photomultiplier tube

A photomultiplier tube is a device for measuring numbers of photons (for example, the flux of photons over the area of the aperture). A photon that enters the tube will be converted to an electrical signal which can be detected.

As a photon enters the tube, it will strike a photocathode. The photocathode emits an electron using the photoelectric effect. The electron emitted will be drawn to an electrode down farther along the photomultiplier tube that is held at a higher potential. (These electrodes are called dynodes in a photomultiplier.) As the electron is drawn towards the dynode, it will accelerate due to the electric field. The electron will gain enough energy to "knock out" other electrons in the target dynode. This process continues all the way down the photomultiplier tube, with the electrons being pulled by stronger and stronger successive dynode potentials. At the end of the tube, there will be a large electric signal, as all of the electrons strike a detector. Because one single photon causes such a large flux of electrons, the photomultiplier tube is an extremely sensitive instrument, which assists in increasing the efficiency of measurements.

In nuclear medicine, PMTs are arrayed (usually in a honeycomb pattern) over a large (40cm or so) flat crystal. When the tracer isotope in the body emits a particle that interacts with the crystal, the photons from that interaction reach the PMTs at slightly different times. After some non-hairy math you can resolve a location where the tracer substance is, roughly, in the body.

The less-crisp images from this process earn the nickname of "unclear medicine", but for lung scans and the like there is no substitute.

The photomultiplier tube(PMT) is the most sensitive photoelectric sensor available at this time, capable of detecting the lowest light intencity possible, a single photon. It consists of a vaccum tube with a light sensitive cathode at one end. When a photon hits this cathode, it will emit an electron. This electron try to fly in a straight line through the tube, but the dynode(Dy1)'s lower electron charge(more positive) will attract it. When the electron strikes the dynode, more secondary electrons are released. These electrons fly off at an angle towards the anode end of the tube. They will be attracted by Dy2, and a new electron multiplication will happen. This continues until the electrons finally hits the anode. By measuring the bursts of electrons an indication of the number of photons can be computed.

Simple schematic of a PMT with four dynodes.


      .----------------------------------.
      |                                  |
      #Cathode                   anode | |
      #                                |-+-.
      #                                | | |
      #                                | || |R6
      #    Dy1   Dy2   Dy3   Dy4         || |
      #    ___   ___   ___   ___         | T
      |     T     T     T     T          | |
      |-----+-----+-----+-----+----------' |
      |  _  |  _  |  _  |  _  |  _         |
      |-|_|-*-|_|-*-|_|-*-|_|-*-|_|--------|
      |  R1    R2    R3    R4    R5        |
      |                                   ---
    High                                  GND
   voltage
   (2000V)

A normal PMT have ten or more dynodes.

An example of the multiplication:
If we have a tube with 12 dynodes, where each have a four times multiplication. We will have:
4^(12-1)=4194304
This means that one photon triggers the flow of over four million electrons.

The tube must be cooled or you might get false readings (due to photons from heat(infrared light)). The tubes accuracy may also be affected by magnetic fields, since magnetic fields deflects the electrons, and may cause them to miss the dynodes. Also the high voltage supply must be very stable, since fluctuation might affect the results.

The PMT is expencive and relatively fragile but is much used in applications where high sensitivity is needed.The PMTs are available for wavelengths between 200nm and 1100nm(from UV to IR).