Barcelona Field Studies Centre

Synchrotron Glossary

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The transfer of energy to a medium, such as body tissues, as a radiation beamGlossary Term passes through the medium.
A device (i.e., machine) used to produce high-energy high-speed beamsGlossary Term of chargedGlossary Term particles, such as electronsGlossary Term, protons, or heavy ions, for research in high-energy and nuclear physics, synchrotron radiation research, medical therapies, and some industrial applications. The accelerator at SLAC is an electron accelerator.
A process in which a particle meets its corresponding antiparticle and both disappear. Their energy and momentum appears in some other form, producing other particles together with their antiparticlesGlossary Term and providing their motion.
A unit of length equal to 10-10 meter.
A material made from antiparticlesGlossary Term. The particles that are common in our universe are defined as matter and their antiparticles as antimatter. In the particle theory there is almost no a priori distinction between matter and antimatter.  Their interactionsGlossary Term are almost identical. The asymmetry of the universe between these two classes of particles is a deep puzzle which is yet to be fully understood.
In particle physics every particle with any type of chargeGlossary Term or fermionGlossary Term label has a corresponding antiparticle type. Any particle and its antiparticle have identical mass and spin but opposite charges. For example the antiparticle of an electronGlossary Term is a positron. It has exactly the same mass as an electron but positive charge.
Some particles are their own antiparticles, the antiparticle of a photon is a photon for instance. Conserved quantities such as baryonGlossary Term number and leptonGlossary Term number are further types of "charges" that are reversed for particle and antiparticle. Thus an electronGlossary Term and an electron neutrino both have electron number +1 while their antiparticles the positron and the anti-electron-neutrino have electron number -1.
The process by which a compound is reduced in concentration over time, through adsorption, degradation, dilution, and/or transformation. Radiologically, it is the reduction of the intensityGlossary Term of radiation upon passage through a medium. The attenuation is caused by absorptionGlossary Term and scattering.
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Primary radiation deflected or secondary radiation emitted in the general direction of the incident radiation beamGlossary Term.
Radiation-absorbing material, such as lead or concrete, used to reduce radiation exposure. A primary barrier attenuates useful beamGlossary Term to the required degree. A secondary barrier attenuates stray radiation to the required degree.
A hadronGlossary Term made from a basic structure of three quarks. The proton and the neutron are both baryonGlossary Term. The antiproton and the antineutron are antibaryons.
A unidirectional or approximately unidirectional flow of electromagnetic radiation or particles.
Bhabda Scattering
Scattering of positrons by electrons.
The general name for any  particle with a spin of an integer number ( 0,1 or 2...) of quantum units of  angular momentum. (named for Indian physicist S.N. Bose). The carrier particlesGlossary Term of all interactionsGlossary Term are bosons. Mesons are also bosons.
Bottom Quark or B Quark
The fifth flavorGlossary Term of quark (in order of increasing mass), with electric chargeGlossary Term -1/3.
Bound State
This is a state in which a particle is confined within a composite system, for example an atom or a nucleus, because it does not have enough energy to escape. An electronGlossary Term in a atom is bound because of its electrical attraction to the nucleus, which makes the mass of the atom slightly less than the sum of the masses of the electron plus the rest of the atom without that electron.
This involves placing the source of radiation directly within the tumor and employs radioactive plaques, needles, tubes, wires, or small "seeds" made of radionuclides. These radioactive materials are placed over the surface of the tumor or implanted within the tumor, or placed within a body cavity surrounded by the tumor.
X-rays emitted when a chargedGlossary Term particle (such as an electron) is decelerated by passing through matter. The word bremsstrahlung is German for "braking radiation".
Bubble Chamber
A chamber filled with liquid at low pressure chosen so that small bubbles form along the path of any chargedGlossary Term particle. After each beam pulse a photographic record is made of the chamber and then it is depressurized to clear the bubbles.
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Carrier Particle
A fundamental bosonGlossary Term associated with quantum excitations of the force field corresponding to some interactionGlossary Term. GluonsGlossary Term are carrier particles for strong interactions (color force fields), photons are carrier particles of electromagnetic interactionsGlossary Term, and the W and Z bosons are carrier particles for weak interactions.
In particle physics, any device that can measure the energy deposited in it by particles (originally a device that measured heat energy deposited, thus a calorie-meter).
