Download L1 THE GEIGER COUNTER

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OPERATING CHARACTERISTICS OF THE GEIGER COUNTER
OBJECTIVE
The objective of this laboratory is to determine the operating voltage for a Geiger tube and to
calculate the effect of the dead time and recovery time of the tube on the counting rate.
INTRODUCTION
In medical and biological research, radioactive isotopes are utilized in three types of
measurements: studies of the various chemical and physical interactions within a living
organism or with its environment, measurements of the distribution of elements and
compounds in the body and measurements of radioactive isotopes in the living organisms
without taking samples. The techniques used in these measurements depend on the fact that the
radioactive isotopes emit ionizing radiations, which can be detected by their effects on a
photographic emulsion, or by electrical methods.
Gases conduct electricity only when a number of their atoms are ionized, i.e. split up into a
number of free electrons and positive ions. Alpha, beta or gamma radiation emitted by
radioactive materials ionizes atoms with which they collide. Hans Geiger, an associate of
Rutherford used this property to invent a sensitive detector for radiation.
Mica
Window
α, β, or γ
radiation
Outer Cylinder ( - Voltage)
Central Wire ( + Voltage)
+
_
R
Voltage spike
to counter
V
Figure 1. The Geiger Counter
A "Geiger counter" usually contains a metal tube with a thin metal wire along its middle; the
space in between them sealed off and filled with a suitable gas, and with the wire at a very high
positive electric potential relative to the tube.
An electron, positive ion, or gamma radiation penetrating the tube through a mica window, will
ionize a number of the atoms in the gas, and because of the high positive voltage of the central
wire, the electrons will be attracted to it while the positive ions will be attracted to the wall.
The high voltage will accelerate the positive and negative charges, they gain more energy and
collide with more atoms to release more electrons and positive ions; the process escalates into
an "avalanche" which produces an easily detectable pulse of current. With a suitable filling gas,
the current quickly drops to zero so that a single voltage spike occurs across the resistor R and
that is registered by an electronic counter. A typical composition of the gas filling in a Geiger
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counter tube was a mixture of argon and ethanol. More recently, tubes filled with ethyl formate
in place of the alcohol are reported to have a longer life and smaller temperature coefficients
than counters filled with ethanol.
Very important is the self suppressing mechanism in a Geiger counter. The counter is triggered
by the pulse from the tube and feeds back a square pulse of 300-500 µsec to the central wire.
This pulse has an opposite polarity and high enough amplitude to extinguish the discharge.
The Geiger detector is usually called a "counter" because every particle passing through it
produces an identical pulse, allowing particles to be counted; however, the detector cannot tell
anything about the type of radiation or its energy/frequency (it can only tell that the radiation
particles have sufficient energy to penetrate the counter. To improve sensitivity to alpha and
beta particles, the ST150 detector has a very thin mica window with a superficial density of
only 1.5 – 2 mg/cm2. This window is therefore extremely fragile and if broken cannot be
repaired. Never allow any object to touch the window!
The Geiger Tube Voltage Characteristic
The most important information about a particular counter tube is the voltage characteristic
curve. The counting rate due to a constant intensity radioactive source is graphed as a function
of the voltage across the counter. A curve of the form shown in fig. 2 is obtained.
Counting
Rate
Continuous
Discharge
Region
Geiger Plateau
Proportional
Discharge
Region
Start
Voltage
Applied
Voltage
Geiger
Threshold
Voltage
Operating
Voltage
V0
Geiger
Breakdown
Voltage
Figure 2: Geiger Tube Voltage Characteristic
The counter starts counting at a point corresponding approximately to the Geiger threshold
voltage; from there follows a “plateau" with little change in the counting rate as the voltage
increases. Finally a point is reached where the self-suppressing mechanism no longer works,
and the counting rate rapidly increases until the counter breaks down into a continuous
discharge. In order to ensure stable operation, the counter is operated at a voltage
corresponding approximately to the mid-point of the plateau.
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Hence a flat plateau is regarded as a desirable characteristic in a counter; a long plateau is also
desirable, but not so important. In practice most counters have a slightly sloping plateau, partly
because of geometrical limitations of the counter design, and partly because of spurious counts
due to an unsatisfactory gas-filling or to undesirable properties of the cathode surface.
The correct operating voltage for the Geiger-Mueller tube will be determined experimentally
using a small radioactive source such as Cs-137 or Co-60. A properly functioning tube will
exhibit a "plateau" effect, where the counting rate remains nearly constant over a long range of
applied voltage.
Resolving Time; Dead Time and Recovery Time.
Geiger-Mueller tubes exhibit dead time effects due to the recombination time of the internal
gas ions after the occurrence of an ionizing event. The actual dead time depends on several
factors including the active volume and shape of the detector and can range from a few
microseconds for miniature tubes, to over 1000 microseconds for large volume devices.
