Transcript Proportional Counters & Geiger

Proportional Counters &
Geiger Mueller counters
by
Hiba Al-Sa’eed
Physics 641
Prof. Nidal Ershaidat
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Proportional Counters
• Introduction
• The proportional counter is a type of gas-filled
detector that was introduced in the late 1940s.
• They rely on the phenomenon of gas multiplication
to amplify the charge represented by the original ion
pairs created within the gas.
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• Proportional counters are used in the
detection and spectroscopy of low-energy Xradiation
• They are also applied in the detection of
neutrons
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Gas Multiplication
• A. Avalanche Formation
• At low values of the field, the electrons and
ions created by incident radiation drift to
there respective collecting electrodes
• Many collisions occur with neutral gas
molecules
• Positive or negative ions achieve very little
average energy between collisions because of
their low mobility
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• Free electrons are easily accelerated by the
applied field and may have significant kinetic
energy when undergoing such a collision
• Now, if this energy is greater than the
ionization energy of the neutral gas molecule,
it is possible for an additional ion pair to be
created in the collision
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• Because the average energy of the electron
between collisions increases with increasing
electric field , there is a threshold value of the
field above which this secondary ionization
will occur .
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• The electron liberated by this secondary
ionization process will also accelerated by the
electric field . This electron undergoes
collisions with other neutral gas molecules
and thus can create additional ionizations .
• So we can say that Gas Multiplication is a
consequence of increasing the electric field
within the gas to sufficiently high value .
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Regions of Detector Operation
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B. Regions of detector operation
• At very low values of the voltage , the field is
insufficient to prevent recombination of the
original ion pairs , and the collected charge is
less than that represented by the original ion
pairs .
• As the voltage is raised recombination is
suppressed and the region of ion saturation is
achieved . This is the normal mode of
operation for ionization chambers .
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• as the voltage increases , the threshold field at
which gas multiplication begins is reached .
The collected charge then to multiply , and the
observed pulse amplitude will increase
• Over some region of the electric field , the gas
multiplication will be linear , and the collected
charge will be proportional to the number of
original ion pairs created by incident radiation
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• That is the region of true proportionality and
represents the mode of operation of
conventional proportional counters
• Increasing the applied voltage still further can
introduce nonlinear effects
Explanation is in the next slide
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• Although the free electrons are quickly
collected , the positive ions move much more
slowly and , during the time it takes to collect
the electrons , they barely move at all.
Therefore , each pulse within the counter
creates a cloud of positive ions , they
represent a space charge that can significantly
alter the shape of the electric field within the
detector
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• Because gas multiplication is dependent on
the magnitude of the electric field , some
nonlinearities will begin to be observed
• These effects mark the onset of the region of
limited proportionality
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• If the applied voltage is made sufficiently high
, the space charge created by the positive ions
can become completely dominant in
determining the subsequent history of the
pulse. Under these conditions the avalanche
proceeds until a sufficient number of positive
ions have been created to reduce the electric
field below the point at which additional gas
multiplication can take place .
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• The process ,here, is self-limiting and will
terminate when the same total number of
positive ions have been formed regardless of
the number of initial ion pairs created by the
incident radiation
This is the Geiger-Mueller region of operation
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Applications
• Proportional counters are used in resonance–electron Mossbauer
spectroscopy.
• The type used is the High‐temperature proportional counter.
• “In order to avoid insulation difficulties due to electric leakage on the
surfaces of heated insulators, the counter is carefully constructed so that
the sensitive volume of the counter can be warmed up while keeping the
insulators at room temperature; all insulating materials between the
anode and cathodes are located outside the electric furnace in the counter
system. The counter operation is influenced by thermal electrons emitted
from the cathode material”. *
• “The high‐temperature counter provides us with a new method to observe
directly surface phenomena at high temperatures”.*
• * http://rsi.aip.org/resource/1/rsinak/v52/i3/p413_s1?isAuthorized=no
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Geiger-Mueller Counters
• Introduction
• The Geiger-Mueller counter is one of the oldest
radiation detector types in existence, having been
introduced by Geiger and Mueller in 1928.
• Commonly referred to as the G-M counter, or simply
Geiger tube.
• The simplicity, low cost, and ease of operation of
these detectors have led to their continued use to
the present time.
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• G-M counters comprise the third general category of
gas-filled detectors based on ionization (first two
types are the ion chambers and the proportional
counters).
• They are used to detect alpha and beta particles,
gamma and X-ray.
• In common with proportional counters, G-M
counters employ gas multiplication to greatly
increase the charge represented by the original ion
pairs, but in a fundamentally different manner.
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• In proportional counter, each original electron leads to an
avalanche that is basically independent of all other
avalanches. Because all avalanches are nearly identical, the
collected charge remains proportional to the number of
original electrons.
• In the G-M counters, substantially higher electric fields are
created that enhance the intensity of each avalanche
• Under proper conditions, a situation is created in which one
avalanche can itself trigger a second avalanche at a different
position within the tube.
