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SLAC Visitor Centre - Physics glossary Category: Education > Physics glossary Date & country: 10/11/2010, USA Words: 164

Absorption
The transfer of energy to a medium, such as body tissues, as a radiation beam passes through the medium.

Accelerator
A device (i.e., machine) used to produce high-energy high-speed beams of charged particles, such as electrons, 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.

Annihilation
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 antiparticles and providing their motion.

Angstrom
A unit of length equal to 10^-10 meter.

Antimatter
A material made from antiparticles. 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 interactions are almost identical. The asymmetry of the universe between these two cla...

Antiparticles
In particle physics every particle with any type of charge or fermion 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 electron is a positron It has exactly the same mass as an electron but positive charge

Attenuation
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 intensity of radiation upon passage through a medium. The attenuation is caused by absorption and scattering.

Backscattering
Primary radiation deflected or secondary radiation emitted in the general direction of the incident radiation beam.

Barrier
Radiation-absorbing material, such as lead or concrete, used to reduce radiation exposure. A primary barrier attenuates useful beam to the required degree. A secondary barrier attenuates stray radiation to the required degree.

Baryon
A hadron made from a basic structure of three quarks. The proton and the neutron are both baryon. The antiproton and the antineutron are antibaryons.

Beam
A unidirectional or approximately unidirectional flow of electromagnetic radiation or particles.

Bhabha Scattering
Scattering of positrons by electrons.

Boson
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 particles of all interactions are bosons. Mesons are also bosons.

Bottom Quark or B Quark
The fifth flavor of quark (in order of increasing mass), with electric charge -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 electron 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 pl...

Brachytherapy
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.

Bremsstrahlung
X-rays emitted when a charged 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 charged particle. After each beam pulse a photographic record is made of the chamber and then it is depressurized to clear the bubbles.

Carrier Particle
A fundamental boson associated with quantum excitations of the force field corresponding to some interaction. Gluons are carrier particles for strong interactions (color force fields), photons are carrier particles of electromagnetic interactions, and the W and Z bosons are carrier particles for weak interactions.

Calorimeter
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).

Cathode Ray Tube
(CRT) An evacuated 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 charged 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 t...

CERN
The major European International Accelerator Laboratory located near Geneva, Switzerland. (European Organisation for Nuclear Research, originally called l'Organisation Européenne pour la Recherche Nucléair) The WWW was created at CERN.

Charge
A quantity carried by a particle that determines its participation in an interactions process. A particle with electric charge has electrical interactions; one with strong charge (or color charge) has strong interactions, etc.

Charm
One "flavor" of quarks. Also known as the C quark.

Clinac®
Varian trade name for a range of linear accelerator models used in cancer treatment and stereotactic radiosurgery.

Coherence
A term that's applied to electromagnetic waves. 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 cohere...

Collimation
The alignment of the direction of the photons, so the beam of radiation can be directed at a well-defined part of a target material.

Collimator
A mechanical device, sometimes called a "slit", installed along the trajectory of a beam to reduce the size of the beam. Collimators are also useful for removing stray radiation.

Color Charge
The charge associated with strong interactions. Quarks and gluons have color charge and consequently participate in strong interactions Leptons, photons and W and Z bosons 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.

Cosmos
The universe regarded as an orderly, harmonious whole.

Crystalline
A regularly repeated crystal-like substructure.

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.

Decay
Any process in which a particle disappears and in its place two or more different particles appear.

Detector
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 events.

Dipole Magnet
Any magnet with one north and one south pole. In an accelerator the dipole magnets are used to steer a particle beam 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 beam 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 cond...

Dose Equivalent
(DE) Parameter used to express the risk of the deleterious effects of ionization radiation 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.

Dosimeter
A radiation sensitive device, e.g., film, monitor ion chamber, TLD, etc., with a known sensitivity that is placed in the beam path of radiation to dose.

Dosimetry
The calculations, measurements and other activities required for determining the radiation dose to be delivered.

Down Quark or D Quark
The second flavor of quark (in order of increasing mass), with electric charge -1/3.

Electric Field
A force field which defines what acceleration an electric charge 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...

Electromagnetic
(em) 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 rays are also electromagnetic waves. ...

Electromagnetic Interaction
The interaction due to electric charge; this includes magnetic effects that have to do with moving electric charges.

Electron
The least massive electrically charged particle, therefore absolutely stable. It is the most common lepton with charge -1. An electron is one of the fundamental particles 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 a...

Electron Accelerator
Electrons carry electrical charge and successful manipulation of 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 s...

