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Photomultiplier
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Photomultiplier tubes (photomultipliers or PMTs for short), members of the class of vacuum tubes, and more specifically phototubes, are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum. These detectors multiply the signal produced by incident light by as much as 100 million times (i.e., 160 dB), in multiple dynode stages, enabling (for example) single photons to be detected individually when the incident flux of light is very low.
The combination of high gain, low noise, high frequency response, and large area of collection has earned photomultipliers an essential place in nuclear and particle physics, astronomy, medical diagnostics including blood tests, medical imaging, motion picture film scanning (telecine), and high-end image scanners known as drum scanners.
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Encyclopedia
Photomultiplier tubes (photomultipliers or PMTs for short), members of the class of vacuum tubes, and more specifically phototubes, are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum. These detectors multiply the signal produced by incident light by as much as 100 million times (i.e., 160 dB), in multiple dynode stages, enabling (for example) single photons to be detected individually when the incident flux of light is very low.
The combination of high gain, low noise, high frequency response, and large area of collection has earned photomultipliers an essential place in nuclear and particle physics, astronomy, medical diagnostics including blood tests, medical imaging, motion picture film scanning (telecine), and high-end image scanners known as drum scanners. Semiconductor devices, particularly avalanche photodiodes, are alternatives to photomultipliers; however, photomultipliers are uniquely well-suited for applications requiring low-noise, high-sensitivity detection of light that is imperfectly collimated. While photomultipliers are extraordinarily sensitive and moderately efficient, research is still underway to create a photon-counting light detection device that is much more than 99% efficient. Such a detector is of interest for applications related to quantum information and quantum cryptography. Elements of photomultiplier technology, when integrated differently, are the basis of night vision devices.
History
Combining Two Scientific Discoveries
The invention of the photomultiplier is predicated upon two prior achievements, firstly discovering the photoelectric effect and secondly discovering secondary emission (i.e., the ability of electrons in a vacuum tube to cause the emission of additional electrons by striking an electrode).
Photoelectric Effect
The first demonstration of the photoelectric effect was carried out in 1887 by Heinrich Hertz who demonstrated it 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 cesium, another alkali metal, 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, who relied upon the phenomenon to establish the fundamental principle of quantum mechanics, in 1905, an accomplishment for which Albert Einstein received the 1921 Nobel Prize. It is worthwhile to note that Heinrich Hertz, working 18 years earlier, had not recognized the proportionality between optical frequency and photon energy. This proportionality implied a discrete nature of light, i.e. the existence of quanta, for the first time.
Secondary Emission
The phenomenon of secondary emission was first limited to purely electronic inventions (i.e., those lacking photosensitivity). 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 by Westinghouse scientist Joseph Slepian in a 1919 patent.
The First Photomultiplier
The ingredients for inventing the photomultiplier were coming together during the 1920s as the pace of vacuum tube technologies accelerated. The stage was set to combine the dual phenomena of photoemission (i.e., the photoelectric effect) with secondary emission to create the photomultiplier.
The first 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 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 (Proc. IRE). The device consisted of a semi-cylindrical photocathode, 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.
In Soviet Russia, RCA-manufactured radio equipment was introduced on a large scale by Joseph Stalin 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. Simultaneously, numerous visits were made by RCA scientific personnel to Soviet Russia (prior to the Cold War) in the 1930s for the purpose of instructing the Russian customers on the capabilities of, and determining the specific needs for, RCA transmission installations and their associated equipment. During one of these visits, in September 1934, RCA's Vladimir Zworykin was shown the first multiple-dynode photomultiplier, or photoelectron multiplier. This pioneering device of 28-year-old Leonid A. Kubetsky 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 (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, 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 a Ag-O-Cs (silver-oxide-cesium) photocathode. They exhibited a peak quantum efficiency of 0.4% at 800 nm.
Also in 1936, a much improved photocathode, Cs3Sb (cesium-antimony), was reported by P. Gorlich. The cesium-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 dynodes. Different photocathodes provided differing spectral responses.
Spectral response of photocathodes
In the early 1940s the JEDEC (Joint Electron Devices 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. The various types of photocathode devices 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 cesium-antimony photocathode, and S-25 uses a so-called "multialkali" photocathode (Na-K-Sb-Cs, or sodium-potassium-antimony-cesium) 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 one micrometer.
Role of RCA
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 photomultiplers. The company compiled and published an authoritative and very-widely used Photomultiplier Handbook. RCA made printed copies available for 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 and disposition of the divisions of RCA to numerous third-parties, RCA's photomultiplier business became an independent company.
