Transcript 151b650e7a25cfd

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 exposurearea
product
detector 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. 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.