![Pmside](https://upload.wikimedia.org/wikipedia/commons/3/36/Pmside.jpg)
Photomultiplier tubes (photomultipliers or PMTs for short) are extremely sensitive detectors of light in the
ultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
,
visible
Visibility, in meteorology, is a measure of the distance at which an object or light can be seen.
Visibility may also refer to:
* A measure of turbidity in water quality control
* Interferometric visibility, which quantifies interference contrast ...
, and
near-infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of Light, visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from ...
ranges of the
electromagnetic spectrum
The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies.
The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from ...
. They are members of the class of
vacuum tube
A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied.
The type kn ...
s, more specifically vacuum
phototube
A phototube or photoelectric cell is a type of gas-filled or vacuum tube that is sensitive to light. Such a tube is more correctly called a 'photoemissive cell' to distinguish it from photovoltaic or photoconductive cells. Phototubes were previ ...
s. These detectors multiply the current produced by incident light by as much as 100 million times or 10
8 (i.e., 160
dB),
[Decibels are power ratios. Power is proportional to I2 (current squared). Thus a current gain of 108 produces a power gain of 1016, or 160 dB] in multiple
dynode
A dynode is an electrode in a vacuum tube
A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential diff ...
stages, enabling (for example) individual
photons
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alway ...
to be detected when the incident
flux
Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ph ...
of light is low.
![Dynodes](https://upload.wikimedia.org/wikipedia/commons/f/f9/Dynodes.jpg)
The combination of high
gain
Gain or GAIN may refer to:
Science and technology
* Gain (electronics), an electronics and signal processing term
* Antenna gain
* Gain (laser), the amplification involved in laser emission
* Gain (projection screens)
* Information gain in de ...
, low
noise
Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference arise ...
, high frequency response or, equivalently, ultra-fast response, and large area of collection has maintained photomultipliers an essential place in
low light level spectroscopy,
confocal microscopy
Confocal microscopy, most frequently confocal laser scanning microscopy (CLSM) or laser confocal scanning microscopy (LCSM), is an optical imaging technique for increasing optical resolution and contrast of a micrograph by means of using a sp ...
,
Raman spectroscopy
Raman spectroscopy () (named after Indian physicist C. V. Raman) is a spectroscopic technique typically used to determine vibrational modes of molecules, although rotational and other low-frequency modes of systems may also be observed. Raman sp ...
,
fluorescence spectroscopy
Fluorescence spectroscopy (also known as fluorimetry or spectrofluorometry) is a type of electromagnetic spectroscopy that analyzes fluorescence from a sample. It involves using a beam of light, usually ultraviolet light, that excites the electron ...
,
nuclear
Nuclear may refer to:
Physics
Relating to the nucleus of the atom:
* Nuclear engineering
*Nuclear physics
*Nuclear power
*Nuclear reactor
*Nuclear weapon
*Nuclear medicine
*Radiation therapy
*Nuclear warfare
Mathematics
*Nuclear space
*Nuclear ...
and
particle physics
Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) an ...
,
astronomy
Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objects of interest ...
, medical
diagnostics
Diagnosis is the identification of the nature and cause of a certain phenomenon. Diagnosis is used in many different disciplines, with variations in the use of logic, analytics, and experience, to determine "cause and effect". In systems engineer ...
including
blood tests
A blood test is a laboratory analysis performed on a blood sample that is usually extracted from a vein in the arm using a hypodermic needle, or via fingerprick. Multiple tests for specific blood components, such as a glucose test or a cholester ...
,
medical imaging
Medical imaging is the technique and process of imaging the interior of a body for clinical analysis and medical intervention, as well as visual representation of the function of some organs or tissues (physiology). Medical imaging seeks to rev ...
, motion picture film scanning (
telecine
Telecine ( or ) is the process of transferring film into video and is performed in a color suite. The term is also used to refer to the equipment used in the post-production process.
Telecine enables a motion picture, captured originally on f ...
),
radar jamming
Radar jamming and deception is a form of electronic countermeasures that intentionally sends out radio frequency signals to interfere with the operation of radar by saturating its receiver with noise or false information. Concepts that blanket the ...
, and high-end image scanners known as
drum scanner
An image scanner—often abbreviated to just scanner—is a device that optically scans images, printed text, handwriting or an object and converts it to a digital image. Commonly used in offices are variations of the desktop ''flatbed scanner'' ...
s. Elements of photomultiplier technology, when integrated differently, are the basis of
night vision device
A night-vision device (NVD), also known as a night optical/observation device (NOD), night-vision goggle (NVG), is an optoelectronic device that allows visualization of images in low levels of light, improving the user's night vision. The dev ...
s. Research that analyzes
light scattering
Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including ...
, such as the study of
polymers
A polymer (; Greek '' poly-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic an ...
in solution, often uses a laser and a PMT to collect the scattered light data.
Semiconductor device
A semiconductor device is an electronic component that relies on the electronic properties of a semiconductor material (primarily silicon, germanium, and gallium arsenide, as well as organic semiconductors) for its function. Its conductivity li ...
s, particularly
silicon photomultiplier
Silicon photomultipliers, often called "SiPM" in the literature, are solid-state single-photon-sensitive devices based on Single-photon avalanche diode (SPAD) implemented on common silicon substrate. The dimension of each single SPAD can vary fro ...
s and
avalanche photodiode
An avalanche photodiode (APD) is a highly sensitive semiconductor photodiode detector that exploits the photoelectric effect to convert light into electricity. From a functional standpoint, they can be regarded as the semiconductor analog of pho ...
s, are alternatives to classical photomultipliers; however, photomultipliers are uniquely well-suited for applications requiring low-noise, high-sensitivity detection of light that is imperfectly
collimated
A collimated beam of light or other electromagnetic radiation has parallel rays, and therefore will spread minimally as it propagates. A perfectly collimated light beam, with no divergence, would not disperse with distance. However, diffraction pr ...
