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====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]].
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