CONTENTS

Page no.

 Abstract 03
 Introduction 03
 Radiation 03
 04
α,Alpha
β∧γ Radiations
particles 04
 Beta particles 05
 Gamma rays 05
 Geiger Muller Counter 06
 Background 06
 Description 07
 Principle 07
 Concept of Quenching 09
 Organic Quenching 10
 Halogen Quenching 10
 Characteristics of G.M. Tube 11
 Dead Time 11
 Recovery Time 12
 Plateau length &Plateau slope 12
 Methods and Materials 13
 Apparatus 13
 Experimet-1: Operating Plateau for Geiger tube. 14
 Experimental Procedure 14
 Result 15
 Experiment-2: Resolving time correction for the Geiger
counter purpose. 16
 Calculating Resolving Time (Dead Time) 16
 Experimental procedure 18
 Result 18
 Discussion 19
 Types and Applications 19
 Particle Detection 29

1
  Gamma And X Ray Detection 20
 Gamma measurement—personnel protection
and process control 20
 Limitations 21
 Conclusion 21
 Advantages 21
 Disadvantages 22
 Sources of Error 22
 Precautions 22
 Preference 22
 Appendix 23

2
 ABSTRACT

The purpose of this experiment is to make the students

familiar with the “Geiger Muller Counter” a widely used pulse counting
instrument that used gas amplification which makes it remarkably simple
and a sensitive but whose simple construction makes it relatively
inexpensive. The experiments that are designed to accomplish this
purpose deal with the resolving-time corrections and the basic nuclear
considerations involved.

INTRODUCTION

RADIATION:

In physics, radiation is the emission or transmission

of energy in the form of waves or particles through space or through a
material medium. This includes:

 Electro-magnetic radiation (also known as "continuum radiation")

such as radio waves, visible light, x-rays, and γ.
 Particle radiation such as α, β, and neutron radiation (discrete rest
energy per particle)
 Acoustic radiation such as ultrasound, sound, and seismic waves.
(dependent on intervening mass for transmission)

Radiation is often categorized as either ionizing or non-

ionizing depending on the energy of the radiated particles. Ionizing
radiation carries more than 10 eV, which is enough to ionize atoms and
molecules, and break chemical bonds. This is an important distinction due
to the large difference in harmfulness to living organisms. A common
source of ionizing radiation is radioactive materials that emit α, β, or γ
radiation, consisting of helium nuclei, electrons or positrons, and photons,

3
 respectively. Other sources include X-rays from
medical radiography examinations
and muons, mesons, positrons, neutrons and other particles that
constitute the secondary cosmic rays that are produced after primary
cosmic rays interact with Earth's atmosphere.

Gamma rays, X-rays and the higher energy range

of ultraviolet light constitute the ionizing part of the electromagnetic
spectrum. The lower-energy, longer-wavelength part of the spectrum
including visible light, infrared light, microwaves, and radio waves is non-
ionizing; its main effect when interacting with tissue is heating. This type
of radiation only damages cells if the intensity is high enough to cause
excessive heating. Ultraviolet radiation has some features of both ionizing
and non-ionizing radiation. While the part of the ultraviolet spectrum that
penetrates the Earth's atmosphere is non-ionizing, this radiation does far
more damage to many molecules in biological systems than can be
accounted for by heating effects, sunburn being a well-known example.
These properties derive from ultraviolet's power to alter chemical bonds,
even without having quite enough energy to ionize atoms.

The word radiation arises from the phenomenon of

waves radiating (i.e., traveling outward in all directions) from a source.
This aspect leads to a system of measurements and physical units that
are applicable to all types of radiation. Because such radiation expands
as it passes through space, and as its energy is conserved (in vacuum),
the intensity of all types of radiation from a point source follows
an inverse-square law in relation to the distance from its source.

α, β∧γ Radiations:
Alpha particles:

Alpha Particles are released by high mass, proton rich

unstable nuclei. The alpha particle is a helium nucleus; it consists of two
protons and two neutrons. It contains no electrons to balance the two
positively charged protons. Alpha particles are therefore positively
charged particles moving at high speeds.

4
 Beta particles:

Beta particles are emitted by neutron rich unstable nuclei.

Beta particles are high energy electrons. These electrons are not
electrons from the electron shells around the nucleus, but are generated
when a neutron in the nucleus splits to form a proton and an
accompanying electron. Beta particles are negatively charged.