.
Structure and operating principles
![PMT Voltage Divider](https://upload.wikimedia.org/wikipedia/commons/1/1a/PMT_Voltage_Divider.jpg)
Photomultipliers are typically constructed with an evacuated glass housing (using an extremely tight and durable
glass-to-metal seal
Glass-to-metal seals are a very important element of the construction of vacuum tubes, electric discharge tubes, incandescent light bulbs, glass encapsulated semiconductor diodes, reed switches, pressure tight glass windows in metal cases, Vacuum I ...
like other
vacuum tubes
A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.
The type known as a ...
), containing a
photocathode
A photocathode is a surface engineered to convert light (photons) into electrons using the photoelectric effect. Photocathodes are important in accelerator physics where they are utilised in a photoinjector to generate high brightness electron be ...
, several
dynode
A dynode is an electrode in a vacuum tube
A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential diff ...
s, and an
anode
An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic is ...
. Incident
photons
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alway ...
strike the
photocathode
A photocathode is a surface engineered to convert light (photons) into electrons using the photoelectric effect. Photocathodes are important in accelerator physics where they are utilised in a photoinjector to generate high brightness electron be ...
material, which is usually a thin
vapor-deposited conducting layer on the inside of the entry window of the device.
Electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
s are ejected from the surface as a consequence of the
photoelectric effect
The photoelectric effect is the emission of electrons when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, and solid st ...
. These electrons are directed by the focusing
electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials de ...
toward the
electron multiplier
An electron multiplier is a vacuum-tube structure that multiplies incident charges. In a process called secondary emission, a single electron can, when bombarded on secondary-emissive material, induce emission of roughly 1 to 3 electrons. If an ele ...
, where electrons are multiplied by the process of
secondary emission
In particle physics, secondary emission is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. The term often refers to the emi ...
.
The electron multiplier consists of a number of electrodes called ''dynodes''. Each dynode is held at a more positive potential, by ≈100 Volts, than the preceding one. A primary electron leaves the photocathode with the energy of the incoming photon, or about 3 eV for "blue" photons, minus the
work function
In solid-state physics, the work function (sometimes spelt workfunction) is the minimum thermodynamic work (i.e., energy) needed to remove an electron from a solid to a point in the vacuum immediately outside the solid surface. Here "immediately" m ...
of the photocathode. A small group of primary electrons is created by the arrival of a group of initial photons. (In Fig. 1, the number of primary electrons in the initial group is proportional to the energy of the incident high energy gamma ray.) The primary electrons move toward the first dynode because they are accelerated by the electric field. They each arrive with ≈100 eV kinetic energy imparted by the potential difference. Upon striking the first dynode, more low energy electrons are emitted, and these electrons are in turn accelerated toward the second dynode. The geometry of the dynode chain is such that a cascade occurs with an exponentially-increasing number of electrons being produced at each stage. For example, if at each stage an average of 5 new electrons are produced for each incoming electron, and if there are 12 dynode stages, then at the last stage one expects for each primary electron about 5
12 ≈ 10
8 electrons. This last stage is called the
anode
An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic is ...
. This large number of electrons reaching the anode results in a sharp current pulse that is easily detectable, for example on an oscilloscope, signaling the arrival of the photon(s) at the photocathode ≈50 nanoseconds earlier.
The necessary distribution of voltage along the series of dynodes is created by a voltage divider chain, as illustrated in Fig. 2. In the example, the photocathode is held at a negative high voltage of order 1000 V, while the anode is
very close to ground potential. The capacitors across the final few dynodes act as local reservoirs of charge to help maintain the voltage on the dynodes while electron avalanches propagate through the tube. Many variations of design are used in practice; the design shown is merely illustrative.
![Photomultiplicateur ancien](https://upload.wikimedia.org/wikipedia/commons/1/17/Photomultiplicateur_ancien_.jpg)
There are two common photomultiplier orientations, the ''head-on'' or ''end-on'' (transmission mode) design, as shown above, where light enters the flat, circular top of the tube and passes the photocathode, and the ''side-on'' design (reflection mode), where light enters at a particular spot on the side of the tube, and impacts on an opaque photocathode. The side-on design is used, for instance, in the
type 931, the first mass-produced PMT. Besides the different photocathode materials, performance is also affected by the transmission of the
window material that the light passes through, and by the arrangement of the dynodes. Many photomultiplier models are available having various combinations of these, and other, design variables. The manufacturers manuals provide the information needed to choose an appropriate design for a particular application.
History
The invention of the photomultiplier is predicated upon two prior achievements, the separate discoveries of the
photoelectric effect
The photoelectric effect is the emission of electrons when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, and solid st ...
and of
secondary emission
In particle physics, secondary emission is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. The term often refers to the emi ...
.
Photoelectric effect
The first demonstration of the
photoelectric effect
The photoelectric effect is the emission of electrons when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, and solid st ...
was carried out in 1887 by
Heinrich Hertz
Heinrich Rudolf Hertz ( ; ; 22 February 1857 – 1 January 1894) was a German physicist who first conclusively proved the existence of the electromagnetic waves predicted by James Clerk Maxwell's Maxwell's equations, equations of electrom ...
using ultraviolet light. Significant for practical applications, Elster and Geitel two years later demonstrated the same effect using ''visible'' light striking alkali metals (potassium and sodium). The addition of
caesium
Caesium (IUPAC spelling) (or cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only five elemental metals that a ...
