Measurements of
radioactive penetration
by Andy Cooper, Juan Tyranowski
2014-2015
1
Abstract
The notion and dangers of radioactivity are known to many; however few people
know all of its properties. The objective of this experiment was to find out more
about radioactivity’s ability to pass through objects. For accurate results, we also
had to search for the causes of variations in our measurements. Our results show an
exponential decrease of radioactivity in function of object thickness, thus
supporting the vast majority of theories.
Introduction
Current theories all agree that the measured radioactivity decreases with distance
between the source and the measuring device, and that it decreases exponentially
when passing through objects. Our goal was to create a setup with which we could
put these theories to the test and find results of our own. In order to contain
radioactive elements and to protect oneself against harmful radiation, a dense metal
such as lead is commonly used. If the source is strong enough, would even lead
prove to be insufficient?
Method
Material:
• Sources
o Strontium 90 (β- emitter)
o Radium 226 (γ emitter)
o Americium 241 (α emitter)
o Caesium 137 (γ emitter)
• Plates
o Aluminium (2 mm thick)
o iron (2 mm thick)
o lead (2 mm thick)
o Aluminium foil (0,01mm thick)
• Other
o Geiger counter
o Stands
2
Setup:
First, the three stands are aligned at a certain measured distance. Next, the Geiger
tube and the source are fixed to the outside stands so that they face one another; the
source needs to reach the membrane on the tube in a straight trajectory. Then the
thin plates (if any) are fastened to the middle stand and block that trajectory.
Finally, the measurement is started and timed.
Geiger counter
Stand
3)Geiger tube
Thin plate
Radioactive source
3
Results
Background average was 0,32 counts per second;
Material
Counts (N)
Strontium 90
Strontium 90
Strontium 90
Strontium 90
Strontium 90
Strontium 90
Strontium 90
Strontium 90
Strontium 90
Strontium 90
Strontium 90
Strontium 90
Strontium 90
Time (t) in s
147238
93082
81246
45446
21239
12631
7876
5289
3973
2919
2378
1904
1630
60
60
60
60
60
60
60
60
60
60
60
60
60
Count rate (n)
in counts/s
2454
1551
1354
757
354
211
131
88
66
49
40
32
27
Medium
Distance (d) in m
air
air
air
air
air
air
air
air
air
air
air
air
air
0
0,003
0,005
0,01
0,02
0,03
0,04
0,05
0,06
0,07
0,08
0,09
0,1
Distance variation
Sr 90 through air
Counts per second
3000
2500
y = 1244,6e-44,02x
R² = 0,9337
2000
Distance variation
1500
Expon. (Distance
variation)
1000
500
0
Thickness (m)
0
0,02
0,04
0,06
0,08
0,1
0,12
The measured radiation drastically decreases by increasing the distance between
the source and the Geiger tube. The traced exponential trendline does not quite
fit the data series; an inverse square (1/x2) would have connected the dots more
accurately.
4
3000,00
Americium
Counts per second
2500,00
2000,00
y = 1846,6e-224,2x
R² = 0,7658
air
aluminium
1500,00
lead
Expon. (air)
1000,00
Expon. (aluminium)
Expon. (lead)
y = 76,254e-156,4x
R² = 0,9883
500,00
y = -1,5159x + 0,0248
R² = 0,5316
0,00
0,000
0,005
0,010
0,015
0,020
0,025
0,030
0,035
Distance (m)
-500,00
Americium, as an alpha emitter, was the only emitter whose particles had trouble
passing air molecules. Penetrating through aluminium or lead was near
impossible for this type of radiation.
