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==Structure and operating principles== [[File:PhotoMultiplierTubeAndScintillator.svg|thumb|500px|left|Fig.1: Schematic of a photomultiplier tube coupled to a [[scintillator]]. This arrangement is for detection of [[gamma rays]].]] [[File:PMT Voltage Divider.jpg|thumb|500px|left|Fig. 2: Typical photomultiplier voltage divider circuit using negative high voltage.]] Photomultipliers are typically constructed with an evacuated glass housing (using an extremely tight and durable [[glass-to-metal seal]] like other [[Vacuum_tube|vacuum tubes]]), containing a [[photocathode]], several [[dynode]]s, and an [[anode]]. Incident [[photons]] strike the [[photocathode]] material, which is usually a thin [[vacuum deposition|vapor-deposited]] conducting layer on the inside of the entry window of the device. [[Electron]]s are ejected from the surface 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 potential, by β100 Volts, than the preceding one. A primary electron leaves the photocathode with the energy of the incoming photon, or about 3 eV for "blue" photons, minus the [[work function]] of the photocathode. A small group of primary electrons is created by the arrival of a group of initial photons. (In Fig. 1, the number of primary electrons in the initial group is proportional to the energy of the incident high energy gamma ray.) The primary electrons move toward the first dynode because they are accelerated by the electric field. They each arrive with β100 eV kinetic energy imparted by the potential difference. Upon striking the first dynode, more low energy electrons are emitted, and these electrons are in turn accelerated toward the second dynode. The geometry of the dynode chain is such that a cascade occurs with an exponentially-increasing number of electrons being produced at each stage. For example, if at each stage an average of 5 new electrons are produced for each incoming electron, and if there are 12 dynode stages, then at the last stage one expects for each primary electron about 5<sup>12</sup> β 10<sup>8</sup> electrons. This last stage is called the [[anode]]. This large number of electrons reaching the anode results in a sharp current pulse that is easily detectable, for example on an oscilloscope, signaling the arrival of the photon(s) at the photocathode β50 nanoseconds earlier. The necessary distribution of voltage along the series of dynodes is created by a voltage divider chain, as illustrated in Fig. 2. In the example, the photocathode is held at a negative high voltage on the order of 1000 V, while the anode is [[#Usage_considerations|very close to ground potential]]. The capacitors across the final few dynodes act as local reservoirs of charge to help maintain the voltage on the dynodes while electron avalanches propagate through the tube. Many variations of design are used in practice; the design shown is merely illustrative. [[File:Photomultiplicateur ancien .jpg|thumb|Internal metallisation as a protective screen against unwanted light sources]] There are two common photomultiplier orientations, the ''head-on'' or ''end-on'' (transmission mode) design, as shown above, where light enters the flat, circular top of the tube and passes the photocathode, and the ''side-on'' design (reflection mode), where light enters at a particular spot on the side of the tube, and impacts on an opaque photocathode. The side-on design is used, for instance, in the [[#Electrostatic photomultipliers (1937βpresent)|type 931]], the first mass-produced PMT. Besides the different photocathode materials, performance is also affected by the transmission of the [[Photomultiplier#Window_materials|window material]] that the light passes through, and by the arrangement of the dynodes. Many photomultiplier models are available having various combinations of these, and other, design variables. The manufacturers manuals provide the information needed to choose an appropriate design for a particular application.
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