, another
alkali metal
The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...
, has permitted the range of sensitive wavelengths to be extended towards longer wavelengths in the red portion of the visible spectrum.
Historically, the photoelectric effect is associated with
Albert Einstein
Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theory ...
, who relied upon the phenomenon to establish the fundamental principle of
quantum mechanics
Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, ...
in 1905, an accomplishment for which Einstein received the 1921
Nobel Prize
The Nobel Prizes ( ; sv, Nobelpriset ; no, Nobelprisen ) are five separate prizes that, according to Alfred Nobel's will of 1895, are awarded to "those who, during the preceding year, have conferred the greatest benefit to humankind." Alfr ...
. It is worthwhile to note that Heinrich Hertz, working 18 years earlier, had not recognized that the kinetic energy of the emitted electrons is proportional to the frequency but independent of the optical intensity. This fact implied a discrete nature of light, i.e. the existence of ''quanta'', for the first time.
Secondary emission
The phenomenon of
secondary emission
In particle physics, secondary emission is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. The term often refers to the emi ...
(the ability of
electrons
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no ...
in a vacuum tube to cause the emission of additional electrons by striking an
electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials de ...
) was, at first, limited to purely electronic phenomena and devices (which lacked
photosensitivity Photosensitivity is the amount to which an object reacts upon receiving photons, especially visible light. In medicine, the term is principally used for abnormal reactions of the skin, and two types are distinguished, photoallergy and phototoxicity. ...
). In 1899 the effect was first reported by Villard. In 1902, Austin and Starke reported that the metal surfaces impacted by electron beams emitted a larger number of electrons than were incident. The application of the newly discovered secondary emission to the amplification of signals was only proposed after
World War I
World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fightin ...
by
Westinghouse scientist
Joseph Slepian
Joseph Slepian
(February 11, 1891 – December 19, 1969)
was an American electrical engineer known for his contributions to the developments of electrical apparatus and theory.
Born in Boston, MA of Jewish Russian immigrants, he studied ma ...
in a 1919 patent.
The race towards a practical electronic television camera
The ingredients for inventing the photomultiplier were coming together during the 1920s as the pace of vacuum tube technology accelerated. The primary goal for many, if not most, workers was the need for a practical television camera technology. Television had been pursued with primitive prototypes for decades prior to the 1934 introduction of the first practical camera (the
iconoscope
The iconoscope (from the Greek: ''εἰκών'' "image" and ''σκοπεῖν'' "to look, to see") was the first practical video camera tube to be used in early television cameras. The iconoscope produced a much stronger signal than earlier mecha ...
). Early prototype television cameras lacked sensitivity. Photomultiplier technology was pursued to enable television camera tubes, such as the iconoscope and (later) the
orthicon
Video camera tubes were devices based on the cathode ray tube that were used in television cameras to capture television images, prior to the introduction of charge-coupled device (CCD) image sensors in the 1980s. Several different types of tubes ...
, to be sensitive enough to be practical. So the stage was set to combine the dual phenomena of
photoemission
The photoelectric effect is the emission of electrons when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, and solid sta ...
(i.e., the photoelectric effect) with
secondary emission
In particle physics, secondary emission is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. The term often refers to the emi ...
, both of which had already been studied and adequately understood, to create a practical photomultiplier.
First photomultiplier, single-stage (early 1934)
The first documented photomultiplier demonstration dates to the early 1934 accomplishments of an RCA group based in Harrison, NJ. Harley Iams and Bernard Salzberg were the first to integrate a photoelectric-effect cathode and single secondary emission amplification stage in a single vacuum envelope and the first to characterize its performance as a photomultiplier with electron amplification gain. These accomplishments were finalized ''prior'' to June 1934 as detailed in the manuscript submitted to
Proceedings of the Institute of Radio Engineers
The ''Proceedings of the IEEE'' is a monthly peer-reviewed scientific journal published by the Institute of Electrical and Electronics Engineers (IEEE). The journal focuses on electrical engineering and computer science. According to the ''Jo ...
(Proc. IRE). The device consisted of a semi-cylindrical
photocathode
A photocathode is a surface engineered to convert light (photons) into electrons using the photoelectric effect. Photocathodes are important in accelerator physics where they are utilised in a photoinjector to generate high brightness electron be ...
, a secondary emitter mounted on the axis, and a collector grid surrounding the secondary emitter. The tube had a gain of about eight and operated at frequencies well above 10 kHz.
Magnetic photomultipliers (mid 1934–1937)
Higher gains were sought than those available from the early single-stage photomultipliers. However, it is an empirical fact that the yield of secondary electrons is limited in any given secondary emission process, regardless of acceleration voltage. Thus, any single-stage photomultiplier is limited in gain. At the time the maximum first-stage gain that could be achieved was approximately 10 (very significant developments in the 1960s permitted gains above 25 to be reached using negative electron affinity
dynode
A dynode is an electrode in a vacuum tube
A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential diff ...
s). For this reason, multiple-stage photomultipliers, in which the photoelectron yield could be multiplied successively in several stages, were an important goal. The challenge was to cause the photoelectrons to impinge on successively higher-voltage electrodes rather than to travel directly to the highest voltage electrode. Initially this challenge was overcome by using strong magnetic fields to bend the electrons' trajectories. Such a scheme had earlier been conceived by inventor J. Slepian by 1919 (see above). Accordingly, leading international research organizations turned their attention towards improving photomultipliers to achieve higher gain with multiple stages.
In the
USSR
The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, it was nominally a federal union of fifteen nationa ...