CT or CAT Scan (also called Computed Tomography or CT)
Detailed pictures of areas of the body created by a computer linked to an x-ray machine. Also called computed tomography (CT) scan or computed axial tomography (CAT) scan.
Cathode Ray Tube (CRT)
An evacuatedGlossary Term tube containing an anode and a cathode that generates cathode rays (electrons) when operated at a high voltage. The cathode rays produce an image on a screen when they strike phosphors on the screen, causing them to glow.
Cerenkov Radiation
A chargedGlossary Term particle emits Cerenkov radiation (light) in a cone around its direction of travel when it travels through any medium faster than the speed of light through that medium. (Cerenkov - is the name of the scientist who first recognized the nature of this effect and its possible use for distinguishing particle types.) Although the speed of light in a vacuum is the fastest speed that any particle or light can have, in a medium of any type light travels more slowly  because of its interactions with the electric fieldsGlossary Term of the atoms in the medium and so it is possible for a high energy particle to be faster than light in some material . The blue light in the pools of water you may have seen in pictures of nuclear power plants is Cerenkov radiation from particles produced in the reactor.
The major European International Accelerator Laboratory located near Geneva, Switzerland. (originally called Centre European pour Rechearche Nucleaire). The WWW was created at CERN.
A quantity carried by a particle that determines its participation in an interactionsGlossary Term process. A particle with electric charge has electrical interactions; one with strong charge (or color chargeGlossary Term) has strong interactions, etc.
One "flavorGlossary Term" of quarks. Also known as the C quark.
Varian trade name for a range of linear acceleratorGlossary Term models used in cancer treatment and stereotactic radiosurgery.
A term that's applied to electromagnetic wavesGlossary Term. When they "wiggle" up and down together (in phase) they are said to be coherent. A laser is a good example of coherent light. An ordinary light bulb produces incoherent light much like the random waves produced when many raindrops hit a puddle. Electromagnetic radiation is coherent when the photons are produced in such a way that they are in phase with one another and incoherent when the phases of the photons are random. Partial coherence is an intermediate situation where there a significant fraction of the photons have related phase, but not all of them.
The alignment of the direction of the photons, so the beamGlossary Term of  radiation can be directed at a well-defined part of a target material.
A mechanical device, sometimes called a "slit", installed along the trajectory of a beamGlossary Term to reduce the size of the beam. Collimators are also useful for removing stray radiation.
Color Charge
The chargeGlossary Term associated with strong interactionsGlossary Term. Quarks and gluonsGlossary Term have color charge and consequently participate in strong interactions. LeptonsGlossary Term, photons and W and Z bosonsGlossary Term do not have color charge and therefore do not participate in strong interactions.
Compton Scattering
The scattering of photons from charged particles is called Compton scattering after Arthur Compton who was the first to measure photon-electron scattering in 1922.
The universe regarded as an orderly, harmonious whole.
A regularly repeated crystal-like substructure.
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Any  process in which a particle disappears and in its place two or more different particles appear.
Any device used to sense the passage of a particle; also a collection of such devices designed so that each serves a particular purpose in allowing physicists to reconstruct particle eventsGlossary Term.
Dipole Magnet
Any  magnet with one north and one south pole. In an accelerator the dipole magnets are used to steer a particle beamGlossary Term to the left or right by placing one pole above and the other below the beam pipe.
Dose (Absorbed Dose)
More specifically referred to as "absorbed dose", this is a measure of the energy deposited within a given mass of a patient. Absorbed dose is quantified by the unit called the "rad".
Dose Calibration
Determining the response of a dosimeter to a known radiation exposure or known absorbed dose. For a beamGlossary Term of radiation, this means determining the absorbed dose rate at a calibrated point in the beam under a specified set of conditions. Normally, such a determination is carried out with a number of beams under different specified conditions.
Dose Equivalent (DE)
Parameter used to express the risk of the deleterious effects of ionization radiationGlossary Term upon living organisms. For radiation protection purposes, the quantity of the effective irradiation incurred by exposed persons, measured on a common scale in sievert (SI) or rem (non-SI).
Dose Rate
A measure of the dose delivered per unit time.
A radiation sensitive device, e.g., film, monitor ion chamber, TLD, etc., with a known sensitivity that is placed in the beamGlossary Term path of radiation to dose.
The calculations, measurements and other activities required for determining the radiation dose to be delivered.