Gamma rays:

Gamma rays are electromagnetic waves of very short

wavelength and high frequency. Gamma rays are emitted by most
radioactive sources along with alpha or beta particles. After alpha or beta
emission the remaining nucleus may still be in an excited energy state. By

5
 releasing a gamma photon, it reduces to a lower energy state. Gamma
rays have no electrical charge associated with them.

We use Geiger Muller Counter for the detection of these radiations.

6
 GEIGER MULLER COUNTER:

BACKGROUND:
In 1908 Hans Geiger, under the supervision of Ernest
Rutherford at the Victoria University of Manchester (now the University of
Manchester), developed an experimental technique for detecting alpha
particles that would later be used in the Geiger-muller tube. This counter
was only capable of detecting alpha particles and was part of a larger
experimental apparatus. The fundamental ionization mechanism used
was discovered by John Sealy Townsend by his work between 1897 and
1901, and is known as the Townsend discharge, which is the ionization of
molecules by ion impact.
It was not until 1928 that Geiger and Walther Müller (a PhD
student of Geiger) developed the sealed Geiger-Müller tube which could
detect more types of ionizing radiation and it became a practical radiation
sensor. Once this was available, Geiger counter instruments could be
produced relatively cheaply because the large output pulse required little
electronic processing to give a count rate reading, which was a distinct
advantage in the thermionic valve era due to valve cost and power
consumption.
Modern versions of the Geiger counter use the halogen
tube invented in 1947 bySidney H. Liebson. It superseded the earlier
Geiger tube because of its much longer life and lower operating voltage,
typically 400-600 volts."

DESCRIPTION:
Basically, the Geiger Counter consist of two electrodes
with a gas at a reduced pressure between the electrodes. The outer
electrode (celled cathode) is usually a cylinder, while the inner electrode
(called anode) is a thin wire positioned in the center of the cylinder. The
voltage between these two electrodes is maintained at such a value that
virtually any ionizing particle entering the Geiger tube will cause an
electrical avalanche within the tube. The Geiger tube used in this
experiment is called an end-window tube because this has a thin window
at one end through which the ionizing radiation enters.
The Geiger counter does not differentiate between kinds of
particles or energies, it tells only that certain number of particles (Betas
and Gammas for this experiment) entered the detector during its
operation. The voltage pulse from the avalanche is typically greater than 1
volt in amplitude. These pulses are large enough that they are counted in
the scalar directly without amplification.

7
 PRINCIPLE:
A Geiger counter (Geiger-Muller tube) is a device used for
the detection and measurement of all types of radiation: alpha, beta and
gamma radiation. Basically it consists of a pair of electrodes surrounded
by a gas. The electrodes have a high voltage across them. The gas used
is usually Helium or Argon. When radiation enters the tube it can ionize
the gas. The ions (and electrons) are attracted to the electrodes and an
electric current is produced. A scalar counts the current pulses, and one
obtains a ”count” whenever radiation ionizes the gas. The apparatus
consists of two parts, the tube and the (counter + power supply). The
Geiger-Mueller tube is usually cylindrical, with a wire down the center.
The (counter + power supply) have voltage controls and timer options. A
high voltage is established across the cylinder and the wire as shown in
the figure. When ionizing radiation such as an alpha, beta or gamma
particle enters the tube, it can ionize some of the gas molecules in the
tube. From these ionized atoms, an electron is knocked out of the atom,
and the remaining atom is positively charged. The high voltage in the tube
produces an electric field inside the tube. The electrons that were
knocked out of the atom are attracted to the positive electrode, and the
positively charged ions are attracted to the negative electrode. This
produces a pulse of current in the wires connecting the electrodes, and
this pulse is counted. After the pulse is counted, the charged ions become
neutralized, and the Geiger counter is ready to record another pulse. In
order for the Geiger counter tube to restore itself quickly to its original
state after radiation has entered, a gas is added to the tube. For proper
use of the Geiger counter, one must have the appropriate voltage across
the electrodes. If the voltage is too low, the electric field in the tube is too
weak to cause a current pulse. If the voltage is too high, the tube will
undergo continuous discharge, and the tube can be damaged. Usually
the manufacture recommends the correct voltage to use for the tube.
Larger tubes require larger voltages to produce the necessary electric

