A radionuclide is an atom that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation. During those processes, the radionuclide is said to undergo radioactive decay; these emissions are considered ionizing radiation because they are powerful enough to liberate an electron from another atom. The radioactive decay can produce a stable nuclide or will sometimes produce a new unstable radionuclide which may undergo further decay. Radioactive decay is a random process at the level of single atoms: it is impossible to predict when one particular atom will decay. However, for a collection of atoms of a single element the decay rate, thus the half-life for that collection can be calculated from their measured decay constants; the range of the half-lives of radioactive atoms have no known limits and span a time range of over 55 orders of magnitude. Radionuclides occur or are artificially produced in nuclear reactors, particle accelerators or radionuclide generators.
There are about 730 radionuclides with half-lives longer than 60 minutes. Thirty-two of those are primordial radionuclides. At least another 60 radionuclides are detectable in nature, either as daughters of primordial radionuclides or as radionuclides produced through natural production on Earth by cosmic radiation. More than 2400 radionuclides have half-lives less than 60 minutes. Most of those are only produced artificially, have short half-lives. For comparison, there are about 253 stable nuclides. All chemical elements can exist as radionuclides; the lightest element, has a well-known radionuclide, tritium. Elements heavier than lead, the elements technetium and promethium, exist only as radionuclides. Unplanned exposure to radionuclides has a harmful effect on living organisms including humans, although low levels of exposure occur without harm; the degree of harm will depend on the nature and extent of the radiation produced, the amount and nature of exposure, the biochemical properties of the element.
However, radionuclides with suitable properties are used in nuclear medicine for both diagnosis and treatment. An imaging tracer made with radionuclides is called a radioactive tracer. A pharmaceutical drug made with radionuclides is called a radiopharmaceutical. On Earth occurring radionuclides fall into three categories: primordial radionuclides, secondary radionuclides, cosmogenic radionuclides. Radionuclides are produced in stellar nucleosynthesis and supernova explosions along with stable nuclides. Most decay but can still be observed astronomically and can play a part in understanding astronomic processes. Primordial radionuclides, such as uranium and thorium, exist in the present time because their half-lives are so long that they have not yet decayed; some radionuclides have half-lives so long that decay has only been detected, for most practical purposes they can be considered stable, most notably bismuth-209: detection of this decay meant that bismuth was no longer considered stable.
It is possible decay may be observed in other nuclides adding to this list of primordial radionuclides. Secondary radionuclides are radiogenic isotopes derived from the decay of primordial radionuclides, they have shorter half-lives than primordial radionuclides. They arise in the decay chain of the primordial isotopes thorium-232, uranium-238 and uranium-235. Examples include the natural isotopes of radium. Cosmogenic isotopes, such as carbon-14, are present because they are continually being formed in the atmosphere due to cosmic rays. Many of these radionuclides exist only in trace amounts in nature, including all cosmogenic nuclides. Secondary radionuclides will occur in proportion to their half-lives, so short-lived ones will be rare, thus polonium can be found in uranium ores at about 0.1 mg per metric ton. Further radionunclides may occur in nature in undetectable amounts as a result of rare events such as spontaneous fission or uncommon cosmic ray interactions. Radionuclides are produced as an unavoidable result of nuclear thermonuclear explosions.
The process of nuclear fission creates a wide range of fission products, most of which are radionuclides. Further radionuclides can be created from irradiation of the nuclear fuel and of the surrounding structures, yielding activation products; this complex mixture of radionuclides with different chemistries and radioactivity makes handling nuclear waste and dealing with nuclear fallout problematic. Synthetic radionuclides are deliberately synthesised using nuclear reactors, particle accelerators or radionuclide generators: As well as being extracted from nuclear waste, radioisotopes can be produced deliberately with nuclear reactors, exploiting the high flux of neutrons present; these neutrons activate elements placed within the reactor. A typical product from a nuclear reactor is iridium-
A technetium-99m generator, or colloquially a technetium cow or moly cow, is a device used to extract the metastable isotope 99mTc of technetium from a decaying sample of molybdenum-99. 99Mo has a half-life of 66 hours and can be transported over long distances to hospitals where its decay product technetium-99m is extracted and used for a variety of nuclear medicine diagnostic procedures, where its short half-life is useful. 99Mo can be obtained by the neutron activation of 98Mo in a high neutron flux reactor. However, the most used method is through fission of uranium-235 in a nuclear reactor. While most reactors engaged in 99Mo production use enriched uranium-235 targets, proliferation concerns have prompted some producers to transition to low-enriched uranium targets; the target is irradiated with neutrons to form 99Mo as a fission product. Molybdenum-99 is separated from unreacted uranium and other fission products in a hot cell. Tc-99m remained a scientific curiosity until the 1950s when Powell Richards realized the potential of technetium-99m as a medical radiotracer and promoted its use among the medical community.