Down Quark or D Quark
The second flavorGlossary Term of quark (in order of increasing mass), with electric chargeGlossary Term -1/3.
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Electric Field
A force field which defines what acceleration an electric chargeGlossary Term placed at rest at any point in space will feel. Electric charges cause electric fields around them, which then apply a force to any other electric charge placed in the field.  The electric field E has both a magnitude and a direction at each point in space, and the magnitude and direction of the resulting force on a charge q at that point is given by F= qE. When you get a shock from a door handle after scuffing your feet on a carpet you feel the effect of an electric field accelerating electronsGlossary Term.
eV (electronvolt)
The basic unit of energy used in high energy physics. It is the energy gained by one electronGlossary Term when it moves through a potential difference of one volt. By definition an eV is equivalent to 1.6 x 10-19 joules. This is a very small amount of energy and the more commonly used multiples are MeV (million eV), GeV (billion eV or giga-electronvolt) and TeV (trillion eV).
Electromagnetic (em) wave
Electromagnetic waves make up the electromagnetic spectrum.Visible light, ultraviolet, infrared, radio and TV signals are all examples of "everyday" em waves.  X-rays, microwaves and  high energy photons or gamma raysGlossary Term are also electromagnetic waves.
Electromagnetic Spectrum Simplified EM spectrum, representing energy per photon, not total beam energy
Electromagnetic Interaction
The interactionGlossary Term due to electric chargeGlossary Term; this includes magnetic effects that have to do with moving electric charges.
The least massive electrically chargedGlossary Term particle, therefore absolutely stable. It is the most common leptonGlossary Term with charge -1. An electron is one of the fundamental particlesGlossary Term in nature. Fundamental means that, as far as we know, an electron cannot be broken down into smaller particles. (This concept is one of the things SLAC physicists always challenge by looking for other particles.) Electrons are responsible for many of the phenomena that we observe in everyday life. Mutual repulsion between electrons in the atoms of the floor and those within your shoes keeps you from sinking and disappearing into the floor!!! Electrons carry electrical current and successful manipulation of electrons allows electronic devices, such as the one you are using, to function.
Electron Accelerator
Electrons carry electrical chargeGlossary Term and successful manipulation of electronsGlossary Term allows electronic devices to function. The picture and text on the video terminal in front of you is caused by electrons being accelerated and focused onto the inside of the screen, where a phosphor absorbs the electrons and light is produced. A television screen is a simple, low-energy example of an electron accelerator. A typical medical electron accelerator used in medical radiation therapy is about 1000 times more powerful than a color television set, while the electron accelerator at SLAC is about 2,000,000 times more powerful than a color TV. One example of an electron accelerator used in radiotherapy is the Clinac, manufactured by Varian Associates in Palo Alto, CA.
Electron Beam
The stream of electrons generated by the electron gun and accelerated by the accelerator guide.
Electron Beam Therapy
Treatment by electrons accelerated to high energies in a linear acceleratorGlossary Term. Primarily used for lesions situated at or near the surface.
Electronic Structure
The distribution of electrons in the material and the energies related to changes in this distribution.
Elementary Particles
The name given to protons, neutrons and electrons before it was discovered that protons and neutrons had substructure (quarks). Today we use the term "fundamental" for the six types of quarks and the six leptonsGlossary Term and their antiparticlesGlossary Term, which  have no known substructure. GluonsGlossary Term, photons and W and Z bosonsGlossary Term are also fundamental particles. All other particles are composite, that is  made from combinations of fundamental particlesGlossary Term.
Sealed and pumped down to a pressure very much below atmospheric pressure. Typical pressures inside accelerators or waveguides are about 10-12 times atmospheric pressure.
An event occurs when two particles collide or a single particle decayGlossary Term. Particle theories predict the probabilities of various events occurring when many similar collisions or decays are studied. They cannot predict the outcome for a single collision or decay.
The addition of energy to a system, transferring it from its ground state to an excited state. Excitation of a nucleus, an atom, or a molecule can result from absorption of photons or from inelastic collisions with other particles.
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Fermi National Accelerator Laboratory in Batavia, Illinois (named for particle physics pioneer Enrico Fermi).
General name for a particle that is a matter constituent, characterized by spin in odd half integer quantum units (1/2,3/2,5/2...). Named for Italian physicist Enrico Fermi. Quarks, leptonsGlossary Term and baryonsGlossary Term are all fermions.