8
 fields inside the tube. In class we will do an experiment to determine the
proper operating voltage. First we will place a radioactive isotope in from
of the Geiger-Mueller tube. Then, we will slowly vary the voltage across
the tube and measure the counting rate. In the figure I have included a
graph of what we might expect to see when the voltage is increased
across the tube. For low voltages, no counts are recorded. This is
because the electric field is too weak for even one pulse to be recorded.
As the voltage is increased, eventually one obtains a counting rate. The
voltage at which the G-M tube just begins to count is called the starting
potential. The counting rate quickly rises as the voltage is increased. For
our equipment, the rise is so fast, that the graph looks like a “step”
potential. After the quick rise, the counting rate levels off. This range of
voltages is termed the “plateau” region. Eventually, the voltage becomes
too high and we have continuous discharge. The threshold voltage is the
voltage where the plateau region begins. Proper operation is when the
voltage is in the plateau region of the curve. For best operation, the
voltage should be selected fairly close to the threshold voltage, and within
the first 1/4 of the way into the plateau region. A rule we follow with the G-
M tubes in our lab is the following: for the larger tubes to set the operating
voltage about 75 Volts above the starting potential; for the smaller tubes
to set the operating voltage about 50 volts above the starting potential. In
the plateau region the graph of counting rate vs. voltage is in general not
completely flat. The plateau is not a perfect plateau. In fact, the slope of
the curve in the plateau region is a measure of the quality of the G-M
tube. For a good G-M tube, the plateau region should rise at a rate less
than 10 percent per 100 volts. That is, for a change of 100 volts,

(∆ counting rate) should be less than 0.1. An excellent tube

(average countingrate)
could have the plateau slope as low as 3 percent per 100 volts.
This counter also works on the principle of ionization
caused by incoming energetic particle in the gas medium filled between
anode and cathode. The electron liberated in the primary ionization event
would get accelerated towards anode because of its high potential. The
electron may gain sufficient energy to cause ionization of other gas
molecule. This leads to a chain of ionizing events which is usually referred
to as Townsend avalanche. During this process, there may be interactions
in which excitation of atoms may occur due to sufficient energy of
impinging electrons. Such atoms while de-exciting may emit photons
which normally fall in UV or visible region. These photons which are
emitted may again lead to photo electrons due to ionization of gas atoms
or due to photoelectric interaction with walls of counter. Each photo
electron would again cause Townsend effect. Such a series of Townsend

9
 avalanches would lead to discharge in the tube called Geiger-discharge.
In such a state there is formation of dense envelope of electron-ion pairs
distributed on either side of anode.
The voltage applied to anode shall be such that it is
enough to trigger the avalanche mechanism and collect total charge
(electrons) pertaining to single event leading to Geiger discharge.

CONCEPT OF QUENCHING:

Practically the process would not be as simple as above.

During the Geiger discharge, there is dense envelope of electrons and
ions. The electrons would drift towards anode and positive ions would drift
towards cathode. The positive ions which drift towards cathode having
ionization potential (E) greater than the work function (W) of cathode
material leads to exchange of electron from cathode and becomes
neutral. The excess energy may be dissipated in two forms, one by
emission of photon or an electron form cathode if excess energy is
greater than the work function of the cathode material. This would again
initiate another Geiger discharge. The result of this is that the tube would
always be in continuous Geiger discharge and hence will not able to
measure any radiation.
The charge migration in the tube leads to reduction in the
potential of the anode and an increase in the potential of the cathode.
Either of which may be detected as a signal by counter electronics. As the
negative charge around the anode increases the effective electric field is
reduced and eventually this reduction is such that further avalanches are
not possible and the tube can no longer detect the radiations. This state
persists until the sufficient electrons have recombine at the anode and
positive gas ions recombine at the cathode so that the field covering the
strength trigger of avalanche. This is so called dead time of the detector
the time after detection that the counter is sensitive for further avalanche
and this existence means that the detector count rate must be corrected
to give the actual count rate. After the detection further detections are
possible this would reduce signal strength. The total time elapses before
the fore strength signal is produced by a subsequent event is called the
recovery time.
To overcome this problem, concept of quenching is
introduced. There are two types of quenching
i) Organic quenching
ii) Halogen quenching

