Chapter V Radiation Detectors

Gas-Filled Detectors

1-Region A Here Vdc is relatively low so that recombination of positive ions and electrons occurs. As a result not all ion pairs are collected and the voltage pulse height is relatively low. 2- Region B Vdc is sufficiently high in this region so that only a negligible amount of recombination occurs. This is the region where a type of detector called the Ionization Chamber operates. 3-Region C Vdc is sufficiently high in this region so that electrons make a collisions with the electrons of gas atoms to produce new ion pairs. Thus the number of electrons is increased so that the electric charge increased to thousand times greater than the charge produced initially by the radiation interaction. This is the region where a type of detector called the Proportional Counter operates.

4- Region D Vdc is so high that even a minimally-ionizing particle will produce a very large voltage pulse. The initial ionization produced by the radiation triggers a complete gas breakdown as an avalanche of electrons heads towards and spreads along the centre wire. This region is called the Geiger-Müller Region, and is exploited in the Geiger Counter. 5-Region F Here Vdc is high enough for the gas to completely breakdown and it cannot be used to detect radiation.

When a beta-particle interacts with the gas the energy required to produce one ion pair is about 30 eV. Therefore when a beta-particle of energy 1 MeV is completely absorbed in the gas the number of ion pairs produced is: The electric charge produced in the gas is therefore Q = n . e Because such a small voltage is generated it is necessary to use a very sensitive amplifier in the electronic circuitry connected to the chamber.

An exposure meter used in radiography An exposure-area product detector used in radiography. An exposure meter used in radiography

Geiger counter The true reading T without going into detail can be obtained using the equation where A is the actual reading and τ is the dead time. Some instruments perform this calculation automatically.

Scintillation detector. m: number of light photons produced in crystal k: optical efficiency of the crystal l: quantum efficiency of the photocathode, that is the efficiency of the photocathode converts photons to electrons n: number of dynodes, e: the electronic charge. R: dynode multiplication factor, that is the number of secondary electrons emitted

For example supposing a 100 keV gamma-ray is absorbed in the crystal For example supposing a 100 keV gamma-ray is absorbed in the crystal. The number of light photons produced, m, might be about 1,000 for a typical scintillation crystal. A typical crystal might have an optical efficiency, k, of 0.5 - in other words 50% of the light produced reaches the photocathode which might have a quantum efficiency of 0.15. A typical PMT has ten dynodes and let us assume that the dynode multiplication factor is 4.5. Therefore  Q = 41 PC This amount of charge is very small. A very sensitive amplifier is therefore needed to amplify this signal.