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===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.<ref>{{cite book|url=https://books.google.com/books?id=pksGCAAAQBAJ&q=Secondary+Emission+1899&pg=PA9|title=Interaction of Atomic Particles with a Solid Surface / Vzaimodeistvie Atomnykh Chastits S Poverkhnost'yu Tverdogo Tela / Взаимодействие Атомных Частиц С Поверхностью Твердого Тела|first=U. A.|last=Arifov|date=14 December 2013|publisher=Springer|via=Google Books|url-status=live|archive-url=https://web.archive.org/web/20170312055330/https://books.google.co.uk/books?id=pksGCAAAQBAJ&pg=PA9&lpg=PA9&dq=Secondary+Emission+1899&source=bl&ots=ejD3YmKcMc&sig=5jk2lIaSN7vbF0wPJfCtOe9gRIs&hl=en&sa=X&ved=0ahUKEwiI_8yN6N3QAhXNFsAKHSq7AzIQ6AEIJzAB#v=onepage&q=Secondary+Emission+1899&f=false|archive-date=12 March 2017|isbn=9781489948090}}</ref> In 1902, Austin and Starke reported that the metal surfaces impacted by electron beams emitted a larger number of electrons than were incident.<ref>H. Bruining, Physics and applications of secondary electron emission, (McGraw-Hill Book Co., Inc.; 1954).</ref> The application of the newly discovered secondary emission to the amplification of signals was only proposed after [[World War I]] by [[Westinghouse Electric (1886)|Westinghouse]] scientist [[Joseph Slepian]] in a 1919 patent.<ref>J. Slepian, Westinghouse Electric, "Hot Cathode Tube" {{US Patent|1450265}}, Issued April 3, 1923 (Filed 1919)</ref> ====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|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).<ref>{{cite journal|last1=Iams|first1=H.|last2=Salzberg|first2=B.|title=The Secondary Emission Phototube|journal=Proceedings of the IRE|volume=23|page=55|year=1935|doi=10.1109/JRPROC.1935.227243 |s2cid=51654002}}</ref> 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 [[dynode]]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]], 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.<ref>A.B. Magoun [http://www.histech.nl/Shot2004/programma/txt/magoun.asp?file=magoun ''Adding Sight to Sound in Stalin's Russia: RCA and the Transfer of Television Technology to the Soviet Union''] {{webarchive|url=https://web.archive.org/web/20110724154432/http://www.histech.nl/Shot2004/programma/txt/magoun.asp?file=magoun |date=2011-07-24 }}, Society for the History of Technology (SHOT), Amsterdam (2004)</ref> 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<ref>{{cite book|script-title=ru:Большая советская энциклопедия|trans-title=[[Great Soviet Encyclopedia]]|chapter=Кубецкий Леонид Александрович|trans-chapter=Kubetsky Leonid Aleksandrovich|language=ru|year=1973|volume=13|edition=3|publisher=Sovetskaya Entsiklopediya|location=Moscow|chapter-url=http://bse.sci-lib.com/article066966.html}}</ref> 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<ref>{{cite journal|last1=Kubetsky|first1=L.A.|title=Multiple Amplifier|journal=Proceedings of the IRE|volume=25|page=421|year=1937|doi=10.1109/JRPROC.1937.229045|issue=4 |s2cid=51643186}}</ref> as emphasized in a recent 2006 publication of the [[Russian Academy of Sciences]] (RAS),<ref>{{cite journal|last1=Lubsandorzhiev|first1=B|doi=10.1016/j.nima.2006.05.221|title=On the history of photomultiplier tube invention|year=2006|page=236|volume=567|issue=1|journal=Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment|arxiv=physics/0601159|bibcode = 2006NIMPA.567..236L }}</ref> 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, [[Zworykin|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''<ref>{{cite journal|last1=Zworykin|first1=V.K.|last2=Morton|first2=G.A.|last3=Malter|first3=L.|title=The Secondary Emission Multiplier-A New Electronic Device|journal=Proceedings of the IRE|volume=24|page=351|year=1936|doi=10.1109/JRPROC.1936.226435|issue=3 |s2cid=51654458}}</ref> — 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 [[nanometer|nm]]. ====Electrostatic photomultipliers (1937–present)==== Whereas these early photomultipliers used the magnetic field principle, electrostatic photomultipliers (with no magnetic field) were demonstrated by [[Jan A. Rajchman|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.<ref>J. Rajchman and E.W. Pike, RCA Technical Report TR-362, "Electrostatic Focusing in Secondary Emission Multipliers," September 9, 1937</ref> ====Improved photocathodes==== Also in 1936, a much improved photocathode, Cs<sub>3</sub>Sb ([[caesium]]-[[antimony]]), was reported by P. Görlich.<ref>{{cite journal|last1=Görlich|first1=P.|title=Über zusammengesetzte, durchsichtige Photokathoden|journal=Zeitschrift für Physik|volume=101|page=335|year=1936|doi=10.1007/BF01342330|bibcode = 1936ZPhy..101..335G|issue=5–6 |s2cid=121613539}}</ref> 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]]s. Different photocathodes provided differing spectral responses.
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