GEIGER-MULLER COUNTER (I)

Introduction

A typical G.M. tube consists essentially of a cylindrical cathode in

the form of a graphite coating on the inner wall of a glass envelope

and an anode in the form of fine tungsten wire which stretches within

and along the axis of the tube. Usually it is filled with a mixture of

an inert gas (argon or neon) at a partial pressure of about 100 torr

and a quenching gas (halogens or organic vapours) at about 10 torr. To

allow 1onis1ng particles to enter the tube, a window covered with a

thin sheet of mica is provided at one end of a tube.

In operation, a sufficiently large potential difference i.e. applied

across the anode and cathode of the tube so that a high radial electric

field near the central wire is obtained. Under this condition,

electrons produced by ionizing collisions between a high-speed particle

entering the tube and the inert gas atoms are accelerated towards the

anode wire by the strong electric field and acquire within a very short

distance a high speed of their own. Because of this speed, they too can

ionize other atoms and free more electrons. This multiplication of

charges repeats itself in rapid succession producing within a very

short interval of time an avalanche of electrons.

The electron avalanche is concentrated near the central wire while the

positive ions, being much heavier, drift slowly toward the cathode. For

a G.M. tube with a cathode of radius 1cm, the time of flight of the

positive ions is roughly about 100 microseconds, which is about 100

times longer than the time necessary to build up the electron

avalanche. The consequence of this is that after the initiation of an

electron avalanche by an entering particle the slowly moving positive

ion sheath around the anode wire increases the effective radius of the

anodes. The electric field round the wire therefore drops to a value

below that which is capable of supporting ionization by collision. The

electron avalanche ceases and a pulse of current due to this avalanche

is subsequently produced.

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The object of the counter is to produce a single pulse for each

particle entering the tube. This can only be achieved if spurious

pulses due to secondary electrons released from the cathode surface by

the bombardment of ions are completely suppressed so that the tube can

recover as quickly as possible to be in a state when it is able to

record the next entering particle. A quenching gas (it must be both

polyatomic and of low ionization potential) introduced into the tube is

to serve this purpose. The idea is to allow the inert gas ions on their

way to the cathode to collide with the heavy molecules thereby transfer

their charges to the molecules and become neutralized - a process known

as quenching. The molecular ions thus produced move slowly to the

cathode and on reaching there, capture electrons from the cathode

surface to become neutral molecules. Any excess energy that the neutral

molecules have will cause them to dissociate into individual atoms

rather than be imparted to the cathode to produce fresh electrons that

would take part in further ionizing collisions.

The usual G.M. counter circuit is as shown in the following block

diagram:

where R, a register of several , is connected in series with the

stabilized H.T. supply and the tube. The current pulse initiated in the

tube by an entering particle produces a voltage pulse across this

resistor. The output pulse is then fed via a capacitor C to a pulse

amplifier, which is followed by an electronic scaling unit for

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recording the number of pulses. The register is usually composed of

decade counting tubes. Sometimes, in additional to decade counting

tubes, mechanical registers are also used.

Typically, the counting rate of a G.M. counter depends on the applied

voltage. Below a minimum voltage, the threshold voltage, no counts will

be registered. This minimum voltage is a function of the gas pressure

and the anode diameter, and may be between 300V and 900V. As the

voltage is increased, more and more counts are registered. Over a range

of voltages, called the plateau range, the counting rate is relatively

insensitive to applied voltage. The change in counting rate over a 100V

range of applied voltage may be as little as 5V. Organic quenched tubes

usually have a flatter plateau than halogen quenched tubes. For still

higher applied voltages the tube may go into continuous discharge. It

is particularly important that an organic-quenched tube not be

permitted to go into continuous discharge, as the quenching gas may be

exhausted in this way.

In this experiment a counter, which incorporates all the decade

necessary components, described above in one single unit is provided.

The G. M. tube connected to the decade counter is of type Mullard

MX168. It has a mica window and uses halogens as quenching gas.

Students are advised to do additional reading and answer the following

questions:

(i) Would the counter perform its normal duty if the polarities of the

central wire and the inner wall of the tube were interchanged?

(ii) Is there any advantage of using halogens rather than organic

vapours as quenching gases? Explain.

