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Atomic Physics Glossary

Absolute Zero The lowest possible temperature. Absolute zero is the temperature at which there is no molecular motion. This is 0 degrees on the Kelvin temperature scale. It is equal to -273° Celsius and -459° Fahrenheit.

Absorb To take in the kinetic energy of a particle or of electromagnetic radiation. To merge a particle or photon with an atom, nucleus, or other particle. This may cause the first particle or photon to disappear, but cause a secondary photon or particle that has a lower level of energy to appear. Also, to reduce the number of particles in a beam as it passes through a substance.

Absorbed Dose The amount of energy that has been absorbed per unit of mass of a material from an ionizing radiation. The standard unit for absorbed dose is the gray. See Gray.

Absorber Any material that stops ionizing radiation. Material in a nuclear reactor that will absorb radiation. A sheet of paper will stop alpha particles and most beta particles. Lead, steel, and concrete will diminish the intensity of gamma rays. See Shielding.

Absorber Rod Another name for control rod. See Control Rod.

Accelerator An apparatus that imparts high energy to subatomic particles by electric or magnetic forces, and then directs a beam of these high speed particles to a target. The collision of these charged particles, such as electrons, protons, or heavy ions, with the target produces other subatomic particles that pass into detectors. The information gathered in the detectors helps physicists to understand the particles' properties and interactions. The two basic designs of particle accelerators are linear (linac) and circular (synchrotron). The longer the linac is, the greater is the energy of the particles that it produces. A synchrotron generates high-energy particles by circulating them many times before they are directed toward their target. See Cyclotron.

Actinide Any of the series of chemically similar and largely man-made radioactive elements range in atomic number from 89 to 103. They are 89 actinium (Ac), 90 thorium (Th), 91 protactinium (Pa), 92 uranium (U), 93 neptunium (Np), 94 plutonium (Pu), 95 americium (Am), 96 curium (Cm), 97 berkelium (Bk), 98 californium (Cf), 99 einsteinium (Es), 100 fermium (Fm), 101 mendelevium (Md), 102 nobelium (No), and 103 lawrencium (Lr).

Actinium Decay Series The natural radioactive decay series of uranium-235 (also called actinouranium), actually beginning with plutonium-239 and ending with lead-207. The series is as follows: Pu-239-U-235-Th-231-Pa-231-Ac-227 -Th-227-Ra-223-Rn-219-Po-215-Pb-211-Bi-211-Ti-207-Pb-207. See also Uranium Decay Series.

Activation The process of making a material radioactive by bombarding it with neutrons, protons, or other radiation.

Activation Product Any nuclide formed as the result of activation. A radioactive isotope of an element that has been formed in a nuclear reactor by created by neutron bombardment. The term usually applies to elements with atomic numbers that are greater than uranium (93). See also Transuranic Element.

Active Fuel Length (AFL) The overall length of the fuel material within the fuel element.

Activity Radioactivity. Also, the rate of disintegration or decay of radioactive material. That is, the number of disintegrations of a radioactive source per unit of time. Activity is expressed in becquerels. See Bequerel.

Alpha Particle A positively charged particle that consists of two protons and two neutrons, which is emitted by certain radioactive sources. Alpha particles can only travel a few inches and lose their energy on impact. The term is used for a 4He nucleus, a helium isotope emitted during radioactive decay or in a nuclear reaction.

Alpha Decay The process of radioactive decay in which the nucleus emits an alpha particle. Alpha particle A small, positively charged particle of very high velocity that is emitted during the radioactive decay of certain radioactive materials, including uranium, plutonium, and radium. The particle is made up of two neutrons and two protons and is identical to the nucleus of the helium-4 isotope. Alpha particles are the least penetrating of the three common forms of radiation and are not dangerous to living things, unless inhaled, ingested, or brought into contact with the lens of the eye. However, isotopes that emit alpha particles within the body can be damaging.

AngstromA unit of measurement for the length of very small things, such as atoms, molecules, wavelengths of light, etc. One angstrom is equal to one × 10-10 meter.

Annihilation The spontaneous process in which a particle and its antiparticle combine and annihilate, converting matter into energy. For example, an electron and a positron combine to form two gamma rays, each of which has energy equivalent to the rest mass of the electron (511 KeV).

In fact, the energy released in annihilation is carried by a force carrier (e.g., a fundamental boson, such as a gluon, photon or Z particle) that can decay into other particles and antiparticles. During any process, the number of particles plus antiparticles of a type are conserved.

Antimatter Matter made up of antiparticles. See Antiparticle.

Antiparticle A particle whose mass is identical to its normal-matter counterpart, but whose electric charge and other properties are the opposite. For example, the positron is the antiparticle of an electron. It has a positive charge instead of a negative charge. When a positron and an electron collide, both annihilate and energy is released. In particle physics, every particle that is a fermion and has an electrical charge has a corresponding antiparticle of identical mass, but opposite electrical charge and other characteristics. Bosons also have antiparticles, except for those that have a zero value for electrical charge. In such cases, there is no difference between the particle and the antiparticle.

Antiquark The antiparticle of a quark. See Quark.

Argon (Ar) A naturally occurring element that can be found in small concentration in the Earth's atmosphere. Argon is inert and will not react with other elements. Consequently, it is used as to replace air in insulators. Argon has an atomic number of 18 and atomic weight of 39.948. Argon's name comes from the Greek word for no reaction, argos.

As Low As Reasonably Achievable (ALARA) The goal of radiation protection - to keep exposures of radiation to levels that are well below the limits, taking into account the state of technology, the expense for improvement, benefits to public health and safety, and other socioeconomic considerations. ALARA is achieved by minimizing exposure times, maximizing shielding, and maximizing distance from the radioactive material.

Atom The smallest particle of an element that retains the characteristics of that element. It consists of a central nucleus and one or more electrons moving in orbits around the nucleus at relatively great distances. The nucleus, which accounts for most of the atom's mass, carries a positive electric charge and consists of a specific number of protons, neutrons, and other particles (mesons). The number of electrons orbiting the nucleus is normally the same as the number of protons in the nucleus. As a result, the electric charges cancel each other out, causing the atom to be neutral. The word "atom" comes from atomos, a Greek word for indivisible. See also Ionization.

Atomic Relating to an atom.

Atomic Bomb An explosive device from which enormous energy is released by the fission of the nuclei of heavy elements, such as uranium-235 or plutonium-239, and/or fusion. Even small nuclear weapons are more powerful than all but the largest of conventional explosives.

Atomic Energy The energy released by a nuclear reaction. In particular, the term refers to the energy released when the nucleus of a heavy atom is split into smaller particles (fission), or when two or more nuclei join together (fusion). In either case, a portion of the nucleus changes into energy. Atomic energy should be more correctly called nuclear energy. See also Fission, Fusion.

Atomic Energy Commission (AEC) The Federal agency created in 1946 to manage and control the development and use of military and civilian applications of nuclear energy. It was replaced in 1975 by the Energy Research and Development Administration and the Nuclear Regulatory Commission, which forms part of the Department of Energy.

Atomic Mass The total number of protons and neutrons in the nucleus of an atom. This is the total number of nucleons in the atom. The atomic mass is generally used to identify different nuclides of the same element. For example, Tritium contains one proton and two neutrons and is represented by the symbol 3H. Although tritium's atomic number of 1, its weight is triple that of the common form of hydrogen.

Atomic Nucleus See Nucleus.

Atomic Number The number of protons in the nucleus of an atom. This is what determines the chemical properties of an element. Each element has a different atomic number. Elements are listed by atomic number in the periodic table. The atomic number of hydrogen is 1. It is 6 for Carbon, 8 for Oxygen, and 92 for Uranium. The number of electrons in an atom is the same as the number of protons in the nucleus. See also Ion.

Atomic Pile Another term for nuclear reactor. See Nuclear Reactor.

Atomic Theory The theory of the matter of nature. The atomic theory holds that all matter is composed of tiny, indivisible particles called atoms than have a positive nucleus and are surrounded by electrons, and that atoms can combine to form molecules. Atoms can't be divided by chemical means, but can be split using nuclear means.

Atomic Weight A measure of the mass of an atom of a particular element. The atomic weight is the average weight of the atoms of all known forms of an element. It is expressed by a scale in which the atomic weight of carbon-12 is exactly 12.000 atomic mass units (AMUs). It is commonly (closely) approximated by the sum of the protons and neutrons in the nucleus of the atom of the element. For example, the atomic weight of oxygen is approximated by 16, although it is actually 15.944 AMU's. See also Neutron, Proton.

Atom Smasher A colloquial term for a particle accelerator. See Accelerator.

Attenuate To reduce or diminish the intensity of radiation by absorbing it or scattering it as a result of passing it through a material. The radiation may be absorbed by molecules, atoms, or nuclei.

Background Radiation The natural radiation present in our environment and that can interfere with low count measurements. This consists of cosmic rays and naturally radioactive elements in the soil, water and air, such as radon and its decay products. It also includes global fallout that exists in the environment as a result of testing of nuclear explosive devices. Background radiation varies with location and time, but can never be eliminated. Other sources of background radiation are radiation due to contamination of samples, incomplete absorption of radiation in the detector, or incomplete charge collection by the detector. It does not include radiation from source material or by-product material.

Barrier See Shielding.

Baryon A massive, composite hadron that consists of three quarks. The most common hadrons are protons and neutrons. Antiprotons and antineutrons are antibaryons.

Becquerel (Bq) An international unit of measurement of radioactive activity. material. One Bq is equal to one disintegration per second. In practice, GBq or TBq are more commonly used than Bq. In some fields, the older unit, the curie, is still used. 1 Ci = 3.7 × 1010 Bq. See also Curie.

Beta Decay The process of radioactive decay in which a neutron becomes a proton, emitting a beta particle (electron) and an antineutrino in the process. When this occurs, the charge of the nucleus increases by one. The atomic number changes, but the atomic mass remains the same. This is the most common form of radioactive decay and is sometimes termed beta minus or beta minus emission.

Alternatively, a proton can become a neutron, emitting a beta plus particle (a high-speed positron). A neutrino is emitted in the process. This results in the nucleus converting a proton into a neutron. The nucleus absorbs an orbital electron. The atomic number decreases by one. This second reaction is rare among naturally occurring isotopes.

