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.
The photoelectric effect is perhaps most associated with Albert Einstein, who educed the fundamental principle of quantization (i.e., the basis of quantum mechanics) for which he received the 1921 Nobel Prize. However, the phenomenon had been known earlier without understanding the quantum mechanical proportionality between optical frequency and photon energy.
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. But the use of secondary emission as a means for signal amplification was not proposed until after World War I, by Joseph Slepian in a 1919 Westinghouse patent.
Sixteen years later, the phenomenon of photoemission (i.e., the photoelectric effect) was combined with secondary emission to create the photomultiplier. In 1935 H.E. Iams and B. Salzberg of RCA reported on a single-stage photomultiplier. 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.
In 1936, Vladimir Zworykin, G.A. Morton, and L. Malter of RCA first reported a tube that amplified the current of photoemitted electrons in multiple stages—a device that was later called a photomultiplier. The first experimental photomultipliers used a Ag-O-Cs (silver-oxide-cesium) photocathode having a typical 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.
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.
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 Photonis Group Following the acquisition, Photonis was comprised 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.
Photomultipliers are constructed from a glass vacuum tube, 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.
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.
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.
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.
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).
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