While Richards was in charge of the radioisotope production at the Hot Lab Division of the Brookhaven National Laboratory, Walter Tucker and Margaret Greene were working on how to improve the separation process purity of the short-lived eluted daughter product iodine-132 from tellurium-132, its 3.2-days parent, produced in the Brookhaven Graphite Research Reactor. They detected a trace contaminant which proved to be Tc-99m, coming from Mo-99 and was following tellurium in the chemistry of the separation process for other fission products. Based on the similarities between the chemistry of the tellurium-iodine parent-daughter pair and Greene developed the first technetium-99m generator in 1958, it was not until 1960 that Richards became the first to suggest the idea of using technetium as a medical tracer. Technetium-99m's short half-life of 6 hours makes storage impossible and would make transport expensive. Instead, its parent nuclide 99Mo is supplied to hospitals after its extraction from the neutron-irradiated uranium targets and its purification in dedicated processing facilities.
It is shipped by specialised radiopharmaceutical companies in the form of technetium-99m generators worldwide or directly distributed to the local market. The generators, colloquially known as moly cows, provide radiation shielding for transport and to minimize the extraction work done at the medical facility. A typical dose rate at 1 metre from Tc-99m generator is 20-50 μSv/h during transport; these generators' output declines with time and must be replaced weekly, since the half-life of 99Mo is still only 66 hours. Since the half-life of the parent nuclide is much longer than that of the daughter nuclide, 50% of equilibrium activity is reached within one daughter half-life, 75% within two daughter half-lives. Hence, removing the daughter nuclide from the generator is reasonably done as as every 6 hours in a 99Mo/99mTc generator. Most commercial 99Mo/99mTc generators use column chromatography, in which 99Mo in the form of molybdate, MoO42− is adsorbed onto acid alumina; when the Mo-99 decays it forms pertechnetate TcO4−, because of its single charge, is less bound to the alumina.
Pouring normal saline solution through the column of immobilized 99Mo elutes the soluble 99mTc, resulting in a saline solution containing the 99mTc as the pertechnetate, with sodium as the counterion. The solution of sodium pertechnetate may be added in an appropriate concentration to the organ-specific pharmaceutical to be used, or sodium pertechnetate can be used directly without pharmaceutical tagging for specific procedures requiring only the 99mTcO4− as the primary radiopharmaceutical. A large percentage of the 99mTc generated by a 99Mo/99mTc generator is produced in the first 3 parent half-lives, or one week. Hence, clinical nuclear medicine units purchase at least one such generator per week or order several in a staggered fashion; when the generator is left unused, 99Mo decays to 99mTc. The half-life of 99Tc is far longer than its metastable isomer, so the ratio of 99Tc to 99mTc increases over time. Both isomers are carried out by the elution process and react well with the ligand, but the 99Tc is an impurity useless to imaging.