10
 ORGANIC QUENCHING: -
This involves addition of small quantity of organic gas
having complex molecule structure. This prevents the continuous Geiger
discharge mechanism by charge transfer collision principle. The positive
ions on their path collide with organic molecules to get neutralized. This
makes only ions of organic gas reach cathode and gets neutralized. If
there is any excess energy released leads to dissociation of organic
molecules. Thus multiple Geiger discharges could be avoided.
A typical filling of organic quenched GM tubes is 90%
Argon (Principal gas) and 10% of ethyl alcohol (organic quenching gas).
When organic gas gets depleted to a sufficient extent there is occurrence
of multiple discharges frequently and thus the plateau length gets
decreased, with slope increased.
Thus the organic quenched GM tubes are characterized by
short life time and thus not suitable for operation in very high fields which
leads to large count rate. To overcome this, technique of Halogen
quenching is introduced.

HALOGEN QUENCHING: -
This involves the addition of small quantity of Halogen gas
such as Chlorine or Bromine. A typical filling is about 0.1% of chlorine to
Neon. The quenching action is same as that in Organic quenching
process. The diatomic halogen gas molecules too gets dissociated in
quenching but gets recombined to replenish the gas molecules and thus
counter life gets extended.
The recombination of positive detection gas ions of the cathode

CHARACTERISTICS OF GM TUBES:-

The important parameters which decide the quality of

functioning of Gm tubes are
i) Dead time
ii) Recovery time
iii) Plateau length &Plateau slope

DEAD TIME: -

As discussed above, the positive ions take considerable

time to reach cathode tube compared to electrons. The reason is that the
mobility of electrons is about 1000 times greater than that of electrons.
Due to the low drift velocity of positive ions, there is formation of cloud of
positive ions which tend to electric field opposite to that of actual field.

11
 This reduces the electric field intensity due to anode potential and thus
affects gas multiplication factor. This in turn affects the pulse heights.
In high count rates, it is more worse that there is formation
of dense positive cloud which makes the electric field intensity in the
vicinity of anode wire reduce by great margin thus multiplication goes
down by big margin. During this phase of detector, any new ionizing event
caused by incoming particle cannot be recorded. Thus the time interval
during which any event caused by newly incoming particle would not get
counted and called as dead time of the country.

RECOVERY TIME: -

After certain time, all the positive ions tend to reach

cathode wall and thus the electric field begins to restore to actual value.
When the electric field goes beyond a critical value there is again
formation for pulses. But the process requires some time to give
maximum pulse heights. Hence the total time required for GM tube to give
maximum pulse height pulses is Recovery time.

12
 PLATEAU LENGTH & SLOPE:

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.
After applying minimum voltage to initiate Geiger discharge, the no. of
pulses shall remain same in fixed radiation field exposure. But due to
formation of short pulses during recovery time there is variation in count
rate. Thus one of the quality parameters deciding the operation of GM
tube is that plateau slope shall be less. Usually 2-3% plateau slope is a
good choice. As we go on applying voltage to the anode, the tube starts
entering continuous discharge region. Thus the slope gets worsened.
The region or length of voltage region during which the plateau slope
remains in desired value is called as plateau length and usually the
operating voltage is chosen at the midpoint of plateau length.

13
 METHODS AND MATERIALS

We shall use the following apparatus.

APPARATUS:

 Geiger Muller Counter

 Supply for counter
 Radiation source ( )
 Teaser
α, β ,γ
 Stopwatch
 Thick sheet to place the source (Aluminum, Lead)

14
 EXPERIMET-1 : OPERATING PLATEAU FOR GEIGER TUBE.

EXPERIMENTAL PROCEDURE

I. Connect the Geiger tube with the scalar.

II. Place the beta source SR90 at a distance of approximately 5-cm
from the window of Geiger tube.
III. Increase the (+ve ) high voltage until the scalar just begins
registering counts. This point is called the starting voltage. Here
starting voltages are greater than 360 volts.
IV. Reset the scalar and note the counts for 1-min. using a stopwatch.
Increase the high voltage by 20 volts and note the counts again
for 1-min.
V. Continue making measurements at 20-volts interval until you have
enough data to plot a curve on a linear graph paper. The region
between V1 and V2 is usually less than 200 volts. A sharp rise in
the counting rate will be observed if you go to just above V2. When
this happens the upper end of the plateau has been reached.
Reduce the voltage to V2 immediately. Choose the operating point
for the tube at about 50% to 70% for plateau range.
VI. Evaluate the Geiger tube by measuring the slope of plateau from
the graph, it should be less than 10%. The slope of the Plateau is
defined as,