Beta Particle A particle emitted by the nucleus of an isotope during radioactive decay. Beta particles emerge at high speeds, sometimes approaching the speed of light. They are more penetrating than alpha particles, but less penetrating than gamma rays or x-rays. Beta particles may be either high-energy electrons (with negative charge) or positrons. They have a short range in air. See also Alpha particle Gamma ray.

Binding Energy The energy associated with the binding of two or more components in a physical system by an attractive force. (e.g., the protons and neutrons in a nucleus). It is also the energy released by the system when the bonds are broken, either by an external agent or an intrinsic instability of the system. For an atom, the binding energy is the energy that would be released if the nucleus is separated into its constituent parts. Similarly, the term describes the energy required to separate a molecule, atom, or nucleus into its constituent particles. Binding energy is also termed Separation Energy.

Biological Shield A radiation shield that surrounds a nuclear reactor or other object that produces radiation, especially gamma rays or neutrons, in order to protect humans. The shield may consist of thick concrete walls, lead plates, or some other or absorbing material. See also Nuclear Reactor.

Boiling Water Reactor (BWR) A common design of light water reactor (LWR), in which coolant water flows to the core where it is heated and permitted to boil. The steam that is so produced in the reactor vessel is passed through a heat exchanger where it is condenses to water. It is then returned to the reactor core.

Bombardment The act of subjecting atomic nuclei to the impact of high-energy particles, such as electrons or alpha particles, in order to split them and form new elements. See Alpha Particle.

Boson Any particle that has a spin of an integer number ( 0,1 or 2...) of quantum units of angular momentum. It is typically a messenger particle. The particles associated with all fundamental forces) are bosons. W- and Z-bosons are the elementary particles that mediate the weak nuclear force. Gluons and mesons are also bosons. All particles are either fermions or bosons.

Breeder Reactor A nuclear reactor that produce more fissile material (fuel) than it consumes. It uses an unfissionable material, such as thorium or the widely available, nonfissionable uranium isotope U-238, together with small amounts of fissionable U-235, to produce a fissionable plutonium, Pu-239. In breeder reactors, the fuel is surrounded by a fertile (non-fissile) material that is converted to a fissile material, when irradiated. Consequently, breeder reactors are sometimes called converter reactors. See also Fast Neutron Reactor.

Bremsstrahlung The electromagnetic radiation produced by the acceleration of a charged particle, such as an electron, when deflected by another charged particle, such as an atomic nucleus. The term is also used for the process of producing the radiation. Consequently, bremsstrahlung radiation is present during all beta decay processes. Similarly, it is a phenomenon present during the production of x-rays or can be used in reference to the x-rays themselves. In the production of x-rays, a high speed electron traveling in a material is slowed or completely stopped by the force of an atom that it meets. The word, bremsstrahlung, is German for breaking radiation.

Bubble Chamber A vessel filled with a superheated transparent liquid to detect electrically charged particles that move through it. The particles leave a trail of bubbles that may be photographed. A bubble chamber is similar to a cloud chamber in application and principle. See Cloud Chamber.

Burnup A measure of the energy obtained from the fuel of a nuclear reactor. Burnup is defined by the percentage of fuel used up or the ratio of the megawatt days obtained per metric ton of initial fissile material.

CANDU Acronym for CANada Deuterium Uranium. This is a Canadian designed reactor that uses natural uranium as fuel and heavy water as a moderator and coolant. In a deuterium-oxide-moderated reactor, fuel made from natural uranium, U-235, can sustain a chain reaction. CANDU reactors are in service in Canada, Pakistan, and Argentina. See also Heavy Water.

Carbon-14 (C) A radioactive isotope of carbon that has six protons and eight neutrons in its nucleus, rather than the six protons and six neutrons of carbon-12, the stable isotope that accounts for almost 99% pf carbon found in nature. High-energy neutrons, produced by cosmic ray interaction in the upper atmosphere, react with 14N to form 14C. In turn, 14C mixes with carbon in the lower atmosphere, making 14C available naturally. 14C. has a half-life of 5,730 years.

Carbon-14 Dating An isotope dating technique that is based on the rate of decay of carbon-14 to nitrogen-14. The 5,730-year half-life of carbon-14 facilitates the estimation of the ages of rocks containing carbon-14 that are between 100 and 100,000 years old. See also Potassium-Argon Dating.

Cation An ion with a positive charge, such as Na+ or NH+. See Ion.

Celsius (Centigrade) Temperature Scale A temperature scale similar to the Kelvin, but shifted by 273 degrees. Absolute zero is -273°C. The freezing and boiling points of water are 0° and 100° respectively on the Celsius scale versus 273° and 373° on the Kelvin scale. To convert a temperature from Celsius to Fahrenheit, subtract 32 from the Fahrenheit value and then multiply the remainder by 5/9.

CERN The major European accelerator laboratory. Originally known as Centre European pour Recherche Nucleaire, it is now called the European Organization for Nuclear Research. CERN is located near Geneva, Switzerland. and is probably best known as the place where the WWW was created.

Cesium (Cs) A soft silver-white ductile metal that is a co-product in the processing of titanium, beryllium, or lithium. It has a melting point of 28.5ş F and ignites if exposed to air. It has an atomic number of 55 and atomic mass of 133. Cesium is an element that is chemically similar to calcium. An isotope, cesium-137, is a radioactive product created by the fission of uranium atoms. Its half-life of 30 years makes it dangerous for a long time.

Chain Reaction A series of nuclear fission reactions that are self-sustaining. A reaction in which a nucleus that is split by a neutron releases energy and one or more neutrons and these neutrons, in turn, split other nuclei, releasing more energy and more neutrons, and so on. The reaction induces further reactions. As a result, the reaction is self-sustaining. Nuclear reactors and nuclear weapons require such chain reactions in order to function.

Charged Particle Another term for ion. See Ion.

Chemical Compound See Compound.

Chemical Separation Another term for reprocessing. See Reprocessing.

Cherenkov Radiation A characteristic, eerie, blue glow seen around strong radioactive sources, such as irradiated fuel sources stored under water. Cherenkov radiation is electromagnetic radiation emitted that is when a charged particle passes through a transparent medium, such as water, at a speed greater than that of light in the medium. Although the speed of light in a vacuum is the fastest speed that any particle or light can have, light travels more slowly in any medium because of its interactions with the electric fields of the atoms in the medium. Therefore, it is possible for a high-energy particle to be having a greater velocity than light in some mediums. Cherenkov, also spelled Cerenkov was the scientist who first recognized the nature of this effect.

Cladding The metal tubes that contain the pellets of a fissile nuclear fuel element in the core of a nuclear reactor. Cladding provides structural support and protection for the fuel. It also maintains a separation between the fuel and the coolant. See also Zircaloy.

Cloud Chamber A sealed, glass-domed chamber containing supercooled, supersaturated water vapor that is used to detect and measure particles of ionizing radiation. When certain types of atomic particles pass through the chamber, they leave a cloud-like track (vapor trail). This enables scientists to see and photograph the visible track of these particles. The cloud chamber is known also as the Wilson chamber. See also Bubble Chamber.

Cobalt-60 An artificially produced radioactive isotope of the naturally occurring cobalt-59. Cobalt is a hard ferromagnetic silver-white element. It is not found as a free metal or mined separately, but is usually produced as a by-product of nickel and copper mining activities. Cobalt has an atomic number of 27. Its isotopes range in atomic weight from 50 amu's to 73 amu's.

Cobalt-60 is an important source of gamma radiation and has many uses as a gamma ray source. It is an important radioactive tracer and is also used in radiotherapy. In addition, it is used to sterilize foods (irradiation) and to detect structural flaws in metal. Cobalt-60 has a half-life of 5.2714 years. It is the most stable of the 22 Cobalt isotopes, most of which have half-lives of less one second.

Coffin A thick-walled shipping container (usually lead) used to transport spent fuel rods. Coffins are heavily shielded with lead and designed to withstand impacts during transportation.

Cold A term for not radioactive.

Collider An accelerator that brings two beams of high-energy particles traveling in opposite directions into collision with each other for purposes of research, rather than into collision with a fixed target. See also Accelerator.

Collimation The act of making parallel. The act of aligning the direction of a beam of photons, so that it can be accurately directed at a particular target.

Color The basic characteristic of a quark that determines its role in the strong interaction and differentiates it from the other varieties of quarks. See also Quark.

Compound Any chemical combination of two or more elements that have been combined in specific and constant proportions. The physical characteristics of a compound differ from those of its constituent elements. Water (hydrogen and oxygen) and carbon dioxide (carbon and oxygen) are two examples of (chemical) compounds.

Compton Effect See Compton Scattering.

Confinement The phenomenon that quarks cannot be isolated. They are never found separately. They are confined by their strong interactions to form pairs or triplets.

Conservation of Energy A principle of science, which was formulated during the nineteenth century. It states that energy (or its equivalent in mass) cannot be created or destroyed, but only changed into other forms of energy. This was modified as a result of Einstein's E = mc², which indicates that matter and energy are interchangeable.

Contamination Deposition of radioactive material in any place where it is not desired, particularly if it will be harmful.

Control Rods Long rods used that are inserted or withdrawn, wholly or partly, from the core of a nuclear reactor in order to control the rate of nuclear reactivity.

The reactor functions by the splitting of nuclei of nuclear fuel by neutrons. Control rods contain strong neutron absorbing material. Therefore, inserting the rods into the reactor causes more neutrons to be absorbed and slows the chain reaction. Withdrawing the rods reduces their absorption of the neutrons produced in the process of fission and accelerates the chain reaction in the reactor.

Shim rods are used for large changes to the rate of reactivity, regulator rods are used for fine adjustments, and safety rods are used to rapidly decrease the chain reactivity in the reactor in the case of an accident. See also Nuclear Reactor.

Coolant The liquid or gas that is circulated through the core of a nuclear reactor to transfer heat from the core to steam generators or directly to turbines. Coolants can also serve as neutron moderators. See also Nuclear Reactor.

Cooldown The cooling of the fuel rods following the shutdown of a nuclear reactor. This is achieved by circulating a coolant through the reactor core.

Core That part of a nuclear reactor that contains the fuel elements and any moderator. Energy is released as a result of the fission that takes place in the nuclear reactor's core. A material to reflect stray neutrons back to the fuel usually surrounds the core. See Nuclear Reactor.

