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

Phosphor
A substance that emits light when excited by radiation.

Photon
The carrier particle of the electromagnetic interaction. Depending on its frequency (and therefore its energy) photons can have different names such as visible light, X rays and gamma rays. We describe light in several ways. When we talk about "photons" we generally think of uncharged particles with out mass that carry energy ...

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 6.626069 x 10^-34 Joule seconds. Symbolized by h.

Polarization
A polarized particle beam 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

Positron
Antiparticle of the electron.

Proton
A baryon with electric charge +1. Protons contain a basic structure of two up quarks and one down quark . 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.

Quantum
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.

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 charge 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 charge given as an integer multiple of the electron's charge is an example of a quantum number.

Quark
A fundamental matter particle that has strong interactions Quarks have an electric charge of either +2/3 (up, charm and top) or -1/3 (down, strange and bottom) in units where the proton charge is 1.

Rad
One rad is equal to an energy absorption of 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
Radiation is energy in transit in the form of high speed particles and electromagnetic waves. 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 electrons from their orbits, causing ...

Radiation Oncology
Treatment of tumors with ionization radiation.

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 radiation. More information about radiotherapy is available through the National Cancer Institute at the National Institutes of Health.

Radiation Transport
The field of nuclear science dealing with the prediction and measurement of the movement of electromagnetic radiation or particles through matter.

Radioactivity
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.

Radiography
The making of shadow images on a photographic emulsion by the action of ionization radiation. The image is the result of the differential attenuation of the radiation in its passage through the object being radiographed.

Radioimmunotherapy
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.

Radiology
The branch of medicine that deals with the diagnostic and therapeutic applications of radiation.

Radionuclide
Radionuclides are materials that produce ionization radiation, such as X-rays, gamma rays, alpha particles, and beta particles

Relativistic
Traveling at a significant fraction of the speed of light.

Residual Interaction
Interaction between objects that do not carry a charge 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 interactions between electrically neutral atoms. The residual strong interaction between protons and neut...

Scalar
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

Shield
A mass of attenuating material used to prevent or reduce the passage of radiation or particles.

Shower
(also called Electromagnetic Cascade Shower) can create photons by interacting with a medium. In a similar way, photons can create electrons and their antiparticles, 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 ...

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 ...

SLAC
SLAC National Accelerator Laboratory - where this virtual visitor center is located, along with many real facilities Visit us!

SPECT
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.

SPEAR
Stanford Positron Electron Accelerating Ring.

Spin
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 particles spin is an intrinsic and inherently quantum property, it cannot be understood in terms o...

Stable
Does not decay.

Standard Model
Physicists' name for the current theory of fundamental particles and their interactions.

Steradian
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 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 accelerators equipped with specially designed sterotactic hardwar...

Storage Ring
A circular (or near circular) structure in which either high energy 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 on...

Strange Quark
The third flavor of quark (in order of increasing mass), with electric charge -1/3.

String Theory
A theory of elementary particles 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 Gluons that makes them combine to form the observed hadrons, such as protons and neutrons. It also causes forces between hadrons, such as the strong nuclear force that makes protons and neutrons bind together to form nuclei.

Strong Interaction
The interaction responsible for binding quarks and Gluons to make hadrons. 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 charged particles is an electromagnetic interaction, the force bet...

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.

Supernova
A star that explodes and becomes extremely luminous in the process.

Synchrotron Radiation
Whenever a charged particle undergoes accelerated motion it radiates electromagnetic energy. A common example is the emission of radio waves when electrons 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 b...

Target
A metallic object placed in the beam of to produce x-rays

Tau
The third charged lepton (in order of increasing mass), with electric charge -1.

TeV
(Tera Electron Volt) Unit of energy equal to that acquired by a particle with one electronic charge in passing through a potential difference of one trillion volts.

Top Quark
The sixth flavor of quark (in order of increasing mass) with electric charge +2/3.

Track
The record of the path of a particle traversing a detector

Tracking Chamber
A section of a particle detector capable of detecting the passage of electrically charged particles.

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 ...

Units
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 On...

Unstable
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 decays into other particles.

Up Quark
The first flavor of quark (in order of increasing mass), with electric charge +2/3.

Vacuum
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.

Vector
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.

Waveguide
An evacuated 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.

Weak Interaction
The interactions responsible for all processes in which flavor changes; hence for the instability of heavy leptons and quarks, and particles that contain them. Weak interactions that do not change flavor have also been observed.

Møller Scattering
Scattering of electrons by electrons.

W Boson
A carrier particle of the weak interaction.

X-rays
X-rays are electromagnetic waves (photons of light) emitted by energy changes of . These energy changes are either in the electron orbital shells that surround an atom or are due to the slowing down (i.e., interaction) of electrons in matter, such as a "target" in an x-ray machine.

Z Boson
Also known as a Z Particle. A carrier particle of weak interactions It is involved in weak processes that do not change flavor.