Lancaster, Pennsylvania facility
The Lancaster, Pennsylvania facility was opened by the U.S. Navy in 1942 and operated by RCA for the manufacture of radio and microwave tubes. Following the Allied victory in World War II, the naval facility was acquired by RCA. RCA Lancaster, as it became known, was the base for development and production of commercial television products. In subsequent years other products were added, such as cathode ray tubes, photomultiplier tubes, motion-sensing light control switches, and closed-circuit television systems.
Transition to Burle Industries
Burle Industries, as a successor to the RCA Corporation, has carried the RCA photomultiplier business forward since 1986, and is based in the Lancaster, Pennsylvania facility. The 1986 acquisition of RCA by General Electric resulted in the divestiture 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.
The RCA Photomultipler Handbook, along with another famous RCA reference work, is available on the Burle Industries website.
In 2005, after eighteen years as an independent enterprise, Burle Industries and a key subsidiary were acquired by Photonis, a European holding company . 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), 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 devices.
Other companies
The Japan-based company Hamamatsu Photonics (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.
Structure and operating principles
Photomultipliers are constructed from a glass envelope with a high vacuum inside, which houses a photocathode, several dynodes, and an anode. Incident photons strike the photocathode material, which is present as a thin deposit on the entry window of the device, with electrons being produced as a consequence of the photoelectric effect. These electrons are directed by the focusing electrode toward the electron multiplier, where electrons are multiplied by the process of secondary emission.
The electron multiplier consists of a number of electrodes, called dynodes. Each dynode is held at a more positive voltage than the previous one. The electrons leave the photocathode, having the energy of the incoming photon (minus the work function of the photocathode). As the electrons move toward the first dynode, they are accelerated by the electric field and arrive with much greater energy. Upon striking the first dynode, more low energy electrons are emitted, and these electrons in turn are accelerated toward the second dynode. The geometry of the dynode chain is such that a cascade occurs with an ever-increasing number of electrons being produced at each stage. Finally, the electrons reach the anode, where the accumulation of charge results in a sharp current pulse indicating the arrival of a photon at the photocathode.
There are two common photomultiplier orientations, the head-on or end-on design, as shown above, where light enters the flat, circular top of the tube, and the side-on design, where light enters at a particular spot on the side of the tube. 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. A large number of photomultiplier models are available having various combinations of these, and other, design variables. Either of the manuals mentioned will provide the information needed to choose an appropriate design for a particular application.
Usage considerations
Photomultiplier tubes typically utilize 1000 to 2000 volts to accelerate electrons within the chain of dynodes. 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 preferred, because this configuration enables the photocurrent to be measured at the low voltage side of the circuit for amplification by subsequent electronic circuits operating at low voltage. Voltages are distributed to the dynodes by a resistive voltage divider, although variations such as active designs (with transistors or diodes) are possible. The divider design, which influences frequency response or rise time, 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. If used in a location with strong magnetic fields, which can curve electron paths, photomultipliers are usually shielded by a layer of mu-metal. 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.
Typical applications
- Photomultipliers were the first electric eye devices, being used to measure interruptions in beams of light.
- Photomultipliers are used in conjunction with scintillators to detect nuclear and particle radiation in physics experiments.
- Photomultipliers are used in research laboratories to measure the intensity and spectrum of light-emitting materials such as compound semiconductors and quantum dots.
- Photomultipliers are used in numerous medical equipment designs. For example, blood analysis devices used by clinical medical laboratories utilize photomultipliers to determine the relative concentration of various components in vials of blood drawn in doctors' offices, in combination with optical filters and incandescent lamps.
High sensitivity applications
After fifty years, during which solid-state 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 owing to the high voltage utilized in extracting the tiny currents associated with weak light signals. There is no Johnson noise 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.
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). 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" 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 for low 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., photons).
See also
Bibliography
- Engstrom, Ralph W., Photomultiplier Handbook, RCA (1980).
- Photomultiplier Tubes: Basics and Applications (Second Edition), Hamamatsu Photonics, Hamamatsu City, Japan, (1999).
- Flyckt, S.O. and Marmonier, C., , Philips Photonics, Brive, France (2002).
External links
- - Java-based simulation and tutorial on photomultiplier tubes
- (4MB PDF) from Burle Industries, essentially the Engstrom-RCA Handbook reprinted
- from Electron Tubes Ltd.
- basics and applications from Hamamatsu Photonics
- - simulation of an electron multiplier tube
- Light pulse generator Blue light pulse generator for simulating a scintillator's output into a PMT.
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