Cosmic Rays A term that describes any high-energy particles (e.g., protons, helium nuclei, heavy atomic nuclei, and leptons) and radiation (e.g., x-rays and gamma rays) that bombard the Earth from all directions at speeds nearly equal to that of light. Some particles originate in the Sun, but most originate beyond the solar system.

Covalent Bond A chemical bond in which one or more electron pairs in the outer shell of atoms are shared equally by two or more atoms in order to achieve chemical stability (a net charge of zero) under the octet rule. The bond may consist of a pair of electrons (single bond), two pairs of electrons (double bond), or three pairs of electrons (triple bond). Atoms tend to share electrons in such a way as to fill their outer shells. Covalent bonds are normally stronger than other bonds.

Critical At, or of, a point at which a system may support a sustained fission chain reaction. See Criticality.

Critical Assembly An arrangement of fissionable material sufficient to be achieve criticality. This normally is used in reference to a reactor that is operated at low power in order to confirm its physical characteristics.

Criticality The state at which a nuclear reactor is just capable of sustaining a chain reaction or conditions necessary to do so. A reaction has achieved criticality if it is generating neutrons sufficient to maintain the reaction at that rate. If it generates fewer neutrons, the state is subcritical.

Critical Mass The least amount of fissile material that will support a self-sustaining chain reaction. If there is too little fuel, or it is not in the proper form, too many neutrons will stray and the reaction will sustain itself.

Curie (Ci) The basic unit of measurement of intensity of radioactivity in a material. A curie is defined as 3.7x1010 disintegrations per second (i.e., 37 billion atoms breaking up each second). Originally a curie was defined as the radon emitted by one gram of radium 226. The name, curie, comes from Pierre and Marie Curie, who discovered radium, radon, and polonium. See also Becquerel.

Cyclotron A circular particle accelerator. Atomic particles are whirled around in a spiral between the ends of a huge magnet in this machine. Their speed increases with each rotation until the particles are finally directed in a beam to the target material. The machine is used for research in which atoms are bombarded by high-speed particles. See also Accelerator.

Daughter Isotope An isotope formed by the radioactive decay of a parent isotope. A daughter isotope may or may not be the same element as its parent. It may be in a permanently stable state or, if radioactive, break down further. If it does decay, it will become the parent of a new daughter isotope. However, the last daughter that will be formed by this process will be stable (not radioactive). Daughter and daughter product are other names for a daughter isotope.

Decay (Radioactive) The spontaneous disintegration of the nucleus of a radioisotope into two or more different particles and the simultaneous emission of alpha or beta and/or gamma rays. The end product is a less energetic, more stable nucleus. Each decay process has a definite half-life. The sum of the masses of the new particles is always less than the mass of the particle that disintegrated. For example, when an atom of polonium decays, an atom of lead is formed and an alpha particle is emitted. See also Half Life.

Decay Product An isotope that the decay of an unstable atom has produced.

Decontamination The reduction or removal of unwanted radioactive contamination by washing, chemical action, or mechanical action.

Depleted Uranium Uranium in which the U-235 content is less than the 0.711% concentration that occurs in nature. The concentration of U-235 is normally 0.25%-0.30% as a result of the enrichment process, which stripped it of much of the U-235 that it once contained. The remaining uranium is U-238. It can be blended with highly enriched uranium (e.g., from weapons) to bring the content up for use as reactor fuel.

Deuterium (2H) A stable isotope of hydrogen that occurs naturally in nature in a ratio of one atom of deuterium to 7,000 atoms of normal hydrogen, 1H. Deuterium is lighter than tritium, but twice as heavy as ordinary hydrogen. Hence, it is often called heavy hydrogen. Regular hydrogen has a single proton and no neutrons as its nucleus, but the nucleus of deuterium contains one proton and one neutron. A deuterium nucleus is often called a deuteron. Deuterium is usually found in the form of heavy water.

Deuterium Oxide (D2O) The chemical name for heavy water. See Heavy Water.

Deuteron (d) A positively charged particle consisting of a proton and a neutron, equivalent to the nucleus of a deuterium atom. The nucleus of the 2H (deuterium) nucleus. The mass of a deuteron is 3.3435860(20)x10-27 kg. See Deuterium.

Dirac Equation Paul Dirac's mathematical description of the electron. It makes use of both quantum mechanics and special relativity.

Dose A quantity of ionizing radiation or energy absorption in a particular substance.

Dosimeter (Dose Meter) A small, pocket-sized instrument used to monitor the exposure of a person to harmful radiation.

Electric charge The property of a particle that causes it to attract (or repel) other particles that have opposite (or similar) charges.

Electromagnetic Force The force of attraction between oppositely charged particles or of repulsion between similarly charged particles. Also, the force of attraction between opposite magnetic poles and repulsion between similar poles. The electromagnetic force is one of the four fundamental forces of nature. The other three are gravity, the weak force, and the strong force.

Electromagnetic Radiation Radiation that consists of electric and magnetic waves that travel at the speed of light. Examples include light, radio waves, gamma rays, x-rays.

Electromagnetic Spectrum The entire range of frequencies of electromagnetic radiation, from the lowest to the highest. This encompasses gamma waves that have a wavelength of only 0.001 angstroms to very long radio waves and includes infrared light, ultraviolet light, visible light, X-ray, microwaves, and cosmic ray waves.

Electron (e) The smallest atomic particle and the lightest component of matter. An electron's mass is 0.00091x10^(-27) kg, It takes 1,836 electrons to equal the mass of a proton. It is a negatively charged subatomic particle that orbits the nucleus of an atom and exists free in space and in stars. It is bound to the nucleus of the atom by electromagnetic forces. The electron is the smallest charged particle and the fundamental charge of electricity. Electrons convey the electric current to our computers and all other lamps, motors, appliances, etc. See also Beta Particle, Positron, Proton, Neutron.

Electron Capture A radioactive decay process in which an orbital electron is absorbed by the nucleus, converting a proton into a neutron and reducing the atomic number by one. Also called electron capture decay. See Beta Decay.

Electron Shells The various layers surrounding the nucleus of an atom in which the electrons orbit. The quantum principle determines the radius of these shells and the Pauli exclusion principle the number of electrons.

Electronic Structure The arrangement of electrons within an atom, molecule, or other body. The placement of electrons within orbits.

Electron Volt (EV) A unit of energy that is equal to the energy acquired by an electron accelerating through a potential difference of one volt in a vacuum. It is (1.602 ) times (10 to the -19th power) joules. The electron volt is used to express the energy of nuclear processes at the atomic level. However, because it is so small, the terms KeV (kilo-thousand), MeV (mega-million), GeV (giga-billion), and TeV (trillion eV) electron volts are often used.

Element A basic substance of matter that cannot be divided into simpler substances by heating, cooling, or chemical means. It consists of a family of naturally occurring isotopes, all of which have the same atomic number and similar chemical properties. Metals comprise three quarters of all elements. Examples of elements include hydrogen, oxygen, carbon, iron, and gold. In contrast, neither water nor salt are elements because they contain two or more atoms having different atomic numbers. There are 115 known elements on Earth, of which 92 are naturally occurring. Each element is represented by different one- or two-letter abbreviations. See also atom, atomic number. See also Atomic Number, Nuclide.

Elementary Particle Any particle that cannot be subdivided into smaller units. A fundamental component of matter. A particle of which other larger particles are composed. For example, atoms are made up of electrons, protons, and neutrons.

The name, elementary particles, was given to protons, neutrons, and electrons before it was discovered that they, in turn, are made up of more elementary particles, which are known as quarks. Today, elementary particle or fundamental particle is used for the six types of quarks and the six leptons and their antiparticles. Gluons, photons and W and Z bosons are also fundamental particles or elementary particles. All other particles are made from combinations of elementary particles. All subatomic particles also exhibit aspects of wavelike behavior.

Enriched Uranium Uranium in which the proportion of U-235 (to U-238) has been increased above the 0.711% that is found in nature. Weapons-grade uranium is more than 90% U-235.

Enrichment The physical process of increasing the proportion of U-235 relative to U-238, which predominates in nature. This is done in the United States by means of the gaseous diffusion process. See Gaseous Diffusion.

Erg A unit of energy that is equal to one dyne-centimeter or 10-7 joules. It takes approximately 13.6 million ergs to equal one foot-pound.

Excited State The state of an atom or nucleus when it has absorbed energy and possesses more than usual. The excess energy is normally emitted as a gamma ray.

Exotic Atom An atom containing one or more an unusual constituents. For example, an electron may have been replaced by a negatively charged fundamental particle. Alternatively, the nucleus may have been replaced by a positively-charged fundamental particle.

Fast Breeder Reactor (FBR) A fast neutron reactor that produces more fissile material than it consumes.

The most common breeding reaction is the conversion of non-fissionable U-238 to plutonium-239. The latter is far more abundant than fissionable U-235 and can be converted into Pu-239 efficiently by the neutrons from a fission chain reaction. No moderator is used in the breeder reactor since fast neutrons are more efficient in converting U-238 to Pu-239. The cooling and transfer of heat is accomplished by a liquid metal, usually liquid sodium or lithium. Use of water would slow the neutrons.

The reactor core consists of thousands of stainless steel tubes that contain a mixture of uranium and plutonium oxides. These are surrounded by a so-called breeder blanket that consists of tubes filled only with uranium oxide. The energy from the nuclear fission heats the sodium, which transfers the energy to a second sodium loop. The second loop heats water to produce steam for generation of electricity.

Fast Fission Fission brought about by the use of fast neutrons.

Fast Neutron A free neutron that has a kinetic energy approaching 1 MeV and a speed of 14,000 kms/second. Fast neutrons are neutrons that have been produced in a nuclear reactor and not yet slowed by a moderator.

Fast Neutron Reactor (FNR) A reactor in which the reaction is sustained primarily by fast neutrons. It uses little or no moderator to slow the neutrons. Consequently, it must use fuel that is relatively rich in fissile material.

Feedwater The water used to remove heat from the fuel rods of a reactor by boiling and becoming steam. The steam is used to drive the turbine and, hence, the generator.

Fermion Any of several elementary particle, such as a proton or neutron, that have half-integral spins (e.g., (1/2,3/2,5/2...). Fermions follow the Pauli exclusion principle, which says that no two fermions can exist in an atom in the same quantum state. This restricts the number of electrons, which are also fermions, in each electron shell. Electrons, protons, and neutrons are all fermions, as are all the fundamental matter particles, both quarks and leptons. Electrons, protons, and neutrons are fermions, as are all quarks and leptons.