The name used for the different quark types and the different leptonGlossary Term types. The six flavors of quarks are up, down, strange, charm, bottom, top, in increasing order of mass. The flavors of chargedGlossary Term leptons are electronsGlossary Term, muon and tau, again in increasing order of mass. For each charged lepton flavor there is a corresponding neutrino flavor.
Fundamental Interaction
The known fundamental interactionsGlossary Term are the strong, electromagnetic, weak and gravitational interactionGlossary Term. These interactions explain all observed physical processes but do not explain particle masses. Any force between two objects is due to one or another of these interactions. All known particle decaysGlossary Term can be understood in terms of these strong, electromagnetic or weak interactions.
Fundamental Particle
A particle with no internal substructure. In the Standard Model, the quarks, leptonsGlossary Term, photons, gluonsGlossary Term ,W-bosonGlossary Term and Z-bosons are fundamental. All other objects are made from these particles.
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Gamma Rays
Gamma rays are electromagnetic wavesGlossary Term or photons emitted from the nucleus (center) of an atom. See also: photon.
Geiger Counter
A device that detects the passage of chargedGlossary Term particles via the ionizationGlossary Term of gas that they cause as they pass through a region. Used to detect the particles produced  in certain forms of radioactivity.
GeV (Giga Electron Volt)
Unit of energy equal to that acquired by a particle with one electronic chargeGlossary Term in passing through a potential difference of one billion volts.
The carrier particleGlossary Term of the strong interactionGlossary Term.
Gravitational Interaction
An attractive force between any two objects or particles. The "charge" that determines the strength of the gravitational interaction is energy. For a static object it is mass-energy but in fact all forms of energy  both cause and feel gravitational effects.
Gray (Gy)
The SI unit for absorbed dose equal to an energy absorbed of one joule/kilogram in the stated medium.
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Any  particle made of quarks and gluonsGlossary Term, i.e. a meson or a baryonGlossary Term. All such particles have no strong charge (i.e. are strong charge neutral objects) but  participate in residual strong interactionsGlossary Term due to the strong charges of their constituents.
The time required for half the nuclei in a sample of a specific isotopic species to undergo radioactive decay.
Materials made from more than one type of  substructure. Opposite of homogeneous, a material with a uniform composition. (Note that a material can be chemically complex yet homogeneous.) 
High-Energy Physics
A branch of science that tries to understand the interactionsGlossary Term of the fundamental particlesGlossary Term, such as electronsGlossary Term, photons, neutrons and protons (and many others than can be created). These particles are the basic building blocks of everyday matter, making up the human body as well as the entire universe. This type of physics is called high-energy because very powerful machines, such as the Two-Mile Accelerator at SLAC, are created to make these particles go very fast so that they can probe deeply into other particles and try to understand what they are made of.
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Induced Radioactivity
Radioactivity produced in materials, especially metals, exposed to high-energy photons or neutrons.
The amount of radiation, for example, the number of photons arriving in a given time   
A process in which a particle decaysGlossary Term or it responds to a force due to the presence of another particle (as in a collision).
Atomic particle, atom, or chemical radical bearing an electrical chargeGlossary Term, either negative or positive.
The process by which a neutral atom or molecule acquires a positive or negative chargeGlossary Term.
Ionizing Radiation
Radiation that has enough energy to eject electronsGlossary Term from electrically neutral atoms, leaving behind chargeGlossary Term atoms or ions. There are four basic types of ionizing radiation: Alpha particles (helium nuclei), beta particles (electrons), neutrons, and gamma raysGlossary Term (high frequency electromagnetic waves, x-rays, are generally identical to gamma rays except for their place of origin.)  Neutrons are not themselves ionizing but their collisions with nuclei lead to the ejection of other charged particles that do cause ionizationGlossary Term.
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The name physicists give to a cluster of particles emerging from a collision or decayGlossary Term eventGlossary Term all traveling in roughly the same direction and carrying a significant fraction of the energy in the event. The particles in the jet are chiefly hadronsGlossary Term.
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An  evacuatedGlossary Term electronGlossary Term tube used as an oscillator or amplifier at microwave frequencies. In the klystron, an electron beamGlossary Term is velocity modulated (periodically bunched) to produce large amounts of power.
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A fundamental matter particle that does not participate in strong interactions. The chargeGlossary Term leptons are the electronGlossary Term (e), the muon (), the tau () and their antiparticlesGlossary Term. Neutral leptons are called neutrinos ( ).