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Experiment

(a) G.M. Tube Characteristics

Using handling forceps, place the radium source on the lowest shelf of

the lead castle directly below the window of the G.M. tube. Switch on

the counter and allow it to warm up for a couple of minutes. Increase

the applied voltage from 320V in steps of 10V up to 450V. At each

setting, note down the number of counts over a period of 2 minutes.

Plot a graph of count rate per minute against the applied voltage.

Indicate on your graph the plateau, the Geiger threshold voltage and

the operating voltage (i.e. the voltage at the middle of the plateau).

(b) Background Count

Remove all radioactive sources from the vicinity of the G.M. tube. Set

the counter voltage at the operating voltage and take a 5-minute

background count.

Note: The background count rate per minute should be subtracted from

all counts in subsequent experiments in order to obtain the true count

rates due to radioactive sources alone.

(c) The Resolution Time of a G.M. Counter

After a pulse is registered, a sheath of positive ions that gradually

increases in radius remains about the anode wire. This effectively

decreases the potential gradient near the wire and not until this space

charge has drifted sufficiently far from the anode will the counter

become sensitive again. The total time taken for the tube to recover to

its fully sensitive state to give the next pulse, is called the

resolution time.

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For a tube having a resolution time t, it means that for each single

count registered. The tube is inoperative for t sec. Thus if we have n

record sounds registered per sec., the lost time in one sec is nt and

the effective operating time is sec. Following from this, if we

assume that the corrected count rate is N counts per sec. Then

The resolution time can be found readily using the "two-source" method.

This is carried out experimentally by counting the two sources one at a

time and then both together. If are the counts registered per

minute for the first source, the second source and the combination of

the two sources respectively, we can write:

and

Since

which follows

From , we have

Substituting into , we obtain after manipulating:

Normally , we can approximate to

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which yields

Using forceps, place a radium source left of center on the bottom shelf

of the lead castle. Then add another radium source symmetrically to the

right of center on the same shaft and finally remove the first source

without disturbing the second source. At each of these stages make a

two-minute count. Correct all the observed counts for background and

calculate the resolution time of the counter.

(d) Verification of Inverse Square Law

Remove the G.M. tube from the lead castle and attach it horizontally to

a stand provided. Using forceps, a place a radium source on

another stand and align it until its active face faces the tube window

and lies along the axis of the tube. Starting with a separation d

between the window and the source equal to 10cm and thereafter increase

d successively by 10cm until it reaches 70cm, note down the number of

counts per minute at each setting.

Correct the observed counts for background and resolution time using

equation (1), and hence plot the corrected count rate against to

verify the inverse square law.

Repeat the above experiment with in place of the

. On the same graph paper, give a plot of the inverse-square law for

the and hence from the gradients of the two linear plots

deduce the strength of .

(e) Attenuation of by Matter

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The attenuation of a beam of passing through matter depends on

photoelectric absorption, Compton scattering and pail production. The

relative importance of each of these processes, in any given case, is a

function of the initial energy of the -photons and the atomic weight

of the absorbing material. Experimentally it has been found that the

attenuation follows closely the exponential law i.e. I is the initial

intensity of the , then after transversing a layer of matter of

thickness , its intensity I is reduced to

where is known as the linear absorption coefficient of the matter.

The value of when the initial intensity is reduced to half is called

the half value layer (HVL).

Note that in experiments using a G.M. counter, I is proportional to N

(the counting rate corrected for background and resolution time), hence

Place the at a distance of about 20cm from the window of the

G.M. tube. Take a one-minute count to determine the initial count rate.

Without disturbing the setup, take a series of one-minute counts as a

succession of aluminum sheets is placed vertically in the region

between the G.M. tube and the source using the data obtained, plot a

suitable graph and hence deduce the and HVL for .

References

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(1) J.B.A. England, Techniques in Nuclear Structure Physics,

Part 1, Chapter 1.

(2) W.E. Burcham, Nuclear Physics An Introduction,

Second Edition, Chapter 6.

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Appendix A

Proper Handling of Radioactive Source

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Source Face

Source in the proper storage box. Source to be handled by spincer only and and face downwards or away from people

Use pincer to remove source from storagebox

Picture of radioactive source. Do not face radioactive source towards yourself or anybody. Source must be put back into proper storage box after use.