The generator is washed of 99Tc and 99mTc at the end of the manufacturing process of the generator, but the ratio of 99Tc to 99mTc builds up again during transport or any other period when the generator is left unused. The first few elutions will have reduced effectiveness because of this high ratio. Celebrating the 60th Anniversary of Technetium-99m, Brookhaven National Laboratory
A nuclear reactor known as an atomic pile, is a device used to initiate and control a self-sustained nuclear chain reaction. Nuclear reactors are used at nuclear power plants for electricity generation and in propulsion of ships. Heat from nuclear fission is passed to a working fluid; these either turn electrical generators' shafts. Nuclear generated steam in principle can be used for industrial process heat or for district heating; some reactors are used to produce isotopes for medical and industrial use, or for production of weapons-grade plutonium. Some are run only for research; as of early 2019, the IAEA reports there are 454 nuclear power reactors and 226 nuclear research reactors in operation around the world. Just as conventional power-stations generate electricity by harnessing the thermal energy released from burning fossil fuels, nuclear reactors convert the energy released by controlled nuclear fission into thermal energy for further conversion to mechanical or electrical forms; when a large fissile atomic nucleus such as uranium-235 or plutonium-239 absorbs a neutron, it may undergo nuclear fission.
The heavy nucleus splits into two or more lighter nuclei, releasing kinetic energy, gamma radiation, free neutrons. A portion of these neutrons may be absorbed by other fissile atoms and trigger further fission events, which release more neutrons, so on; this is known as a nuclear chain reaction. To control such a nuclear chain reaction, neutron poisons and neutron moderators can change the portion of neutrons that will go on to cause more fission. Nuclear reactors have automatic and manual systems to shut the fission reaction down if monitoring detects unsafe conditions. Used moderators include regular water, solid graphite and heavy water; some experimental types of reactor have used beryllium, hydrocarbons have been suggested as another possibility. The reactor core generates heat in a number of ways: The kinetic energy of fission products is converted to thermal energy when these nuclei collide with nearby atoms; the reactor absorbs some of the gamma rays produced during fission and converts their energy into heat.
Heat is produced by the radioactive decay of fission products and materials that have been activated by neutron absorption. This decay heat-source will remain for some time after the reactor is shut down. A kilogram of uranium-235 converted via nuclear processes releases three million times more energy than a kilogram of coal burned conventionally. A nuclear reactor coolant — water but sometimes a gas or a liquid metal or molten salt — is circulated past the reactor core to absorb the heat that it generates; the heat is carried away from the reactor and is used to generate steam. Most reactor systems employ a cooling system, physically separated from the water that will be boiled to produce pressurized steam for the turbines, like the pressurized water reactor. However, in some reactors the water for the steam turbines is boiled directly by the reactor core; the rate of fission reactions within a reactor core can be adjusted by controlling the quantity of neutrons that are able to induce further fission events.
Nuclear reactors employ several methods of neutron control to adjust the reactor's power output. Some of these methods arising from the physics of radioactive decay and are accounted for during the reactor's operation, while others are mechanisms engineered into the reactor design for a distinct purpose; the fastest method for adjusting levels of fission-inducing neutrons in a reactor is via movement of the control rods. Control rods therefore tend to absorb neutrons; when a control rod is inserted deeper into the reactor, it absorbs more neutrons than the material it displaces—often the moderator. This action results in fewer neutrons available to cause fission and reduces the reactor's power output. Conversely, extracting the control rod will result in an increase in the rate of fission events and an increase in power; the physics of radioactive decay affects neutron populations in a reactor. One such process is delayed neutron emission by a number of neutron-rich fission isotopes; these delayed neutrons account for about 0.65% of the total neutrons produced in fission, with the remainder released upon fission.
The fission products which produce delayed neutrons have half lives for their decay by neutron emission that range from milliseconds to as long as several minutes, so considerable time is required to determine when a reactor reaches the critical point. Keeping the reactor in the zone of chain-reactivity where delayed neutrons are necessary to achieve a critical mass state allows mechanical devices or human operators to control a chain reaction in "real time"; this last stage, where delayed neutrons are no longer required to maintain criticality, is known as the prompt critical point. There is a scale for describing criticality in numerical form, in which bare criticality is known as zero dollars and the prompt critical point is one dollar, other points in the process interpolated in cents. In some reactors, the coolant acts as a neutron moderator. A moderator increases the power of the reactor by causin
A gamma ray or gamma radiation, is a penetrating 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 strong penetration of matter. Gamma rays from radioactive decay are in the energy range from a few keV to ~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 decaying radionuclides using gamma spectroscopy. Very-high-energy gamma rays in the 100–1000 TeV range have been observed from sources such as the Cygnus X-3 microquasar. Natural sources of gamma rays originating on Earth are as a result of radioactive decay and secondary radiation from atmospheric interactions with cosmic ray particles.
