Transcript Geiger- Muller counter

Geiger- Muller counter
GM counter
construction
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A GM counter consists of a fine wire
mounted along the axis of a cylindrical
cathode made of glass or metal with a
metalized inner coating n filled with a
suitable gas mixture.
The gas mixture usually consists of
argon (90%) at 10cm of Hg n ethyl
alcohol (10%) at 1 cm of Hg.
The diameter of the cylindrical
cathode varies from 1cm to 5cm n its
length varies from 2cm to 100cm
depending on the purpose for which it
is to be used.The circuit to which GM
tube is connected to detect the
particles is shown below.
working
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When an ionizing particles passes through the gas in the GM tube ionisation takes place.
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The electrons produced are attracted towards the central wire n counted as a single voltage
pulse.
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The wire (anode) is surrounded by the slow moving positive ions sheath which in turn
reduces the electric field.
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As a result of decreased electric field the discharge stops. The discharge continues to cease
till the positive ions around the wire move away from it.
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Since the positive ions take enough time to move away from the anode so during this
period GM counter remains dead.
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Dead time is the time during which the counter fails to record any other ionising particle
entering the counter.
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Positive ions move away from anode towards cathode, the value of electric field increases n
another discharge takes place.
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It is essential that the electric discharge caused by the first ionizing particle is completely
quenched before the arrival of new particle.
Plateau and over voltage
In order to decide the operating
voltage of the GM tube, a graph
between anode voltage (X axis) and
count rate (Y axis) is plotted.
 The counting rate rises n soon Geiger
region is reached when pulses due to
all ionizing particles become of same
size n are recorded. This is called
Geiger threshold.
 When voltage is increased beyond
threshold the counting rate remains
the same for large variation. This
straight portion of the curve is called a
plateau.
 This plateau continues, till the excess
potential over the threshold, called the
over voltage becomes so high that the
counter breaks down into a continous
discharge.
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quenching
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The grid bias is adjusted in such a way that
no plate current flows before the arrival of
the ionizing agent. On the arrival of ionizing
particle, the gas gets ionized n the current
lows in the plate circuit coz the grid becomes
less negative. If the change in the grid voltage
is sufficient, the voltage drop in Ŕ will result in
effective counter voltage dropping below
threshold value.Thus the counter discharge is
extinguished n the circuit recovers to its
Dead time and Recovery time
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The counter remains dead till the positive
ions have moved sufficiently away from the
avalanche sites to put the wire back at the
Geiger threshold potential.
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The time for which the counter is dead is
called dead time.
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The recovery time is the time after which the
original pulse levels are restored.
efficiency
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Suppose 2000 particles pass through and 1600
are recorded then efficiency η is given by
η=1600/2000 *100%
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Therefore efficiency of counting is the ratio of the
number of counts observed per unit (n°) to the
number of ionizing particles (n) which pass
through the counter during that time
η=n°/n
applications
The first historical uses of the
Geiger principle were for the
detection of alpha and beta
particles, and the instrument is
still used for this purpose.
 Geiger counters can be used to
detect gamma radiation.
 GM counters are very useful for
detecting nuclear particles.
 In measuring cosmic ray
intensities and recording cosmic
ray events.
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Disadvantages
The resolving power of GM counter is limited
due to a large dead time and recovery time so
the counter cannot rates greater particles.
 GM counter have a very low intrinsic efficiency
for the detection of γ-radiation.
 GM tube have a very limited life as the
quenching gases dissociate and change the
pressure of the inert gases filled inside.
 It cannot detect uncharged particles like
neutron.
 Because it is heavy , so it cannot be sent in
balloons to sky to study cosmic rays.
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DEFINITION: Scintillation
counter
A scintillation counter measures ionizing radiation.
The sensor, called a scintillator, consists of a
transparent crystal, usually phosphor, plastic (usually
containing anthracene) or organic liquid (see liquid
scintillation counting) that fluoresces when struck
by ionizing radiation.
A sensitive photomultiplier tube (PMT) measures the light
from the crystal, and the output signal is fed to
an electronic amplifier and other electronic equipment to
count and possibly quantify the amplitude of the signals
produced by the photomultiplier.
Scintillation counters are widely used because they can be
made inexpensively yet with good quantum efficiency.
