Experiment – 2

(This should be carried out only by RADLab software)

Objective : Study of absorption in matter by -ray source.

Sources : -ray source = Cs137

Scope of the Expt : In principle the experiment should have four parts. But in RADLab software

one can skip the first and second part of the experiment. The first part is already done in Expt-1.

i. To find out the operating voltage of the supplied GM tube. Keep the source in front of the

GM tube and record different values of the voltage vs the number of counts. From the

graph between the count rate and the applied voltage find out the operating voltage of the

supplied GM tube.

ii. To find background counts. Operate the GM tube with the operating voltage. Take

reading for about 10 min without the source.

iii. To calculate the linear attenuation coefficient, mass attenuation coefficient and half value

thickness. Now put the source and then the absorber [Aluminum (Al)]. Change the

thickness of Al block and determine the count rate at different thickness.

iv. To verify the inverse square law of radiation.

Theory : Write about GM counter in details. Describe about mass attenuation coefficient and

inverse square law of radiation.

How to setup experiment in RADLab :

Go to the folder called "RADlab" under "Program Files"

Right click on "run.bat" and "Run as Administrator". REDLab will start and it will show User Login icon.

Give User Login Creditential : Use “New User” to create your own ID and Password. Or use the following dafult ID and Password. User Name: demo Password: demo

Now, you can construct your Expt-2. You need the following items. 1) A Gamma-ray source 2) A GM tube 3) High Voltage source 4) A counter 5) Aluminium Foil Now go to Administration, then click on “Create Experiment”

Now, Select Experiment Type :

o Gamma Experiment Now

Now, Select Source Type :

o Cs-137 Now All sources are designed with Monte Carlo

Simulation. Monte Carlo Simulation is a

mathematical technique that generates random

variables for modeling uncertainty of a certain

system. The random variables or inputs are modeled

on the basis of probability distributions such as

normal distribution to be followed in a typical

radioactive source.

> Next

> Next

Now, Select Detectors :

o Geiger Muller Now How does a GM tube work ? To know more read

supplied literatures or from reference books or from

internet sources.

You can also read about all detectors, modules etc. from Help of RADLab as shown below. It has a nice description about all instrument. Otherwise follow reference Books.

Radiation Detection and Measurement; By G. F. Knoll

Nuclear Radiation Detection, Measurements and Analysis; By K. M. Varier

Nuclear Radiation Detectors; By S. S. Kapoor and V. S. Ramamurthy

Now, Select NIM Module :

o HV Supplier o Aluminium Foil o Counter

Now You can select / choose multiple NIM module just by

pressing ctrl + NIM Module

> Next

> Next

You can give Experimental Name : Expt 2 Also choose Experimental Sheet as text : Expt 2 A documentation file in .html is not available

currently for Expt 2 and in future we shall make it. In

that case you can upload / choose the other option i.e

Experiment Sheet (as html).

Now, verify your selection i.e Detector, Source, NIM Modules etc. Now

Now go to File --> Select Experiment --> Expt 2 Now After that click on You can transfer all of them to your work bench just by clicking on “Cs-137” then clicking on “work bench”. Transfer all (except the Al foil) instruments to your work bench.

After that click on You can connect the cables to your modules available in “work bench”. Use the appropriate cables as mentioned below

Red Cable : HV Signal Cable

Gray Cable : Signal Cable

Now you have all your required Source, Detector, NIM Modules, and Cables in your work bench. Now connect the cables as described below. Carry out the experiment with the designed parameters. You can click on to start the experiment (use the parameters).

Finish

Select

Instrument

Cables

Run

A) Operational characteristics curve of GM Tube (Optional as you have done the same in Expt-1)

(i) Set counting time for 1000 unit (it’s not exact unit of second) or more. (ii) Clear the counter so that it should show 0. (ii) Set your source in line with GM tube. (iii) Set your HV to 0.0 kV by turning the HV key. Don’t forget to switch ON HV supply. (iv) Now go to view and click on Draw particle path (Without this you cannot do Expt-2). (v) Now RUN. Note your counts. This is RUN-1 (vi) Repeat the same for RUN-2 and note down your counts. (vii) Increase your HV in a step (see the step size for different region) and take RUN-1 and RUN-2. Note it down in your record. (viii) Repeat (vii) in a step of 0.1 kV [from 0 to 0.75 kV], step of 0.01 [from 0.75 kV to 0.95 kV], step of 0.1 kV [from 1.0 kV t 3.0 kV]. (vii) Make proper table and draw the graph.

HV

(kV)

No of Counts Average

Counts

HV No of Counts Average

Counts Run-1 Run-2 Run-

1

Run-2

0 Step : 0.05

0.1 1.0

0.2 1.05 Step : 0.1 1.10

0.75 Step : 0.05

0.76 2.0 Step : 0.01 Step : 0.05

0.94 Step : 0.05

0.95 3.0

Plot characteristic graph of GM Tube:

Now you decide the operating voltage from the above graph i.e may be 1.5 kV

(B) Determination of background readings for long time (may be 10 minutes) without

sources:

Now go to “View” and click on “Draw particle path” (Without this you cannot do Expt-2)”. By doing this

you can avoid “counts” without source. [RADLab does not give any counts but in laboratory you shall

find some background noise. No need to do this in RADLab.] RADLab sometimes consider the

presence of source even though you have not kept the source. It’s a bug in the software. This can be

avoided by clicking on “Draw Particle Path” in View Tab.