However there are other rare natural sources, such as terrestrial gamma-ray flashes, that produce gamma rays from electron action upon the nucleus. Notable artificial sources of gamma rays include fission, such as occurs in nuclear reactors, as well as high energy physics experiments, such as neutral pion decay and nuclear fusion. Gamma rays and X-rays are both electromagnetic radiation and they overlap in the electromagnetic spectrum, the terminology varies between scientific disciplines. In some fields of physics, they are distinguished by their origin: Gamma rays are created by nuclear decay, while in the case of X-rays, the origin is outside the nucleus. In astrophysics, gamma rays are conventionally defined as having photon energies above 100 keV and are the subject of gamma ray astronomy, while radiation below 100 keV is classified as X-rays and is the subject of X-ray astronomy; this convention stems from the early man-made X-rays, which had energies only up to 100 keV, whereas many gamma rays could go to higher energies.
A large fraction of astronomical gamma rays are screened by Earth's atmosphere. Gamma rays are thus biologically hazardous. Due to their high penetration power, they can damage internal organs. Unlike alpha and beta rays, they pass through the body and thus pose a formidable radiation protection challenge, requiring shielding made from dense materials such as lead or concrete; 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 immediately upon formation. 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 described types of rays from radium, which included beta rays, first noted as "radioactivity" by Henri Becquerel in 1896, 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.
In 1903, Villard's radiation was recognized as being of a type fundamentally different from 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 noted that gamma rays were not deflected by a magnetic field, another property making them unlike alpha and beta rays. Gamma rays were first thought to be particles like alpha and beta rays. Rutherford believed that they might be 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, found that they were similar to X-rays, but with shorter wavelengths and higher frequency.
This was recognized as giving them more energy per photon, as soon as the latter term became accepted. A gamma decay was understood to emit a gamma photon. Natural sources of gamma rays on Earth include gamma decay from occurring radioisotopes such as potassium-40, as a secondary radiation from various atmospheric interactions with cosmic ray particles; some rare terrestrial natural sources that produce gamma rays that are not of a nuclear origin, are lightning strikes and terrestrial gamma-ray flashes, which produce high energy emissions from natural high-energy voltages. Gamma rays are produced by a number of astronomical processes in which high-energy electrons are produced; such electrons produce secondary gamma rays by the mechanisms of bremsstrahlung, inverse Compton scattering and synchrotron radiation. A large fraction of such astronomical gamma rays are screened by Earth's atmosphere. Notable artificial sources of gamma rays include fission, such as occurs in nuclear reactors, as well as high energy physics experiments, such as neutral pion decay and nuclear fusion.
A sample of gamma ray-emitting material, used for irradiating or imaging is known as a gamma source. It is called a radioactive sou
A cyclotron is a type of particle accelerator invented by Ernest O. Lawrence in 1929-1930 at the University of California and patented in 1932. A cyclotron accelerates charged particles outwards from the center along a spiral path; the particles are held to a spiral trajectory by a static magnetic field and accelerated by a varying electric field. Lawrence was awarded the 1939 Nobel prize in physics for this invention. Cyclotrons were the most powerful particle accelerator technology until the 1950s when they were superseded by the synchrotron, are still used to produce particle beams in physics and nuclear medicine; the largest single-magnet cyclotron was the 4.67 m synchrocyclotron built between 1940 and 1946 by Lawrence at the University of California at Berkeley, which could accelerate protons to 730 million electron volts. The largest cyclotron is the 17.1 m multimagnet TRIUMF accelerator at the University of British Columbia in Vancouver, British Columbia which can produce 500 MeV protons.
