Lecture 37

Health Effects – IV Radiation Detection
 

Proportional Counter

  • As the voltage is increased, the pulse height increases and the output is proportional to the energy deposited and hence called proportional counter.
  • It may be noticed that the ratio of response for the higher energy to the lower energy keeps decreasing, till both almost give the same response.
  • This is the beginning of a new region called the Geiger Muller region or GM region.
  • Counters operating in this region are called GM counters.

GM Counter

  • At a characteristic voltage, an avalanche of electrons are formed all over the anode, called “Townsend Avalanche”.
  • At this point the pulse height is constant irrespective of the energy of the radiation that triggers the count.
  • If we define a term called amplification (A) that represents the ratio of the total ion-electron pairs collected to the number primarily formed by the radiation, its value in GM region reaches ~ 106.
  • The value of A in the Ionisation chamber plateau is one.
  • It gradually increases in the proportional region.
  • The reason for the pulse height being independent of the initial number of ion-electron pair is that the amplification is so high that it saturates the detector, cripples the electric field and prevents further multiplication.
  • This plateau is called the GM Plateau.
  • It has been argued that as the voltage is increased, the avalanche spreads radially and increases the count rate or the pulse height, though the increase is small.
  • Thus, the plateau has a slight positive slope.
  • As the voltage is increased, at a characteristic voltage, the gas breaks down and goes into a continuous discharge region.
  • The detector will fail if taken to continuous discharge region.
  • The slope of the GM Plateau is defined as a percentage increase in count rate per 100 V.
  • It is given as.