Radiation Detectors
Content
How Geiger Mueller Counter Work
A Geiger Mueller Counter is a device used for the detection and measurement of all
types of radiation: alpha, beta and gamma radiation.The counter works on the "principle
of gas multiplication". Ionization in the gas is caused by the entry of photon or a
particulate radiation. The ions are attracted to their appropriate electrode (i.e. cation to
cathode, anion to anode) and thy gain sufficient energy to eject electrons from the gas
atoms as they pass through the gas. This causes the atoms to ionize. Therefore
electrons are produced continuously by this process and rapid gas multiplication takes
place (especially in the central electrode because of its strong electric field strength).
The effect of "gas multiplication" is that more than one million electrons are collected by
the central electrode for every ion produced in the primary absorption process. Theses
"electron avalanches" create electric pulses which then can be amplified electronically
and counted by a meter to calculate the number of initial ionization events. Therefore a
Geiger counter is able to detect low-energy radiation because even one ionized particle
produces a full pulse on the central wire.
Application
1.
2.
3.
4.
Particle detection
Gamma and X-ray detection
Neutron detection
Gamma measurement—personnel protection and process control
Advantages
1. Geiger counters are even more useful because they can be fine-tuned to the type
of radiation that is expected.
2. Can detect and measure all types of radiation.
Disadvantages
1. Inability to measure high radiation rates.
2. Geiger-Muller tube cannot differentiate between radiation types.
How Ionization chamber work
When ionising radiation interacts with air, it produces ions (typically electrons
andpositively charged atoms from which they arrived). In the presence of charge, the
positive particles will migrate towards a negative terminal and vice versa.
Ionisation chambers utilise this principle. All types contain a positive and negative
terminal, separated by a gas (typically air). As ions reach their respective terminals, they
produce a current which is detectable by an electrometer. The charge used within an
ionisation chamber (100-400 V) is sufficient to pull ions towards the terminals, but not
excessive enough to accelerate them and create further ionisations (see Geiger Muller
Counters ).
Ionisation chambers need to have correction factors applied for different beam energies.
They are highly sensitive (down to individual ionisations) which makes them useful in a
variety of applications.
Application
1. Nuclear industry
2. Smoke detectors
3. Medical radiation measurement
Advantages
1. Simple and easy to build.
2. Good in measuring high levels of gamma radiation.
Disadvantages
1. Electrical current produced is extremely weak, and must be amplified with
sophisticated electronic circuitry.
2. The output current tends to drift with time, and the system must be frequently rezeroed.
How Proportional counter work
The proportionalcounter is a type of gaseous ionization detector device used to
count particles of ionizing radiation that mean ability to measure the energy of incident
radiation and it is widely used where discrimination between radiation types is required,
such as between alpha and beta particles.
A proportional counter uses a combination of the mechanisms of a Geiger-Muller
tube and an ionization chamber that operates in an intermediate voltage region between
these. Considering a gas-filled chamber with a wire anode, if the field strength
everywhere in the volume is below a critical value, Townsend avalanches do not occur
at all, and the detector operates as an ionization chamber. If the applied voltage is too
high, complete ionisation of the fill gas occurs with almost each ion pair and the detector
operates as a Geiger-Müller counter, with the consequent loss of incident particle
energy information. The accompanying plot shows the proportional operating region for
a co-axial cylinder arrangement
In a proportional counter the fill gas of the chamber is an inert gas which is ionised by
incident radiation, and a quench gas to ensure each pulse discharge terminates; a
common mixture is 90% argon, 10% methane, known as P-10. An ionizing particle
entering the gas collides with a molecule of the inert gas and ionises it to produce an
electron and a positively charged atom, commonly known as an "ion pair". As the
charged particle travels through the chamber it leaves a trail of ion pairs along its
trajectory, the number of which is proportional to the energy of the particle if it is fully
stopped within the gas. Typically a 1 MeV stopped particle will create about 30,000 ion
pairs.
Applications
i.
ii.
iii.
Spectroscopy .
Photon detection.
Radioactive contamination detector.