A type of particle accelerator in which chargedGlossary Term particles are accelerated in a straight line, either by a steady electrical field or by means of radiofrequency electric fieldsGlossary Term. In the latter variety, the passage of the particle is synchronized with the phase of the accelerating field. The SLAC Linear Accelerator (linac) is a two-mile long accelerator, consisting of a cylindrical, disc-loaded, copper waveguide placed on concrete girders in a tunnel about 25 feet underground.
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Mammography is a low-doseGlossary Term x-ray procedure that creates an image of the breast. The x-ray image is called a mammogram.
We call the commonly observed particles such as protons, neutrons and electronsGlossary Term matter particles, and their antiparticles are then antimatterGlossary Term.
The average time during which a system, such as an atom, nucleus, or elementary particle, exists in a specified form. Also known as the average life.
Medical Physics
This is a very diverse field that applies the knowledge gained in other areas of physics (such as high-energy PhysicsGlossary Term) to heal people. Radiation therapy is one example. CAT scansGlossary Term, mammography, and other x-ray imaging techniques are diagnostic techniques that have also been developed by physicists working in medicine. Another important example is the technology that went into building the particle accelerator at SLAC, which has been adapted for use in hospitals to treat cancer patients with beams of electrons and x-rays.
A hadronGlossary Term with the basic structure of one quark and one antiquark.
Energy equal to that acquired by a particle with one electronic chargeGlossary Term in passing through a potential difference of one million volts.
One millionth of a meter; also known as a micrometer.
Waves of electromagnetic radiation that oscillate from approximately 109 to 3 x 1011Hz. (cycles per second). SLAC's klystronsGlossary Term produce microwaves of 2,856 MHz.
Møller Scattering
Scattering of electrons by electrons.
Momentum is a property of any moving object. For a slow moving object it is given by the mass times the velocity of the object. For an object moving at close to the speed of light this definition gets modified. The total momentum is a conserved quantity in any process. Physicists use the letter p to represent momentum, presumably because m was already used for mass, n for number, and o is too much like zero.
Monte Carlo Calculations
There is a gaming aspect to Monte Carlo calculations. Every simulation is based up events that happen randomly, and so the outcome of a calculation is not always absolutely predictable. This element of chance reminds one of gambling and so the originators of the Monte Carlo technique, Ulam and von Neumann, both respectable scientists, called the technique Monte Carlo to emphasize its gaming aspect.
The second leptonGlossary Term (in order of increasing mass), with electric chargeGlossary Term -1.
Muon Chamber
The outer layers of a particle detectorGlossary Term capable of registering tracks of chargedGlossary Term particles. The detector is designed so that the only charged particles that can get out to this layer are muons.
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A leptonGlossary Term with no electric chargeGlossary Term. Neutrinos participate only in weak (and gravitational) interactionsGlossary Term and therefore are very difficult to detect. There are three known types of neutrino, all of which have very low or possibly even zero mass.
A baryonGlossary Term with electric chargeGlossary Term zero. Its basic structure is two down quarksGlossary Term and one up quark.
A collection of protons and neutrons that form the core of an atom (plural: nuclei).
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Pair Production and Annihilation
Whenever sufficient energy is available to provide the mass-energy, a particle and its matching antiparticle can be produced (pair production). When a particle collides with its matching antiparticle they may annihilate -- which means they both disappear and their energy appears as some other particles -- with balanced number of particles and antiparticlesGlossary Term for each type.  All conservation laws are obeyed in these processes.
In "particle physics", a subatomic object with definite mass and chargeGlossary Term.
Pauli Exclusion Principle
No two fermionsGlossary Term of the same type can exist in the same state at the same place and time.
PET Scan
Positron Emission Tomography scanning uses an array of stationary detectors around the patient and using the spatial 180 degree opposing properties of the 0.511-MeV annihilation radiation from positron-emitting radiopharmaceuticals deposited in the organ or region of interest. The name tomography refers to the fact that the scanner computes a “slice” of the scanned object, not just a flat image. Each slice really is a volumetric (tomo-) image (-graphy). For more details, see See also, SPECT.
A substance that emits light when excited by radiation.