Fertile Capable of becoming fissile, by capturing neutrons. An example is provided by 238U. Its capture of a neutron produces 239U, which then decays to 239Pu, which has a large fission cross-section.

Film Badge A small badge that is worn by personnel to monitor radiation exposure. It consists of masked photographic film that darkens on exposure to Gamma rays and beta particles. The extent to which the film darkens is a measure of the dosage of radiation received.

Fissile (of an isotope) Any nucleus that is capable of being split by a slow-energy neutron. The most common fissile isotopes are uranium-235 and plutonium-239.

Fission The splitting of the nucleus of a heavy atom, like uranium or plutonium, into two roughly equal parts, which are the nuclei of lighter elements. This is accompanied by the release of a large amount of energy and usually one or more neutrons. The fission may be spontaneous radioactive decay for some nuclides. However, it is usually is the result of a nucleus being bombarded by neutrons and absorbing one or more of them.

The energy appears in the form of kinetic energy of the two new, lighter nuclei and the emission of gamma rays and neutrons. The energy released as heat can be used to produce heat and electricity. Fission, also termed nuclear fission, occurs only with heavy elements, such as uranium and plutonium. It is used in nuclear power plants and is the basis of the atomic bomb. Toxic radioactive waste is also produced by the nuclear fission. See also nuclear fusion.

Fissionable Capable of undergoing nuclear fission. Any nucleus that is capable of undergoing fission as a result of any nuclear reaction. The property of the nuclei of some elements that permits them to split into smaller atoms. Cobalt-60 is fissionable, as is plutonium.

Fission Gases Products of fission that exist in the gaseous state. Krypton and Xenon are the main fission gases. The products of the fission of uranium in the nuclear reactor include nine stable and unstable isotopes of xenon (including xenon-135) and 17, mostly unstable, isotopes of krypton (including krypton-85).

Fission Neutron A neutron that was emitted from a nuclide as a result of fission.

Fission Products The nuclei (fission fragments) formed by the fission of heavy elements. The great variety of smaller atoms that are left over by splitting uranium and plutonium. Most of these are highly radioactive and decay into other isotopes. More than 200 isotopes of 35 elements are in this category. They include strontium-90 and cesium-130. Most of the fission products can be found in spent nuclear fuel and high-level waste.

Flavor The name that distinguishes the different types of quarks (up, down, strange, charm, bottom, top) and different types of lepton types (electron, muon, tau). Each charged lepton flavor has a corresponding neutrino flavor. Each flavor of quark and charged lepton has a different mass.

Flux The rate of flow of particles through a given surface. It is measured as the number of particles per unit of surface area per unit time.

Frequency The number of cycles or wave oscillations per second, or the number of wave crests passing a fixed point in one second or another specific period. Frequency is measured in hertz (Hz).

Fuel Assembly An assembly of fuel rods that are inserted into a nuclear reactor to form its core. Each assembly may consist of 60 to 300 fuel rods. Also called fuel element.

Fuel Rod A long, hollow cylinder made of a zirconium alloy and containing uranium oxide pellets. The building block of nuclear fuel. Also called fuel pin.

Fundamental Particle An elementary particle from which all other particles are made. Fundamental particles have no internal structure. They include photons, gluons, leptons, quarks, W-bosons, and Z-bosons.

Fusion A nuclear process in which the atomic nuclei of certain lighter elements, especially the isotopes of hydrogen (deuterium or tritium), combine to form a heavier nucleus under conditions of extreme heat (millions of degrees). This is accompanied by the release of an enormous amount of energy that is comparable to that realized by nuclear fission. The principal byproduct of nuclear fusion is helium. The fusion produced by high temperatures is also called a thermonuclear reaction. The word, thermonuclear, indicates that the reaction occurs only at very high temperatures. This is the source of energy for stars like our Sun.

Gamma Rays High-energy electromagnetic radiation of very short wavelength (<10 picometers) and very similar to x-rays. They are highly penetrating and potentially dangerous. They are the most energetic form of electromagnetic radiation, followed by x-rays, visible light, infrared radiation, and radio waves. The individual gamma ray quanta are called photons.

Gamma rays are emitted from the nuclei of atoms during the decay of certain radioactive materials (e.g., Cobalt-60) or by collisions of certain elementary particles. They travel in straight lines and at the speed of light and penetrate material easily, but do not make the material radioactive. They have the greatest penetrating power of all radioactive emissions. They require more shielding material, such as lead or steel or concrete, than do alpha or beta particles.

All gamma rays from a particular isotope possess the same energy. Gamma rays can be used in radiation therapy to kill cancer cells. See also Photon, X-Rays.

Gas-Cooled Reactor A nuclear reactor that uses an inert gas that has good thermal conduction as a coolant.

Gaseous Diffusion A process used to make enriched uranium It is a process in which refined natural uranium is fluorinated in order to produce uranium hexafluoride, a gas. Then, the slightly lighter fractions, U-234 and U-235, are separated from the slightly heavier U-238 isotope by the difference in their diffusion rates through semi-permeable membranes. The lighter isotopes diffuse faster through the porous membranes than do heavier isotopes. See Enriched Uranium.

Geiger Counter An instrument used to detect and measure radiation, although it cannot identify the type of radiation. It is a gas ionization detector that detects radioactivity by counting the formation of ions that it produces in a gas-filled tube when ionizing radiation passes through it. It is also called a Geiger-Müller detector.

GeV One giga electron volts. This is equal to one billion electron volts (109.Ev). See Electron Volt.

Giga A prefix that denotes one billion units. For example, a gigawatt is equal to a million kilowatts or 109 watts.

Gluon (g) A boson that carries the strong nuclear force. A particle that holds quarks together. There are eight different types of gluons. They have no mass and are electrically neutral.

Graphite A soft crystalline form of carbon that appears naturally in metamorphic rocks, such as marble, schist, and gneiss. It is used in gas-cooled nuclear reactors as a moderator and absorber of neutrons. It has a very low neutron capture cross section and a moderate scattering cross section.

Gray (Gy) An absorbed dose of energy in the International System of Units that is equal to one joule per kilogram of tissue. One gray is equivalent to 100 rads, a previous standard that it has replaced. See also Rad.

Hadron A strongly interacting, heavy, subatomic particle. A hadron is not a fundamental particle, but is, instead, composed of fermions and bosons. The fermions are called quarks and antiquarks. The bosons are called gluons. The gluons mediate the color force that binds the quarks together. The best-known hadrons are protons and neutrons.

Half-Life The amount of time it takes for one half of the atoms of a radioactive isotope to decay. The disintegrated atoms become daughter isotopes. Each radioactive isotope has a different half-life that is a characteristic of it. The half-lives of various isotopes range from a small fraction of a second to many years. For example, the half-lives of Cobalt-60, Uranium-238, radium-226, and Tritium are 5.333 years, 4.47 years, 1,600 years and 122.33 years respectively. In contrast, the half-life of Radon-222 is 38 seconds. See also Daughter Isotope.

Heavy Hydrogen An isotope of hydrogen, such as deuterium, or tritium. The nucleus of each of these isotopes has a greater weight than does the nucleus of regular hydrogen because they contain one or two neutrons in addition to a proton. See Deuterium, Tritium.

Heavy Ion The atom of a heavy metal, but with fewer (or more) electrons than usual. Alternatively, the ion of a heavy metal.

Heavy Metals Elements that have high atomic weights, such as uranium-238, thorium-232, or radium-226. Some define heavy metals as those metallic elements that have specific gravities of five or more times that of water. In any event, very heavy metallic elements can cause damage to living things at low concentrations. See also Plutonium, Radium, Uranium.

Heavy Water Water containing a much higher concentration of deuterium oxide than occurs naturally. Deuterium replaces hydrogen in most of the water molecules. Heavy water is used as a moderator in some nuclear reactors because it slows neutrons and also has a slow cross-section for absorption of neutrons. See CANDU.

Heavy Water Reactor (HWR) A nuclear reactor that uses heavy water as its moderator. The Canadian CANDU reactor is an example. See CANDU.

Helium (He) The second most abundant element in the universe, helium is an inert gas. It is gaseous at most temperatures, having a boiling point of -268.6ş C. The atom of the common isotope has a nucleus consisting of two protons and two neutrons. This is the same as an alpha particle. Two electrons orbit the nucleus. Helium also has a stable isotope of atomic mass of 3. Helium's chief use is probably in inflating blimps, scientific balloons, balloons, and other inflatables. See Fusion.

Hertz A unit of frequency equal to one cycle (complete wave) per second. The term is named after Heinrich Rudolph Hertz, who was born in 1887 and won a Nobel prize in 1925.

High-Energy Physics The branch of physics that studies subatomic particles and their interactions.

High-Level Wastes The highly radioactive spent nuclear fuel or solid materials that result from the reprocessing of spent nuclear fuel. It contains various fission products. Also, any liquid wastes produced in reprocessing and solids into which the liquid wastes have been converted. High-level wastes must be handled, stored, and disposed of with great care.

Highly-Enriched Uranium (HEW) Uranium in which the concentration of the isotope, Uranium-235, is increased from the 0.711% concentration in which it occurs in its natural state to a level more than 20% for use as a fuel in isotope production and electrical power generation. In the case of nuclear weapons, uranium is enriched to about 90% U-235.

Hot Radioactive. A hot material is a radioactive material.

Hot-Cell A heavily shielded room in which highly radioactive or contaminated materials can be viewed through shielded windows and handled remotely by electromechanical means to avoid risk to personnel.

Hot Spot An area in which the level of radiation is significantly higher than in the surrounding areas.

Hydrogen (H) The lightest and most abundant element in the universe. Hydrogen is a highly flammable gas at ambient temperatures. It has an atomic number of one and an atomic mass of one. Hydrogen is the first element in the Periodic Table. The hydrogen atom consists of one proton and one electron. Hydrogen appears most commonly on Earth in water. Its name is Greek for forming water.

Deuterium and Tritium are isotopes: They have atomic numbers of one and atomic masses of two and three, respectively. Both deuterium and tritium have been used in nuclear weapons. See also Deuterium, Tritium.

Hyperon Any of several elementary particles having a mass intermediate between that of a neutron and a deuteron.

Induced Radioactivity Radioactivity that has been created in a material, particularly a metal, by radiation or bombarding it with neutrons in a reactor or charged particles from a particle accelerator. For example, by irradiating fluorine-19, the stable form, fluorine-20, an unstable isotope with a half-life of 11 seconds, can be formed.

