Photomultiplier tube

Photomultiplier tube

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  Photomultiplier Tube Memoona Rafique
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  What is it? Extremely sensitive detector of light in the ultraviolet, visible and near infrared  Multiplies the signal produced by incident light by as much as 108  single photons can be resolved  High gain, low noise, high frequency response, and large area of collection  A tiny and normally undetectable current becomes a much larger and easily measurable current
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  Vs. Phototube  Inboth, the photon strikes the photocathode and emits electrons (photoelectric effect)  In a phototube only these few electrons are collected  It can only be used for very low signals  In a photomultiplier tube these electrons are multiplied  This process is called secondary emission
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  Components  Made ofa glass vacuum tube  Photocathode  Several dynodes  One anode
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  How it works
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  OPERATING PRINCIPLES  Aphotomultiplier converts light into an electrical signal then amplifies that signal to a useful level by emission of secondary electrons.  The operating principle is that – caused by the photoelectric effect – photons striking a photocathode at the entrance window of a PMT produce electrons, which are then accelerated by a high-voltage field and multiplied in number within a chain of dynodes by the process of secondary emission.
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  Elements  − aphotocathode which converts light flux into electron flux;  − an electron-optical input system which focuses and accelerates the electron flux;  − an electron multiplier consisting of a series of secondary-emission electrodes  (dynodes); and, finally  − an anode which collects the electron flux from the multiplier and supplies the output signals
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  How it works Electron is released by the photocathode  Electron is then multiplied by the electrodes  Metal channel dynodes  At the end of the chain is the collection electrode  Anode  The current flowing from the anode to ground is directly proportional to the photoelectron flux generated by the photocathode
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  The Variables  Photocathodethickness  Too thick and more photons will be absorbed, less electrons will be emitted  Too thin and too many photons will pass through without being absorbed  Semitransparent Photocathode  Multiplies the electrons to up to 100 million
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  Usage  Require 1000to 2000 volts  Negative voltage is connected to cathode and the positive to the anode  Distributed to the dynodes by a resistive voltage divider (series of resistors)  Must be shielded from ambient light  To prevent destruction through over excitation  If used in an area of high magnetic fields it must be shielded by a layer of mu-metal  Mu-metal is a nickel-iron alloy with a very high magnetic permeability
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  Cost  The priceranges from $175-300  Depends on size  Type of photocathode  Voltage  Number of dynodes  Cathode sensitivity  Anode sensitivity
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  Sources - Websites http://elchem.kaist.ac.kr/vt/chem- ed/optics/detector/pmt.htm  http://micro.magnet.fsu.edu/primer/digitalimagi ng/concepts/photomultipliers.html  http://en.wikipedia.org/wiki/Photomultiplier  http://www.aditpmt.com/products/b29b02h.htm