Over 1200 cyclotrons are used in nuclear medicine worldwide for the production of radionuclides. The first cyclotron was developed and patented by Ernest Lawrence in 1932 at the University of California, Berkeley, he used large electromagnets recycled from obsolete Poulsen arc radio transmitters provided by the Federal Telegraph Company. A graduate student, M. Stanley Livingston, did much of the work of translating the idea into working hardware. Lawrence read an article about the concept of a drift tube linac by Rolf Widerøe, working along similar lines with the betatron concept. At the Radiation Laboratory of the University of California at Berkeley Lawrence constructed a series of cyclotrons which were the most powerful accelerators in the world at the time, he developed a 467 cm synchrocyclotron. Lawrence received the 1939 Nobel prize in physics for this work; the first European cyclotron was constructed in Leningrad in the physics department of the Radium Institute, headed by Vitaly Khlopin.
This Leningrad instrument was first proposed in 1932 by George Gamow and Lev Mysovskii and was installed and became operative by 1937. In Nazi Germany a cyclotron was built in Heidelberg under supervision of Walther Bothe and Wolfgang Gentner, with support from the Heereswaffenamt, became operative in 1943. A cyclotron accelerates a charged particle beam using a high frequency alternating voltage, applied between two hollow "D"-shaped sheet metal electrodes called "dees" inside a vacuum chamber; the dees are placed face to face with a narrow gap between them, creating a cylindrical space within them for the particles to move. The particles are injected into the center of this space; the dees are located between the poles of a large electromagnet which applies a static magnetic field B perpendicular to the electrode plane. The magnetic field causes the particles' path to bend in a circle due to the Lorentz force perpendicular to their direction of motion. If the particles' speeds were constant, they would travel in a circular path within the dees under the influence of the magnetic field.
However a radio frequency alternating voltage of several thousand volts is applied between the dees. The voltage creates an oscillating electric field in the gap between the dees that accelerates the particles; the frequency is set. To achieve this, the frequency must match the particle's cyclotron resonance frequency f = q B 2 π m,where B is the magnetic field strength, q is the electric charge of the particle and m is the relativistic mass of the charged particle; each time after the particles pass to the other dee electrode the polarity of the RF voltage reverses. Therefore, each time the particles cross the gap from one dee electrode to the other, the electric field is in the correct direction to accelerate them; the particles' increasing speed due to these pushes causes them to move in a larger radius circle with each rotation, so the particles move in a spiral path outward from the center to the rim of the dees. When they reach the rim a small voltage on a metal plate deflects the beam so it exits the dees through a small gap between them, hits a target located at the exit point at the rim of the chamber, or leaves the cyclotron through an evacuated beam tube to hit a remote target.
Various materials may be used for the target, the nuclear reactions due to the collisions will create secondary particles which may be guided outside of the cyclotron and into instruments for analysis. The cyclotron was the first "cyclical" accelerator; the advantage of the cyclotron design over the existing "electrostatic" accelerators of the time such as the Cockcroft-Walton accelerator and Van de Graaff generator, was that in these machines the particles were only accelerated once by the voltage, so the particles' energy was equal to the accelerating voltage on the machine, limited by air breakdown to a few million volts. In the cyclotron, in contrast, the particles encounter the accelerating voltage many times during their spiral path, so are accelerated many times, so the output energy can be many times the accelerating voltage. Since the particles are accelerated by the voltage many times, the final energy of the particles is not dependent on the accelerating voltage but on the strength of the magnetic field and the diameter of the accelerating chamber, the dees.
Cyclotrons can only accelerate particles to speeds much slower than the speed of light, nonrelativistic sp
A beta particle called beta ray or beta radiation, is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus during the process of beta decay. There are two forms of beta decay, β− decay and β+ decay, which produce electrons and positrons respectively. Beta particles with an energy of 0.5 MeV have a range of about one metre in air. Beta particles are a type of ionizing radiation and for radiation protection purposes are regarded as being less ionising than alpha particles, but more ionising than gamma rays; the higher the ionising effect, the greater the damage to living tissue. An unstable atomic nucleus with an excess of neutrons may undergo β− decay, where a neutron is converted into a proton, an electron, an electron antineutrino: n → p + e− + νeThis process is mediated by the weak interaction; the neutron turns into a proton through the emission of a virtual W− boson. At the quark level, W− emission turns a down quark into an up quark, turning a neutron into a proton.
