Radiation astronomy/Detectors

Radiation detectors provide a signal that is converted to an electric current. The device is designed so that the current provided is proportional to the characteristics of the incident radiation.

This tree diagram shows the relationship between types and classification of most common particle detectors. Credit: Wdcf.{{free media}}

There are detectors that provide a change in substance as the signal and these may be automated to provide an electric current or quantified proportional to the amount of new substance.

Theoretical radiation detectorsEdit

Def. a "device capable of registering a specific substance or physical phenomenon"[1] is called a detector.

Def. "a device that recovers information of interest contained in a modulated wave"[2] is called a radio detector.

Def. "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 particle accelerator"[3] is called a particle detector or radiation detector.

Def. "a device or organ that detects certain external stimuli and responds in a distinctive manner"[4] is called a sensor.


This is an overview of electromagnetic radiation absorption. Credit: Jon Chui.{{free media}}
An example of applying Absorption spectroscopy is the first direct detection and chemical analysis of the atmosphere of a planet outside our solar system in 2001. Sodium filters the alien star light of HD 209458 as the hot Jupiter planet passes in front. The process and absorption spectrum are illustrated above. Credit: A. Feild, STScI and NASA website.{{free media}}

In the image at center visible light is used as a specific example of absorption spectroscopy. A white beam source – emitting light of multiple wavelengths – is focused on a sample (the complementary color pairs are indicated by the yellow dotted lines). Upon striking the sample, photons that match the energy gap of the molecules present (green light in this example) are absorbed in order to excite the molecule. Other photons transmit unaffected and, if the radiation is in the visible region (400-700nm), the sample color is the complementary color of the absorbed light. By comparing the attenuation of the transmitted light with the incident, an absorption spectrum can be obtained.

Active galactic nucleiEdit

The image contains a series of radio images at successive epochs using the VLBA of the jet in the broad-line radio galaxy 3C 111. Credit: M. Kadler, E. Ros, M. Perucho, Y. Y. Kovalev, D. C. Homan, I. Agudo, K. I. Kellermann, M. F. Aller, H. D. Aller, M. L. Lister, and J. A. Zensus.{{fairuse}}
Swedish crystal radio made by Radiola with earphones. Credit: Holger.Ellgaard.{{free media}}
A germanium diode is used in modern crystal radios (about 3 mm long). Credit: Aomorikuma.{{free media}}
Eastern end of the Very Long Baseline Array (VLBA), St. Croix, U.S. Virgin Islands, is shown. Credit: Cumulus Clouds.{{free media}}

A crystal radio receiver is in the top right image. The device at its top is the cat's whisker detector, with a pair of earphone jacks provided.

A crystal radio receiver uses only the power of the received radio signal and is named for its most important component, a crystal detector, originally made from a piece of crystalline mineral such as galena.[5]

Galena (lead sulfide) was the most common crystal used,[6][7][8] but various other types of crystals were also used, the most common being iron pyrite (fool's gold, FeS2), silicon, molybdenite (MoS2), silicon carbide (carborundum, SiC), and a zincite-bornite (ZnO-Cu5FeS4) crystal-to-crystal junction trade-named Perikon.[9][10]

In modern sets, a semiconductor diode is used for the detector, which is much more reliable than a crystal detector and requires no adjustments.[9][11][12] Germanium diodes (or sometimes Schottky diodes) are used instead of silicon diodes, because their lower forward voltage drop (roughly 0.3 V compared to 0.6 V[13]) makes them more sensitive.[11][14]

Radio receivers are essential components of all systems that use radio, where the information produced by the receiver may be in the form of sound, moving images (television), or digital data.[15]

A radio telescope is a specialized antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky.[16][17][18]

Each receiver (lowest image on the right) in the VLBA consists of a parabolic dish antenna 25 m (82 feet) in diameter, is about as tall as a ten-story building when the antenna is pointed straight up, where each antenna weighs about 218 metric tons (240 short tons), with its adjacent control building, containing the supporting electronics and machinery for the receiver, including low-noise electronics, digital computers, data storage units, and the antenna-pointing machinery for each of the antennas.[19]

Alpha particlesEdit

A Bragg curve of 5.49 MeV alpha particles in air is illustrated. Credit: Helmut Paul.{{free media}}
Forming of alcohol condensation trails is described in a diffusion cloud chamber. Credit: Kotarak71.{{free media}}
Rutherford scattering of an alpha particle from a 210
source is shown in a diffusion cloud chamber. Credit: Qwerty123uiop.{{free media}}

An alpha particle from a 210
source near point 1 undergoes Rutherford scattering near point 2 of about 30° and scatters near point 3 coming to rest in the gas. The gas nucleus received enough kinetic energy in the elastic collision at point 2 to cause a short visible recoiling track near point 2.

Breakdown voltagesEdit

Photo of long-flashover arrester (LFA) stressed by lighting stroke; a LFA is a lightning protection device for medium-voltage overhead power lines. Credit: Maremoto.{{free media}}

Def. the "minimum voltage that causes part of an insulator to become electrically conductive"[20] is called a breakdown voltage.

For sensitive electronics, excessive current can flow if a voltage spike exceeds a material's breakdown voltage, or if it causes avalanche breakdown.

Within rarefied gases found in certain types of lamps, breakdown voltage is also sometimes called the striking voltage.[21]

Bubble chambersEdit

In this photograph is recorded "[t]he first use of a hydrogen bubble chamber to detect neutrinos, on November 13, 1970. Credit: Argonne National Laboratory.{{free media}}

In the first use of a hydrogen bubble chamber to detect neutrinos, a neutrino hit a proton in a hydrogen atom. The collision occurred at the point where three tracks emanate on the right of the photograph.

