Particle shower
Encyclopedia
In particle physics
, a shower is a cascade of secondary particles
produced as the result of a high-energy
particle interacting with dense matter. The incoming particle interacts, producing multiple new particles with lesser energy; each of these then interacts in the same way, a process that continues until many thousands, millions, or even billions of low-energy particles are produced. These are then stopped in the matter and absorbed.
or electron
. Hadronic showers are produced by hadron
s (i.e. nucleon
s and other particles made of quark
s), and proceed mostly via the strong nuclear force.
and Compton scattering
are dominant), photons interact with matter primarily via pair production
— that is, they convert into an electron-positron
pair, interacting with an atomic nucleus
or electron in order to conserve momentum
. High-energy electrons and positrons primarily emit photons, a process called bremsstrahlung
. These two processes continue in turn, until the remaining particles have lower energy. Electrons and photons then lose energy via scattering until they are absorbed by atoms.
The "shower depth" is approximately determined by the relation
where
is the radiation length
of the matter, and
is the critical energy
. It logarithmically depends on the initial energy.
The characteristic radius of the shower is the Molière radius
.
, pion
, or atomic nucleus. Some such particles have electric charge
, and so produce showers that are partially electromagnetic, but all also interact with nuclei via the strong force. Although the details are more complex for this force, such an interaction involves one hadron interacting with a nucleus and producing several lower-energy hadrons. This continues, as with the electromagnetic shower, until all particles are stopped or absorbed in the material.
s hit earth's atmosphere on a regular basis, and they produce showers as they proceed through the atmosphere. It was from these air showers
that the first muon
s and pion
s were detected experimentally, and they are used today by a number of experiments as a means of observing ultra-high-energy cosmic ray
s. Some experiments, like Fly's Eye, have observed the visible atmospheric fluorescence
produced at the peak intensity of the shower; others, like Haverah Park experiment
, have detected the remains of a shower by sampling the energy deposited over a large area on the ground.
In particle detector
s built at high-energy particle accelerator
s, a device called a calorimeter
records the energy of particles by causing them to produce a shower and then measuring the energy deposited as a result. Many large modern detectors have both an electromagnetic calorimeter and a hadronic calorimeter, with each designed specially to produce that particular kind of shower and measure the energy of the associated type of particle.
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...
, a shower is a cascade of secondary particles
Subatomic particle
In physics or chemistry, subatomic particles are the smaller particles composing nucleons and atoms. There are two types of subatomic particles: elementary particles, which are not made of other particles, and composite particles...
produced as the result of a high-energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
particle interacting with dense matter. The incoming particle interacts, producing multiple new particles with lesser energy; each of these then interacts in the same way, a process that continues until many thousands, millions, or even billions of low-energy particles are produced. These are then stopped in the matter and absorbed.
Types of showers
There are two basic types of showers. Electromagnetic showers are produced by a particle that interacts primarily or exclusively via the electromagnetic force, usually a photonPhoton
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
or electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
. Hadronic showers are produced by hadron
Hadron
In particle physics, a hadron is a composite particle made of quarks held together by the strong force...
s (i.e. nucleon
Nucleon
In physics, a nucleon is a collective name for two particles: the neutron and the proton. These are the two constituents of the atomic nucleus. Until the 1960s, the nucleons were thought to be elementary particles...
s and other particles made of quark
Quark
A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Due to a phenomenon known as color confinement, quarks are never directly...
s), and proceed mostly via the strong nuclear force.
Electromagnetic showers
An electromagnetic shower begins when a high-energy electron, positron or photon enters a material. At high energies (above a few MeV, below which photoelectric effectPhotoelectric effect
In the photoelectric effect, electrons are emitted from matter as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as photoelectrons...
and Compton scattering
Compton scattering
In physics, Compton scattering is a type of scattering that X-rays and gamma rays undergo in matter. The inelastic scattering of photons in matter results in a decrease in energy of an X-ray or gamma ray photon, called the Compton effect...
are dominant), photons interact with matter primarily via pair production
Pair production
Pair production refers to the creation of an elementary particle and its antiparticle, usually from a photon . For example an electron and its antiparticle, the positron, may be created...
— that is, they convert into an electron-positron
Positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...
pair, interacting with an atomic nucleus
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
or electron in order to conserve momentum
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
. High-energy electrons and positrons primarily emit photons, a process called bremsstrahlung
Bremsstrahlung
Bremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon because energy is conserved. The term is...
. These two processes continue in turn, until the remaining particles have lower energy. Electrons and photons then lose energy via scattering until they are absorbed by atoms.
The "shower depth" is approximately determined by the relation
where
is the radiation lengthRadiation length
In physics, the radiation length is a characteristic of a material, related to the energy loss of high energy, electromagnetic-interacting particles with it.-Definition:High-energy electrons predominantly lose energy in matter...
of the matter, and
is the critical energyCritical Energy
Critical Energy is a live album by Threshold, released in 2004.- Disc 1:# "Phenomenon" # "Oceanbound" # "Choices" # "Angels" # "Falling Away" # "Virtual Isolation" # "Innocent"...