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 accelerator. 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 leptons and their antiparticles, which have no known substructure. Gluons, photons and W and Z bosons are also funda...

eV
(electronvolt) The basic unit of energy used in high energy physics It is the energy gained by one electron 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 multiple...

Evacuated
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.

Event
An event occurs when two particles collide or a single particle decay. 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.

Excitation
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.

Fermilab
Fermi National Accelerator Laboratory in Batavia, Illinois (named for particle physics pioneer Enrico Fermi).

Fermion
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, leptons and baryons are all fermions.

Flavor
The name used for the different quark types and the different lepton types. The six flavors of quarks are up, down, strange, charm, bottom, top, in increasing order of mass. The flavors of charged leptons are , 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 interactions are the strong, electromagnetic, weak and gravitational interaction. 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 decays can be understood in terms of the...

Fundamental Particle
A particle with no internal substructure. In the Standard Model, the quarks, leptons, photons, Gluons ,W-boson and Z-bosons are fundamental. All other objects are made from these particles.

Gamma Rays
Gamma rays are electromagnetic waves or photons emitted from the nucleus (center) of an atom. See also: photon.

Geiger Counter
A device that detects the passage of charged particles via the ionization 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 charge in passing through a potential difference of one billion volts.

Gluon
The carrier particle of the strong interaction.

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.

Hadron
Any particle made of quarks and Gluons, i.e. a meson or a baryon. All such particles have no strong charge (i.e. are strong charge neutral objects) but participate in residual strong interactions due to the strong charges of their constituents

Half-life
The time required for half the nuclei in a sample of a specific isotopic species to undergo radioactive decay.

Heterogeneous
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 interactions of the fundamental particles, such as , 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 ...

Intensity
The amount of radiation, for example, the number of photons arriving in a given time

Interaction
A process in which a particle decays or it responds to a force due to the presence of another particle (as in a collision).

Ion
Atomic particle, atom, or chemical radical bearing an electrical charge, either negative or positive.

Ionization
The process by which a neutral atom or molecule acquires a positive or negative charge.

Ionizing Radiation
Radiation that has enough energy to eject from electrically neutral atoms, leaving behind charge atoms or ions. There are four basic types of ionizing radiation: Alpha particles (helium nuclei), beta particles (electrons), neutrons, and gamma rays (high frequency electromagnetic waves, x-rays, are generally identical to gamma rays ex...

Jet
The name physicists give to a cluster of particles emerging from a collision or decay event 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 hadrons.

Klystron
An evacuated tube used as an oscillator or amplifier at microwave frequencies. In the klystron, an electron beam is velocity modulated (periodically bunched) to produce large amounts of power

Lepton
A fundamental matter particle that does not participate in strong interactions. The charge leptons are the (e), the muon (), the tau () and their antiparticles. Neutral leptons are called neutrinos ( ).

Linear Accelerator
A type of particle accelerator in which charged particles are accelerated in a straight line, either by a steady electrical field or by means of radiofrequency electric fields. 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 ...

Mammography
Mammography is a low-dose x-ray procedure that creates an image of the breast. The x-ray image is called a mammogram.

Matter
We call the commonly observed particles such as protons, neutrons and matter particles, and their antiparticles are then antimatter.

Mean-life
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 Physics) to heal people. Radiation therapy is one example. CAT scans, mammography, and other x-ray imaging techniques are diagnostic techniques that have also been developed by physicists working in medicine. Another importan...

Meson
A hadron with the basic structure of one quark and one antiquark.

MeV
(Mega Electron Volt) Energy equal to that acquired by a particle with one electronic charge in passing through a potential difference of one million volts.

Micron
One millionth of a meter; also known as a micrometer.

Microwaves
Waves of electromagnetic radiation that oscillate from approximately 10⁹ to 3 x 10¹¹Hz. (cycles per second). SLAC's klystrons produce microwaves of 2,856 MHz.

Momentum
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 beca...

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, ...

Muon
The second lepton (in order of increasing mass), with electric charge -1.

Muon Chamber
The outer layers of a particle detector capable of registering tracks of charged particles. The detector is designed so that the only charged particles that can get out to this layer are muons

Neutrino
A lepton with no electric charge. Neutrinos participate only in weak (and gravitational) interactions 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.

Neutron
A baryon with electric charge zero. Its basic structure is two down quarks and one up quark.

Nucleus
A collection of protons and neutrons that form the core of an atom (plural: nuclei).

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 particle...

Particle
In "particle physics", a subatomic object with definite mass and charge.

Pauli Exclusion Principle
No two fermions 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 compu...