¿
¿
¿

[ ][ ]
R −R
slope= 2 1
R1
100
V 2 −V 1 ¿
1¿

15
 RESULT:

After calculating the values we obtain the following result for Sr90
Voltage Counts without Counts with source Net counts
source
300 0 3 3
320 0 5 5
340 27 18192 18165
360 31 26856 26825
380 39 28159 28120
400 42 28179 28137
420 47 28220 28173
440 49 28354 28305
460 55 28374 28319
480 58 28812 28760
500 63 30244 30181

16
 Graph b/w Counts/min & Voltage
35000
30000
25000
20000
Counts/min 15000

10000
5000
0
300 320 340 360 380 400 420 440 460 480 500
Voltage

From the graph we calculate the values of R1, R2, V1 & V2.

The calculated values are as follow:

V1 = 360v

V2 = 500v

R1= 26825 counts/min

R2 = 30181 counts/min

As,
slope=
[ ][ ]
R2−R1 100
R 1 V 2 −V 1

Slope= [ 30181−26825
26825 ][
100
500−360 ]
Slope=(0.1252)(0.7142)
Slope=0.08942

17
 EXPERIMENT-2: RESOLVING TIME CORRECTION FOR THE GEIGER
COUNTER PURPOSE.

Once an ionization avalanche has produced a discharge in

the tube no pulse can be generated at the output of the tube until the
ionization is cleared from the tube. This time interval during which no
pulse can be recorded is called the dead time of the counter.
During the dead time no current can be received and the
particles coming in this interval are lost. An account has to be made of
these lost counts to get the true counts.
In some experiments we use fast electronics (nano-
seconds). The Geiger counter, however, is a slow device, when used for
counting rates above 5000 counts/min. it is necessary to make a dead-
time correction to obtain the true counting rate. The double source
technique is used to determine tis dead time.

CALCULATING RESOLVING TIME (DEAD TIME)

After a count has been recorded, it takes the G-M tube a

certain amount of time to reset itself to be ready to record the next count.
The resolving time or “dead time”, TR, of a detector is the time it takes for
the detector to”reset” itself. Since the detector is”not operating” while it is
being reset, the measured activity is not the true activity of the sample. If
the counting rate is high, then the effect of dead time is very important. we
will estimate the dead time by examining the discharge pulse of the tube.
If there is time, we can also examine the series of times between
successive Geiger Counter pulses. This is somewhat different than the
conventional methods used in other student labs which don’t have the
capability to examine the discharge or measure the time between
successive pulses.Correcting for the Resolving time:
We define the following variables:
TR = the resolving time or dead time of the detector
Tr = the real time that the detector is operating. This is the actual time that
the detector is on. It is our counting time. Tr does not depend on the dead
time of the detector, but on how long we actually record counts.
Tl = the live time that the detector is operating. This is the time that the
detector is able to record counts. Tl depends on the dead time of the
detector.
C = the total number of counts that we record.
RO = the measured counting rate, RO = C/Tr
R = the true counting rate, R = C/Tl
Note that the ratio n/N is equal to:

18
 C
RO T r Tl
= =
R C Tr
T
This means that lthe fraction of the counts that we record is
the ratio of the ”live time” to the ”real time”. This ratio is the fraction of the
time that the detector is able to record counts. The key relationship we
need is between the real time, live time, and dead time. To a good
approximation, the live time is equal to the real time minus C times the
dead time TR:

T =T −CT
R

Thisl is rtrue since CT

is the total time that the detector is
R

unable to record counts during the counting time T r. We can solve for R in
terms of RO and TR by combining the two equations above. First divide the
second equation by

Tl CTR
=1− =1−n T R
T r T r
From the first equation, we see that the left side is equal to R /R:
O

Ro
=1−RO T R
R
Solving for N, we obtain the equation:

RO
R=
This is
1−RO T R
the equation we need to determine the true
counting rate from the measured one. Notice that R is always larger than
RO. Also note that the product ROTR is the key parameter in determining
by how much the true counting rate increases from the measured
counting rate. For small values of the R OTR, the product ROTR (unit less) is
the fractional increase that R is of RO. For values of ROTR < 0.01 dead
time is not important, and are less than a 1% effect. Dead time changes
the measured value for the counting rate by 5% when R OTR = 0.05. The
product ROTR is small when either the counting rate RO is small, or the
deat time TR is small.