Inert Gases See Noble Gases.

Intensity The amount of energy transmitted per unit of area per unit of time. For example, the number of photons that strike a specified area of surface in a given time

Inverted Atom An atom consisting completely of antimatter. The nucleus is made up of antiprotons and antineutrons. Positrons orbit the nucleus.

Ion An atom that has lost or gained one or more electrons, thereby acquiring a net positive or negative charge. Also, any molecule or molecular fragment that has so acquired a positive or negative charge.

Ionization The process of forming ions, electrically charged atoms, from neutral atoms or molecules by exposure to ionizing radiation or collision with other atoms or subatomic particles. See also Atom, Ion.

Ionization Chamber An instrument that measures the strength of an ionizing radiation by measuring the electric current created when the radiation ionizes the gas within the chamber. A potential of several hundred thousand volts is created between the chamber wall and an insulated electrode centrally located in the chamber. The ion pairs created in the chamber formed by the radiation are attracted to the positively and negatively charged electrons and measured.

Ionizing Radiation - Radiation that has sufficient energy to eject the electrons from the outer orbits of the atoms of substances through which it passes, thereby creating ions. There are four types of radiation that are capable of breaking chemical bonds. They are: alpha particles (helium nuclei), beta particles (electrons), and gamma rays (high frequency electromagnetic waves) or x-rays (X-rays are generally identical to gamma rays, except for their origin.) Alpha particles are high-speed protons. High-speed neutrons are not ionizing, but their collisions with nuclei lead to the ejection of other particles from nuclei that do cause ionization. In contrast, microwaves, infrared light, and visible light are non-ionizing. Ionizing radiation can damage living tissue.

Ionized A term that describes the status of atoms that have gained or lost electrons, leaving them with electrical charges.

Irradiate To expose to ionizing radiation, often in a nuclear reactor. Targets are often irradiated to produce isotopes.

Isomers Two or more nuclides that have the same numbers of neutrons and protons, but arranged differently. As a result, the nuclides have different half-lives.

Isotone Two nuclides are isotones if they have the same number of neutrons. This differs from isotopes, which have the same number of protons. Helium-4 and Tritium are examples of isotones.

Isotope Another atom of an element that has the same atomic number (number of protons), but a different atomic mass (number of protons and neutrons). Isotopes of a particular element differ only in the numbers of neutrons within the nucleus. It is identical in its electrical and chemical properties, but differs in atomic weight and physical (nuclear) properties.

Elements occurring in Nature are usually mixtures of a common form and one or more uncommon forms (isotopes). Hydrogen and its two isotopes, deuterium and tritium, provides an example, as does carbon-12 and carbon-14, and uranium 235- and uranium 238. Some isotopes are unstable and decay to form isotopes of other elements. For example, unstable isotopes of uranium decay to become lead. The word isotope, meaning at the same place, reflects the fact that all isotopes of an element are located at the same spot on the periodic table

Isotropy The quality or state of having equal physical properties along all lines; being the same in all directions.

Jet A cluster of particles, primarily hadrons, emerging from a decay event or collision and traveling in the same approximate direction.

Kaon (K) A subatomic particle, formerly called the K-meson. A kaon consists of a meson that contains a strange quark and an anti-up (or anti-down) quark, or an anti-strange quark and an up (or down) quark.

K-Capture The capture of an electron from the interior K-shell (orbit) closest to the nucleus of an atom by the nucleus. This usually results in the emission of highly energetic radiation.

Kelvin Temperature Scale A temperature scale commonly used in science, particularly by those in astronomy or who study what happens to things when they become very cold. Zero degrees Kelvin are equal to absolute zero. This is the only temperature scale of the three temperature scales (Fahrenheit, Celsius, Kelvin) in which absolute zero appears as zero on the scale. Water freezes at 273 and boils at 373 on this scale. Convert degrees of Kelvin to degrees Celsius by subtracting 273. See Absolute Zero.

KeV One thousand electron volts. See Electron Volt.

Kinetic Energy The energy of a moving body or a system with respect to the motion of the body, or of a nucleon, nucleus, atom, or other particle in the system. See also Potential Energy. For example, a speeding bullet contains kinetic energy, which will be transferred upon impact to its target.

Lead (Pb) A heavy, soft, gray, malleable, corrosion resistant, metallic element. It has an atomic number 82. There are four stable isotopes of lead. They have atomic weights of 204, 206, 207, and 208 and appear in nature in the following relative concentrations: 1.4%, 24.1%, 22.1%, and 52.4%, respectively. Most lead is obtained from ores: galena, anglesite, cerrusite, and minum. In addition, there are nine unstable isotopes of lead: Pb-202, Pb-203, Pb-204m, Pb-205, Pb-209, Pb-210, Pb-211, Pb-212, and Pb-214. Lead-206 is the stable end point of the Uranium Decay Series. Lead-207 is the stable end point of the Actinium Decay Series.

Lead's high density makes it ideal as a shield against x-ray and gamma ray radiation. However, most lead is used in the production of lead-acid storage batteries, as used in automobiles. See Actinium Decay Series, Uranium Decay series.

Lepton A fundamental matter particle that is susceptible to the weak force, but not to the strong force (that binds atoms together). There are six leptons. The three charged leptons are the electron, the muon, the tau (and their antiparticles). They have a negative electric charge of -1. Three leptons (electron neutrino, muon neutrino, tau neutrino) have no electric charge. In decreasing order of mass (most to least), the six leptons are: tau (tau-minus), muon (muon-minus), tau nutrino, electron, muon neutrino, and electron nutrino.

Light Visible electromagnetic radiation that has a wavelength ranging from 4000 to 7700 angstrom units. Light has a velocity of about 185,300 miles per second. At various times it is considered to be a wave, corpuscular, or quantum phenomenon. Light is also termed luminous energy and radiant energy.

Light Water Ordinary water (H20), rather than deuterium oxide, heavy water.

Light-Water Reactor (LWR) A nuclear reactor that uses ordinary water as a neutron moderator and a coolant. It uses low-enriched uranium as fuel and must be shut down to change the fuel. The LWR is the most commonly used type of power reactor. See also Pressurized Water Reactor.

Liquid Sodium Sodium is a soft silvery metal that that melts at 97.8° C (208° F) and boils at 552.9° C (1,027° F). The stable isotope, Na-23, has an atomic number of 11 and atomic mass of 23. The other isotopes are Na-22 (half life of 2.6 years) and Na-24 (half-life of 14.96 hours). Liquid sodium is used as the coolant and heat transfer medium in the LMFBR reactor (Liquid Metal Fast Breeder Reactor).

Sodium's melting and boiling points bracket the range of operating temperatures for the nuclear reactor. Consequently, liquid sodium does not need to be pressurized, as does a water-steam coolant system. In addition, sodium has a large specific heat, which makes it an efficient heat-transfer fluid. However, liquid sodium must be kept in a sealed system to prevent contact with air or water as it is highly reactive.

Lithium (Li) The third lightest element. Lithium is a silvery and highly reactive metal that has an atomic number of 3. It has two naturally occurring isotopes, lithium-6 and lithium-7. Lithium-7 is the most commonly available one. Lithium-6 targets are irradiated in order to produce tritium, an isotope of hydrogen. See Tritium.

Low-Enriched Uranium (LEW) Uranium in which the concentration of the isotope, Uranium-235, is increased from the 0.711% concentration in which it occurs in its natural state to a level below 20% and usually in the range of 2%-4%. LEU is suitable for use as a fuel in reactors that use natural water as a moderator and coolant.

Low-Level Waste (LLW) Any radioactive material that contains a low level of radioactivity. It is not classified as high-level radioactive waste, spent nuclear fuel, or by-product material.

Mass The bulk quantity of matter in an object. Its inertia or resistance to acceleration. The term, mass, usually refers to the mass of the object at rest. See Rest Mass.

Matter The substance of which the physical objects of the Universe are composed. Matter is made of molecules, atoms, and atomic particles. Matter exists in solid, liquid, gaseous, and plasma states.

Meson A hadron that has been made from an even number of quark, usually one quark and one antiquark.

Metalloids Elements, such as arsenic and selenium, which have both metallic and non-metallic properties. These elements form a sort of dividing line in the periodic chart between metals and non-metals. They include boron, silicon, germanium, arsenic, antimony, tellurium, and polonium. (The atomic numbers of the preceding elements are 5, 14, 32, 33, 51, 52, and 84.) Metalloids are usually semi-conductors rather than conductors.

Metallic Bonding The process in which two or more atoms of electron-donating elements are crowded so closely together that some electrons wander among the nuclei rather than orbiting the nucleus of a single atom. It is the sharing of electrons within a lattice of metal elements.

MeV One million (10 to the 6th power) electron volts. See Electron Volt.

Microcurie (µCi) One millionth (1.0x10-6 curies) of a curie, a unit of measurement of radioactivity. See Curie.

Micron A unit of length equal to one millionth of a meter or 0.00004 inches. A micron is also known as a micrometer or micrometer.

Milling The crushing of stone extracted from a mine to facilitate the removal of valuable minerals from it. The mechanical reduction in size of pieces of ore.

Millicurie One thousandth (0.001) of a curie, a unit of measurement of radioactivity. See Curie.

Millirem A unit of measurement of radiation exposure that is equal to one thousandth of a rem. See REM.

Mixed-Oxide Fuel (MOX) Fuel for a nuclear reactor consisting of a blend of plutonium-239 and natural or depleted uranium. Using plutonium reduces the amount of enriched uranium required.

Moderator A material used in a nuclear reactor to slow fast neutrons in order to increase the probability that they will cause further fission. Slow neutrons are particularly effective in causing fission. Neutrons are slowed when they collide with atoms of light elements, such as hydrogen and carbon. As a result, materials, such as light water (natural water), heavy water, or graphite are commonly used as moderators in nuclear reactors.

Molecule The smallest unit of a chemical compound that can exist independently while retaining the properties of the original substance. It is a chemical combination of two or more tightly linked atoms from one or several elements. For example, a molecule of oxygen consists of two oxygen atoms. A molecule of water consists of two atoms of hydrogen and one atom of oxygen. A molecule is electrically neutral.

Muon A short-lived lepton that has an electric charge of -1. It is 207 times more massive than an electron. See also Lepton.

