Download Absorption of Gamma Rays - Ryerson Department of Physics

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Absorption of Gamma Rays
The object of this experiment is to measure the absorption coefficient for gamma rays
passing through lead, copper, and aluminum. The materials will be compared as to their ability
to block the rays.
Introduction
Three types of radiation are observed to be emitted by naturally radioactive materials:
alpha particles, beta particles, and gamma rays. Gamma rays are created by processes that occur
in radioactive nuclei. The gamma rays to be used in this experiment originate in two radioactive
nuclides: 137Cs and 60Co. The energy of the gamma rays used in this experiment are,
respectively, 0.662 MeV from 137Cs and 1.17 MeV and 1.33 MeV from the 60 Co source. The
corresponding wavelengths are: 0.0019 nm, 0.00106 nm, and 0.00093 nm. The energies of the
60
Co gamma photons are in the order of a million times greater than those of visible light!
Gamma rays are very penetrating. Given their high energy and lack of electric charge,
this is hardly surprising. When they do interact with matter they do so in one of three possible
processes. The first possibility is the photoelectric process in which the gamma rays interact
with electrons initially bound to an atom to eject an electron from the atom. The second
possibility is the Compton process in which they make collisions with the free electrons in an
absorbing material. The third possibility can only occur if the gammas have energies over 1.022
MeV. In this process the gamma interacts in the electromagnetic field of an atomic nucleus to
materialize itself as an electron-positron pair.
Theory
When a beam of gamma radiation passes through an absorbing material the number of
photons absorbed (dn) in a thickness of the material (dx) is proportional to the number of photons
(n) arriving at that layer of material. This is expressed in the following equation:
dn = − μndx
(1)
where the proportionality constant, µ, is called the linear absorption coefficient. Equation (1) is
a differential equation that can be solved to obtain the following exponential equation:
n = n0 e − μ x
(2)
where n0 is the number of gamma rays per unit of time incident on the layer of absorber, n is the
number of gammas getting through the absorber in the same time interval, and x is the thickness
of the material in centimetres.
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The linear absorption coefficient will be seen to be a function of the material used to
block the gamma rays. In particular, it strongly depends on the material density. This
dependence is revealed by dividing the linear absorption coefficient by the material density. The
number obtained by this division is called the mass absorption coefficient and should have
roughly the same value for all materials. If the linear absorption coefficient is in units of 1/cm,
the mass absorption coefficient will have units of cm2/g.
Apparatus
The detector is a Geiger-Muller tube. Radiation incident upon the tube enters through a
thin (and very delicate) mica window and into a gas filled cylinder that contains two terminals
with a high potential difference between them: one of the terminals is the outside of the cylinder
(or cathode) which is negatively charged, and the other terminal is a wire that runs down the
centre of the tube (the anode) which is positively charged. Radiation moving through the gasfilled cylinder will ionise the gas. When these ions move towards either the cathode or the
anode, the electric circuit is closed and a signal is sent to the scaler. The scaler is simply an
adding machine that counts the number of times that a particle of radiation is detected.
Figure 1 Diagram of the scaler-timer unit
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Precautions
1.
The detector is located at the top of the opening located in the lower right hand side of the
apparatus. The mica window is very thin and fragile. Do not touch it under any circumstances.
2.
Treat the radioactive sources with respect. They are quite weak as radioactive sources
go, but nevertheless, they are radioactive. Handle them only with the tongs provided. Do not
put one in your pocket. Do not attempt to break their plastic containers. Keep them behind lead
shielding as much as possible. Return them to the lab instructor as soon as you are finished with
them.
3.
One of the materials you will be using in this experiment is lead. Lead is a toxic
substance, especially if ingested, so be sure to wash your hands immediately after the lab.
Procedure
1.
Setting the voltage: Turn on the scaler unit. Push the H.V. (high voltage) button once.
The LED screen will now show you the voltage of the anode. Using the UP and DOWN buttons
adjust the anode voltage to 360 Volts. Push the H.V. button again to have the LED screen put
back on count mode (i.e. the light above the H.V. button is off).
2.
Setting the timing interval: You will be counting the gamma ray incidents in one minute
intervals. Push the button marked TIME. The LED screen will now show the time (in seconds)
that the timer is set for. Adjust this time until it reads 60 seconds. Push the TIME button to go
back into the COUNT mode (i.e. light above the TIME button is off).
3.
Sign out a 137Cs source from your lab demonstrator (make sure you record the source as
well as the time you signed it out and the time of its return when you are done with it). Place the
source in its plastic tray at the bottom of the detector cavity. Place the plastic sheet with a hole
in the centre of it four or five levels away from the 137Cs source. This plastic sheet will act as a
holder for the various blockers that you will be using in the lab, and it should be located as close
to the detector as possible.
4.
The scaler is now ready to start counting the incident radiation. To start counting simply
press the COUNT button. The scaler will count for the amount of time the timing interval was
set for (60 seconds in this case) and the number of radiation events will be displayed on the LED
screen. When the scaler is done counting, the light above the COUNT button will go out, and
the number of radiation events will be displayed on the LED screen. Take three one-minute
counts of the amount of radiation emitted by the 137Cs source without any blocking (n0).
5.
Place one sheet of lead on the plastic holder above the source and take three counts with
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the blocker. Repeat this for multiple layers of lead up to four sheets. Record the thickness of
each lead sheet using a micrometer.
6.
Repeat step 5. for copper and aluminum. Note: for each of these materials n0 will be
same as the value recorded in step 3.
7.
Return the 137Cs source to your lab demonstrator and sign out a 60Co source. Repeat
steps 3. and 4. for the 60Co (i.e. lead only).
8.
Return the 60Co source to your lab demonstrator. Remove all the blockers from the
detector cavity and take three one-minute counts for the background radiation.
9.
Turn down the high voltage to zero and turn off the scaler timer.
Analysis
Take the average of each set of three counts per minute. Subtract the background average from
each average to correct for background radiation. Plot all the results on a single graph page,
plotting the natural logarithm of the average counts/min against the absorber thickness in
centimetres. Draw the best straight line through the points for each material. The slopes of these
lines are the linear absorption coefficients. Look up the densities of the absorbing materials used.
Divide each of their linear absorption coefficients by the appropriate density in g/cm3. The result
is called the mass absorption coefficient. Compare the results for each absorber and comment.
Also compare the results for lead using the different sources and comment.