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THE PHOTOMULTIPLIER TUBE-Biosensors and Transducers

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MUKESH SUNDARARAJAN

This document discusses different types of photoelectric transducers, including photoemissive, photoconductive, and photovoltaic devices. It focuses on photomultiplier tubes, describing their construction and working principle of electron multiplication through secondary emission at dynode stages. Photomultiplier tubes can amplify current by 105 to 109 times, achieving high luminous sensitivity down to 10-5 lumens. The document also covers photoconductive cells, whose resistance varies with light intensity, allowing their use in light-controlled circuits.

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PHOTO ELECTRIC TRANSDUCERS - MUKESH SUNDARARAJAN
PHOTO ELECTRIC TRANSDUCERS:  A photoelectric transducer can be categorized as photoemissive, photoconductive, or photovoltaic. In photoemissive devices, radiation falling on a cathode causes electrons to be emitted from the cathode surface.  In photoconductive devices, the resistance of a material is changed when it is illuminated. Photovoltaic cells generate an output voltage proportional to radiation intensity.  The incident radiation may be infrared, ultraviolet, gamma rays, or X rays as well as visible light.
THE PHOTOMULTIPLIER TUBE  The Photomultiplier tube consists of an evacuated glass envelope containing a photo cathode, an anode and several additional electrodes caller dynodes, each at a higher voltage. Figure illustrates the principle of IN Photomultiplier.  Electrons emitted by the cathode are attracted to the firs anode. Here a phenomenon known as secondary emission takes place. When electrons moving at a high velocity strike an appropriate material, the material emits a greater number of electrons than it was struck with.
The Photomultiplier Tube Fig (17) Principle of Photomultiplier tube
 In this device the high velocity is achieved by using a high voltage between the first anode and the cathode. The electrons emitted by the first anode are then attracted to the second anode, where the same thing takes place again.  Each anode is at a higher voltage, in order to achieve the requisite electron velocity each time. Thus, secondary emission and a resulting "electron multiplication" occur at each step, with an overall increase in electron flow that may be very great.
 Amplification of the original current by as much as 105 to 109 is common. Luminous sensitivities range from 1 A per lumen or less, to over 2000 A per lumen. Typical anode current ratings are 100,uA minimum to 1 A maximum.  The extreme luminous sensitivity possible with these devices is illustrated by the fact that with a sensitivity of 100 A per lumen, only 10-5 lumen is needed to produce a 1-mA output current.
 Magnetic fields affect the gain of the Photomultiplier because some electrons may be deflected from their normal path between stages and therefore never reach a dynode or, eventually, the anode.  In scintillation counting applications this effect may be disturbing, and mu-metal magnetic shields are often placed around the Photomultiplier tube.
Photo Electric Transducers (cont’d):- 8.2. Photoconductive Cells or Photocells  Another photoelectric effect that has proved very useful is the photoconductive effect, which is used in photoconductive cells or photocells.  In this type of device the electrical resistance of the material varies with the amount of light striking it.A typical form of construction is shown in Fig. (18-a).
(cont’d):-  The photoconductive material, typically cadmium sulfide, cadmium selenide, or cadmium sulfoselenide, is deposited in a zigzag pattern, to obtain a desired resistance value and power rating. Fig (18) Photoconductive cell. (a) Construction. (b) Typical curves of resistance versus illumination.
CONTIN…  The material separates two metal-coated areas acting as electrodes, all on an insulating base such as ceramic. The assembly enclosed in a metal case with a glass window over the photoconductive material. Photocells of this type are made in a range of sizes, having diameters of one-eighth inch to over one inch..  The small sizes are suitable where spa is critical, for example, in equipment for reading punched cards and similar applications. However, the very small units have very low power dissipation ratings.  A typical control circuit utilizing a photoconductive cell is illustrated in Fig. The potentiometer is used to make adjustments to compensate for manufacturing tolerances in photocell sensitivity and relay-operating sensitivity.
 When the photocell has the appropriate light shining on it, its resistance will be low and the current through the relay will consequently be high enough to operate the relay. When the light is interrupted, the resistance will rise, causing the relay current to decrease enough to deenergize the relay.