The counter discharge occurs very close to the wire, and the negative particles, usually
electrons, are collected very rapidly. The positive ions move relatively slowly, so that as the
discharge proceeds a positively charged sheath forms around the wire. This has the effect of
reducing' the field around the wire to a value below that corresponding to the threshold voltage,
and the discharge ceases. The positive ion sheath then moves outwards until the critical radius
r is reached, when the field at the wire is restored to the threshold value. This marks the end of
the true "dead time". If another ionizing event triggers the counter at this stage, a pulse smaller
than normal is obtained, as the full voltage across the counter is not operative. However, if the
positive ions have reached the cathode before the next particle arrives, the pulse will be of full
size. This effect can be demonstrated with a triggered oscilloscope, as shown in fig. 3, the
period during which only partially developed pulses are formed being termed the "recovery
time". The effective resolving time or insensitive time following a recorded pulse, is
determined by both the dead time and the recovery time, and will depend not only on a number
of parameters associated with the counter dimensions and gas filling, but in principle, also on
the operating voltage of the counter, on the sensitivity of the electronic recording equipment
and on the counting rate. It is necessary to apply appropriate corrections to observed counting
rates to compensate for this resolving time.
Pulse Level
Minimum
Input
Sensitivity
Level of
Counter
Second Geiger
Discharge
Counted
Initial
Geiger
Discharge
Counted
Possible
Events Not
Counted
Time
Dead
Time
Recovery
Time
Resolving Time = t
Figure 3: Dead time and Recovery Time
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When making absolute measurements it is important to compensate for dead time losses at
higher counting rates. If the resolving time Tr of the detector is known, the true counting rate Rt
may be calculated from the measured rate Rm using the following expression:
(Eq. 1)
If the detector resolving time is unknown, it may be determined experimentally using two
radioactive sources. Maintaining constant counting geometry is important throughout the
experiment. A special source split into two halves is available for making the measurement, but
good results may be obtained by careful positioning of two standard check sources. With the
high voltage correctly set for the GM tube, and denoting the count rate for the two sources
(a+b) side by side as R(a+b), the count rate for source a alone R(a) and the count rate for the
source b alone R(b), the resolving time is given by
(Eq. 2)
Because of the solid state electronics used in the circuitry of the ST150 Nuclear Lab Station its
own resolving time is very short - one microsecond or less – and so, not significant compared
to that of the GM tube. Therefore, only the resolving time of the GM tube affects the true count
rate.
EQUIPMENT
•
•
The ST150 Nuclear Lab Station provides a self-contained unit that includes a
versatile timer/counter, GM tube and source stand; High voltage is fully variable
from 0 to +800 volts.
Two types of radioactive sources: 137Cs and 60Co.
Figure 4: The ST150 Nuclear Lab Station
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PROCEDURE
PART I. GEIGER PLATEAU AND NORMAL OPERATING VOLTAGE
1. Sign out one radioactive source from your TA and place it on the transparent source
stand close to the window of the GM detector.
2. Slowly increase the high voltage until radiation events just begin to be detected.
3. Now increase the voltage in 20 volt steps recording the counting rate at each increment.
The rate should remain fairly constant over a large range of voltage and then increase
rapidly as the high voltage is further raised indicating that the tube is entering the
breakdown region. Record your data in the table below.
4. Do not continue to operate the tube in this breakdown condition (you may irreversibly
damage the tube); instead reduce the high voltage to zero.
Table 1: Data for Constructing a Plateau Curve
Run No
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
Voltage (V)
Counts
Time (min)
Counting Rate
(Counts/min)
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DATA ANALYSIS
1. Using the data in table 1 make a plot of the counting rate versus the applied voltage.
2. The recommended Geiger operating voltage may be determined as the center of
the plateau region. In the example below the plateau extends from approximately
350V to 600V. A reasonable operating voltage in this case would be 500V.
(V)
Figure 3: Determination of Operating Voltage
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PART II. THE RECOVERY TIME OF THE TUBE
1. Set the high voltage to the operating voltage found in part I and the time interval for
each count to 5 minutes.
2. Record all your data in the table below to calculate the recovery time t
3. Record the count rate for the first source (a) you have already used in part a as R(a).
4. Sign out a second radioactive source (b) from your TA and place both sources side by
side on the transparent source stand close to the window of the GM detector.
5. With the two sources (a+b) side by side, and the high voltage as the normal Geiger
operating voltage obtained in part I, record the count rate as R(a+b) for another 5
minutes.
6. Remove the first source (a), and return it to your TA
7. Record the count rate of the second source (b) as R(b).
Table 2: Data for Recovery Time
Voltage (V)
Time (min)
R(a)
R(b)
R(a+b)
DATA ANALYSIS
3. Using the recovery time find the adjusted true counting rates for the two sources using
(Eq. 1).
4. Find the percentage difference between the measured and the true rates for both sources
used.
5. Redraw the graph of counts vs. voltage in part one, using the adjusted true rates and
determine the value of the normal operating voltage. How does the resolving time
affect the value of this normal operating voltage determined in part I?