"If neutrinos have negligible rest mass, the present density expected for relic neutrinos from the big bang is nν = 110 (Tγ/2.7 K)3 cm–3 for each two-component species. This is of order the photon density nγ, differing just by a factor 3/11 (i.e. a factor 3/4 because neutrinos are fermions rather than bosons, multiplied by 4/11, the factor by which the neutrinos are diluted when e+–e annihilation boosts the photon density). This conclusion holds for non-zero masses, provided that mvc2 is far below the thermal energy (~ 5 MeV) at which neutrinos decoupled from other species and that the neutrinos are stable for the Hubble time. Comparison with the baryon density, related to Ω via nb = 1.5 x 10–5 Ωb h2 cm–3, shows that neutrinos outnumber baryons by such a big factor that they can be dynamically dominant over baryons even if their masses are only a few electron volts. In fact, a single species of neutrino would yield a contribution to Ω of Ωv = 0.01 h–2 (mv)eV, so if h = 0.5, only 25 eV is sufficient to provide the critical density."[22]

"Neutrinos of nonzero mass would be dynamically important not only for the expanding universe as a whole but also for large bound systems such as clusters of galaxies. This is because they would now be moving slowly: if the universe had cooled homogeneously, primordial neutrinos would now be moving at around 200 (mv)-1eV km s–1. They would be influenced even by the weak (~ 10–5 c2) gravitational potential fluctuations of galaxies and clusters. If the three (or more) types of neutrinos have different masses, then the heaviest will obviously be gravitationally dominant, since the numbers of each species should be the same."[22]


The Solar Occultation for Ice Experiment uses solar occultation to measure cloud particles, temperature and atmospheric gases. Credit: NASA.{{free media}}

The Solar Occultation for Ice Experiment (SOFIE) uses solar occultation to measure cloud particles, temperature and atmospheric gases involved in forming the clouds to reveal the mixture of chemicals that prompt noctilucent cloud (NLC) formation, as well as the environment in which the clouds form.[23]

Cosmic raysEdit

Cloud chamber shows visible tracks from α-particles (short, thick) and β-particles (long, thin). Credit: Bionerd.{{free media}}

"The CMS [of the Galileo Orbiter Energetic Particles Detector (EPD)] contained two types of energetic particle telescopes. A small time-of-flight (TOF) telescope was oriented in the 0 degree direction while a pair of delta-E x E solid-state detector telescopes (covering higher energy ranges) were oriented in the opposite direction. The TOF portion of the CMS was modified during the post-Challenger period to permit a lower-energy threshold for composition measurements. The TOF telescope permitted the measurement of H ions from 80 keV-1.25 MeV, He from 27 keV/nucleon-1.0 MeV/nucleon, medium nuclei (O) between 12-522 keV/nucleon, intermediate nuclei (S) between 16-310 keV/nucleon, and heavy nuclei (Fe) between 20-200 keV/nucleon. The delta-E x E telescopes were designed to permit the measurement of Z>=2 ions to higher energies than those attained by the CMS TOF telescope. These instruments measured He (0.19-1.4 MeV/nucleon), medium nuclei (O: 0.16-10.7 MeV/nucleon), intermediate nuclei (Na: 1.0-11.7 MeV/nucleon), and heavy nuclei (Fe: 0.22-15.0 MeV/nucleon). The TOF telescope measured ions in 13 composition channels as did the delta-E x E telescopes."[24]


This drawing shows the Cloud Imaging and Particle Size (CIPS) detector of the AIM or Explorer 90 satellite. Credit: NASA.{{free media}}

The CIPS instrument has four cameras positioned at different angles, which provide multiple views of clouds from different angles and allow a determination of the sizes of the ice particles that make up the cloud,[25] and can be used to infer gravity waves in the atmosphere.[26]


This is an image of the Energetic Particles Detector (EPD) aboard the Galileo Orbiter. Credit: NASA.{{free media}}

"The Energetic Particles Detector (EPD) was designed to: (1) measure the energy and angular distribution, composition, and stability of trapped radiation at Jupiter; (2) study the interaction of these particles with the Galilean satellites and the solar wind; (3) derive thermal plasma flow velocities and temperatures; and, (4) examine adiabatic and non-thermal processes in the trapped radiation."[24]

The EPD "used two instruments: the Low-Energy Magnetospheric Measurements System (LEMMS) and the Composition Measurements System (CMS)."[24]

"[The] two bi-directional, solid-state detector telescopes [are] mounted on a platform which [is] rotated by a stepper motor into one of eight positions. This rotation of the platform, combined with the spinning of the orbiter in a plane perpendicular to the platform rotation, [permits] a 4-pi [or 4π] steradian coverage of incoming [electrons]. The forward (0 degree) ends of the two telescopes [have] an unobstructed view over the [4π] sphere or [can] be positioned behind a shield which not only [prevents] the entrance of incoming radiation, but [contains] a source, thus allowing background corrections and in-flight calibrations to be made."[24]

"The LEMMS was a double-ended telescope containing eight heavily shielded silicon solid-state surface barrier totally-depleted detectors. The 0 degree end of the LEMMS used magnetic deflection to separate incoming electrons and ions. The 180 degree end used absorbers in combination with the detectors to provide measurements of higher-energy electrons and ions. The LEMMS provided measurements of electrons from 15 keV to greater than 11 MeV and of ions from 22 keV to about 55 MeV in 32 rate channels."[24]