The carrier particleGlossary Term of the electromagnetic interactionGlossary Term. Depending on its frequency (and therefore its energy) photons can have different names such as visible light, X rays and gamma raysGlossary Term. We describe light in several ways. When we talk about "photons" we generally think of uncharged particles with out mass that carry energy (but be careful, there are other particles like this!). Photons of light are known by other names too, such as gamma rays and x-rays. Low-energy forms are called ultraviolet rays, infrared rays, even radio waves! A photon is one of the fundamental particleGlossary Term in nature and it plays an important role involving electron interactions. Photons are the most familiar particles in everyday existence. The light we see, the radiant heat we feel, microwaves we cook with, are make use of photons of different energies. An x-ray is simply a name given to the most energetic of these particles.
The lightest type of mesonsGlossary Term. They are copiously produced in high energy particle collisions.
Planck's Constant
A fundamental physical constant, the elementary quantum of action, It is the ratio of the energy of a photon to its frequency and is equal to 6.62620 x 10-34 joule-second. Symbolized by h.
A polarized particle beamGlossary Term is a beam of particles whose spins are aligned in a particular direction. The polarization of the beam is the  fraction of the particles with the desired alignment.
Antiparticle of the electronsGlossary Term.
A baryonGlossary Term with electric chargeGlossary Term +1. Protons contain a basic structure of two up quarks and one down quarkGlossary Term . The nucleus of a hydrogen atom is a proton. A nucleus with atomic number Z contains Z protons; therefore the number of protons is what distinguishes the different chemical elements.
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When used as a noun (plural quanta): a discrete quantity of energy, momentum or angular momentum, given in units involving Planck's constant h. For example electromagnetic radiation of a given frequency f is composed of quanta (also called photons) with energy hf.
When used as an adjective (as in quantum theory, quantum mechanics, quantum field theory): defines the theory as involving quantities which depend on Planck's constant h. In such theories radiation comes in discrete quanta as described above; angular momenta must be integer units of h, except that the intrinsic angular momenta of fundamental particles are integer multiples of 1/2h; and solutions for the possible states of a particle in a potential (such as the states of an electron the electrical potential due to an atomic nucleus) occur only for certain discrete energies. 
Quantum Mechanics
The laws of physics that apply on very small scales. The essential feature is that energy, momentum and angular momentum as well as chargeGlossary Term come in discrete amounts called quanta.
Quantum Number
A number that labels a state, it denotes  the number of quanta of a particular type that the state contains. Electric chargeGlossary Term given as an integer multiple of  the electron's charge is an example of a quantum number.
A fundamental matter particle that has strong interactionsGlossary Term. Quarks have an electric chargeGlossary Term of either +2/3 (up, charmGlossary Term and top) or -1/3 (down, strange and bottom) in units where the proton charge is 1.
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One rad is equal to an energy absorptionGlossary Termof 100 ergs in a gram of any material. An "erg" is a unit for quantifying energy (just like a "mile" is used for measuring distance.)
Radiation is energy in transit in the form of high speed particles and electromagnetic wavesGlossary Term. Radiation is further defined into ionizing and non-ionizing radiation.
  • Ionizing radiation is radiation with enough energy so that during an interaction with an atom, it can remove tightly bound electronsGlossary Term from their orbits, causing the atom to become chargedGlossary Term or ionized. Examples are X-rays and electrons.
  • Non-ionizing radiation is radiation without enough energy to remove tightly bound electrons from their orbits around atoms. Examples are microwaves and visible light.
Radiation Oncology
Treatment of tumors with ionization radiationGlossary Term.
Radiation Sickness
The syndrome associated with intense acute exposure to ionizing radiation.
Radiotherapy (also called Radiation Therapy)
Medical therapy consisting of one or more treatments with ionization radiationGlossary Term.
Radiation Transport
The field of nuclear science dealing with the prediction and measurement of the movement of electromagnetic radiation or particles through matter.
The property of spontaneously emitting alpha, beta, and/or gamma radiation as a result of nuclear disintegration.
Radio Frequency (RF)
Any ac frequency at which coherent electromagnetic radiation of energy is possible. Usually considered to denote frequencies above 150 kilohertz and extending up to the infrared range.
The making of shadow images on a photographic emulsion by the action of ionization radiationGlossary Term. The image is the result of the differential attenuationGlossary Term of the radiation in its passage through the object being radiographed.
This type of radiotherapy is the application of monoclonal antibodies that have been tagged with high activities of suitable radionuclides. These tumor-specific antibodies are derived from the patient's own cancer and, hence, they selectively target this tumor when injected into the patient. Also known as monoclonoal antibody therapy.