Nanocurie One billionth (10x10-9) of a curie. See Curie.

Neutrino An elementary subatomic particle that has no electric charge and a negligible mass. It travels at the speed of light. Neutrinos are affected only by the weak nuclear force and gravity and not the strong nuclear and electromagnetic forces. Consequently, they are very difficult to detect.

Neutrinos are leptons, fundamental matter particles that do not participate in strong interactions. They are produced in many nuclear reactions, such as in beta decay. Stars like the Sun produce staggering numbers of neutrinos every second. However, they interact so weakly with other matter that they pass straight through the Earth. The existence of neutrinos has been assumed in order to explain nuclear reactions according to the laws of conservation of energy and mass.

Neutron One of the basic particles in an atomic nucleus (the other is the proton). A neutron has no electrical charge. Its structure is composed of two down quarks and one up quark. Its mass is slightly greater than that of a proton - 1.6749 x 10^(-27) kg) vs. 1.67264 x 10^(-27) kg. Neutrons are constituents of the nuclei of all atoms, except those of hydrogen. The number of neutrons in a nucleus differentiates the various isotopes of an element.

A neutron is the bullet that causes nuclear fission. Uranium and plutonium atoms fission when they absorb neutrons. Consequently, the chain reactions required by nuclear reactors and atomic weapons require neutrons. Although stable within the nucleus of an atom, outside of the nucleus it has a half-life of about 10 minutes and decays into a proton, an electron, and an antineutrino. See Quark.

Neutron Absorption Another term for Neutron Capture. See Neutron Capture.

Neutron Activation The process in which neutrons induce radioactivity in an element. Nuclei capture free electrons, becoming heavier nuclei that are often radioactive and emit radiation.

Neutron Capture The absorption of a neutron by an atomic nucleus. This is the principal method by which very massive nuclei are formed during the violent aftermath of a supernova. Heavy elements are formed not by the fusion of like nuclei, but by the addition of nuclei to existing nuclei. Neutron capture is also called neutron absorption.

Noble Gases The noble gases are any of the inert gaseous elements of the helium group in the Periodic Table. The nuclei of noble gases have completed outer shells of electrons and are thus relatively inert chemically. The noble gases are helium, neon, argon, krypton, xenon, and radon. These elements were previously referred to as inert gases. However, this is not accurate as because several of them do take part in chemical reactions. Another older term for these elements was rare gases. However, They account for 0.93% of the volume of the Earth's atmosphere and 1.29% of its mass.

Non-Ionizing Radiation Electromagnetic radiation, such as radio or television waves, that lacks sufficient energy to eject the electrons in the outer orbits around the nuclei of atoms. Microwaves and visible light are examples of non-ionizing radiation.

Nuclear Energy The energy released by radioactive decay or a nuclear reaction, such as fission or fusion. See Atomic Energy, Fission, Fusion.

Nuclear Fission The disintegration of atomic nuclei in a chain reaction when struck by neutrons. The process of fission releases further neutrons and energy. See Fission, Nuclear Fusion.

Nuclear Force The strong force between two or more nucleons. It affects the binding of nucleons into nuclei. The strong force has a short range.

Nuclear Fuel Fissionable materials that will sustain a self-sustaining chain reaction in a nuclear reactor. The term can also describe the fuel assembly or the pellets of uranium oxide that are contained in the fuel rods.

Nuclear Fusion The fusion of atoms of low atomic weight (e.g., deuterium or tritium, hydrogen isotopes) at high temperatures to create a new, heavier atomic nucleus (e.g., helium) and release energy. Like nuclear fission, nuclear fusion is a thermonuclear reaction in which matter is changed to energy. See Nuclear Fission.

Nuclear Incident An unexpected event, or series of events, leading to the discharge into the environment of dangerous radioactive material in a quantity sufficient to warrant concern for safety and consideration of protective steps.

Nuclear Physics The branch of physics that is concerned with the study of the nuclei of atoms and the interactions of their parts.

Nuclear Radiation Particles (alpha, beta, or neutrons) or gamma rays (photons) that are emitted from the nuclei of radioactive atoms.

Nuclear Reaction Any reaction involving an atomic nucleus. It is usually caused by the bombardment of a nucleus with an atomic or sub-atomic particle or very high-energy radiation. It often results in the splitting of the nucleus or the emission of other particles. Radioactive decay is also a nuclear reaction. All nuclear radiations are ionizing radiations. However, not all ionizing radiations are nuclear reactions.

Nuclear Reactor An atomic furnace. A device in which a sustained fission chain reaction can be maintained and controlled. Most reactors consist of a core that contains the fuel elements, shielding, the moderator, control rods, and a coolant. The core is made of a fissile material such as uranium enriched in the isotope 235U. A material to reflect stray neutrons back to the fuel usually surrounds the core. The coolant is a liquid or gas that circulates through the core to moderate the neutrons and carry the heat away. Fission takes place in the nuclear reactor's core. The nuclei of the fuel split under bombardment by neutrons. This fission produces new neutrons, and hence a chain reaction. This releases large amounts of energy. This energy, which is removed as heat, may be used to make steam for use in generation of electricity. See also Biological Shield, Core, Control Rods, Coolant, Shielding.

Nucleon Any particle, such as proton or neutron, that is a constituent of the nucleus of an atom.

Nucleonics The study of the uses of nuclear energy and radioactivity.

Nucleus The central part of the atom around which electrons orbit. The nucleus contains most of the atom's mass. It is composed of protons and, except for hydrogen, neutrons bound together by the strong nuclear force. The nucleus has a positive electrical charge. (plural: nuclei)

Nuclide A general term used for an atom, or a collection of atoms, that has the same atomic number (number of protons) and atomic mass (sum of the number of protons and neutrons). A radioactive atom is termed a radionuclide.

Octet Rule The scientific law that atoms of all elements require eight electrons in their outermost orbits in order to maintain chemical stability. The two exceptions to this rule are hydrogen and helium.

Orbit The path of an electron around the nucleus of an atom. Also, the path that any object makes around another object while under the influence of a source of centripetal force.

Parent Isotope A radioactive isotope that decays to create a different isotope or element, known as a daughter isotope. The daughter isotope may be stable or radioactive. See also Daughter Isotope.

Particle A subatomic object that has a definite mass and charge. Examples of particles include electrons, protons, and ions. See also Elementary Particle.

Particle Accelerator See Accelerator.

Particle Physics The branch of science concerned with the smallest-known constructions of matter and energy. Also called High-Energy Physics.

Periodic Table A chart of all known chemical elements arranged by atomic number, the number of protons in the nucleus, in a systematic arrangement of columns and rows. Elements in any column have similar properties to others in the same column.

Photon An elementary particle of light. A quantum of electromagnetic radiation, such as light, gamma rays or x-rays. It is a quantity of energy and has no mass or electric charge. A photon is regarded as a stable particle without measurable mass, but possessing momentum, that travels at 300,000 km/s (186,000 miles per second) in a vacuum (the speed of light).

Depending on its frequency (and thus its energy), photons may be known as X-rays or gamma rays. Lower energy forms are known as ultraviolet rays, infrared rays, and radio waves. Electromagnetic radiation exhibits so-called wave-particle duality, behaving waves at times or as particle at other times, depending on the situation. For examples, gamma rays behave as photons in certain processes.

Physics The science that deals with the interactions of matter and energy.

Pico (p) A prefix in the SI system of units that means one trillionth (one million million in the British system) or 10-12.

Picocurie A unit of measure of radioactivity, particularly radon concentration, that is equivalent to one trillionth of a curie (1.0x10-12 curie). This is equal to 0.037 nuclear disintegrations per second. See also Becquerel.

Pile A nuclear reactor. The name, pile, arose because the earliest reactors were constructed by stacking graphite blocks and uranium slugs.

Pion The least massive known spin-0 meson. A subatomic particle that is composed of a quark and an anti-quark and involved in holding the nucleus together. Pions can have electric charges of +1, -1, or 0. See also Quark.

Pitchblende A dark, lustrous heavy mineral that is a source of uranium and radium. It is largely uranium dioxide or trioxide, but does contain lead and trace amounts of thorium, polonium and radium. It is greenish, brownish, or black in color with a pitchy to submetallic luster. It is massive in form and frequently has a grape-cluster appearance. It has a 50%-80% uranium content and a specific gravity of 6-9.

Plasma Any ionized gas. Any extremely hot gas composed of ions (atomic nuclei) and electrons, and possibly some neutral particles. Plasma is often called the "fourth state" because its properties differ from those of solids, liquids, and (normal) gases. The temperature of the gas is too high for complete atoms to exist. Matter consists only of subatomic particles. The Sun is made up of plasma.

Plutonium (Pu) A heavy, silvery, chemically reactive, radioactive, heavier-than-lead metal that is similar to uranium. It is man-made. This fissile element is produced in a nuclear reactor by bombardment of natural uranium with neutrons. It has an atomic number of 94. The atomic masses of the reported 15 unstable isotopes range upwards from 236. Their half-lives range from a few hours to 375,000 years. Some are fissile and some of these undergo spontaneous fission, releasing neutrons. The main isotope used in reactors and for atomic weapons is Pu-239. It has a half-life of 24,100 years. This is the main isotope of value recovered from reprocessing used fuel.

Because plutonium is readily fissionable by neutrons bombardment and availability in quantity, it is important. However, it presents a dangerous health risk due to its high rate of emission of alpha particles and its high rate of absorption by bone marrow. In addition, particular care must be taken to prevent the unintentional formation of a critical mass. The complete detonation of one kilogram of the metal would produce an explosion equal in power to 20,00 tons of T.N.T.

Poison A substance that by absorbing neutrons can prevent further fission in a reactor. If sufficient poisons are present in a reactor core, the chain reaction will die out.

Positron (e+) The electron's antiparticle. The positron has the same mass and spin as the electron, but possesses a positive electrical charge of the same magnitude as the electron's negative charge. When a positron and an electron collide, they mutually annihilate. Their mass is converted to energy, which appears in the form of two gamma rays. See also Electron, Antiparticle, Antimatter.

Potassium-Argon Dating An isotope dating technique that is based on the rate of decay of potassium 40 to argon 40. The very long half-life of potassium-40 facilitates the estimation of the ages of rocks that are between 100,000 and 4 billion years old. See also Carbon-14 Dating.

Potential Energy The energy possessed by a body due to its location within a field of gravity.