"The LEMMS detectors were capable of handling particle incidence rates up to 600,000 counts/s without requiring significant rate corrections for detector dead time. The CMS electronics were more rate restricted, with the TOF telescope capable of operating at rates well above 150,000 counts/s and the delta-E x E telescopes experiencing problems around 50,000 counts/s."[24]

Emission detectorsEdit

Cross-sectional view shows the ZEPLIN-III experiment (CAD) illustrating WIMP and neutron interactions in the liquid xenon target and neutron capture and detection in the anti-coincidence veto detector. Credit: Wimpybuns.{{free media}}
Event from ZEPLIN-III liquid xenon detector with optical simulation insets are shown. Credit: Wimpybuns.{{free media}}

In two-phase xenon – so called since it involves liquid and gas phases in equilibrium – the scintillation light produced by an interaction in the liquid is detected directly with photomultiplier tubes; the ionisation electrons released at the interaction site are drifted up to the liquid surface under an external electric field, and subsequently emitted into a thin layer of xenon vapour: once in the gas, they generate a second, larger pulse of light (electroluminescence or proportional scintillation), which is detected by the same array of photomultipliers, where such systems are known as xenon 'emission detectors'.[27]

"Liquid xenon emission detectors [1–3] have proven extremely useful for rare-event search experiments, such as for direct detection of galactic dark matter."[28]

In the image at the right, the ZEPLIN-III experiment: the WIMP detector, built mainly out of copper, included two chambers within a cryostat vessel: the upper one contained 12 kg of active liquid xenon; an array of 31 photomultipliers operated immersed in the liquid to detect prompt scintillation as well as delayed electroluminescence from a thin gas layer above the liquid. The lower chamber contained liquid nitrogen to provide cooling. The detector was surrounded by Gd-loaded polypropylene to moderate and capture neutrons, a potential source of background. The gamma-rays from neutron capture were detected by 52 modules of plastic scintillator placed around the moderator. The shielding was completed by a 20-cm thick lead castle.

Exclusion limits on the spin-independent WIMP-nucleon elastic scattering cross-section were above 3.9 × 10−8 pb for a 50 GeV WIMP mass.[29] Although not as stringent as results from XENON100,[30] this was achieved with a 10 times smaller fiducial mass and demonstrated the best background discrimination ever achieved in these detectors. The WIMP-neutron spin-dependent cross-section was excluded above 8.0 × 10−3 pb.[31][32] It also ruled out an inelastic WIMP scattering model which attempted to reconcile a positive claim from DAMA with the absence of signal in other experiments.[33]


This Galaxy Evolution Explorer (GALEX) image of the spiral galaxy Messier 81 is in ultraviolet light. Credit: NASA/JPL-Caltech/J. Huchra (Harvard-Smithsonian CfA).{{free media}}
Schematic diagram shows the operation of a microchannel plate. Credit: Andreas 06.{{free media}}

The telescope aboard the Galaxy Evolution Explorer (GALEX or Explorer 83 or SMEX-7) was a 50 cm (20 in) modified Ritchey–Chrétien telescope with a rotating grating prism, a combination of a prism and grating arranged so that light at a chosen central wavelength passes straight through. GALEX used the first ever UV light dichroic beam-splitter flown in space to direct photons to the Near UV (175-280 nanometers) and Far UV (135-174 nanometers) microchannel plate detectors. Each of the two detectors has a 65 mm (2.6 in) diameter.

Galaxy clustersEdit

The ACIS focal plane array appears from the vantage point of looking down the telescope. Credit: NASA.{{free media}}
The pseudo-colour image is of the large-scale radio structure of the FRII radio galaxy 3C98. Lobes, jet and hotspot are labelled. Credit: Mhardcastle.{{free media}}

The CCD chips are in the square (imaging on the right) and rectangular (spectroscoping) arrays. In a flight configuration, you would not see the arrays, as the optical blocking filters would be in place.

ACIS is a focal plane instrument that uses an array of charge-coupled devices (microchannel plate detector) that serve as an X-ray integral field spectrograph for Chandra capable of measuring both the position and energy of incoming X-rays.[34]

The CCD sensors of ACIS operate at −120 °C (−184 °F) and its filters at −60 and −50 °C (−76 and −58 °F) by having a special heater that allows contamination from Chandra to be baked off; the spacecraft contains lubricants, and the ACIS design took this into account in order to clean its sensors of contamination buildup that can reduce the instrument's sensitivity.[35] Radiation in space is another potential danger to the sensor.[36]

The intergalactic medium (IGM) is a rarefied plasma.[37]

"The Chandra observations found evidence for the massive and hot intergalactic medium filaments by noting a slight dimming in distant quasar X-rays likely caused by hot gas absorption."[38]

Gamma raysEdit

High-purity germanium detector (disconnected from liquid nitrogen dewar) is imaged. Credit: Sergio.ballestrero.{{free media}}
A Germanium detector spectrum shows the electron-positron annihilation radiation peak (under the arrow). Note the width of the peak compared to the other gamma rays visible in the spectrum. Credit: Hidesert.{{free media}}

In order to achieve a high probability of detection, semiconductors having a high atomic number such as germanium, gallium arsenide or cadmium telluride, with a relatively large thickness of the single crystal, are used for gamma radiation.[39] Semiconductor detectors made of germanium, such as the HP-Ge detector shown (right), must be cooled to liquid nitrogen temperature (77 K) because they have a very high leakage current at room temperature, which would lead to the destruction of the detector at the necessary operating voltage.[39]