PARTS :
The complete scintillation
counter consist s
of
three basic parts:
1)The scintillating material or
phosphor which produces a
tiny light flash when a charged
particle strikes it.
 2)The photo-multiplier tube
which detects the light flash
and produce electrical pulse.
 3)Amplifier
and electronic
circuits which record and
count the electrical pulses
from the photo-multiplier.
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Working
When a charge particle strikes the scintillator, the phosphor's
atoms are excited and emit photons, which are directed at the
photomultiplier tube's photocathode which is connected to the
negative of a high voltage source. Each incident photon releases
an electron. A number of accelerating electrodes
called dynodes are arranged in the tube at increasing positive
potentials and the electron is accelerated by this electric field
towards the first dynode. The incident electron causes multiple
secondary electrons to be emitted, which accelerate towards and
hit the second dynode. More electrons are emitted and the
electron multiplication chain continues through the increasing
potentials of the dynodes, with increasing numbers of electrons
generated each time. By the time the electrons reach the anode,
enough have been released to generate a measurable voltage
pulse across external resistors. This voltage pulse is amplified
and recorded by the processing electronics.
Schematic showing incident particles hitting a scintillating
crystal, triggering the release of photons which are then
converted
into
photoelectrons
and
multiplied
in
the photomultiplier.
Detection materials:Cesium iodide (CsI) in crystalline form is used as the scintillator
for the detection of protons and alpha particles. sodium
iodide (NaI) containing a small amount of thallium is used as a
scintillator for the detection of gamma waves and Zinc Sulphide
is widely used as a detector of alpha particles.
Detector efficiencies:The quantum efficiency of a gamma-ray detector (per unit
volume) depends upon the density of electrons in the detector,
and certain scintillating materials, such as sodium
iodide and bismuth germanate, achieve high electron densities
as a result of the high atomic numbers of some of the elements
of which they are composed. However, detectors based on
semiconductors, notably hyperpure germanium, have better
intrinsic energy resolution than scintillators, and are preferred
where feasible for gamma-ray spectrometry. In the case
of neutron detectors, high efficiency is gained through the use of
scintillating materials rich in hydrogen that scatter neutrons
efficiently. Liquid scintillation counters are an efficient and
practical means of quantifying beta radiation.
Producing of a scintillation flash by the incoming
ionizing particle and subsequent generation of an
electrical pulse in a photomultiplier are divided
into five distinct events.
1)The incident radiation is first absorbed in the phosphor
material and its atoms or molecules are excited.
2).The excited atoms or molecules of the fluorescent
material of the phosphor decay and produce a light flash of
short duration.
3).The emitted photons are transmitted to the photocathode of the photo-multiplier.
4).Photo-electrons are produced due to absorption of light
photons.
5).Electron multiplication takes place very quickly and all
these operations takes place with in about 10ˉ8 seconds.
1).
GENERALLY USED:1)
Sodium Iodide:-This is most commonly used
scintillator in the study of gamma rays.it has one
drawback, it is hygroscopic and therefore has to be
sealed in an aluminium can with reflecting or diffusing
walls.
2)
Zinc Sulphide:-It is extensively used for the
detection of those particles which have short ranges.
It can not be used in thick layers because it rapidly
becomes opaque to its own radiation.
3)
CsI:-This is not hygroscopic and is therefore
preferred over sodium iodide.
4)Anthracene and stilbene:-These are organic
phosphors. For heavy particles, these have very poor
efficiency.
These are useful for the detection of 𝛽 particles.
5)Plastic and Liquid Scintillators:-In these
scintillators the energy of excitation is transferred from the
solvent to solute. These are used in counter telescope which
are generally used in high energy physics.
6)Gases:-For counting heavy charge particles in the presence
of 𝛾- radiation, Xenon is used which emits radiation in the
UV
region.The most outstanding feature of scintillation counter
over proportional counter is its extremely short duration
pulses and higher resolution.
Applications
Scintillation counters are used to measur
radiation in a variety of applications.
•Hand held radiation survey meters
•Personnel and environmental monitoring
for Radioactive contamination
•Medical imaging
•National and homeland security
•Border security
•Nuclear plant safety
•Radon levels in water