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• At a critical value of the electric field, each
avalanche can create, on the average, at least
one more avalanche, and a self-propagating
chain reaction results.
• At still greater values of the electric field, the
process becomes rapidly divergent and, in
principle, an exponentially growing number of
avalanches could be created within a very
short time.
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• Once this Geiger discharge reaches a certain size, however,
collective effects of all the individual avalanches come into
play and ultimately terminate the chain reaction.
• Because this limiting point is always reached after about the
same number of avalanches have been created, all pulses
from a Geiger tube are of the same amplitude regardless of
the number of original ion pairs that initiated the process.
• A Geiger tube can therefore function only as a simple counter
of radiation-induced events and cannot be applied in direct
radiation spectroscopy because all information on the amount
of energy deposited by the incident radiation is lost.
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• 1. The Geiger Discharge
When excited molecules produced by an
avalanche return to their ground state they
emit photons whose wavelength may be in
the visible or ultraviolet region. These photons
are the key element in the propagation of the
chain reaction that makes up the Geiger
discharge.
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• These photons may be reabsorbed elsewhere
in the gas by photoelectric absorption,
creating a new free electron which will
migrate toward the anode and trigger another
avalanche.
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• The termination of Geiger discharge.
Positive ions are created along with each electron in
an avalanche. The mobility of these ions is much less
than that of the free electrons, so they remain
motionless during the time necessary to collect all
the free electrons. When the concentration of these
positive ions is sufficiently high, their presence
begins to reduce the magnitude of the electric field
in the vicinity of the anode wire.
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• Each Geiger discharge is terminated after
developing about the same total charge,
regardless of the number of original ion pairs
created by the incident radiation. All output
pulses are therefore about the same size, and
their amplitude can provide no information
about the properties of the incident radiation.
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2. Quenching
• After the primary Geiger discharge is terminated, the
positive ions slowly drift away from the anode wire
and ultimately arrive at cathode of the counter.
• Here they are neutralized by combining with an
electron from the cathode surface.
• In this process, an amount of energy equal to the
ionization energy of the gas minus the energy
required to extract the electron from the cathode
surface (the work function) is liberated.
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• If this liberated energy also exceeds the
cathode work function, it is energetically
possible for another free electron to emerge
from the cathode surface . This will be the
case if the gas ionization energy exceeds twice
the value of the work function.
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• The probability is always small that any given ion will liberate
an electron in its neutralization, but if the total number of
ions is large enough, there will likely be at least one such free
electron generated. This electron will trigger another
avalanche, leading to a second Geiger discharge. The entire
cycle will now be repeated and this will produce a continuous
output of multiple pulses.
• Special precautions must be taken in Geiger counters to
prevent the possibility of excessive multiple pulsing.
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• External quenching consists of some method for reducing the
high voltage applied to the tube, for a fixed time after each
pulse, to a value that is too low to support further gas
multiplication.
• One method of external quenching is simply to choose R (see
the next slide) to be a large enough value (108 ohms) so that
the time constant of the charge collection circuit is of the
order of a millisecond.
• This method has the disadvantage of requiring several
milliseconds for the anode to return to near its normal
voltage, and thus Geiger discharges for each event are
produced only at very low counting rates.
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• Another method is the internal quenching,
which is accomplished by adding a second
component called the quench gas to the
primary fill gas.
• It is chosen to have a lower ionization
potential and a more complex molecular
structure than the primary gas component
and is present with a concentration of 5-10%.
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• This gas prevents multiple pulsing through the
mechanism of charge transfer collision.
• When the positive ions make collisions, some of
these collisions will be with molecules of the quench
gas and, because of the difference in ionization
energies, there will be a tendency to transfer the
positive charge to the quench gas molecule.
• The original positive ion is thus neutralized by
transfer of an electron and a positive ion of the
quench gas begins to drift in its place.
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• If the concentration of the of the quench gas is
sufficiently high, these charge-transfer collisions
ensure that all the ions that eventually arrive at the
cathode will be those of the quench gas.
• When they are neutralized, the excess energy may
now go into dissociation of the more complex
molecules in preference to liberating a free electron
from the cathode surface. Thus no second
avalanche will occur.
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Applications
• Geiger – Mueller tubes are used in monitoring the
environment near nuclear power sources.
• “The environment near nuclear power sources is generally
monitored by a number of peripheral instrument stations,
each with two Geiger- Muller tubes. lf release of activity
occurs, it is immediately detected and a signal is fed to a
computer-controlled monitoring system. The computers
trigger alarm warnings and provide an indication of the
location of probable 'downwind' areas where precautions
should be taken”. *
* www.centronic.co.uk/tube_applications.htm
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THE END
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References
• Radiation Detection and Measurement ,Third
Edition, Glenn F . Knoll
• http://rsi.aip.org/resource/1/rsinak/v52/i3/p4
13_s1?isAuthorized=no
• www.centronic.co.uk/tube_applications.htm
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