High-velocity galaxiesEdit

FORS1 is on the telescope simulator during flexure tests. Credit: I. Appenzeller, K. Fricke, W. Fürtig, W. Gässler, R. Häfner, R. Harke, H.-J. Hess, W. Hummel, P. Jürgens, R.-P. Kudritzki, K.-H. Mantel, W. Meisl, B. Muschielok, H. Nicklas, G. Rupprecht, W. Seifert, O. Stahl, T. Szeifert, K. Tarantik.{{fairuse}}
FORS1 is installed on Unit Telescope 1 of the VLT. Credit: I. Appenzeller, K. Fricke, W. Fürtig, W. Gässler, R. Häfner, R. Harke, H.-J. Hess, W. Hummel, P. Jürgens, R.-P. Kudritzki, K.-H. Mantel, W. Meisl, B. Muschielok, H. Nicklas, G. Rupprecht, W. Seifert, O. Stahl, T. Szeifert, K. Tarantik.{{fairuse}}
The optical spectrum is for the BL Lac object PG 1553+11. Credit: Rfalomo.{{free media}}
The BL Lac object H 0323+022 (z=0.147) is imaged at ESO NTT (R filter). The host galaxy and close companions are visible. Credit: Renato Falomo.{{free media}}

In contrast to other types of active galactic nuclei, BL Lacs are characterized by rapid and large-amplitude flux variability and significant optical polarization.[40] Because of these properties, the prototype of the class (BL Lacertae, BL Lac) was originally thought to be a variable star, but when compared to the more luminous active nuclei (quasars) with strong emission lines, BL Lac objects have spectra dominated by a relatively featureless non-thermal emission continuum over the entire electromagnetic range.[41] This lack of spectral lines historically hindered BL Lac's identification of their nature and proved to be a hurdle in the determination of their distance.[41]

The second image down on the right was obtained using the Very Large Telescope (VLT) with the FOcal Reducer/low dispersion Spectrograph (FORS1) attached.


Def. "an effect caused by the superposition of two systems of waves"[42] or "a distortion on a broadcast signal due to atmospheric or other effects"[42] is called an interference.

Def. "undesirable signals from a neighbouring transmission circuit; undesired coupling between circuits"[43] or the "situation where one or more components of a signal transduction pathway affect another pathway"[44] is called crosstalk.

Def. "[blocking] or [confusing] a broadcast signal"[45] is called jamming.

Def. the "deliberate radiation or reradiation of mechanical or electroacoustic signals with the objectives of obliterating or obscuring signals that an enemy is attempting to receive and of disrupting enemy weapons systems"[46] is called acoustic jamming (JP 1-02 Department of Defense Dictionary of Military and Associated Terms).


The in situ dust detector from the AIM-Satellite is called the Cosmic Dust Experiment. Credit: Dieter K. Bilitza, NASA.{{free media}}

"The AIM scientific objectives will be achieved by measuring near simultaneous [Polar Mesospheric Clouds] PMC abundances, PMC spatial distributions, cloud particle size distributions, gravity wave activity, cosmic dust influx to the atmosphere needed to study the role of these particles as nucleation sites and precise, vertical profile measurements of temperature, H2O, OH, CH4, O3, CO2, NO, and aerosols. AIM carries three instruments: an infrared solar occultation differential absorption radiometer, ... (Solar Occultation for Ice Experiment, SOFIE); a panoramic ultraviolet imager (Cloud Imaging and particle Size Experiment, CIPS); and, an in-situ dust detector (Cosmic Dust Experiment, CDE)".[47]

The instrument records impacts from cosmic dust particles as they enter Earth's upper atmosphere using fourteen polyvinylidene fluoride detectors, which emit a pulse of charge when impacted by a hypervelocity dust particle (velocity 1 km/s (0.62 mi/s)), measuring the value and variability of the cosmic dust input, where the CDE is a nearly identical replica to the Student Dust Counter on the New Horizons mission.[48]


Diagram shows a continuous operation cloud chamber. Credit: Nuledo.{{free media}}

Alcohol (typically isopropanol) is evaporated by a heater in a duct in the upper part of the cloud chamber. Vapour descents to the black refrigerated plate and saturates. Due to the temperature gradient a thin layer of oversaturated vapour is formed above the bottom plate. In this region, radiation particles can induce condensation and create cloud tracks.

The discovery of the kaon was made using a cloud chamber as the detector.[49]


The white spot on this image of the Earth side of the Moon is the impact site of a meteor from March 17, 2013. Credit: NASA.{{free media}}
This is a Hubble Space Telescope image taken on July 23, 2009, showing a blemish of about 5,000 miles long left by the 2009 Jupiter impact.[50] Credit: NASA, ESA, and H. Hammel (Space Science Institute, Boulder, Colo.), and the Jupiter Impact Team.{{free media}}

Usually, a meteor detector is designed for another form of radiation that the meteor may radiate.

In the image at right, a 0.3 m meteor has impacted a meteor detector, in this case the Moon, and created a scintillation event that in turn is detected by a photoelectronic detector system.

In the image at left, a meteor has impacted another detector, here Jupiter, but instead of a scintillation event has created a lowering of albedo as detected by the photoelectronic system, the Hubble Space Telescope.