(C) Determination of count rate after putting the absorber with source:

Now go to “View” and click on “Draw particle path” (Without this you cannot do this

experiment !!)”. Now put the Aluminium foil in the work bench as shown below.

1) Right click on source and select “Properties”. Then Set Source Activity (Bq) = 1,50,000 at the

bottom. Then “Update”. Now you have a strong source to carry out the Expt – 2.

2) Fix Source and Detector distances so that you have x = ~ 0.0 cm and y = ~ 25 cm.

3) Right click on the Aluminium Foil and select Properties. Now you can change the thickness of

the Foil / Slab. Keep “Thickness (x-axis)” = 400 mm, “Width (z-axis)” = 400 mm as fixed

dimensions. Now only change the Height (y-axis) = 0 mm ~ 200 mm.

4) Set time in counter to 1000. Clear counter.

5) Be sure you have click on “Draw particle path” (Without this you cannot do this experiment

!!)”.

6) How to take Data ?

A. Set Height of Al Foil to Zero. Update

B. Clear counter and it will set count to “Zero”

C. RUN the source (It will start the timer) (See left side bar for RUN symbol)

D. Counter will record counts and Timer will STOP at preset time (1000 sec) when it

reaches the same.

E. STOP the Source (See left side bar for STOP symbol)

F. Now record your data and change the “Height” of Al Foil / slab. Then “Update”

G. Go to “B” for new reading.

Follow the following step: From 0 mm ~ 5 mm; Step size = 1 mm

From 6 mm ~ 25 mm; Step size = 3 mm

From 26 mm ~ 80 mm; Step size = 5 mm

From 80 mm ~ 200 mm; Step size = 10 mm

Sl

No

Voltage

(kV)

Time

(sec)

Al

Thickness (mm)

Count

(2 Run)

Avg. Count Count

Rate/sec

1 1.5 1000 0 11,170

11,256 11.25 11,342

2

3

You can take two (2) RUN. Time can be more than 1000 sec. You decide your No of RUN and

Time if you donot have proper computer resources. One can clearly see the gamma ray blocking

/ attenuating in the Aluminium slab for higher thickness.

Graph : Plot of log(count rate) vs thickness (in mm)

Thickness (in mm)

Absorber mass thickness

In terms of mass attenuation coefficient, the attenuation law for gamma rays takes the form

I/I0 = e-(µ/ρ) ρt

I = intensity that comes out

I0=Incident intensity

µ= linear attenution coefficient

ρ= density of the absorber

t = thickness

(μ / ρ) = the mass attenuation coefficient

Half value thickness

Half value thickness is the thickness of the absorber at which the value of the incident intensity attenuated

to half of its initial value.

Calculation :

From above graph we have µ=linear attenuation coefficient=0.0461 mm-1

Now as ρaluminium = 2.7 g/cm3 or 2.7 mg/mm

3

We have mass attenuation coefficient

μ / ρ = 0.0461/2.7 mm2mg

-1 = 1.7074x10

-2 mm

2mg

-1

Calculating half value thickness

At half value thickness log(I/I0)=ln(1/2)= -0.693= -µt

t = -0.693/-0.0461 = 15.0325 = 1.5033

Note: All the numerical value (except ρaluminium = 2.7 g/cm3) are just examples and not real value

of expt. Put your own experimental value and find the true value.

(D) Inverse square law for gamma radiation :

Gamma radiation obeys an inverse square law in air since absorption is negligible.

(Radiation spreads out over an increasing sphere. Area of a sphere = 4 r2, so as r gets larger,

intensity will decrease as 1/r2. The effect of absorption by the air will be relatively small.

Fix the GM Tube and move the source and take reading. Repeat 2 or 3 times and take

average of the same. Do background subtraction for correct reading. Then plot counts vs 1/r2

(straight line fitting) or counts vs ln r. You should get a coefficient of -2 in counts vs ln r graph.

Due to the absorption and scattering of radiations by air molecules, inverse square law

deviates slightly from ideal behavior. Error associated with isotropy also contributed in the

deviation of inverse square law.

How to take Data ?

A. Be sure Al foil / slab is not in the work bench. If its there then delete it.

B. Detector and Source should be properly aligned i.e x = ~ 0 cm

C. Keep source 5 cm (this is the minimum distance available) away from Detector.

D. Clear counter and it will set count to “Zero”

E. RUN the source (It will start the timer) (See left side bar for RUN symbol)

F. Counter will record counts and Timer will STOP at preset time (1000 sec) when it

reaches the same.

G. STOP the Source (See left side bar for STOP symbol)

H. Now record your data and change the source position by selecting it and then moving by

arrow key”

I. Go to “D” to record new reading for a new position of source.

Follow the following step: From 5 cm ~ 20 cm; Step size = 1 cm

From 21 cm ~ 50 cm; Step size = 3 cm

Source Activity = 1,50,000 Bq (Mention the activity). Be sure you have clicked on “Draw

particle path” in View Tab.

Sl.

No

Time

(sec)

Source Distance from

Detector “r” (cm)

1/r2

cm-2

Counts Avg. counts

(per sec)

1 1000 5.00 -- 66365

66.175 65986

Result :

The threshold voltage of the GM counter was found to be ………… volts.

The linear attenuation coefficient of aluminum was found to be ……….. mm-1

The mass attenuation coefficient = …………….. mm2mg

-1

The half value thickness = ……………. mm

Have you verified the 1 / r2 law? Write down your observations.

Error: Write possible errors.