, RCA-manufactured radio equipment was introduced on a large scale by
Joseph Stalin
Joseph Vissarionovich Stalin (born Ioseb Besarionis dze Jughashvili; – 5 March 1953) was a Georgian revolutionary and Soviet political leader who led the Soviet Union from 1924 until his death in 1953. He held power as General Secreta ...
to construct broadcast networks, and the newly formed All-Union Scientific Research Institute for Television was gearing up a research program in vacuum tubes that was advanced for its time and place. Numerous visits were made by RCA scientific personnel to the
USSR
The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, it was nominally a federal union of fifteen nationa ...
in the 1930s, prior to the
Cold War
The Cold War is a term commonly used to refer to a period of geopolitical tension between the United States and the Soviet Union and their respective allies, the Western Bloc and the Eastern Bloc. The term '' cold war'' is used because the ...
, to instruct the Soviet customers on the capabilities of RCA equipment and to investigate customer needs. During one of these visits, in September 1934, RCA's
Vladimir Zworykin
Vladimir Kosma Zworykin; or with the patronymic as ''Kosmich''; or russian: Кузьмич, translit=Kuz'mich, label=none. Zworykin anglicized his name to ''Vladimir Kosma Zworykin'', replacing the patronymic with the name ''Kosma'' as a middle na ...
was shown the first multiple-dynode photomultiplier, or ''photoelectron multiplier''. This pioneering device was proposed by Leonid A. Kubetsky in 1930 which he subsequently built in 1934. The device achieved gains of 1000x or more when demonstrated in June 1934. The work was submitted for print publication only two years later, in July 1936 as emphasized in a recent 2006 publication of the
Russian Academy of Sciences
The Russian Academy of Sciences (RAS; russian: Росси́йская акаде́мия нау́к (РАН) ''Rossíyskaya akadémiya naúk'') consists of the national academy of Russia; a network of scientific research institutes from across t ...
(RAS), which terms it "Kubetsky's Tube." The Soviet device used a magnetic field to confine the secondary electrons and relied on the Ag-O-Cs photocathode which had been demonstrated by General Electric in the 1920s.
By October 1935,
Vladimir Zworykin
Vladimir Kosma Zworykin; or with the patronymic as ''Kosmich''; or russian: Кузьмич, translit=Kuz'mich, label=none. Zworykin anglicized his name to ''Vladimir Kosma Zworykin'', replacing the patronymic with the name ''Kosma'' as a middle na ...
, George Ashmun Morton, and Louis Malter of RCA in Camden, NJ submitted their manuscript describing the first comprehensive experimental and theoretical analysis of a multiple dynode tube — the device later called a ''photomultiplier'' — to Proc. IRE. The RCA prototype photomultipliers also used an Ag-O-Cs (
silver oxide
Silver oxide is the chemical compound with the formula Ag2O. It is a fine black or dark brown powder that is used to prepare other silver compounds.
Preparation
Silver oxide can be prepared by combining aqueous solutions of silver nitrate and a ...
-
caesium
Caesium (IUPAC spelling) (or cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only five elemental metals that a ...
) photocathode. They exhibited a peak
quantum efficiency
The term quantum efficiency (QE) may apply to incident photon to converted electron (IPCE) ratio of a photosensitive device, or it may refer to the TMR effect of a Magnetic Tunnel Junction.
This article deals with the term as a measurement of ...
of 0.4% at 800
nm.
Electrostatic photomultipliers (1937–present)
Whereas these early photomultipliers used the magnetic field principle, electrostatic photomultipliers (with no magnetic field) were demonstrated by
Jan Rajchman of RCA Laboratories in Princeton, NJ in the late 1930s and became the standard for all future commercial photomultipliers. The first mass-produced photomultiplier, the Type 931, was of this design and is still commercially produced today.
Improved photocathodes
Also in 1936, a much improved photocathode, Cs
3Sb (
caesium
Caesium (IUPAC spelling) (or cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only five elemental metals that a ...
-
antimony
Antimony is a chemical element with the symbol Sb (from la, stibium) and atomic number 51. A lustrous gray metalloid, it is found in nature mainly as the sulfide mineral stibnite (Sb2S3). Antimony compounds have been known since ancient time ...
), was reported by P. Görlich. The caesium-antimony photocathode had a dramatically improved quantum efficiency of 12% at 400 nm, and was used in the first commercially successful photomultipliers manufactured by RCA (i.e., the 931-type) both as a photocathode and as a secondary-emitting material for the
dynode
A dynode is an electrode in a vacuum tube
A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential diff ...
s. Different photocathodes provided differing spectral responses.
Spectral response of photocathodes
In the early 1940s, the
JEDEC
The JEDEC Solid State Technology Association is an independent semiconductor engineering trade organization and standardization body headquartered in Arlington County, Virginia, United States.
JEDEC has over 300 members, including some of the w ...
(Joint Electron Device Engineering Council), an industry committee on standardization, developed a system of designating spectral responses. The philosophy included the idea that the product's user need only be concerned about the response of the device rather than how the device may be fabricated. Various combinations of photocathode and window materials were assigned "S-numbers" (spectral numbers) ranging from S-1 through S-40, which are still in use today. For example, S-11 uses the caesium-antimony photocathode with a lime glass window, S-13 uses the same photocathode with a fused silica window, and S-25 uses a so-called "multialkali" photocathode (Na-K-Sb-Cs, or
sodium
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable iso ...
-
potassium
Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosphe ...
-
antimony
Antimony is a chemical element with the symbol Sb (from la, stibium) and atomic number 51. A lustrous gray metalloid, it is found in nature mainly as the sulfide mineral stibnite (Sb2S3). Antimony compounds have been known since ancient time ...
-
caesium
Caesium (IUPAC spelling) (or cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only five elemental metals that a ...
