By Kishok Inderaraj 12G
Gamma radiation, also known as gamma rays, and denoted by the Greek letter γ, refers to electromagnetic radiation of an extremely high frequency and is therefore made up of high-energy photons. Gamma rays are ionizing radiation, and are thus biologically hazardous. They are classically produced by the decay of atomic nuclei as they transition from a high energy state to a lower state known as gamma decay, but may also be produced by other processes. 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.
Paul Ulrich Villard (28 September 1860 – 13 January 1934) was a French chemist and physicist, born in Saint-Germain-au-Mont-d’Or, Rhône, 28 September 1860. He discovered gamma rays in 1900 while studying the radiation emanating from radium.
Ernest Rutherford, 1st Baron Rutherford of Nelson, OM FRS (30 August 1871 – 19 October 1937) was a New Zealand-born British physicist who became known as the father of nuclear physics
Gamma rays typically have frequencies above 10 exahertz (or > 1 x E19 Hz), and therefore have energies above 100 keV and wavelengths less than 10 picometres (1 x E−12 metre), which is less than the diameter of an atom.
Gamma waves have the highest frequency and smallest wavelengths of the electromagnetic spectrum. There is some overlap with X-rays but what sets each type apart are the methods by which they are produced. X-rays are emitted by definition by electrons outside the nucleus, while gamma rays are emitted by the nucleus. Exceptions to this convention occur in astronomy, where gamma decay is seen in the afterglow of certain supernovas, but other high energy processes known to involve other than radioactive decay are still classed as sources of gamma radiation.
They have the most energy of any wave in the electromagnetic spectrum. They are produced by the hottest and most energetic objects in the universe, such as neutron stars and pulsars, supernova explosions, and regions around black holes. On Earth, gamma waves are generated by nuclear explosions, lightning, and, as mentioned before, the less dramatic activity of radioactive decay.
Above is an illustration of an emission of a gamma ray (γ) from an atomic nucleus. Gamma radiation occurs when the energy level of the nucleus decreases.
Gamma rays are emitted during nuclear fission in nuclear explosions.
Gamma radiation cannot be seen or felt. It mostly passes through skin and soft tissue, but some of it is absorbed by cells. Gamma radiation is used for things such as the sterilisation of surgical instruments, killing harmful bacteria in food and to destroy cancer cells.
Unlike light and x-rays, gamma rays cannot be captured or reflected by mirrors. Gamma-ray wavelengths are so short that they can pass through the space within the atoms of a detector. Gamma-ray detectors typically contain densely packed crystal blocks. As gamma rays pass through, they collide with electrons in the crystal. This process is called Compton scattering, where a gamma ray strikes an electron and loses energy. These collisions create charged particles that can be detected by the sensor.
Compton scattering is the inelastic scattering of a photon by a semi-free charged particle, usually an electron. It results in a decrease in energy (increase in wavelength) of the photon (which may be an X-ray or gamma ray photon), called the Compton effect. Part of the energy of the photon is transferred to the recoiling electron.
Gamma-ray bursts are the most energetic and luminous electromagnetic events since the Big Bang and can release more energy in 10 seconds than our Sun will emit in its entire 10-billion-year expected lifetime.
If we could see gamma rays, the night sky would look strange and unfamiliar. The familiar view would be replaced by ever-changing bursts of high-energy gamma radiation that last fractions of a second to minutes, popping like cosmic flashbulbs, dominating the gamma-ray sky and then fading.
The sky at energies above 100 MeV observed by the Energetic Gamma Ray Experiment Telescope (EGRET) of the Compton Gamma Ray Observatory (CGRO) satellite (1991–2000)
Gamma rays provide information about some of the most energetic phenomena in the universe; however, they are largely absorbed by the Earth’s atmosphere. Instruments aboard high-altitude balloons and satellites missions such as the Fermi Gamma-ray Space Telescope provide our only view of the universe in gamma rays. Scientists can also use gamma rays to determine the elements on other planets.
Gamma-induced 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.
Non-contact industrial sensors commonly use sources of gamma radiation in the refining, mining, chemical, food, soaps and detergents, and pulp and paper industries, for the measurement of levels, density, and thicknesses. Typically, these use Co-60 or Cs-137 isotopes as the radiation source.
In the US, gamma ray detectors are beginning to be used as part of the Container Security Initiative (CSI). These machines are advertised to be able to scan 30 containers per hour.
Gamma-ray image of a truck with two stowaways taken with a VACIS (vehicle and container imaging system)
Gamma radiation is often used to kill living organisms, in a process called irradiation. Applications of this include sterilization of medical equipment (as an alternative to autoclaves or chemical means), the removal of decay-causing bacteria from many foods and the prevention of sprouting of fruit and vegetables to maintain freshness and flavour.
Despite their cancer-causing properties, gamma rays are also used to treat some types of cancer, since the rays kill cancer cells also. In the procedure called gamma-knife surgery, multiple concentrated beams of gamma rays are directed to the growth in order to kill the cancerous cells. The beams are aimed from different angles to concentrate the radiation on the growth while minimizing damage to surrounding tissues.
Gamma rays are also used for diagnostic purposes in nuclear medicine in imaging techniques. A number of different gamma-emitting radioisotopes are used. For example, in a PET scan a radiolabeled sugar called fludeoxyglucose emits positrons that are annihilated by electrons, producing pairs of gamma rays that highlight cancer as the cancer often has a higher metabolic rate than the surrounding tissues.
When struck by gamma cosmic rays, chemical elements in soils and rocks emit uniquely identifiable signatures of energy in the form of gamma rays. These data can help scientists look for geologically important elements such as hydrogen, magnesium, silicon, oxygen, iron, titanium, sodium, and calcium.
An acute full-body equivalent single exposure dose of 1 Sv (1000 mSv) causes slight blood changes, but 2.0–3.5 Sv (2.0–3.5 Gy) causes very severe syndrome of nausea, hair loss, and hemorrhaging, and will cause death in a sizable number of cases—-about 10% to 35% without medical treatment. A dose of 5 Sv (5 Gy) is considered approximately the LD50 (lethal dose for 50% of exposed population) for an acute exposure to radiation even with standard medical treatment. A dose higher than 5 Sv (5 Gy) brings an increasing chance of death above 50%. Above 7.5–10 Sv (7.5–10 Gy) to the entire body, even extraordinary treatment, such as bone-marrow transplants, will not prevent the death of the individual exposed (see Radiation poisoning). (Doses much larger than this may, however, be delivered to selected parts of the body in the course of radiation therapy.)
For low dose exposure, for example among nuclear workers, who receive an average yearly radiation dose of 19 mSv, the risk of dying from cancer (excluding leukaemia) increases by 2 percent. For a dose of 100 mSv, the risk increase is 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.