. It logarithmically depends on the initial energy.
The characteristic radius of the shower is the Molière radius
Molière radius
The Molière radius is a characteristic constant of a material giving the scale of the transverse dimension of the fully contained electromagnetic showers initiated by an incident high energy electron or photon. By definition, it is the radius of a cylinder containing on average 90% of the shower's...
.
Hadronic showers
A hadronic shower is produced by a high-energy hadron such as a nucleonNucleon
In physics, a nucleon is a collective name for two particles: the neutron and the proton. These are the two constituents of the atomic nucleus. Until the 1960s, the nucleons were thought to be elementary particles...
, pion
Pion
In particle physics, a pion is any of three subatomic particles: , , and . Pions are the lightest mesons and they play an important role in explaining the low-energy properties of the strong nuclear force....
, or atomic nucleus. Some such particles have electric charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
, and so produce showers that are partially electromagnetic, but all also interact with nuclei via the strong force. Although the details are more complex for this force, such an interaction involves one hadron interacting with a nucleus and producing several lower-energy hadrons. This continues, as with the electromagnetic shower, until all particles are stopped or absorbed in the material.
Examples of showers
Cosmic rayCosmic ray
Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...
s hit earth's atmosphere on a regular basis, and they produce showers as they proceed through the atmosphere. It was from these air showers
Air shower (physics)
An air shower is an extensive cascade of ionized particles and electromagnetic radiation produced in the atmosphere when a primary cosmic ray enters the atmosphere...
that the first muon
Muon
The muon |mu]] used to represent it) is an elementary particle similar to the electron, with a unitary negative electric charge and a spin of ½. Together with the electron, the tau, and the three neutrinos, it is classified as a lepton...
s and pion
Pion
In particle physics, a pion is any of three subatomic particles: , , and . Pions are the lightest mesons and they play an important role in explaining the low-energy properties of the strong nuclear force....
s were detected experimentally, and they are used today by a number of experiments as a means of observing ultra-high-energy cosmic ray
Ultra-high-energy cosmic ray
In astroparticle physics, an ultra-high-energy cosmic ray or extreme-energy cosmic ray is a cosmic ray with an extreme kinetic energy, far beyond both its rest mass and energies typical of other cosmic rays....
s. Some experiments, like Fly's Eye, have observed the visible atmospheric fluorescence
Fluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
produced at the peak intensity of the shower; others, like Haverah Park experiment
Haverah Park experiment
The Haverah Park experiment was a cosmic ray air shower detection array consisting of water Cherenkov detectors distributed over an area of 12 km2on Haverah Park on the Pennine moorland near Harrogate, North Yorkshire...
, have detected the remains of a shower by sampling the energy deposited over a large area on the ground.
In particle detector
Particle detector
In experimental and applied particle physics, nuclear physics, and nuclear engineering, a particle detector, also known as a radiation detector, is a device used to detect, track, and/or identify high-energy particles, such as those produced by nuclear decay, cosmic radiation, or reactions in a...
s built at high-energy particle accelerator
Particle accelerator
A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...
s, a device called a calorimeter
Calorimeter (particle physics)
In particle physics, a calorimeter is an experimental apparatus that measures the energy of particles. Most particles enter the calorimeter and initiate a particle shower and the particles' energy is deposited in the calorimeter, collected, and measured. The energy may be measured in its...
records the energy of particles by causing them to produce a shower and then measuring the energy deposited as a result. Many large modern detectors have both an electromagnetic calorimeter and a hadronic calorimeter, with each designed specially to produce that particular kind of shower and measure the energy of the associated type of particle.
See also
- Air shower (physics)Air shower (physics)An air shower is an extensive cascade of ionized particles and electromagnetic radiation produced in the atmosphere when a primary cosmic ray enters the atmosphere...
, an extensive (many kilometres wide) cascade of ionized particles and electromagnetic radiationElectromagnetic radiationElectromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
produced in the atmosphereEarth's atmosphereThe atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
when a primary cosmic rayCosmic rayCosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...
(i.e. one of extraterrestrial origin) enters our atmosphere. - MAGIC Cherenkov TelescopeMAGIC (telescope)MAGIC is a system of two Imaging Atmospheric Cherenkov telescopes situated at the Roque de los Muchachos Observatory on La Palma, one of the Canary Islands, at about 2200 m above sea level...
- Pierre Auger ObservatoryPierre Auger ObservatoryThe Pierre Auger Observatory is an international cosmic ray observatory designed to detect ultra-high-energy cosmic rays: single sub-atomic particles with energies beyond 1020 eV...
- ATLAS experiment calorimeters