Advantages
1. They are versatile in that they can be used for a variety of different applications.
2. Proportional counters can detect a variety of radiations.
3. They can also distinguish among radiation types.
Disadvantages
1. Proportional counters require a stable high voltage supply.
2. In general, these detectors are more expensive than GM countersiii.
A Geiger Mueller Counter is a device used for the detection and measurement of all
types of radiation: alpha, beta and gamma radiation.The counter works on the "principle
of gas multiplication". Ionization in the gas is caused by the entry of photon or a
particulate radiation. The ions are attracted to their appropriate electrode (i.e. cation to
cathode, anion to anode) and thy gain sufficient energy to eject electrons from the gas
atoms as they pass through the gas. This causes the atoms to ionize. Therefore
electrons are produced continuously by this process and rapid gas multiplication takes
place (especially in the central electrode because of its strong electric field strength).
The effect of "gas multiplication" is that more than one million electrons are collected by
the central electrode for every ion produced in the primary absorption process. Theses
"electron avalanches" create electric pulses which then can be amplified electronically
and counted by a meter to calculate the number of initial ionization events. Therefore a
Geiger counter is able to detect low-energy radiation because even one ionized particle
produces a full pulse on the central wire.
Application
1.
2.
3.
4.
Particle detection
Gamma and X-ray detection
Neutron detection
Gamma measurement—personnel protection and process control
Advantages
1. Geiger counters are even more useful because they can be fine-tuned to the type
of radiation that is expected.
2. Can detect and measure all types of radiation.
Disadvantages
1. Inability to measure high radiation rates.
2. Geiger-Muller tube cannot differentiate between radiation types.
How Ionization chamber work
When ionising radiation interacts with air, it produces ions (typically electrons
andpositively charged atoms from which they arrived). In the presence of charge, the
positive particles will migrate towards a negative terminal and vice versa.
Ionisation chambers utilise this principle. All types contain a positive and negative
terminal, separated by a gas (typically air). As ions reach their respective terminals, they
produce a current which is detectable by an electrometer. The charge used within an
ionisation chamber (100-400 V) is sufficient to pull ions towards the terminals, but not
excessive enough to accelerate them and create further ionisations (see Geiger Muller
Counters ).
Ionisation chambers need to have correction factors applied for different beam energies.
They are highly sensitive (down to individual ionisations) which makes them useful in a
variety of applications.
Application
1. Nuclear industry
2. Smoke detectors
3. Medical radiation measurement
Advantages
1. Simple and easy to build.
2. Good in measuring high levels of gamma radiation.
Disadvantages
1. Electrical current produced is extremely weak, and must be amplified with
sophisticated electronic circuitry.
2. The output current tends to drift with time, and the system must be frequently rezeroed.
How Proportional counter work
The proportionalcounter is a type of gaseous ionization detector device used to
count particles of ionizing radiation that mean ability to measure the energy of incident
radiation and it is widely used where discrimination between radiation types is required,
such as between alpha and beta particles.
A proportional counter uses a combination of the mechanisms of a Geiger-Muller
tube and an ionization chamber that operates in an intermediate voltage region between
these. Considering a gas-filled chamber with a wire anode, if the field strength
everywhere in the volume is below a critical value, Townsend avalanches do not occur
at all, and the detector operates as an ionization chamber. If the applied voltage is too
high, complete ionisation of the fill gas occurs with almost each ion pair and the detector
operates as a Geiger-Müller counter, with the consequent loss of incident particle
energy information. The accompanying plot shows the proportional operating region for
a co-axial cylinder arrangement
In a proportional counter the fill gas of the chamber is an inert gas which is ionised by
incident radiation, and a quench gas to ensure each pulse discharge terminates; a
common mixture is 90% argon, 10% methane, known as P-10. An ionizing particle
entering the gas collides with a molecule of the inert gas and ionises it to produce an
electron and a positively charged atom, commonly known as an "ion pair". As the
charged particle travels through the chamber it leaves a trail of ion pairs along its
trajectory, the number of which is proportional to the energy of the particle if it is fully
stopped within the gas. Typically a 1 MeV stopped particle will create about 30,000 ion
pairs.
Applications
i.
ii.
iii.
Spectroscopy .
Photon detection.
Radioactive contamination detector.
Advantages
1. They are versatile in that they can be used for a variety of different applications.
2. Proportional counters can detect a variety of radiations.
3. They can also distinguish among radiation types.
Disadvantages
1. Proportional counters require a stable high voltage supply.
2. In general, these detectors are more expensive than GM countersiii.