) that provides extended response in the red portion of the visible light spectrum. No suitable photoemissive surfaces have yet been reported to detect wavelengths longer than approximately 1700 nanometers, which can be approached by a special (InP/InGaAs(Cs)) photocathode.
[ p. 34, Table 4-1: Typical Spectral Response Characteristics, Transmission Mode Photocathodes]
RCA Corporation
For decades, RCA was responsible for performing the most important work in developing and refining photomultipliers. RCA was also largely responsible for the commercialization of photomultipliers. The company compiled and published an authoritative and widely used ''Photomultiplier Handbook''. RCA provided printed copies free upon request. The handbook, which continues to be made available online at no cost by the successors to RCA, is considered to be an essential reference.
Following a corporate break-up in the late 1980s involving the acquisition of RCA by
General Electric
General Electric Company (GE) is an American multinational conglomerate founded in 1892, and incorporated in New York state and headquartered in Boston. The company operated in sectors including healthcare, aviation, power, renewable energ ...
and disposition of the divisions of RCA to numerous third parties,
RCA
The RCA Corporation was a major American electronics company, which was founded as the Radio Corporation of America in 1919. It was initially a patent trust owned by General Electric (GE), Westinghouse, AT&T Corporation and United Fruit Comp ...
's photomultiplier business became an independent company.
Lancaster, Pennsylvania facility
The
Lancaster, Pennsylvania
Lancaster, ( ; pdc, Lengeschder) is a city in and the county seat of Lancaster County, Pennsylvania. It is one of the oldest inland cities in the United States. With a population at the 2020 census of 58,039, it ranks 11th in population amon ...
facility was opened by the
U.S. Navy
The United States Navy (USN) is the maritime service branch of the United States Armed Forces and one of the eight uniformed services of the United States. It is the largest and most powerful navy in the world, with the estimated tonnage o ...
in 1942 and operated by RCA for the manufacture of
radio
Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmit ...
and
microwave tube
Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ran ...
s. Following
World War II
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
, the naval facility was acquired by RCA. ''RCA Lancaster,'' as it became known, was the base for the development and the production of commercial
television
Television, sometimes shortened to TV, is a telecommunication medium for transmitting moving images and sound. The term can refer to a television set, or the medium of television transmission. Television is a mass medium for advertisin ...
products. In subsequent years other products were added, such as
"cathode-ray" tubes, photomultiplier tubes,
motion-sensing light control switches, and
closed-circuit television
Closed-circuit television (CCTV), also known as video surveillance, is the use of video cameras to transmit a signal to a specific place, on a limited set of monitors. It differs from broadcast television in that the signal is not openly t ...
systems.
Burle Industries
Burle Industries, as a successor to the RCA Corporation, carried the RCA photomultiplier business forward after 1986, based in the Lancaster, Pennsylvania facility. The 1986 acquisition of RCA by
General Electric
General Electric Company (GE) is an American multinational conglomerate founded in 1892, and incorporated in New York state and headquartered in Boston. The company operated in sectors including healthcare, aviation, power, renewable energ ...
resulted in the
divestiture
In finance and economics, divestment or divestiture is the reduction of some kind of asset for financial, ethical, or political objectives or sale of an existing business by a firm. A divestment is the opposite of an investment. Divestiture is a ...
of the RCA Lancaster New Products Division. Hence, 45 years after being founded by the U.S. Navy, its management team, led by Erich Burlefinger, purchased the division and in 1987 founded Burle Industries.
In 2005, after eighteen years as an independent enterprise, Burle Industries and a key subsidiary were acquired by Photonis, a European holding company
Photonis Group. Following the acquisition, Photonis was composed of Photonis Netherlands, Photonis France, Photonis USA, and Burle Industries. Photonis USA operates the former Galileo Corporation Scientific Detector Products Group (
Sturbridge, Massachusetts
Sturbridge is a town in Worcester County, Massachusetts, United States. It is home to Old Sturbridge Village living history museum and other sites of historical interest such as Tantiusques.
The population was 9,867 at the 2020 census, with mo ...
), which had been purchased by Burle Industries in 1999. The group is known for
microchannel plate detector (MCP) electron multipliers—an integrated micro-vacuum tube version of photomultipliers. MCPs are used for imaging and scientific applications, including
night vision device
A night-vision device (NVD), also known as a night optical/observation device (NOD), night-vision goggle (NVG), is an optoelectronic device that allows visualization of images in low levels of light, improving the user's night vision. The dev ...
s.
On 9 March 2009, Photonis announced that it would cease all production of photomultipliers at both the Lancaster, Pennsylvania and the Brive, France plants.
Hamamatsu
The
Japan
Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
-based company
Hamamatsu Photonics
is a Japanese manufacturer of optical sensors (including photomultiplier tubes), electric light sources, and other optical devices and their applied instruments for scientific, technical and medical use.
The company was founded in 1953 by Hei ...
(also known as Hamamatsu) has emerged since the 1950s as a leader in the photomultiplier industry. Hamamatsu, in the tradition of RCA, has published its own handbook, which is available without cost on the company's website. Hamamatsu uses different designations for particular photocathode formulations and introduces modifications to these designations based on Hamamatsu's proprietary research and development.
Photocathode materials
The photocathodes can be made of a variety of materials, with different properties. Typically the materials have low
work function
In solid-state physics, the work function (sometimes spelt workfunction) is the minimum thermodynamic work (i.e., energy) needed to remove an electron from a solid to a point in the vacuum immediately outside the solid surface. Here "immediately" m ...
and are therefore prone to
thermionic emission
Thermionic emission is the liberation of electrons from an electrode by virtue of its temperature (releasing of energy supplied by heat). This occurs because the thermal energy given to the charge carrier overcomes the work function of the mate ...