Pressurized Water Reactor (PWR) A common type of light water reactor in which the water that flows through, and is heated by, the nuclear reactor is kept under high pressure to prevent it from boiling in the core. Steam formed in a secondary loop turns turbines to generate electricity. See also Light Water Reactor.

Proton (p) One of the basic particles of the atomic nucleus. (The other is the neutron.) The proton's charge is the same in magnitude as that of the electron, but positive. The proton's mass is very slightly less than that of a neutron - 1.67264 x 10^(-27) kg) vs. 1.6749 x 10^(-27) kg, but is 1,836 times that of an electron. A proton is a hadron that is composed of two up quarks and one down quark bound together by gluons. The number of protons in a nucleus is the atomic number for that element. This is what is distinguishes the various elements that appear in the Periodic Table. See also: Electron, Neutron, Quark.

Proton Number See Atomic Number.

Quantum The smallest possible unit or quantity of something, especially of energy, angular momentum, or electromagnetic radiation. Quanta is the plural form.

Quantum Chromodynamics (QCD) The theory of strong interactions between elementary particles. It assumes that strongly interacting particles (hadrons) are made up of quarks and that gluons bind the quarks together.

Quantum Mechanics A modern physical theory that deals with the composition and behavior of atoms and subatomic particles. It explains the duality of light as wave and particle and the existence of chemical bonds, as well as radioactivity.

Quark (q) The elementary particle from which all hadrons are made. An elementary particle from which the proton, the neutron and other heavy particles are believed to be made, but not particles, such as the electron or the neutrino. Quarks have combined with gluons to make all protons and neutrons in the universe. There are six identified varieties (flavors) of quark. They are: up, down, top, bottom, strange, and charm quarks. The first two make up most matter. In order of greatest (to least) mass, the quarks are top, bottom, charm, strange, down, up.

The quark is a particle that has strong interactions. It feels the strong force. They have electric charges of either +2/3 (up, charm, top) or -1/3 (down, strange, bottom) in units where the proton charge is 1. The proton consists of two up quarks and one down quark. The neutron consists of two down quarks and one up quark.

Q-Value The amount of energy released in a nuclear reaction or in radioactive decay. It is released primarily in the form of gamma rays, alpha particles, beta particles, and neutrinos. It is expressed in atomic mass units (amu's) or million electron volts (MeV's).

Radiation Energy that is emitted and propagated in the form of electromagnetic waves or subatomic particles, or the act or process of emitting and/or transmitting such energy. Radiation may be ionizing radiation (capable of breaking up atoms or molecules) or non-ionizing radiation. Some elements are naturally radioactive, whereas others become radioactive when bombarded by neutrons or other particles. See also Electromagnetic Spectrum, Ionizing Radiation, Non-Ionizing Radiation.

Radiation Absorbed Dose (RAD) A unit of measurement of energy that has been absorbed by tissue from an ionizing radiation by tissue. One rad is equal to energy absorption of 100 ergs per gram of tissue exposed to the radiation. See Gray.

Radiation Detector Any device or material that is sufficiently sensitive to radiation or atomic particles that it can produce a record of it or a signal. Photographic film is one form of simple detector. Other types include Geiger counters (gas-filled counters), scintillation counters (e.g., sodium iodide crystal), and semiconductor detectors (e.g., based on silicon or germanium).

Radioactive Emitting radiation or characterized by radiation. Capable of undergoing spontaneous disintegration due to radioactive decay and giving off radiant energy in the form of particles or gamma rays.

Radioactive Decay See Decay.

Radioactive Dating A technique used to estimate the age of an object by measuring the amounts of various radioisotopes in it. This indicates the extent of change that has occurred in the radioactive nuclei in a material as they decay to stable nuclei, and therefore the time required to have reached the present stage of decay. See also Carbon Dating, Half Life.

Radioactive Equilibrium The condition in which the rate of radioactive decay of a particular nuclide is equaled by the rate at which more of the same nuclides are produced as a result of radioactive decay.

Radioactive Isotope Any nuclide that is radioactive. See Nuclide, Radio Nuclide.

Radioactive Nuclide See Radionuclide.

Radioactive Waste Radioactive materials for which there is no longer a use. However, because they are radioactive, storage and disposal of them require special procedures and oversight.

Radioactivity The spontaneous disintegration of the atomic nuclei of unstable elements into those of another, accompanied by the emission of alpha, beta, or gamma rays. Radioactivity occurs naturally in many elements and can be produced artificially in others.

Radioisotope A radioactive isotope. A radionuclide. See Radionuclide.

Radiology The branch of medicine that is concerned with the diagnostic and therapeutic applications of radiation, including x-rays and radioisotopes, for the treatment of disease.

Radionuclide A radioactive nuclide. An unstable isotope of an element that decays spontaneously, emitting ionizing radiation, such as X-rays, gamma rays, alpha particles, or beta particles. Tritium and Cobalt-60 are radionuclides of hydrogen and cobalt respectively. Also termed Radioactive Nuclide or Radioactive Isotope. See also Isotope. .

Radiotoxicity The potential of an isotope to cause damage to living tissue by the absorption of energy from the radioactive material into the body

Radium (Ra) Radium is an element that is a natural radioactive decay product of uranium and is often found in uranium ore. It has an atomic number of 88 and an atomic mass of 226. Radium-226 has a half-life of 1600 years and decays to radon-222, which has a half-life of 38 seconds. There are four other radioactive isotopes of radium with half lives ranging from 11 days to 15 years. Radium is one of the earliest known naturally radioactive elements. It is far more radioactive than uranium and is found in the same ores. It was discovered in 1898 by Marie Sklodowska Curie in pitchblende. See also Uraniun Decay Series,

Radon (Rn) One of the six noble gases, radon is a radioactive element that has seven isotopes. Its atomic number is 86 and its atomic mass varies from 211 to 222. Radon-222, the main isotope, has the longest half-life (3.82 days). The half-lives of four of the remaining six isotopes are less than one minute. Radon-222 is formed by the decay of radium and is given off as a gas from rocks containing radium (or thorium). In turn, radium is a link in the decay chain of uranium. Radon is a hazard of uranium mill tailings. See also Radium, Uranium Decay Series.

Rare Earths The metals known as the rare earths consist of scandium-21, yttrium-39, lanthanum-57, and the 14 lanthanides: cerium-58, praseodymium- 59, neodymium-60, promethium-61, samarium-62, europium-63, gadolinium-64, terbium-65, dysprosium-66, homium-67, erbium-68, thulium-69, ytterbium-70 and lutetium-71.

All the elements are available as pure metals. They are typical transition metals and are silver-white on a fresh surface, tarnish rapidly, are soft and usually ductile, and burn when heated. They are not useful as structural metals and are employed mainly in alloys and as compounds. The valence of all the rare earths is most commonly +3.

The rare earths are not very rare. All occur together in varying amounts. The principal ore of the rare earths is monazite, a dark, hard, heavy mineral, which is chiefly cerous phosphate, but usually contains a considerable amount of the rare earths. The chemical behavior of the rare earths is so similar that they are difficult to separate.

Rare Gases See Noble Gases.

Reactor See Nuclear Reactor.

Reactor Core The core or central part of a nuclear reactor. The core contains the fissionable material, the moderator, the coolant, the control mechanism, and supporting structure.

Reactor Vessel The cylindrical, steel pressure vessel that contains the core, control rods, moderator, coolant, and control systems necessary for safe operation of the reactor.

Regulator Rod See Control Rod.

Relativistic Relating to, or subject to, the general or special theory of relativity. Also, anything traveling at nearly the speed of light and obeying the laws that apply at such speeds.

REM A common unit of measurement of human radiation doses. Rem stands for roentgen equivalent man and is the dose equivalent of the health risk posed by 1 roentgen of x-rays. Doses are usually expressed in millirems, rather than rems (1 rem = 1,000 millirems). See also Sievert.

Reprocessing The processing of spent reactor fuel and irradiated targets, to recover fissile materials. The extraction of uranium and plutonium from spent fuel for use in fuel or atomic weapons. The highly radioactive fuel elements and toxic chemicals necessary make reprocessing dangerous. Special facilities are required.

Rest Mass The energy of the free particle divided by the square of the speed of light (derived from E = mc2). In particle physics, the word mass always means the rest mass (resting mass or mass at rest) of the object.

Roentgen (R) A unit of measurement of the ionization produced in dry air by x-rays or gamma rays. One roentgen is equal to 2.54 × 10-4 coulombs per kilogram of air. It represents an effort to measure the exposure of someone or something to x-ray or gamma rays. See also Gray, Rad, Rem, Sievert.

Scram An emergency shutdown of a nuclear reactor. This is usually accomplished by rapidly inserting the control rods, either automatically or manually. Scram is an acronym for Safety Control Rod Axe Man. During the early nuclear reactor tests in Chicago, Scram was the title of the person who stood by the rope that suspended the rods. He held an axe ready to cut the rope in the case of an accident. If he cut the rope, the rods would fall into the core and stop the reaction.

Secondary Ionization The production of ions from the interaction of primary ions with matter. Secondary ionization occurs when ions have sufficient energy to induce further ionization.

Secondary Radiation Particles or x-rays produced by the bombardment of any matter by charged particles or radiation.

Self-Sustaining Chain Reaction Another term for a chain reaction. See Chain Reaction.

Separation Energy See Binding Energy.

Shield See Shielding.

Shielding Material used as a shield against harmful radiation. A barrier constructed between a radioactive source and employees to absorb radiation or greatly reduce its intensity. A dense material, such as lead or concrete, is normally used. See also Biological Shield.

Shim Rod See Control Rods.

Sievert (Sv) The System International (SI) unit of measurement of radiation absorbed by the human body. It is defined as one joule of radiation absorbed per kilogram of mass (body tissue) or, more specifically, a loss of gamma ray or x-ray energy equal to one joule per kilogram of body mass. It is an attempt to measure the effect of radiation on human tissue. One sievert = 100 rem. See Rem.

Slug An element of uranium that will be used as fissionable material or fuel for a nuclear reactor. It is coated with aluminum to prevent corrosion. See also Cladding, Uranium.

Source Any substance that emits radiation. Also, a radioactive material that emits radiation for scientific or industrial use.

Source Material Any physical or chemical of any material, except special nuclear material or byproducts, in which the uranium or thorium concentration is at least 0.05% by weight.

