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 classes of particles is a deep puzzle which is yet to
be fully understood. More Information: Antiparticles,
Experimental Facility, Accelerator
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.
Some particles are their own antiparticles, the antiparticle of a photon is
a photon for instance. Conserved
quantities such as baryon number and lepton number are further types of "charges" that are
reversed for particle and antiparticle. Thus an electron and an electron neutrino both have electron
number +1 while their antiparticles the positron and the anti-electron-neutrino
have electron number -1. More Information: Antiparticles
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.
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.
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 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.
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. More
Station A Experiment
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.
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 fields 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. More
Tracking: Cerenkov Detectors
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. More
Information: Force and
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 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.
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. More Information: Detectors.
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". More
Information: EGS: The
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
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).
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
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 electrons. More Information: Electrons
are Accelerated in a Copper Structure
The basic unitof 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 multiples are MeV (million eV), GeV (billion
eV or giga-electronvolt) and TeV (trillion eV).
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 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.
More Information: Fundamental
Electrons carry electrical charge and successful manipulation of electrons 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.
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 fundamental particles. All other
particles are composite, that is made from combinations of fundamental particles. More Information: Fundamental
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. More Information: Waveguide,
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. More Information: What is an Event,
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.
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. More Information: Spin
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 electrons, muon and tau, again in
increasing order of mass. For each charged lepton flavor there is a
corresponding neutrino flavor. More Information: Quarks, Leptons
The known fundamental interactions are the strong, electromagnetic, weak
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 these strong,
electromagnetic or weak interactions. More Information: Forces and
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. More Information: Gravitational
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. More
A branch of science that tries to understand the interactions of the fundamental
particles, such as electrons, 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.
Radiation that has enough energy to eject electrons from electrically neutral atoms, leaving
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 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 ionization.
A fundamental matter particle that does not participate in strong
interactions. The charge leptons are the electron (e), the muon (), the tau () and their antiparticles.
Neutral leptons are called neutrinos ( ). More
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 long accelerator, consisting of a cylindrical,
disc-loaded, copper waveguide placed on concrete girders in a tunnel about 25
feet underground. More Information: Accelerator Form and
Linear Collider, Next Linear
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 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. More Information: EGS and
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.
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.
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. More Information: Leptons
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 antiparticles for each type. All conservation laws
are obeyed in these processes. More Information: Antiparticles
Pair Production and Annihilation
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 http://www.triumf.ca/welcome/petscan.html.
See also, SPECT.
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 (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 particle 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. More Information: Electromagnetic
A baryon with
+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. More Information: Hadrons
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.
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. More
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. More Information: Antiparticles
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. More Information: Quarks, Strong
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 the atom to
become charged 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.
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.
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.
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 neutrons, due
to the strong charges of their quark constituents, is responsible for the
binding of the nucleus.
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
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 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. More Information: Liquid
Argon Calorimeter Electromagnetic Shower, Why was
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.More Information: Vertex Detector
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.
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 of motions internal to the object. More Information: Spin
This involves the use of multiple small pencil beams 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 hardware.
A circular (or near circular) structure in which either high energy electrons 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. More
SPEAR Storage Ring, Circular
Accelerators, Positron Electron
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. More Information: Strong
Interactions, Forces and
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 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. More Information: Stanford Synchrotron
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
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 electron 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. More Information: Units
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.
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. More Information: Vacuum System
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. More Information: Waveguide