The QUIET module is a pseudo-correlation receiver comprising low noise amplifiers, phase shifters, detector diodes, and passive components. Credit: Immanuel Buder and the QUIET collaboration.{{fairuse}}

At right is an image of the QUIET module, a pseudo-correlation receiver comprising low noise amplifiers, phase shifters, detector diodes, and passive components. On the left is the first QUIET module which includes the "low noise amplifiers[, an] InP monolithic microwave integrated circuit (MMIC) high electron mobility transistor (HEMT) amplifiers."[51] The upper right shows "an earlier prototype 90 GHz module. The modules are 1.25 x 1.14."[51] The lower right is "the interior of a (2 x 2) 40 GHz module."[51]


The Compact Muon Solenoid (CMS) is an example of a large particle detector. Notice the person for scale. Credit: CERN.{{free media}}

"With γ ray energy 50 times higher than the muon energy and a probability of muon production by the γ's of about 1%, muon detectors can match the detection efficiency of a GeV satellite detector if their effective area is larger by 104."[52]


This color picture was made by combining several exposures taken on the night of December 28th 1994 at the 0.9 m telescope of the Kitt Peak National Observatory. Credit: N.A.Sharp/NOAO/AURA/NSF.{{free media}}
A specially developed CCD in a wire-bonded package is used for ultraviolet imaging. Credit: NASA.{{free media}}

"This color picture was made by combining several exposures taken on the night of December 28th 1994 (UT of observation 29/12/94 around 04:00) with a 2048x2048 CCD detector at the 0.9m telescope of the Kitt Peak National Observatory. Observing conditions were not ideal throughout, and so only a select few of the original observations were used. The final tally used five frames in the B (blue) filter for a total of 22 minutes, three frames with the V (green) filter, 15 minutes, and two with the R (red), total 10 minutes. Each frame was carefully cleaned, a particularly difficult task for the blue filter due to internal reflection problems in the telescope, and then aligned and combined by computer to create this (approximately) true color picture."[53]

The basis for the CCD is the metal–oxide–semiconductor (MOS) structure,[54] with MOS capacitors being the basic building blocks of a CCD,[55][56] and a depleted MOS structure used as the photodetector in early CCD devices.[54][57]


The Sudbury Neutrino Observatory, a 12-meter sphere filled with heavy water surrounded by light detectors located 2000 meters below the ground in Sudbury, Ontario, Canada. Credit: NASA.

Neutrino detectors are often built underground to isolate the detector from cosmic rays and other background radiation.[58]

The advantages of using heavy water as a detector for solar neutrinos is that it makes the detector sensitive to two reactions, one reaction sensitive to all neutrino flavours, the other reaction sensitive to only electron neutrino; thus, such a detector could measure neutrino oscillations directly.[59]

The Sudbury Neutrino Observatory detector target consisted of 1,000 tonnes (1,102 short tons) of heavy water contained in a 6-metre-radius (20 ft) polymethyl methacrylate vessel, with the detector cavity outside the vessel filled with normal water to provide both buoyancy for the vessel and radiation shielding, where the heavy water was viewed by approximately 9,600 photomultiplier tubes (PMTs) mounted on a geodesic sphere at a radius of about 850 centimetres (28 ft), where he cavity housing the detector was the largest in the world at such a depth,[60] requiring a variety of high-performance rock bolting techniques to prevent rock bursts.


This image shows a Bonner Ball Neutron Detector which is housed inside the small plastic ball when the top is put back on. Credit: Tony Choy of NASA{{free media}}.

Because free neutrons are unstable, they can be obtained only from nuclear disintegrations, nuclear reactions, and high-energy reactions (such as in cosmic radiation showers or accelerator collisions).

Detection approaches for neutrons fall into several major categories:[61]

  • Absorptive reactions with prompt reactions - Low energy neutrons are typically detected indirectly through absorption reactions. Typical absorber materials used have high cross sections for absorption of neutrons and include Helium-3, Lithium-6, Boron-10, and Uranium-235. Each of these reacts by emission of high energy ionized particles, the ionization track of which can be detected by a number of means. Commonly used reactions include 3He(n,p) 3H, 6Li(n,α) 3H, 10B(n,α) 7Li and the fission of uranium.[61]
  • Activation processes - Neutrons may be detected by reacting with absorbers in a radiative capture, spallation or similar reaction, producing reaction products which then decay at some later time, releasing beta particles or gamma rays. Selected materials (e.g., indium, gold, rhodium, iron (56Fe(n,p)56Mn), aluminum27Al(n,α)24Na), niobium (93Nb(n,2n)92mNb), & silicon (28Si(n,p)28Al)) have extremely large cross sections for the capture of neutrons within a very narrow band of energy. Use of multiple absorber samples allows characterization of the neutron energy spectrum. Activation also allows recreation of an historic neutron exposure (e.g., forensic recreation of neutron exposures during an accidental criticality).[61]
  • Elastic scattering reactions (also referred to as proton-recoil) - High energy neutrons are typically detected indirectly through elastic scattering reactions. Neutron collide with the nucleus of atoms in the detector, transferring energy to that nucleus and creating an ion, which is detected. Since the maximum transfer of energy occurs when the mass of the atom with which the neutron collides is comparable to the neutron mass, hydrogenous [materials with a high hydrogen content such as water or plastic] materials are often the preferred medium for such detectors.[61]

The Bonner Ball Neutron Detector "BBND ... determined that galactic cosmic rays were the major cause of secondary neutrons measured inside ISS. The neutron energy spectrum was measured from March 23, 2001 through November 14, 2001 in the U.S. Laboratory Module of the ISS. The time frame enabled neutron measurements to be made during a time of increased solar activity (solar maximum) as well as observe the results of a solar flare on November 4, 2001."[62]