19
 EXPERIMENTAL PROCEDURE:

I. Place the source S1 (SR90) 5cm from the window and make a 1-
min count. Record the number of counts. Define this count to be
R1.
II. Place the source S2 (CO60) 5cm from the window and make a 1-
min count. Record the number of counts. Define this count to be
R2.
III. Place both sources S1 and S2 simultaneously at the same distance
and observe the counts for 1-min. define this quantity to be RT.
IV. Calculate the resolving time, TR, of the G.M. Tube with the
following formula:

¿
¿
¿
(2)
R +R −R
T R= 1 2 T min /count¿
2 R1 R 2
The true counting rate then can be determined for an observed counting
rate RO from the following formula,

¿
¿
¿
(3)
Ro
R= counts/min ⁡¿
1−R o T R
Note: Dead time correction should be used to correct any counting rate
that is above 5000 counts/min.

RESULTS

From experiment 1, we find the operating voltage = 430v

So, We perform our next experiment on this operating voltage.
Source Counts without Counts with Net counts
source source
Sr90 48 27671 27623
Co60 60 80 20
Sr90+Co60 61 27558 27497

Here, R1 = 27623
R2 = 20
RT = 27497

20
 As, R1 +R2−R T
T R= min /count
2 R1 R 2
So, ( 27623+20 )−27497
T R=
2(27623)(20)
146
T R=
1104920
T = 1.32 × 10-4
R

Now we find the true counting rate by the given formula:

Ro counts
R=
1−R o T R min
Here we have, c = 27623+20
RO=
Tr 45
So, 27643
R O= =614.28
45
And,
Ro T R=( 614.28 ) ( 1.32×10−4 )=0.079
614.28
R= =666.97
1−0.079
DISCUSSION

TYPES AND APPLICATIONS:

G_M Counter with Pancake Type Probe Laboratory use of a Geiger counter with end window
probe to measure beta radiation from a radioactive
source.

21
 A Geiger counter and metal detector combined for security use.
The application and use of a Geiger counter is dictated entirely by the
design of the tube.

PARTICLE DETECTION
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
today. For alpha particles and low energy beta particles the "end window"
type of GM tube is used as these particles have a limited range even in
free air and are easily stopped by a solid material.
The end window is designed to be thin enough to allow these particles
through with minimal attenuation, and normally has a density of about 1.5
- 2.0 mg/cm2. For efficient detection of alpha particles, the GM tube
window should ideally be within 10mm of the radiation source due to the
particle attenuation in free air. However, the G-M tube produces a pulse
output which is the same magnitude for all detected radiation, so a Geiger
counter with an end window tube cannot distinguish between alpha and
beta particles.
High energy beta particles can also be detected by a thin walled
"windowless" tube; which has no dedicated end window. Although the
tube walls have a greater stopping power than an end window, they still
allow these more energetic particles to reach the fill gas.
However, for discrimination between alpha and beta particles or provision
of particle energy information, proportional counters must be used. These
instrument types can also have much larger detector areas, which means
that checking of surfaces for contamination is much quicker.

22
 GAMMA AND X RAY DETECTION
Geiger counters can be used to detect gamma radiation, and for this the
windowless tube is used. Efficiency is only about 1%, due to low
interaction of gamma with the tube, and the article on the Geiger-Muller
tube carries an account of the technique used to detect photon radiation.
For high energy gamma, this relies on interaction of the photon radiation
with the tube wall material, usually 1-2mm of chrome steel to produce
electrons which can enter and ionize the fill gas. This is necessary as the
gas density in the tube is usually low, and most high energy gamma
photons pass through undetected.
For lower energy photons a different technique is used. This is the direct
interaction with gas in a long thin-walled tube. This design gives an
additional gas volume, and thereby increased chance of particle
interaction, but still allows low energy photons to enter the gas through
the thin wall. Energy compensation of the tube is normally applied to
increase the accuracy over a range of photon energies.