, causing noise and dark current, especially the materials sensitive in infrared; cooling the photocathode lowers this thermal noise. The most common photocathode materials are
[Photomultiplier Tubes. Construction and Operating Characteristics. Connections to External Circuits](_blank)
Hamamatsu Ag-O-Cs (also called S1) transmission-mode, sensitive from 300–1200 nm. High dark current; used mainly in near-infrared, with the photocathode cooled; GaAs:Cs,
caesium
Caesium (IUPAC spelling) (or cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only five elemental metals that a ...
-
activated
"Activated" is a song by English singer Cher Lloyd. It was released on 22 July 2016 through Vixen Records. The song was made available to stream exclusively on ''Rolling Stone'' a day before to release (on 21 July 2016).
Background
In an interv ...
gallium arsenide
Gallium arsenide (GaAs) is a III-V direct band gap semiconductor with a Zincblende (crystal structure), zinc blende crystal structure.
Gallium arsenide is used in the manufacture of devices such as microwave frequency integrated circuits, monoli ...
, flat response from 300 to 850 nm, fading towards ultraviolet and to 930 nm; InGaAs:Cs, caesium-activated
indium gallium arsenide
Indium gallium arsenide (InGaAs) (alternatively gallium indium arsenide, GaInAs) is a ternary alloy (chemical compound) of indium arsenide (InAs) and gallium arsenide (GaAs). Indium and gallium are ( group III) elements of the periodic table whil ...
, higher infrared sensitivity than GaAs:Cs, between 900–1000 nm much higher
signal-to-noise ratio
Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in deci ...
than Ag-O-Cs; Sb-Cs, (also called S11) caesium-activated
antimony
Antimony is a chemical element with the symbol Sb (from la, stibium) and atomic number 51. A lustrous gray metalloid, it is found in nature mainly as the sulfide mineral stibnite (Sb2S3). Antimony compounds have been known since ancient time ...
, used for reflective mode photocathodes; response range from ultraviolet to visible, widely used; bialkali (Sb-K-Cs, Sb-Rb-Cs), caesium-activated antimony-rubidium or antimony-potassium alloy, similar to Sb:Cs, with higher sensitivity and lower noise. can be used for transmission-mode; favorable response to a NaI:Tl
scintillator
A scintillator is a material that exhibits scintillation, the property of luminescence, when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate (i.e. re-emit the absorbed ...
flashes makes them widely used in
gamma spectroscopy
Gamma-ray spectroscopy is the quantitative study of the energy spectra of gamma-ray sources, such as in the nuclear industry, geochemical investigation, and astrophysics.
Most radioactive sources produce gamma rays, which are of various energie ...
and radiation detection; high-temperature bialkali (Na-K-Sb), can operate up to 175 °C, used in
well logging
Well logging, also known as borehole logging is the practice of making a detailed record (a ''well log'') of the geologic formations penetrated by a borehole. The log may be based either on visual inspection of samples brought to the surface (' ...
, low dark current at room temperature; multialkali (Na-K-Sb-Cs), (also called S20), wide spectral response from ultraviolet to near-infrared, special cathode processing can extend range to 930 nm, used in broadband
spectrophotometers;
solar-blind (Cs-Te, Cs-I), sensitive to vacuum-UV and ultraviolet, insensitive to visible light and infrared (Cs-Te has cutoff at 320 nm, Cs-I at 200 nm).
Window materials
The windows of the photomultipliers act as wavelength filters; this may be irrelevant if the cutoff wavelengths are outside of the application range or outside of the photocathode sensitivity range, but special care has to be taken for uncommon wavelengths.
Borosilicate glass
Borosilicate glass is a type of glass with silica and boron trioxide as the main glass-forming constituents. Borosilicate glasses are known for having very low coefficients of thermal expansion (≈3 × 10−6 K−1 at 20 °C), ma ...
is commonly used for near-infrared to about 300 nm.
High borate borosilicate glasses exist also in high UV transmission versions with high transmission also at 254 nm. Glass with very low content of
potassium
Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosphe ...
can be used with bialkali photocathodes to lower the background radiation from the
potassium-40
Potassium-40 (40K) is a radioactive isotope of potassium which has a long half-life of 1.25 billion years. It makes up about 0.012% (120 ppm) of the total amount of potassium found in nature.
Potassium-40 undergoes three types of radioactive de ...
isotope. Ultraviolet glass transmits visible and ultraviolet down to 185 nm. Used in spectroscopy. Synthetic
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
transmits down to 160 nm, absorbs less UV than fused silica. Different thermal expansion than
kovar
Kovar (trademark of CRS Holdings, inc., Delaware) is a nickel–cobalt ferrous alloy compositionally identical to Fernico 1, designed to have substantially the same thermal expansion characteristics as borosilicate glass (~5 × 10−6 /K between ...
(and than borosilicate glass that's
expansion-matched to kovar), a graded seal needed between the window and the rest of the tube. The seal is vulnerable to mechanical shocks.
Magnesium fluoride
Magnesium fluoride is an inorganic compound with the formula MgF2. The compound is a white crystalline salt and is transparent over a wide range of wavelengths, with commercial uses in optics that are also used in space telescopes. It occurs natur ...
transmits ultraviolet down to 115 nm.
Hygroscopic
Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance ...
, though less than other alkali halides usable for UV windows.
Usage considerations
Photomultiplier tubes typically utilize 1000 to 2000
volts
The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827).