Spallation A nuclear reaction in which an energetic heavy nucleus is hit by a low energy proton and breaks into many particles. Alternatively, a low-energy heavy nucleus is hit by an energetic proton and breaks into many particles. This accounts for the relatively high proportion of beryllium, lithium, and boron in cosmic rays.

Specific Activity A measure of the radioactivity of a material. Specific activity is the number of atoms of a radioactive substance that disintegrate per unit of time per unit of mass of a radioisotope.

Speed Of Light (c) The speed at which electromagnetic waves can move in a vacuum. This is equal to 186,000 miles per second or 299,792,458 meters per second (approximately 3 × 108 meters per second). This is the speed of photons of light energy through space.

Spent Nuclear Fuel Fuel that has been used in a nuclear reactor for a year or more, reached the end of its usefulness, and withdrawn permanently. Spent nuclear fuel has not been separated into its constituent elements for reprocessing. It contains the special nuclear material, by-products, source material, and other radioactive materials associated with fuel assemblies, and is highly radioactive.

Spontaneous Fission The spontaneous splitting of the nucleus into two or more nuclei, generally accompanied by the release of energy and the emission of one or more neutrons. This form of radioactive decay is a characteristic of very heavy isotopes, such as the trans-actinide elements. See also Actinide.

Stable Not radioactive. Will not decay spontaneously.

Strong Force The fundamental force of nature that binds quarks and gluons together to form hadrons, such as neutrons and protons. The strong force is the strongest of the four fundamental forces, but has the shortest range. It is effective at distances of less than 10?13 centimeters. Hadrons are also bound to one another by the residual strong force between the quarks and gluons form them. The strong force is approximately one hundred times more powerful than the electric repulsion of protons within the nucleus.

Strontium (Sr) Strontium is a soft, silvery metal that ignites spontaneously when exposed to air when finely divided. It has an atomic number of 38 and four naturally occurring isotopes of atomic mass 84, 86, 87, and 88. Strontium 88 is the most common isotope representing 82.5% of strontium found in nature. There are an additional ten unstable isotopes of strontium. Isotope strontium 90, one of the most common fission products, is the most stable of these ten isotopes, with a half-life of 29.1 years. Strontium is chemically similar to calcium.

Subatomic Of smaller size than an atom.

Subatomic Particle Any particle that is smaller in size than an atom. See Particle.

Subcritical A state that is neither supercritical nor critical. A state in which a fissionable mass generates insufficient free neutrons to sustain a fission chain reaction. There is too little fissionable material to sustain the reaction.

Subcritical Mass A mass of fissile material that is incapable of sustaining a chain reaction because of its composition or its the quantity. See Critical Mass.

Supercritical A state in which the amount of fissionable mass exceeds the critical mass. The amount of energy being produced by nuclear fission because there is more than enough fissionable mass. This usually leads to an explosion.

Swimming Pool Reactor A nuclear reactor in which the fuel elements are suspended in a pool of water. The water serves as the reflector, moderator, and coolant. The reactor is used for research and training, not for electrical power generation

Synchrotron A type of circular accelerator. See Accelerator.

Synchrotron Radiation The electromagnetic radiation emitted by charged particles when they travel in circular orbits at speeds close to that of light in a magnetic field. This occurs in the synchrotron, a circular accelerator. See Accelerator.

Target Material placed in a nuclear reactor for bombardment by neutrons. This is done to produce nuclear fuel, plutonium, radioactive isotopes for medical or other use, or new man-made materials, etc. The word, target, also describes a metallic object placed before a beam of electrons to produce x-rays.

TeV Tera Electron Volts (1,000,000,000,000 electron volts). See Electron Volt.

Thermal fission Fission caused by thermal neutrons.

Thermal Neutron A neutron of kinetic energy approximately equal to 0.025 electron volts. This is equivalent to the kinetic energy possessed by molecules in a gas at room temperature. Neutrons reach this energy level after having experienced several collisions with nuclei.

Thermonuclear Relating to nuclear reactions that occur only at very high temperatures.

Thermonuclear Reaction See Fusion.

Thorium (Th) A soft, very ductile, silvery-gray heavy metallic element that is used in the manufacture of sun lamps. Thorium has an atomic number of 90. The 33 isotopes have atomic masses ranging from 209 to 238 and very short half-lives. Thorium is a byproduct of the decay of uranium. Thorium 232 is used as a power source for nuclear reactors. See Uranium Decay Series.

Tracer A small amount of a radio-active isotope that is introduced into a system in order to be able to observe by radiation its path or the behavior of some component of that system. For example, if radioactive phosphorous is mixed with a chemical fertilizer, its path can be traced as the plant grows.

Transmutation The process of changing one element into another by bombarding the first with nuclear particles, such as neutrons. Fission or neutron capture occurs, modifying the nucleus of the bombarded element.

Transuranic Element An element that has an atomic number greater than 92, the atomic number of uranium. All elements that have atomic numbers greater than 92 are radioactive. Transuranic elements are not naturally occurring in nature. They are artificially produced by neutron capture. Neptunium, plutonium, and curium with atomic numbers of 93, 94, and 96, respectively, are examples of these heavy metals. Such heavy elements are called transuranic because they are man-made and derived from uranium. They are also called transuranium elements. See Activation Product.

Transuranic Waste Waste materials that are contaminated with isotopes that appear above uranium in the periodic table (i.e., have atomic numbers greater than 92). Transuranic wastes are only moderately radioactive, although long-lived.

Tritium The heaviest isotope of the three hydrogen isotopes. Tritium has an atomic number of one and atomic mass of three. It has three times the weight of hydrogen and is radioactive. Its nucleus contains one proton and two neutrons. Tritium is created in nature in the upper atmosphere by bombardment by cosmic rays. It is also produced in production reactors. Tritium has a half-life of 122.33 years. It is used in nuclear weapons to increase the explosive power. Tritium is often called hydrogen three.

Triton The nucleus of the element, tritium 3H.

Unstable Capable of spontaneously disintegrating. A radioactive isotope that spontaneously decays into other particles, emitting radiation.

Uranium (U) A heavy, hard, dense, malleable, ductile, silvery-white, radioactive metal of the actinide series. It has an atomic number of 92 and an atomic weight of 238. Uranium is highly reactive and reacts with nearly all of the nonmetallic elements and their compounds. It has 32 isotopes, of which 26 are radioactive.

Uranium occurs in numerous minerals, such as pitchblende, uraninite, and carnotite, Naturally occurring uranium is a mixture of three isotopes in the following concentrations: 99.283% uranium-238, 0.711% uranium-235, and 0.005% uranium-234. Uranium-235 is important as the basic material of nuclear technology because it is the only isotope that has a readily fissionable nucleus and occurs naturally in significant quantities. However, two isotopes are fissile (U-235 and U-233) and two are fertile (U-238 and U-234). See Uranium Decay Series.

Finely divided uranium metal is pyrophoric and presents a fire hazard. In addition, inhaling or ingesting uranium is dangerous.

Uranium Decay Series The natural radioactive decay series of uranium-238 that begins with uranium-238 and ends with lead-206. The series is as follows: Uranium -238 - Thorium - 234 - Uranium -234 - Protactinium - 234 - Thorium - 230 - Radium - 226 - Radon - 222 - Polonium - 218 - Lead - 214 - Bismuth - 214 - Poloniumi - 214 - Lead - 210-Bismuth - 210 - Polonium - 210 - Lead-206. This decay series accounts for 95% of radioactivity. See also Actinium Decay series.

Uranium Hexafluoride (UF6) A gaseous form of uranium used in the gaseous-diffusion uranium enrichment process. It is highly toxic, reacts violently with water, is corrosive to most metals, and forms solid gray crystals at standard temperature and pressure. It reacts with aluminum to form a thin coating of Aluminum fluoride that resists further reaction.

Uranium Oxide Concentrate (U308) See Yellowcake.

Valence Electron An electron in the outermost (valence) shell of an atom. Valence electrons are the most active in bonding with other atoms or ions.

Wave An up and down or back and forth movement or vibration that propagates, transferring energy from one place to another. For example, sound waves propagate in air.

Wavelength The distance between two consecutive troughs or two consecutive crests of a wave. The wavelength of a radio wave can be several feet whereas the wavelengths of X-rays are atom-like in size.

Wave-Particle Duality Electromagnetic radiation and matter can exhibit both particle-like and wavelike behavior, depending on circumstances. There is no distinction between waves and particles in quantum mechanics. At times, particles may behave like waves and vice-versa. The term, wave-particle duality, provides recognition of this. However, wave-particle duality is only relevant in the case of small objects, such as atoms and subatomic particles, where behavior is determined by quantum mechanics.

Weak Force The second weakest of the four fundamental forces. The weak force is the force that governs the change of one kind of particle into another. It has a very short range. It affects all matter particles, but not force-carrying particles.

Weapons-Grade Uranium This is generally considered to be uranium, at least 90 percent of which is made up of the uranium-235 isotope. In contrast, naturally occurring uranium ore is only 0.711% uranium-235. Uranium-238 and uranium-234 are 99.283% and 0.005%, respectively

Weight The force exerted on a body by gravitation. It is proportional to mass, but not the same.

X-Rays A form of invisible electromagnetic radiation, that is similar to light, but of much shorter wavelength (less than 100 Angstroms). X-rays are capable of penetrating solids, destroying living tissues, and ionizing gases. The x-ray band of the electromagnetic spectrum lies between ultraviolet light and gamma rays. X-rays are produced when high-energy charged particles collide with other charged particles or atoms. They are used in x-ray therapy to destroy diseased cells, as well as in diagnostic procedures. See also Angstrom, Gamma Radiation.

Yellowcake A natural uranium concentrate that takes its name from its color and texture. Yellowcake is normally 70-90% uranium oxide (U3O8 ) by weight. It is used as feedstock for uranium enrichment and fuel pellet fabrication. Uranium oxide (U3O8 ) is produced by crushing and grinding uranium ore and then bathing it in sulfuric acid to leach out the uranium, and drying the solution. Yellowcake is also called urania and uranic oxide.

Zircaloy A zirconium alloy that is used for the tubes that contain uranium oxide fuel pellets in a reactor fuel assembly, as well as cladding and structural materials in water-cooled reactors. The alloy provides high mechanical strength, corrosion resistance, and a low neutron absorption cross section. See also Cladding.

Science Glossaries

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