The branch of medicine that deals with the diagnostic and therapeutic applications of radiation.
Radionuclides are materials that produce ionization radiationGlossary Term, such as X-rays, gamma raysGlossary Term, alpha particles, and beta particles.
The branch of medicine that deals with the diagnostic and therapeutic applications of radiation.
Traveling at a significant fraction of the speed of light.
Residual Interaction
InteractionGlossary Term between objects that do not carry a chargeGlossary Term but that contain constituents that do have a charge. Although some chemical substances involve electrically-charged ions, much of chemistry is due to residual electromagnetic interactionsGlossary Term between electrically neutral atoms. The residual strong interaction between protons and neutrons, due to the strong charges of their quark constituents, is responsible for the binding of the nucleus.
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Any quantity that has only magnitude as opposed to both magnitude and direction. For example mass is scalar quantity. By convention in physics the word speed is a scalar quantity, having only magnitude, while the word velocity is used to denote both the speed and the direction of the motion and is thus a vector quantity.
A mass of attenuating material used to prevent or reduce the passage of radiation or particles.
Shower (also called Electromagnetic Cascade Shower)
ElectronsGlossary Term can create photons by interacting with a medium. In a similar way, photons can create electrons and their antiparticlesGlossary Term, positrons, by interacting with a medium. So, imagine a very high-energy electron, of the sort used at SLAC, impinging on some material. The electron can set photons into motion and these photons can, in turn, set electrons and positrons into motion, and this process can continue to repeat. One high-energy electron can set thousands of particles into motion. Albert Einstein's famous relation governing the equivalence of matter and energy (E = mc²) governs this process -- namely, matter (electrons and positrons) can be creased from pure energy and vice versa. The particle creation process only stops when the energy runs out.
Silicon Vertex Detector
This is a silicon based detector similar to that in a digital camera. It provides precision particle tracking by connecting the dots due to a particle passing through its multiple layers. This allows one to reconstruct any vertex from which two or more tracks emerge. Such a vertex, if outside the beam collision region, indicates the position of a particle decay.
Stanford Linear Accelerator Center - where this virtual visitor center is located, along with many real facilities. Visit us!
Single-Photon Emission Computerized Tomography involves scanning involving the rotation of detectors around a patient and acquires information on the concentration of radionuclides introduced to the patient's body. This is analogous to CT imaging with x-rays. See also PET.
Spectral Range
The range of wavelengths of the electromagnetic radiation that can be produced.
Stanford Positron Electron Accelerating Ring.
The name given to the angular momentum carried by a particle. For composite particles the spin is made up from the combination of the spins of the constituents plus the angular momentum of their motion around one-another. For fundamental particlesGlossary Term spin is an intrinsic and inherently quantum property, it cannot be understood in terms of motions internal to the object.
Does not decayGlossary Term.
Standard Model
Physicists' name for the current theory of fundamental particlesGlossary Term and their interactionsGlossary Term.
The unit of "stereo angle" or solid angle, the angle in three dimensions. It is related to the radian, which is the unit of angle in two dimensions (abbreviated "sterad").
Sterotactic Radiosurgery
This involves the use of multiple small pencil beamsGlossary Term of radiation fired from many different directions and all aimed at the tumor. Machines used include the "gamma-knife," with several hundred small, high-activity Cobalt-60 sources and conventional medical linear acceleratorsGlossary Term equipped with specially designed sterotactic hardware.
Storage Ring
A circular (or near circular) structure  in which either high energy electronsGlossary Term and/or positrons, or protons and/or antiprotons  can be circulated many times and thus "stored".  Used to achieve high energy collisions.  Because of the very different masses of protons and electrons a storage ring must be designed for one or the other type and cannot work for both.
Strange Quark
The third flavorGlossary Term of quark (in order of increasing mass), with electric chargeGlossary Term -1/3.
String Theory
A theory of elementary particlesGlossary Term incorporating relativity and quantum mechanics in which the particles are viewed not as points but as extended  objects. String theory is a possible framework for constructing unified theories which include both the microscopic forces and gravity.
Strong Force
The fundamental strong force is the force between quarks and gluonsGlossary Term that makes them combine to form the observed hadrons, such as protons and neutrons. It also causes forces between hadronsGlossary Term, such as the strong nuclear force that makes protons and neutrons bind together to form nuclei.