Definit ...
to accelerate electrons within the chain of dynodes. (See Figure near top of article.) The most negative voltage is connected to the cathode, and the most positive voltage is connected to the anode. Negative high-voltage supplies (with the positive terminal grounded) are often preferred, because this configuration enables the
photocurrent
Photocurrent is the electric current through a photosensitive device, such as a photodiode, as the result of exposure to radiant power. The photocurrent may occur as a result of the photoelectric, photoemissive, or photovoltaic effect. The photo ...
to be measured at the low voltage side of the circuit for amplification by subsequent electronic circuits operating at low voltage. However, with the photocathode at high voltage, leakage currents sometimes result in unwanted "dark current" pulses that may affect the operation. Voltages are distributed to the dynodes by a resistive
voltage divider
In electronics, a voltage divider (also known as a potential divider) is a passive linear circuit that produces an output voltage (''V''out) that is a fraction of its input voltage (''V''in). Voltage division is the result of distributing the in ...
, although variations such as active designs (with
transistors
upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink).
A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...
or
diode
A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other.
A diode ...
s) are possible. The divider design, which influences frequency response or
rise time In electronics, when describing a voltage or current step function, rise time is the time taken by a signal to change from a specified low value to a specified high value. These values may be expressed as ratiosSee for example , and . or, equivalen ...
, can be selected to suit varying applications. Some instruments that use photomultipliers have provisions to vary the anode voltage to control the gain of the system.
While powered (energized), photomultipliers must be shielded from ambient light to prevent their destruction through overexcitation. In some applications this protection is accomplished mechanically by electrical interlocks or shutters that protect the tube when the photomultiplier compartment is opened. Another option is to add overcurrent protection in the external circuit, so that when the measured anode current exceeds a safe limit, the high voltage is reduced.
If used in a location with strong
magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
s, which can curve electron paths, steer the electrons away from the dynodes and cause loss of gain, photomultipliers are usually magnetically shielded by a layer of soft iron or
mu-metal
Mu-metal is a nickel–iron soft ferromagnetic alloy with very high permeability, which is used for shielding sensitive electronic equipment against static or low-frequency magnetic fields. It has several compositions. One such composition ...
. This magnetic shield is often maintained at cathode potential. When this is the case, the external shield must also be electrically insulated because of the high voltage on it. Photomultipliers with large distances between the photocathode and the first dynode are especially sensitive to magnetic fields.
Applications
Photomultipliers were the first
electric eye
An electric eye is a photodetector used for detecting obstruction of a light beam. An example is the door safety system used on garage door openers that use a light transmitter and receiver at the bottom of the door to prevent closing if there is ...
devices, being used to measure interruptions in beams of light. Photomultipliers are used in conjunction with
scintillator
A scintillator is a material that exhibits scintillation, the property of luminescence, when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate (i.e. re-emit the absorbed ...
s to detect
Ionizing radiation
Ionizing radiation (or ionising radiation), including nuclear radiation, consists of subatomic particles or electromagnetic waves that have sufficient energy to ionize atoms or molecules by detaching electrons from them. Some particles can travel ...
by means of hand held and fixed radiation protection instruments, and
particle radiation
Particle radiation is the radiation of energy by means of fast-moving subatomic particles. Particle radiation is referred to as a particle beam if the particles are all moving in the same direction, similar to a light beam.
Due to the wave–par ...
in physics experiments. Photomultipliers are used in research laboratories to measure the intensity and spectrum of light-emitting materials such as
compound semiconductor
Semiconductor materials are nominally small band gap insulators. The defining property of a semiconductor material is that it can be compromised by doping it with impurities that alter its electronic properties in a controllable way.
Because of t ...
s and
quantum dots
Quantum dots (QDs) are semiconductor particles a few nanometres in size, having optical and electronic properties that differ from those of larger particles as a result of quantum mechanics. They are a central topic in nanotechnology. When the ...
. Photomultipliers are used as the detector in many
spectrophotometers
Spectrophotometry is a branch of electromagnetic spectroscopy concerned with the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength. Spectrophotometry uses photometers, known as spec ...
. This allows an instrument design that escapes the
thermal noise limit on sensitivity, and which can therefore substantially increase the
dynamic range
Dynamic range (abbreviated DR, DNR, or DYR) is the ratio between the largest and smallest values that a certain quantity can assume. It is often used in the context of signals, like sound and light. It is measured either as a ratio or as a base-1 ...
of the instrument.
Photomultipliers are used in numerous medical equipment designs. For example,
blood analysis
A blood test is a laboratory analysis performed on a blood sample that is usually extracted from a vein in the arm using a hypodermic needle, or via fingerprick. Multiple tests for specific blood components, such as a glucose test or a cholester ...
devices used by clinical medical laboratories, such as
flow cytometers, utilize photomultipliers to determine the relative concentration of various components in blood samples, in combination with
optical filter
An optical filter is a device that selectively transmits light of different wavelengths, usually implemented as a glass plane or plastic device in the optical path, which are either dyed in the bulk or have interference coatings. The optical p ...
s and
incandescent lamps
An incandescent light bulb, incandescent lamp or incandescent light globe is an electric light with a wire filament heated until it glows. The filament is enclosed in a glass bulb with a vacuum or inert gas to protect the filament from oxidat ...
. An array of photomultipliers is used in a
gamma camera
A gamma camera (γ-camera), also called a scintillation camera or Anger camera, is a device used to image gamma radiation emitting radioisotopes, a technique known as scintigraphy. The applications of scintigraphy include early drug development ...
. Photomultipliers are typically used as the detectors in
flying-spot scanner
A flying-spot scanner (FSS) uses a scanning source of a spot of light, such as a high-resolution, high-light-output, low-persistence cathode ray tube (CRT), to scan an image. Usually the image to be scanned is on photographic film, such as motion ...
s.
