The invention of the photomultiplier is predicated upon two prior achievements, the separate discoveries of the
photoelectric effect and of
secondary emission.
Photoelectric effect The first demonstration of the
photoelectric effect was carried out in 1887 by
Heinrich Hertz 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, 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 Einstein received the 1921
Nobel Prize. 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 (the ability of
electrons in a vacuum tube to cause the emission of additional electrons by striking an
electrode) was, at first, limited to purely electronic phenomena and devices (which lacked
photosensitivity). 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 by
Westinghouse scientist
Joseph Slepian 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 video camera (the
iconoscope). Early prototype television cameras lacked sensitivity. Photomultiplier technology was pursued to enable television camera tubes, such as the iconoscope and (later) the
orthicon, to be sensitive enough to be practical. So the stage was set to combine the dual phenomena of
photoemission (i.e., the photoelectric effect) with
secondary emission, 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 (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.
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
dynodes). 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, 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. Numerous visits were made by RCA scientific personnel to the
USSR in the 1930s, prior to the
Cold War, 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 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 (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 an Ag-O-Cs (
silver oxide-
caesium) photocathode. They exhibited a peak
quantum efficiency 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, Cs3Sb (
caesium-
antimony), 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
dynodes. Different photocathodes provided differing spectral responses.
Spectral response of photocathodes In the early 1940s, the
JEDEC (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-
potassium-
antimony-
caesium) 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.
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 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
World War II, 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 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.
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 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. 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), 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. 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-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.
Photocathode materials The photocathodes can be made of a variety of materials, with different properties. Typically the materials have low
work function and are therefore prone to
thermionic emission, causing noise and dark current, especially the materials sensitive in infrared; cooling the photocathode lowers this thermal noise. The most common photocathode materials are 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-
activated gallium arsenide, flat response from 300 to 850 nm, fading towards ultraviolet and to 930 nm; InGaAs:Cs, caesium-activated
indium gallium arsenide, higher infrared sensitivity than GaAs:Cs, between 900–1000 nm much higher
signal-to-noise ratio than Ag-O-Cs; Sb-Cs, (also called S11) caesium-activated
antimony, 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 flashes makes them widely used in
gamma spectroscopy and radiation detection; high-temperature bialkali (Na-K-Sb), can operate up to 175 °C, used in
well logging, 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 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 can be used with bialkali photocathodes to lower the background radiation from the
potassium-40 isotope. Ultraviolet glass transmits visible and ultraviolet down to 185 nm. Used in spectroscopy. Synthetic
silica transmits down to 160 nm, absorbs less UV than fused silica. Different thermal expansion than
kovar (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 transmits ultraviolet down to 115 nm.
Hygroscopic, though less than other alkali halides usable for UV windows. ==Usage considerations==