"BBND results show the overall neutron environment at the ISS orbital altitude is influenced by highly energetic galactic cosmic rays, except in the South Atlantic Anomaly (SAA) region where protons trapped in the Earth's magnetic field cause a more severe neutron environment. However, the number of particles measured per second per square cm per MeV obtained by BBND is consistently lower than that of the precursor investigations. The average dose-equivalent rate observed through the investigation was 3.9 micro Sv/hour or about 10 times the rate of radiological exposure to the average US citizen. In general, radiation damage to the human body is indicated by the amount of energy deposited in living tissue, modified by the type of radiation causing the damage; this is measured in units of Sieverts (Sv). The background radiation dose received by an average person in the United States is approximately 3.5 milliSv/year. Conversely, an exposure of 1 Sv can result in radiation poisoning and a dose of five Sv will result in death in 50 percent of exposed individuals. The average dose-equivalent rate observed through the BBND investigation is 3.9 micro Sv/hour, or about ten times the average US surface rate. The highest rate, 96 microSv/hour was observed in the SAA region."[62]


Graph shows burst noise. Credit: B.J. Gross and C.G. Sodini.{{fairuse}}
Simulated power spectral densities as a function of frequency for various colors of noise (violet, blue, white, pink, brown/red). Credit: Mwchalmers.{{free media}}
Without Gaussian noise is shown. Credit: DrWalencia.{{free media}}
With Gaussian noise is shown. Credit: DrWalencia.{{free media}}
Two-dimensional slice is through 3D Perlin noise. Credit: Reedbeta.{{free media}}
Analog display shows random fluctuations in voltage in pink noise. Credit: Bautsch.{{free media}}
The simplest way to quantize a signal is to choose the digital amplitude value closest to the original analog amplitude. Credit: Gregory Maxwell.{{free media}}
A picture generated with Worley noise's basic algorithm. Credit: Rocchini.{{free media}}

Def. sound "or signal generated by random fluctuations",[63] any "part of a signal or data that reduces the clarity, precision, or quality of the desired output",[64] or the "measured level of variation in gene expression among cells, regardless of source, within a supposedly identical population"[65] is called noise.

In electronics, noise is an unwanted disturbance in an electrical signal.[66]

Def. "[N]oise that has a frequency spectrum of predominantly zero power level over all frequencies except for a few narrow bands or spikes"[67] is called black noise.

Def. a "signal or process with a frequency spectrum such that the spectral energy density is proportional to the frequency"[68] is called blue noise.

Def. "sudden step-like transitions between two or more [discrete voltage or current] levels, as high as several hundred microvolts, at random and unpredictable times"[69] is called burst noise.

The color of noises: The power spectral densities are arbitrarily normalized such that the value of the spectra are approximately equivalent near 1 kHz. Note the slope of the power spectral density for each spectrum provides the context for the respective electromagnetic/color analogy.

Def. "[n]oise or other artifacts caused in the electronic reproduction of sound or music"[70] is called distortion.

Def. "a signal or process with a frequency spectrum that falls off steadily into the higher frequencies,[71] with a pink spectrum"[72] is called a flicker noise.

Def. "1/f α noises for which the exponent α is not an even integer"[73] is called fractal noise.

Def. "statistical noise having a probability density function (PDF) equal to that of the normal distribution"[74][75] is called a Gaussian noise.

The probability density function   of a Gaussian random variable   is given by:


where   represents the grey level,   the mean grey value and   its standard deviation.[76]

Def. "a type of noise commonly used as a procedural texture primitive in computer graphics"[77] is called a gradient noise.

Def. a "signal or process with a frequency spectrum such that the spectral energy density is such that the listener perceives that it is [perceived by the listener as][78] equally loud at all frequencies"[79] is called a grey noise.

Def. an "abrupt and unwanted variation of one or more signal characteristics"[80] is called jitter.

Def. the "electronic noise generated by the thermal agitation of the charge carriers (usually the electrons) inside an electrical conductor at equilibrium, which happens regardless of any applied voltage"[81] is called Johnson-Nyquist noise, Johnson noise, or thermal noise.

Def. noise that occurs where current divides between two (or more) paths as a result of random fluctuations during this division,[82] is called partition noise.

Def. random-"looking visual noise generated by a function and widely used in computer graphics to simulate effects such as fire and clouds"[83] is called Perlin noise.

Def. a "signal or process with a frequency spectrum such that the spectral energy density is proportional to the reciprocal of the frequency"[84] is called pink noise.

Def. the "difference between the original signal and the reconstructed signal"[85] is called the quantization noise.

The example in the image (seven down on the right) shows the original analog signal (green), the quantized signal (black dots), the signal reconstructed from the quantized signal (yellow) and the difference between the original signal and the reconstructed signal (red).

Def. a "signal or process with a frequency spectrum such that the spectral energy density is proportional to the reciprocal of the frequency squared (1/f2)"[86] is called red noise.

Def. "noise due to random variations in the number and velocity of electrons or photons in a device"[87] is called shotnoise or shot noise.

Noise that depends mostly on device type is called shot noise.[66][88]

Shot noise has been demonstrated in mesoscopic resistors when the size of the resistive element becomes shorter than the electron–phonon scattering length.[89]

Def. "a method for constructing an n-dimensional noise function comparable to Perlin noise ("classic" noise) but with [fewer directional artifacts and, in higher dimensions,][90] a lower computational overhead"[91] is called simplex noise.

Def. "a function that creates a divergence-free field"[92] is called simulation noise.

Def. "a type of noise commonly used as a procedural texture primitive in computer graphics"[93] is called value noise.

Def. a "signal or process with a frequency spectrum such that the spectral energy density is proportional to the frequency squared"[94] is called a violet noise.

Def. a "random signal (or process) with a flat power spectral density; a signal with a power spectral density that has equal power in any band, at any centre frequency, having a given bandwidth"[95] or any "nondescript noise used for background or to mask or drown out other noise"[96] is called white noise.