Strong Interaction
The interactionGlossary Term responsible for binding quarks and gluonsGlossary Term to make hadronsGlossary Term. Residual strong interactions provide the nuclear binding force. In nuclear physics the term strong interaction is also used for this residual effect. (As a parallel, the force between electrically chargedGlossary Term particles is an electromagnetic interactionGlossary Term, the force between neutral atoms that leads to the formation of molecules is a residual electromagnetic effect.) 
Subatomic Particle
Any particle that is small compared to the size of the atom.
Sun's Corona
The luminous irregular envelope of highly ionized gas outside the chromosphere of the sun.
A star that explodes and becomes extremely luminous in the process.
Synchrotron Radiation
Whenever a chargedGlossary Term particle undergoes accelerated motion it radiates electromagnetic energy. A common example is the emission of radio waves when electronsGlossary Term move back and forth in a radio antenna. A charged particle traveling in the arc of a circle is also undergoing acceleration, due to its change in direction. The radiation emitted by such particles is called synchrotron radiation and it is particularly intense and very directional when electrons traveling at close to the speed of light are bent in magnetic fields.
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A metallic object placed in the beamGlossary Term of electronsGlossary Term to produce x-rays.
The third chargedGlossary Term leptonGlossary Term (in order of increasing mass), with electric charge -1.
Unit of energy equal to that acquired by a particle with one electronic chargeGlossary Term in passing through a potential difference of one trillion volts.
Top Quark
The sixth flavorGlossary Term of quark (in order of increasing mass) with electric chargeGlossary Term +2/3.
The record of the path of a particle traversing a detectorGlossary Term.
Tracking Chamber
A section of a particle detectorGlossary Term capable of detecting the passage of electrically chargedGlossary Term particles.
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Unified Field Theory
A unified field theory is one that attempts to combine any two or more of the known interaction types (strong, electromagnetic, weak and gravitational) in a single theory so that the two distinct types of interaction are seen as two different aspects of a single mathematical structure. A 'grand unified' theory (or GUT) unifies three of the four types (strong, weak and electromagnetic interactions) in this way. The benefit is that the unification gives a simpler overall theory and predicts relationships between parameters that are otherwise independent.
The units one uses should be of a size that makes sense for the particular subject at hand. It is easiest to define units in each area of science and then relate them to one another than to go around measuring particle masses in grams or cheese in proton mass units.

In particle physics the standard unit is the unit of energy GeV. One eV (electron Volt) is the amount of energy that an electronGlossary Term gains when it moves through a potential difference of 1 Volt (in a vacuum). G stands for Giga, or 109. Thus a GeV is a billion (in US counting) electron Volts. The mass-energy of a proton or neutron is approximately 1 GeV.

Matter that is capable of undergoing spontaneous change, as in a radioactive nuclide or an excited nuclear system. An unstable particle is any elementary particle that spontaneously decaysGlossary Term into other particles.
Up Quark
The first flavorGlossary Term of quark (in order of increasing mass), with electric chargeGlossary Term +2/3.
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A space entirely devoid of matter (called also, by way of distinction, absolute vacuum). In a more general sense, a space, as the interior of a closed vessel, which has been exhausted to a high or the highest degree by an air pump or other artificial means.
Any quantity that has both magnitude and direction. Velocity is a vector. An example might be 55 mph south. 55 is the magnitude and south is the direction.
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An evacuatedGlossary Term rectangular copper tube that provides a path for microwaves to travel along. They are very carefully designed for a particular wavelength microwave, so as to transmit as much energy as possible.
A carrier particleGlossary Term of the weak interactionGlossary Term.
Weak Interaction
The interactionsGlossary Term responsible for all processes in which flavorGlossary Term changes; hence for the instability of heavy leptonsGlossary Term and quarks, and particles that contain them. Weak interactions that do not change flavor have also been observed.
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X-rays are electromagnetic wavesGlossary Term (photons of light) emitted by energy changes of electronsGlossary Term. These energy changes are either in the electron orbital shells that surround an atom or are due to the slowing down (i.e., interactionGlossary Term) of electrons in matter, such as a "target" in an x-ray machine.
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Z Boson
Also known as a Z Particle. A carrier particleGlossary Term of weak interactionsGlossary Term. It is involved in weak processes that do not change flavorGlossary Term.
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