![]() ![]() ![]() The first demonstration of the photoelectric effect was carried out in 1887 by Heinrich Hertz using ultraviolet light. The invention of the photomultiplier is predicated upon two prior achievements, the separate discoveries of the photoelectric effect and of secondary emission. The manufacturers manuals provide the information needed to choose an appropriate design for a particular application. Many photomultiplier models are available having various combinations of these, and other, design variables. Besides the different photocathode materials, performance is also affected by the transmission of the window material that the light passes through, and by the arrangement of the dynodes. The side-on design is used, for instance, in the type 931, the first mass-produced PMT. 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. Internal metallisation as a protective screen against unwanted lights sources Many variations of design are used in practice the design shown is merely illustrative. 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. ![]() In the example, the photocathode is held at a negative high voltage of order 1000 V, while the anode is very close to ground potential. The necessary distribution of voltage along the series of dynodes is created by a voltage divider chain, as illustrated in Fig. 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 ≈50 nanoseconds earlier. 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 12 ≈ 10 8 electrons. The geometry of the dynode chain is such that a cascade occurs with an exponentially-increasing number of electrons being produced at each stage. Upon striking the first dynode, more low energy electrons are emitted, and these electrons are in turn accelerated toward the second dynode. They each arrive with ≈100 eV kinetic energy imparted by the potential difference. 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. A small group of primary electrons is created by the arrival of a group of initial photons. 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. Each dynode is held at a more positive potential, by ≈100 Volts, than the preceding one. The electron multiplier consists of a number of electrodes called dynodes. These electrons are directed by the focusing electrode toward the electron multiplier, where electrons are multiplied by the process of secondary emission. Electrons are ejected from the surface as a consequence of the photoelectric effect. Incident photons strike the photocathode material, which is usually a thin vapor-deposited conducting layer on the inside of the entry window of the device. Photomultipliers are typically constructed with an evacuated glass housing (using an extremely tight and durable glass-to-metal seal like other vacuum tubes), containing a photocathode, several dynodes, and an anode. 2: Typical photomultiplier voltage divider circuit using negative high voltage. Semiconductor devices, particularly silicon photomultipliers and avalanche photodiodes, are alternatives to classical photomultipliers however, photomultipliers are uniquely well-suited for applications requiring low-noise, high-sensitivity detection of light that is imperfectly collimated. Research that analyzes light scattering, such as the study of polymers in solution, often uses a laser and a PMT to collect the scattered light data. Elements of photomultiplier technology, when integrated differently, are the basis of night vision devices. The combination of high gain, low noise, high frequency response or, equivalently, ultra-fast response, and large area of collection has maintained photomultipliers an essential place in low light level spectroscopy, confocal microscopy, Raman spectroscopy, fluorescence spectroscopy, nuclear and particle physics, astronomy, medical diagnostics including blood tests, medical imaging, motion picture film scanning ( telecine), radar jamming, and high-end image scanners known as drum scanners. ![]()
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