The virtual W− boson decays into an electron and an antineutrino. Β− decay occurs among the neutron-rich fission byproducts produced in nuclear reactors. Free neutrons decay via this process. Both of these processes contribute to the copious quantities of beta rays and electron antineutrinos produced by fission-reactor fuel rods. Unstable atomic nuclei with an excess of protons may undergo β+ decay called positron decay, where a proton is converted into a neutron, a positron, an electron neutrino: p → n + e+ + νeBeta-plus decay can only happen inside nuclei when the absolute value of the binding energy of the daughter nucleus is greater than that of the parent nucleus, i.e. the daughter nucleus is a lower-energy state. The accompanying decay scheme diagram shows the beta decay of Cs-137. Cs-137 is noted for a characteristic gamma peak at 661 KeV, but this is emitted by the daughter radionuclide Ba-137m; the diagram shows the type and energy of the emitted radiation, its relative abundance, the daughter nuclides after decay.
Phosphorus-32 is a beta emitter used in medicine and has a short half-life of 14.29 days and decays into sulfur-32 by beta decay as shown in this nuclear equation: 1.709 MeV of energy is released during the decay. The kinetic energy of the electron varies with an average of 0.5 MeV and the remainder of the energy is carried by the nearly undetectable electron antineutrino. In comparison to other beta radiation-emitting nuclides the electron is moderately energetic, it is blocked by 5 mm of acrylic glass. Of the three common types of radiation given off by radioactive materials, alpha and gamma, beta has the medium penetrating power and the medium ionising power. Although the beta particles given off by different radioactive materials vary in energy, most beta particles can be stopped by a few millimeters of aluminium. However, this does not mean that beta-emitting isotopes can be shielded by such thin shields: as they decelerate in matter, beta electrons emit secondary gamma rays, which are more penetrating than betas per se.
Shielding composed of materials with lower atomic weight generates gammas with lower energy, making such shields somewhat more effective per unit mass than ones made of high-Z materials such as lead. Being composed of charged particles, beta radiation is more ionizing than gamma radiation; when passing through matter, a beta particle is decelerated by electromagnetic interactions and may give off bremsstrahlung x-rays. In water, beta radiation from many nuclear fission products exceeds the speed of light in that material, thus generates blue Cherenkov radiation when it passes through water; the intense beta radiation from the fuel rods of pool-type reactors can thus be visualized through the transparent water that covers and shields the reactor. The ionizing or excitation effects of beta particles on matter are the fundamental processes by which radiometric detection instruments detect and measure beta radiation; the ionization of gas is used in ion chambers and Geiger-Müller counters, the excitation of scintillators is used in scintillation counters.
The following table shows radiation quantities in SI and non-SI units: The gray, is the SI unit of absorbed dose, the amount of radiation energy deposited in the irradiated material. For beta radiation this is numerically equal to the equivalent dose measured by the sievert, which indicates the stochastic biological effect of low levels of radiation on human tissue; the radiation weighting conversion factor from absorbed dose to equivalent dose is 1 for beta, whereas alpha particles have a factor of 20, reflecting their greater ionising effect on tissue. The rad is the deprecated CGS unit for absorbed dose and the rem is the deprecated CGS unit of equivalent dose, used in the USA. Beta particles can be used to treat health conditions such as eye and bone cancer and are used as tracers. Strontium-90 is the material most used to produce beta particles. Beta particles are used in quality control to test the thickness of an item, such as paper, coming through a system of rollers; some of the beta radiation is absorbed while passing through the product.
