Gamma rays (denoted as γ) are electromagnetic radiation of high energy. They are produced by sub-atomic particle interactions, such as electron-positron annihilation, neutral pion decay, radioactive decay, fusion, fission or inverse Compton scattering in astrophysical processes. Gamma rays typically have frequencies above 1019 Hz and therefore energies above 100 keV and wavelength less than 10 picometers, often smaller than an atom. Paul Villard, a French chemist and physicist, discovered Gamma radiation in 1900, while studying uranium.
Hard X-rays can have higher energy than low energy gamma rays. In the past, distinction between the X- and gamma rays was arbitrarily based on wavelengths. Now the two types of radiation are usually defined by their origin: X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus.
As a form of ionizing radiation, gamma rays can cause serious damage when absorbed by living tissue, and they are therefore a health hazard.
Contents [hide] 1 Uses 2 Health effects 2.1 Body response 2.2 Risk assessment 3 See also 4 References5 External links
Gamma-ray Image of a truck taken with a VACIS (Vehicle and Container Imaging System)This property means that gamma radiation is often used to kill living organisms, in a process called irradiation. Applications of this include sterilizing medical equipment (as an alternative to autoclaves or chemical means), removing decay-causing bacteria from many foods or preventing fruit and vegetables from sprouting to maintain freshness and flavor.
Due to their tissue penetrating property, gamma rays/X-rays have a wide variety of medical uses such as in CT Scans and radiation therapy (see X-ray). However, as a form of ionizing radiation they have the ability to effect molecular changes, giving them the potential to cause cancer when DNA is affected. The molecular changes can also be used to alter the properties of semi-precious stones, and is often used to change white topaz into blue topaz.
Despite their cancer-causing properties, gamma rays are also used to treat some types of cancer. In the procedure called gamma-knife surgery, multiple concentrated beams of gamma rays are directed on the growth in order to kill the cancerous cells. The beams are aimed from different angles to focus the radiation on the growth while minimizing damage to the surrounding tissues.
The Moon as seen in gamma rays by the Compton Gamma Ray Observatory. Surprisingly, the Moon is actually brighter than the quiet Sun at gamma ray wavelengths.Gamma rays are also used for diagnostic purposes in nuclear medicine. Several gamma-emitting radioisotopes are used, one of which is technetium-99m. When administered to a patient, a gamma camera can be used to form an image of the radioisotope's distribution by detecting the gamma radiation emitted. Such a technique can be employed to diagnose a wide range of conditions (e.g. spread of cancer to the bones).
In the US, gamma ray detectors are starting to be used as part of the Container Security Initiative (CSI). These US$5 million machines are advertised to scan 30 containers per hour. The objective of this technique is to screen merchant ship containers before they enter US ports.
Health effects Edit
Gamma rays are the most dangerous form of electromagnetic radiation emitted by something such as a nuclear explosion because they are highly penetrating, highly energetic ionizing radiation. Gamma rays have the shortest wavelength of all waves in the electromagnetic spectrum, and therefore have the greatest ability to penetrate through any gap, even a subatomic one, in what might otherwise be an effective shield. The most biological damaging forms of gamma radiation occur in the gamma ray window, between 3 and 10 MeV. See cobalt-60.
Gamma-rays are not stopped by the skin. They can induce DNA alteration by effect of whole-body gamma-irradiation on localized beta-irradiation-induced skin reactions in mice.
Body response Edit
After gamma-irradiation, and the breaking of DNA double-strands, a cell can repair the damaged genetic material to the limit of its capability. However, a study of Rothkamm and Lobrich has shown that the repairing process works well after high-dose exposure but is much slower in the case of a low-dose exposure.  This could mean that a chronic low-dose exposure cannot be fought by the body. The probability of detecting small alterations or of a detectable defect occurring is most likely small enough that the cell would replicate before initiating a full repair. Some cells cannot detect their own genetic defects.
Risk assessment Edit
The natural outdoor exposure in Great Britain ranges from 2 × 10–7 to 4 × 10–7 cSv/h (centisieverts per hour). Natural exposure to gamma rays is about 0.1 to 0.2 cSv per year, and the average total amount of radiation received in one year per inhabitant in the USA is 0.36 cSv.
By comparison, the radiation dose from chest radiography is a fraction of the annual naturally occurring background radiation dose, and the dose from fluoroscopy of the stomach is, at most, 5 cSv on the skin of the back.
For acute full-body equivalent dose, 100 cSv causes slight blood changes; 200–350 cSv causes nausea, hair loss, hemorrhaging and will cause death in a sizable number of cases (10%–35%) without medical treatment; 500 cSv is considered approximately the LD50 (lethal dose for 50% of exposed population) for an acute exposure to radiation even with standard medical treatment; more than 500 cSv brings an increasing chance of death; eventually, above 750–1000 cSv, even extraordinary treatment, such as bone-marrow transplants, will not prevent the death of the individual exposed (see Radiation poisoning).
For low dose exposure, for example among nuclear workers, who receive an average yearly radiation dose of 1.9 cSv,[clarification needed] the risk of dying from cancer (excluding leukemia) increases by 2 percent. For a dose of 10 cSv, that risk increase is at 10 percent. By comparison, risk of dying from cancer was increased by 32 percent for the survivors of the Atomic bombing of Hiroshima and Nagasaki.