Material
Caesium 137
Caesium 137
Caesium 137
Caesium 137
Caesium 137
Caesium 137
Caesium 137
Caesium 137
Counts (N)
time (t) in s
331
319
320
293
295
239
225
178
60
60
60
60
60
60
60
60
Count rate (n)
5,12
4,92
4,93
4,48
4,52
3,58
3,35
2,57
Medium
aluminium
aluminium
aluminium
aluminium
aluminium
aluminium
aluminium
aluminium
Distance (d) in mm Plate thickness
in counts/s
40
40
40
40
40
40
40
40
5
2
4
6
8
10
20
30
40
Material
Counts (N)
Caesium 137
Caesium 137
Caesium 137
Caesium 137
Caesium 137
Caesium 137
Caesium 137
Caesium 137
Material
time (t) in s
272
250
231
207
205
126
111
87
Counts (N)
Caesium 137
Caesium 137
Caesium 137
Caesium 137
Caesium 137
Caesium 137
Caesium 137
Caesium 137
Medium
60
60
60
60
60
60
60
60
Count rate (n)
in counts/s
4,13
3,77
3,45
3,05
3,02
1,70
1,45
1,05
Medium
60
60
60
60
60
60
60
60
Count rate (n)
in counts/s
3,48
3,05
2,58
2,15
1,77
0,63
0,10
0,08
time (t) in s
233
207
179
153
130
62
30
29
Distance (d) in mm
iron
iron
iron
iron
iron
iron
iron
iron
Plate thickness
40
40
40
40
40
40
40
40
Distance (d) in mm
lead
lead
lead
lead
lead
lead
lead
lead
2
4
6
8
10
20
30
40
Plate thickness
40
40
40
40
40
40
40
40
2
4
6
8
10
20
30
40
6,00
Counts per second
Caesium
5,00
y = 5,3082e-0,017x
R² = 0,9786
4,00
aluminium
iron
lead
3,00
Expon. (aluminium)
Expon. (iron)
2,00
Expon. (lead)
y = 4,2224e-0,036x
R² = 0,9757
1,00
y = 4,7766e-0,109x
R² = 0,969
0,00
0
5
10
15
20
25
30
35
40
45
Distance (mm)
6
Material
Radioisotope
Calculation
Half-value thickness d½
Al
Cs 137
d½ = ln(2)/ 0,017
40,77 mm
Fe
Cs 137
d½ = ln(2)/ 0,036
19,25 mm
Pb
Cs 137
d½ = ln(2)/0,109
6,36 mm
This is our most precise measurement, showing an exponential decrease and a
plausible half-value thickness. The graph clearly shows that iron blocks more
particles than aluminium, and lead blocks more particles than iron.
Discussion:
Information used for better comprehension
Theory:
A
α decay:
B
β- decay:
B
β+ decay:
B
γ decay:
Z
A
A
A
B
A-4
B-2
Z
B+1
Z
B-1
Z
A
B-1
Y+
4
2
He + γ (low energy γ)
A
Y + e¯ + ve + γ (low energy γ)
A
Y + e+ + ve + γ (low energy γ)
Y + γ (high energy γ)
α, β, and γ radiation are also called ionizing radiation because their high kinetic
energy, if transferred to an electron of an atom, can knock it out of its orbit,
effectively ionizing the atom. This property is used in various ways for detecting
the particles.
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Background radiation:
While measuring radiation in the absence of large quantities of radioisotopes, the
Geiger counter still measures incoming particles. This radiation originates from
the sun and the earth’s crust, where large quantities of radioactive material emit
particles that can sometimes penetrate through to the troposphere.
The Geiger counter:
The Geiger counter tube consists of an insulating tube
(usually plastic), an input window, an anode as an axis
of the cylinder, an inert gas at low pressure (neon,
argon, or helium) and a cathode layering the inside of
the plastic tube.
An incident α, β, or γ particle entering through the
input window has a chance of ionizing a gas molecule
by separating an electron e¯ from the atom. Because of its negative charge, the
electron is pushed away from the cathode and attracted by the anode,
accelerating it to the point where it can ionize other atoms along the way
(Townsend discharge). Once the cascade of electrons has reached the anode, the
signal is amplified, creating an electric current which is then detected by the
Geiger counter. The battery then pushes other electrons into the cathode,
allowing the previously ionized gas particle to return to their neutral state.
This method of measuring radioactivity is well-known and widely used, however it
has a low precision. First of all, it cannot tell the difference between radiation
types or energies. Second of all, even if multiple particles ionize the gas in the
tube at the same time, it is counted as if just a single particle had arrived.
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Half-life and half-value thickness:
The radioactive half-life for a given radioisotope is a measure of the tendency of
the nucleus to "decay" or "disintegrate" and as such is based purely upon that
probability. The half-life of a radioisotope is the time needed for half of the
particles of a given quantity to decay. It can vary from a fraction of a second to a
few thousand years depending on its stability.
The half-value thickness, similarly to the half-life, is the thickness of the material
needed to reduce the entering radiation intensity by half.
Mistakes Made:
Our most common mistake was that of false measurements, meaning
measurements with impossibly high or low results, usually due to a slight change
of position of a stand. The largest mistake we made was estimating that air
resistance blocked more particles than were lost by increasing the distance
between the Geiger tube and the source. Furthermore, we abandoned the
measurements with radium because it proved to be inconsistent due to it
emitting two gamma rays of different energy.
Conclusion:
Conclusively, the detection of radioactive particles depends on the distance
travelled, the medium they have to travel through and the particle size, charge
and energy. Equipment precision is another factor; however we have not taken it
into account.
We can also confirm that radioactivity decreases exponentially in function of
distance travelled through a homogenous medium.
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Epilogue:
We want to thank the following persons for their help:
Mr Georges Haupt, technical assistant in the Physics Department of LAML
Mr Claude Schmitz, physics teacher in LAML
Mr Gaston Ternes, director of LAML
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