High-sensitivity applications
After 50 years, during which
solid-state
Solid state, or solid matter, is one of the four fundamental states of matter.
Solid state may also refer to:
Electronics
* Solid-state electronics, circuits built of solid materials
* Solid state ionics, study of ionic conductors and their use ...
electronic components have largely displaced the vacuum tube, the photomultiplier remains a unique and important optoelectronic component. Perhaps its most useful quality is that it acts, electronically, as a nearly perfect
current source
A current source is an electronic circuit that delivers or absorbs an electric current which is independent of the voltage across it.
A current source is the dual of a voltage source. The term ''current sink'' is sometimes used for sources f ...
, owing to the high voltage utilized in extracting the tiny currents associated with weak light signals. There is no
Johnson noise
Johnson is a surname of Anglo-Norman origin meaning "Son of John". It is the second most common in the United States and 154th most common in the world. As a common family name in Scotland, Johnson is occasionally a variation of ''Johnston'', a ...
associated with photomultiplier signal currents, even though they are greatly amplified, e.g., by 100 thousand times (i.e., 100 dB) or more. The photocurrent still contains
shot noise
Shot noise or Poisson noise is a type of noise which can be modeled by a Poisson process.
In electronics shot noise originates from the discrete nature of electric charge. Shot noise also occurs in photon counting in optical devices, where shot ...
.
Photomultiplier-amplified photocurrents can be electronically amplified by a high-input-impedance electronic amplifier (in the signal path subsequent to the photomultiplier), thus producing appreciable voltages even for nearly infinitesimally small photon fluxes. Photomultipliers offer the best possible opportunity to exceed the Johnson noise for many configurations. The aforementioned refers to measurement of light fluxes that, while small, nonetheless amount to a continuous stream of multiple photons.
For smaller photon fluxes, the photomultiplier can be operated in photon-counting, or
Geiger, mode (see also
Single-photon avalanche diode
A single-photon avalanche diode (SPAD) is a solid-state photodetector within the same family as photodiodes and avalanche photodiodes (APDs), while also being fundamentally linked with basic diode behaviours. As with photodiodes and APDs, a SPAD ...
). In Geiger mode the photomultiplier gain is set so high (using high voltage) that a single photo-electron resulting from a single photon incident on the primary surface generates a very large current at the output circuit. However, owing to the avalanche of current, a reset of the photomultiplier is required. In either case, the photomultiplier can detect individual photons. The drawback, however, is that not every photon incident on the primary surface is counted either because of less-than-perfect efficiency of the photomultiplier, or because a second photon can arrive at the photomultiplier during the "
dead time
For detection systems that record discrete events, such as particle and nuclear detectors, the dead time is the time after each event during which the system is not able to record another event.
An everyday life example of this is what happens when ...
" associated with a first photon and never be noticed.
A photomultiplier will produce a small current even without incident photons; this is called the
''dark current''. Photon-counting applications generally demand photomultipliers designed to minimise dark current.
Nonetheless, the ability to detect single photons striking the primary photosensitive surface itself reveals the quantization principle that
Einstein put forth. Photon counting (as it is called) reveals that light, not only being a wave, consists of discrete particles (i.e.,
photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...
s).
Temperature range
It is known that at cryogenic temperatures photo multipliers demonstrate increase in (bursting) electrons emission as temperature lowers. The phenomena is still
unexplained by any physics theory.
See also
*
Lucas cell
A Lucas cell is a type of scintillation counter. It is used to acquire a gas sample, filter out the radioactive particulates through a special filter and then count the radioactive decay. The inside of the gas chamber is coated with ZnS( Ag) - a ...
*
Scintillation counter
A scintillation counter is an instrument for detecting and measuring ionizing radiation by using the excitation effect of incident radiation on a scintillating material, and detecting the resultant light pulses.
It consists of a scintillator w ...
*
Silicon photomultiplier
Silicon photomultipliers, often called "SiPM" in the literature, are solid-state single-photon-sensitive devices based on Single-photon avalanche diode (SPAD) implemented on common silicon substrate. The dimension of each single SPAD can vary fro ...
*
Total absorption spectroscopy
Total absorption spectroscopy is a measurement technique that allows the measurement of the gamma radiation emitted in the different nuclear gamma transitions that may take place in the daughter nucleus after its unstable parent has decayed by mean ...
References
Bibliography
* Wright, A.G., https://global.oup.com/academic/product/the-photomultiplier-handbook-9780199565092 "The Photomultiplier Handbook", 616pp, Oxford University Press, Oxford, England (2017).
* Engstrom, Ralph W.
''Photomultiplier Handbook'' RCA/Burle (1980).
* ''Photomultiplier Tubes: Basics and Applications (Second Edition)'', Hamamatsu Photonics, Hamamatsu City, Japan, (1999).
* Flyckt, S.O. and Marmonier, C.
''Photomultiplier Tubes: Principles and Applications'' Philips Photonics, Brive, France (2002).
External links
Molecular Expressions– Java-based simulation and tutorial on photomultiplier tubes
Photomultiplier Handbook(4MB PDF) from Burle Industries, essentially the Engstrom-RCA Handbook reprinted
Photomultiplier technical papersfrom ET-Enterprises
Photomultiplier tubesbasics and applications from
Hamamatsu Photonics
is a Japanese manufacturer of optical sensors (including photomultiplier tubes), electric light sources, and other optical devices and their applied instruments for scientific, technical and medical use.
The company was founded in 1953 by Hei ...
Electron Multiplier– simulation of an electron multiplier tube
{{Thermionic valves
Ionising radiation detectors
Medical imaging
Particle detectors
Photodetectors
Sensors
Vacuum tubes
Photomultipliers