Def. a noise function used to create procedural textures automatically with arbitrary precision and do not have to be drawn by hand[97] is called Worley noise.

In the image on the right of Worley noise, tweaking of seed points and colors would be necessary to make this look like stone.


Array of 30 CCDs used on the Sloan Digital Sky Survey telescope imaging camera, an example of "drift-scanning". Credit: Sloan Digital Sky Survey.{{free media}}
Sloan Digital Sky Survey (SDSS) image of quasar 3C 273, illustrating the object's star-like appearance. The quasar's jet can be seen extending downward and to the right from the quasar. Credit: Sloan Digital Sky Survey.

“Maps of the radio structure of the quasar 3C273 provide evidence of a superluminal expansion during the period 1977-1980. The superluminal expansion might be attributed to the movement of a single knot away from the nucleus along the jet. The apparent constant velocity of 10 times the speed of light is an important constraint on theories of apparent superluminal expansion.”[98]


This type of main surge arrestor is used in telecom devices such as phones, faxes and modems. Credit: Miikka Raninen.{{free media}}

Def. "the difference between the electric potential of an electrode or cell under the passage of a current and the thermodynamic value of the electrode or cell potential in the absence of electrolysis"[99] or "the hazardous condition that occurs when the voltage in a circuit is raised above that for which it was designed"[99] is called an overvoltage.

Fast, short duration electrical transients (overvoltages) in the electric potential of a circuit are typically caused by

  • Lightning strikes,
  • Power outages,
  • Tripped circuit breakers,
  • Short circuits,
  • Power transitions in other large equipment on the same power line,
  • Malfunctions caused by the power source,
  • Electromagnetic pulses (EMP) with electromagnetic energy distributed typically up to the 100 kHz and 1 MHz frequency range, or
  • Inductive spikes.

An avalanche diode, transient voltage suppression diode, transil, varistor, overvoltage crowbar (circuit), or a range of other overvoltage protective devices can divert (shunt) this transient current thereby minimizing voltage.[100]

Photopeak efficiencyEdit

Def. "the fraction of photoelectric events which end up in the photopeak of the measured energy spectrum"[101] is called the photopeak efficiency (ε).

Ending up in the photopeak means within ± 1 full-width at half maximum (FWHM) of the peak of the distribution.[101]

"The peak to valley ratio is commonly used as a token for ε."[101]

"Another common practice is to fit an exponential function to the “valley” and to extrapolate the fit to lower pulse heights to estimate the fraction of counts hidden in the Compton continuum."[101]

"We have used a calibrated Cs137 source to determine the absolute photopeak efficiency at 662 keV. The source was placed at a sufficiently large distance from the detector so that the event rate was low and the dead time was less than 20%. Based on a log-histogram of the time intervals between events, the dead-time has been estimated to a fractional accuracy of better than 5%. We determine the photopeak efficiency by comparing the dead-time corrected event rate in the photopeak with the theoretical expectation assuming a perfect detector."[101]


Cloud chamber photograph is used to prove the existence of the positron. Credit: Carl D. Anderson.{{free media}}

"A 63 million volt positron (Hρ = 2.1×105 gauss-cm) passing through a 6 mm lead plate and emerging as a 23 million volt positron (Hρ = 7.5×104 gauss-cm). The length of this latter path is at least ten times greater than the possible length of a proton path of this curvature."[102] The thick horizontal line is a lead plate. The positron entered the cloud chamber in the lower left, was slowed down by the lead plane, and curved to the upper left. The curvature of the path is caused by an applied magnetic field that acts perpendicular to the image plane. The higher energy of the entering positron resulted in lower curvature of its path.

"In the first 18 months of operations, AMS-02 recorded 6.8 million positron (an antimatter particle with the mass of an electron but a positive charge) and electron events produced from cosmic ray collisions with the interstellar medium in the energy range between 0.5 giga-electron volt (GeV) and 350 GeV. These events were used to determine the positron fraction, the ratio of positrons to the total number of electrons and positrons. Below 10 GeV, the positron fraction decreased with increasing energy, as expected. However, the positron fraction increased steadily from 10 GeV to 250 GeV. This increase, seen previously though less precisely by instruments such as the Payload for Matter/antimatter Exploration and Light-nuclei Astrophysics (PAMELA) and the Fermi Gamma-ray Space Telescope, conflicts with the predicted decrease of the positron fraction and indicates the existence of a currently unidentified source of positrons, such as pulsars or the annihilation of dark matter particles. Furthermore, researchers observed an unexpected decrease in slope from 20 GeV to 250 GeV. The measured positron to electron ratio is isotropic, the same in all directions."[103]


This is an image of the alpha particle X-ray spectrometer (APXS). Credit: NASA/JPL-Caltech.{{free media}}
The stopping power of aluminum for protons is plotted versus proton energy. Credit: H.Paul.

Some of the alpha particles are absorbed by the atomic nuclei. The [alpha,proton] process produces protons of a defined energy which are detected. Sodium, magnesium, silicon, aluminium and sulfur can be detected by this method. This method was only used in the Mars Pathfinder APXS.

At right, the second figure shows the stopping power of aluminum metal single crystal for protons.

"Choosing materials with the largest stopping powers enables thinner detectors to be produced with resulting benefits in radiation tolerance (which is a bulk effect) and lower leakage currents. Alternatively, choosing smaller stopping powers will increase scattering efficiency, which is a requirement for polarimetry, or say, the upper detection plane of a double Compton telescope."[104]

Signal-to-noise ratiosEdit

Def. a "figure of merit comparing the strength of a signal carrying information to the noise interfering with it"[105] is called a signal-to-noise ratio.