GAMMA MEASUREMENT—PERSONNEL
PROTECTION AND PROCESS CONTROL:
The term "Geiger counter" is commonly used to mean a hand-held survey
type meter, however the Geiger principle is in wide use in installed "area
gamma" alarms for personnel protection, and in process measurement
and interlock applications. A Geiger tube is still the sensing device, but
the processing electronics will have a higher degree of sophistication and
reliability than that used in a hand held survey meter.

LIMITATIONS

There are two main limitations of the Geiger counter. Because the
output pulse from a Geiger-Muller tube is always the same
magnitude regardless of the energy of the incident radiation, the
tube cannot differentiate between radiation types.
A further limitation is the inability to measure high radiation rates
due to the "dead time" of the tube. This is an insensitive period after
each ionization of the gas during which any further incident
radiation will not result in a count.

23
 CONCLUSION

Throughout the course of lab we have become

untimely with our G.M counter. We analysis many of the
characteristics of G.M counter. We prove that the pulse height of
G.M counter is indeed agreement with the theory that independent
from radiation that is detected. We also close look at the
abnormally large dead time using two different method .We
calculate this value and found agreement between them.
We have performed two experiments and ontained
the results. In our 1st experiment we obtain the plateau voltage from
the graph and we were able to calculate the value of slope which is
. in our second experiment we deeply analyze some of
0.08942
the important terms, named as Dead time, Recovery time, Plateau
length and slope. So after calculation we find the result. So from
our calculation we obtain the value for resolving time which is equal
to TR = 1.32 × 10-4 and we calculate the value for true counting rate
which is equal to R = .
666.97
There are some advantages as well as disadvantages of Geiger
Muller Counter which are as follow.

ADVANTAGES

1. They are relatively inexpensive

2. They are durable and easily portable
3. They can detect all types of radiation

DISADVANTAGES:

Some of the disadvantages of using a Geiger counter are:

1. They cannot differentiate which type of radiation is being
detected.
2. They cannot be used to determine the exact energy of the
detected radiation
3. They have a very low efficiency
These values are not completely correct as here is some sort of
error which inflect our result. Some types of errors are as follow.

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 SOURCES OF ERROR

Some possible errors may be:

 Instrument resolution
 Failure to calibrate or check zero of instrument
 Failure to account for a factor
 Instrument drift
 Environmental Factors

PRECAUTIONS:
 The operating voltage should correspond to the midpoint of flat
plateau region.
 In case the continuous discharge is produced, the voltage should
be lowered.
 The applied voltage must be relatively stabilized.
 Introduction of light should be prevented to avoid photo electric
effect.
 Place the source at 5cm from the window.

REFERENCE

 Internet
 https://www.cpp.edu/~pbsiegel/phy432/labman/geiger.pdf
 Course Manual
 Wikipedia

APPENDIX

Cosmic rays:
Cosmic rays are immensely high-energy radiation, mainly
originating outside the System. They may produce showers of
secondary particles that penetrate and impact the Earth's
atmosphere and sometimes even reach the surface. Composed
primarily of high-energy protons and atomic nuclei, they are of
mysterious origin.

Penetrating power of alpha, beta and gamma rays:

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 The penetrating power of alpha rays, beta rays, and gamma rays
varies greatly. Alpha particles can be blocked by a few pieces of
paper. Beta particles pass through paper but are stopped by
aluminum foil. Gamma rays are the most difficult to stop and
require concrete, lead, or other heavy shielding to block them.
Electron avalanche:
An electron avalanche is a process in which a number of free
electrons in a transmission medium are subjected to strong
acceleration by an electric field and subsequently collide with other
atoms of the medium, thereby ionizing them (impact ionization).
This releases additional electrons which accelerate and collide with
further atoms, releasing more electrons—a chain reaction.
End Window:
Here with the side window GM, the cathode is a stainless steel
cylinder. The anode is supported at one end and extends only part
way along the tube axis. The tip of the anode is typically covered
with a small glass bead. The window, covering one end of the tube,
is usually made of mica and typically has a density thickness of 1.5
to 2.0 mg/cm2.
Concentration on Quenching and Principal Gas:
In the Geiger Muller tube the Principal gas (Ne or Ar) is 90% and
quenching gas (N2) is 10% added.

X___________________________________________________
M. Usman Mustafa Group 5 (Leader)

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