If the product is made too thick or thin, a correspondingly different amount of radiation will be absorbed. A computer program monitoring the quality of the manufactured paper will move the rollers to change the thickness of the final product. An illumination device called a betalight contains a phosphor; as tritium dec
Food irradiation is the process of exposing food and food packaging to ionizing radiation. Ionizing radiation, such as from gamma rays, x-rays, or electron beams, is energy that can be transmitted without direct contact to the source of the energy capable of freeing electrons from their atomic bonds in the targeted food; the radiation can be generated electrically. This treatment is used to improve food safety by extending product shelf-life, reducing the risk of foodborne illness, delaying or eliminating sprouting or ripening, by sterilization of foods, as a means of controlling insects and invasive pests. Food irradiation extends the shelf-life of irradiated foods by destroying organisms responsible for spoilage and foodborne illness and inhibiting sprouting. Although consumer perception of foods treated with irradiation is more negative than those processed by other means, because people imagine that the food is radioactive or mutated, these thoughts don't agree with the understood mechanism by which irradiation works.
The food itself is not alive, so irradiation will not affect it meaningfully. Irradiation will kill the living bacteria, however. Additionally, all independent research, the U. S. Food and Drug Administration, the World Health Organization, the Centers for Disease Control and Prevention, U. S. Department of Agriculture have performed studies. In order for a food to be irradiated in the US, the FDA will still require that the specific food be tested for irradiation safety. Food irradiation is permitted by over 60 countries, with about 500,000 metric tons of food annually processed worldwide; the regulations that dictate how food is to be irradiated, as well as the food allowed to be irradiated, vary from country to country. In Austria and many other countries of the European Union only dried herbs and seasonings can be processed with irradiation and only at a specific dose, while in Brazil all foods are allowed at any dose. Irradiation is used to reduce or eliminate the risk of food-borne illnesses, prevent or slow down spoilage, arrest maturation or sprouting and as a treatment against pests.
Depending on the dose, some or all of the pathogenic organisms, microorganisms and viruses present are destroyed, slowed down, or rendered incapable of reproduction. Irradiation can not return over-ripe food to a fresh state. If this food was processed by irradiation, further spoilage would cease and ripening would slow down, yet the irradiation would not destroy the toxins or repair the texture, color, or taste of the food; when targeting bacteria, most foods are irradiated to reduce the number of active microbes, not to sterilize all microbes in the product. In this respect it is similar to pasteurization. Irradiation is used to create safe foods for people at high risk of infection, or for conditions where food must be stored for long periods of time and proper storage conditions are not available. Foods that can tolerate irradiation at sufficient doses are treated to ensure that the product is sterilized; this is most done with rations for astronauts, special diets for hospital patients. Irradiation is used to create shelf-stable products.
Since irradiation reduces the populations of spoilage microorganisms, because pre-packed food can be irradiated, the packaging prevents recontamination of the final product. Irradiation is used to reduce post-harvest losses, it reduces populations of spoilage micro-organisms in the food and can slow down the speed at which enzymes change the food, therefore slows spoilage and ripening, inhibits sprouting. Food is irradiated to prevent the spread of invasive pest species through trade in fresh vegetables and fruits, either within countries, or trade across international boundaries. Pests such as insects could be transported to new habitats through trade in fresh produce which could affect agricultural production and the environment were they to establish themselves; this "phytosanitary irradiation" aims to render any hitch-hiking pest incapable of breeding. The pests are sterilized. In general, the higher doses required to destroy pests such as insects, mites and butterflies either affect the look or taste, or cannot be tolerated by fresh produce.
Low dosage treatments enables trade across quarantine boundaries and may help reduce spoilage. Irradiation reduces the risk of infection and spoilage, does not make food radioactive, the food is shown to be safe, but it does cause chemical reactions that alter the food and therefore alters the chemical makeup, nutritional content, the sensory qualities of the food; some of the potential secondary impacts of irradiation are hypothetical, while others are demonstrated. These effects include cumulative impacts to pathogens and the environment due to the reduction of food quality, the transportation and storage of radioactive goods, destruction of pathogens, changes in the way we relate to food and how irradiation changes the food production and shipping industries; the radiation source supplies energetic waves. As these waves/particles pass through a target material they collide with other particles. Around the sites of these collisions chemical bonds are broken; these radicals cause further chemical changes by bonding with and or stripping particles from nearby molecules.
When collisions damage DNA or RNA, effective reproduction becomes unlike