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THE PHOTOMULTIPLIER TUBE-Biosensors and Transducers

  • 2. PHOTO ELECTRIC TRANSDUCERS:  A photoelectric transducer can be categorized as photoemissive, photoconductive, or photovoltaic. In photoemissive devices, radiation falling on a cathode causes electrons to be emitted from the cathode surface.  In photoconductive devices, the resistance of a material is changed when it is illuminated. Photovoltaic cells generate an output voltage proportional to radiation intensity.  The incident radiation may be infrared, ultraviolet, gamma rays, or X rays as well as visible light.
  • 3. THE PHOTOMULTIPLIER TUBE  The Photomultiplier tube consists of an evacuated glass envelope containing a photo cathode, an anode and several additional electrodes caller dynodes, each at a higher voltage. Figure illustrates the principle of IN Photomultiplier.  Electrons emitted by the cathode are attracted to the firs anode. Here a phenomenon known as secondary emission takes place. When electrons moving at a high velocity strike an appropriate material, the material emits a greater number of electrons than it was struck with.
  • 4. The Photomultiplier Tube Fig (17) Principle of Photomultiplier tube
  • 5.  In this device the high velocity is achieved by using a high voltage between the first anode and the cathode. The electrons emitted by the first anode are then attracted to the second anode, where the same thing takes place again.  Each anode is at a higher voltage, in order to achieve the requisite electron velocity each time. Thus, secondary emission and a resulting "electron multiplication" occur at each step, with an overall increase in electron flow that may be very great.
  • 6.  Amplification of the original current by as much as 105 to 109 is common. Luminous sensitivities range from 1 A per lumen or less, to over 2000 A per lumen. Typical anode current ratings are 100,uA minimum to 1 A maximum.  The extreme luminous sensitivity possible with these devices is illustrated by the fact that with a sensitivity of 100 A per lumen, only 10-5 lumen is needed to produce a 1-mA output current.
  • 7.  Magnetic fields affect the gain of the Photomultiplier because some electrons may be deflected from their normal path between stages and therefore never reach a dynode or, eventually, the anode.  In scintillation counting applications this effect may be disturbing, and mu-metal magnetic shields are often placed around the Photomultiplier tube.
  • 8. Photo Electric Transducers (cont’d):- 8.2. Photoconductive Cells or Photocells  Another photoelectric effect that has proved very useful is the photoconductive effect, which is used in photoconductive cells or photocells.  In this type of device the electrical resistance of the material varies with the amount of light striking it.A typical form of construction is shown in Fig. (18-a).
  • 9. (cont’d):-  The photoconductive material, typically cadmium sulfide, cadmium selenide, or cadmium sulfoselenide, is deposited in a zigzag pattern, to obtain a desired resistance value and power rating. Fig (18) Photoconductive cell. (a) Construction. (b) Typical curves of resistance versus illumination.
  • 10. CONTIN…  The material separates two metal-coated areas acting as electrodes, all on an insulating base such as ceramic. The assembly enclosed in a metal case with a glass window over the photoconductive material. Photocells of this type are made in a range of sizes, having diameters of one-eighth inch to over one inch..  The small sizes are suitable where spa is critical, for example, in equipment for reading punched cards and similar applications. However, the very small units have very low power dissipation ratings.  A typical control circuit utilizing a photoconductive cell is illustrated in Fig. The potentiometer is used to make adjustments to compensate for manufacturing tolerances in photocell sensitivity and relay-operating sensitivity.
  • 11.  When the photocell has the appropriate light shining on it, its resistance will be low and the current through the relay will consequently be high enough to operate the relay. When the light is interrupted, the resistance will rise, causing the relay current to decrease enough to deenergize the relay.