wavelengths and (thus) higher frequency. This was eventually recognized as giving them more energy per photon, as soon as the latter term became generally accepted. A gamma decay was then understood to usually emit a gamma photon.

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Gamma ray

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A gamma ray , or gamma radiation (symbol γ or γ {\displaystyle \gamma } ), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves and so imparts the highest photon energy. Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900 while studying radiation emitted by radium. In 1903, Ernest Rutherford named this radiation gamma rays based on their relatively strong penetration of matter; in 1900 he had already named two less penetrating types of decay radiation (discovered by Henri Becquerel) alpha rays and beta rays in ascending order of penetrating power. Gamma rays from radioactive decay are in the energy range from a few kiloelectronvolts (keV) to approximately 8 megaelectronvolts (~8 MeV), corresponding to the typical energy levels in nuclei with reasonably long lifetimes. The energy spectrum of gamma rays can be used to identify the decayi...
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Properties

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Penetration of matter edit Due to their penetrating nature, gamma rays require large amounts of shielding mass to reduce them to levels which are not harmful to living cells, in contrast to alpha particles, which can be stopped by paper or skin, and beta particles, which can be shielded by thin aluminium. Gamma rays are best absorbed by materials with high atomic numbers ( Z ) and high density, which contribute to the total stopping power. Because of this, a lead (high Z ) shield is 20–30% better as a gamma shield than an equal mass of another low- Z shielding material, such as aluminium, concrete, water, or soil; lead's major advantage is not in lower weight, but rather its compactness due to its higher density. Protective clothing, goggles and respirators can protect from internal contact with or ingestion of alpha or beta emitting particles, but provide no protection from gamma radiation from external sources. The higher the energy of the gamma rays, the thicker the shielding m...
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Units of measurement and exposure

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The following table shows radiation quantities in SI and non-SI units: Ionizing radiation related quantities view  ‧  talk  ‧   edit Quantity Unit Symbol Derivation Year SI equivalence Activity ( A ) becquerel Bq s−1 1974 SI unit curie Ci 3.7 × 1010 s−1 1953 3.7 × 1010 Bq rutherford Rd 106 s−1 1946 1,000,000 Bq Exposure ( X ) coulomb per kilogram C/kg C⋅kg−1 of air 1974 SI unit röntgen R esu / 0.001293 g of air 1928 2.58 × 10−4 C/kg Absorbed dose ( D ) gray Gy J⋅kg−1 1974 SI unit erg per gram erg/g erg⋅g−1 1950 1.0 × 10−4 Gy rad rad 100 erg⋅g−1 1953 0.010 Gy Equivalent dose ( H ) sievert Sv J⋅kg−1 × W R 1977 SI unit röntgen equivalent man rem 100 erg⋅g−1 x W R 1971 0.010 Sv Effective dose ( E ) sievert Sv J⋅kg−1 × W R x W T 1977 SI unit röntgen equivalent man rem 100 erg⋅g−1 x W R x W T 1971 0.010 Sv The measure of the ionizing effect of gamma and X-rays in dry air is called the exposure, for w...
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