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Photomultiplier Tube

Photomultiplier Tube. 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 10 8 single photons can be resolved High gain, low noise, high frequency response, and large area of collection

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Photomultiplier Tube

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  1. Photomultiplier Tube

  2. 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

  3. Vs. Phototube • In both, 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

  4. Components • Made of a glass vacuum tube • Photocathode • Several dynodes • One anode

  5. How it works

  6. 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

  7. Dynodes • Each is held at a more positive voltage than the previous one • As the electron approaches the dynode, it is accelerated by the electric field • On striking it, more low energy electrons are emitted and accelerated to the next dynode in the series

  8. Anode • The final electrode in the chain of dynodes • The electrons accumulate here • This charge results in a sharp current pulse indicating the arrival of a photon

  9. The Variables • Photocathode thickness • 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

  10. Usage • Require 1000 to 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

  11. Cost • The price ranges from $175-300 • Depends on size • Type of photocathode • Voltage • Number of dynodes • Cathode sensitivity • Anode sensitivity

  12. Sources - Websites • http://elchem.kaist.ac.kr/vt/chem-ed/optics/detector/pmt.htm • http://micro.magnet.fsu.edu/primer/digitalimaging/concepts/photomultipliers.html • http://en.wikipedia.org/wiki/Photomultiplier • http://www.aditpmt.com/products/b29b02h.htm

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