Noise is also typically distinguished from distortion, which is an unwanted alteration of the signal waveform, for example in the signal-to-noise and distortion ratio (SINAD). In a carrier-modulated passband analog communication system, a certain carrier-to-noise ratio (CNR) at the radio receiver input would result in a certain signal-to-noise ratio in the detected message signal. In a digital communications system, a certain Eb/N0 (normalized signal-to-noise ratio) would result in a certain bit error rate (BER).


Voltage spikes are illustrated. Credit: JHPrime.{{free media}}

Def. a "sharp peak on a graph"[106] is called a spike.

Def. "a fast, short duration surge (overvoltage) in the electric potential of a circuit"[107] or "a power surge"[107] is called a voltage spike.

Clamping voltage, also known as the let-through voltage specifies what spike voltage will cause the protective components inside a surge protector to short or clamp.[108] The standard let-through voltage for 120 V AC devices is 330 volts.[109]

An average surge (spike) is of short duration, lasting for nanoseconds to microseconds, and experimentally modeled surge energy can be less than 100 joules.[110]

The effective surge energy absorption capacity of the entire system is dependent on the MOV matching so derating by 20% or more is usually required, which can be managed by using carefully matched sets of MOVs, matched according to manufacturer's specification.[111][112]

Surge protectors don't operate instantaneously; a slight delay exists, some few nanoseconds. With longer response time and depending on system impedance, the connected equipment may be exposed to some of the surge; however, surges typically are much slower and take around a few microseconds to reach their peak voltage, and a surge protector with a nanosecond response time would kick in fast enough to suppress the most damaging portion of the spike.[113]

Slower-responding technologies (notably, GDTs) may have difficulty protecting against fast spikes; therefore, good designs incorporating slower but otherwise useful technologies usually combine them with faster-acting components, to provide more comprehensive protection.[114]

Systems used to reduce or limit high-voltage surges[115][116] can include one or more of the following types of electronic components. Some surge suppression systems use multiple technologies, since each method has its strong and weak points.[114][117][118]

A metal oxide varistor (MOV) consists of a bulk semiconductor material (typically sintered granular zinc oxide) that can conduct large currents when presented with a voltage above its rated voltage.[109][119]

MOVs have finite life expectancy and degrade when exposed to a few large transients, or many small transients.[120][121] In a power circuit, you may get a dramatic meltdown or even a fire if not protected by a fuse of some kind.[122]

Transient-voltage-suppression diode (TVS) diodes are often used in high-speed but low-power circuits, such as occur in data communications and can be paired in series with another diode to provide low capacitance.[123]

Stopping powerEdit

Diagram shows the electronic and nuclear stopping power for aluminum ions in aluminum. Credit: HPaul.{{free media}}
This is an illustration of the slowing down of a single ion in a solid material. Credit: Kai Nordlund.{{free media}}
Graphic shows relationships between radioactivity and detected ionizing radiation. Credit: Doug Sim.{{free media}}

Def the retarding force acting on charged particles, typically alpha and beta particles, due to interaction with matter, resulting in loss of particle energy[124][125] is called stopping power.

Its application is important in areas such as radiation protection, ion implantation and nuclear medicine.[126]

The stopping power depends on the type and energy of the radiation and on the properties of the material through which it passes; e.g., since the production of an ion pair (usually a positive ion and a (negative) electron) requires a fixed amount of energy (say, 33.97 eV in dry air[127]:305), the number of ionizations per path length is proportional to the stopping power, where the stopping power of the material is numerically equal to the loss of energy E per unit path length, x:


Instead of energy transfer, some models consider the electronic stopping power as momentum transfer between electron gas and energetic ion which is consistent with the Bethe formula in the high energy range.[128]

Def. the slowing down by momentum transfer of a projectile ion due to the inelastic collisions between bound electrons in the medium and the ion moving through it is called the electronic stopping power.[128]

Since the number of collisions an ion experiences with electrons is large, and since the charge state of the ion while traversing the medium may change frequently, it is very difficult to describe all possible interactions for all possible ion charge states; instead, the electronic stopping power is often given as a simple function of energy   which is an average taken over all energy loss processes for different charge states.[129] It can be theoretically determined to an accuracy of a few % in the energy range above several hundred keV per nucleon from theoretical treatments, the best known being the Bethe formula.[129] At energies lower than about 100 keV per nucleon, it becomes more difficult to determine the electronic stopping using analytical models.[129] Real-time Time-dependent density functional theory has been successfully used to accurately determine the electronic stopping for various ion-target systems over a wide range of energies including the low energy regime.[130][131]

Def. the elastic collisions between the projectile ion and atoms in the sample involving the interaction of the ion with the nuclei in the target is called the nuclear stopping power.[132]


"The existence of superluminal energy transfer has not been established so far, and one may ask why. There is the possibility that superluminal quanta just do not exist, the vacuum speed of light being the definitive upper bound. There is another explanation, the interaction of superluminal radiation with matter is very small, the quotient of tachyonic and electric fine-structure constants being q2/e2 ≈ 1.4 x 10-11 [5], and therefore superluminal quanta are hard to detect."[133]


  1. The use of satellites should provide ten times the information as sounding rockets or balloons.

A control group for a radiation satellite would contain

  1. a radiation astronomy telescope,
  2. a two-way communication system,
  3. a positional locator,
  4. an orientation propulsion system, and
  5. power supplies and energy sources for all components.

A control group for radiation astronomy satellites may include an ideal or rigorously stable orbit so that the satellite observes the radiation at or to a much higher resolution than an Earth-based ground-level observatory is capable of.

See alsoEdit


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External linksEdit

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