Tracking Cosmic Ray Muons Using
a Cloud Chamber
Leah Wilson and Lori Wilson
duPont Manual High School
and
Dr. Akhtar Mahmood
Bellarmine University
KAPT 2009 SPRING MEETING
MARCH 7, 2009
BELLARMINE UNIVERSITY
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Purpose and Hypothesis
• Cosmic rays are all around us, one type of cosmic ray that
strikes the earth is a muon (µ).
• On average, one muon strikes the fingertip every single minute.
• In order to determine the muon flux count (number of muons
hitting the earth’s surface in a given area per minute), a cloud
chamber was constructed out of a basketball display case to
determine the muon flux in the Louisville area.
• This experiment was conducted at Bellarmine University in a
Physics Lab.
• The muon flux count obtained by our cloud chamber was also
compared with the muon flux data obtained on-line from the
Cosmic Ray Detector located at SLAC’s (Stanford Linear
Accelerator Center) Visitor’s Center.
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Background Information
• The history of cosmic rays started in the
beginning of the 20th century.
• In 1912, Victor Hess was in his hot air
balloon soaring at an altitude of about
5,000 meters. When he was sailing
around he noticed “penetrating radiation”
coming from outer space.
• Following the ideas of Hess was Robert
Millikan in 1925 who introduced the name
“cosmic rays.”
• In 1929 Dimitry Skobelzyn built the first
cloud chamber to test the theory of cosmic
rays.
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Background Information
• In 1935, the Explorer II balloon mission
ascended to 22,066 meters in space while
collecting data about cosmic rays.
• In 1937, Seth Neddermeyer and Carl
Anderson discovered the muon using a
cloud chamber.
• In a major discovery in 1938, Pierre Auger
discovered "extensive air showers" in the
outer atmosphere. These showers were
made up of secondary subatomic particles
caused by the collision of high-energy
cosmic rays with air molecules, which is
now defined as a cosmic ray shower.
• In 1947, Cecil Powell of Bristol University in
the United Kingdom, discovered a new type
of cosmic ray called the pion ().
Explorer II balloon
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Background Information (cont’d)
• Most muons come from what are
known as cosmic rays. A muon is
roughly 200 times heavier than an
electron.
• There are two categories of cosmic
rays: primary and secondary cosmic
rays.
• Primary cosmic rays can generally
be defined as all particles that come
to earth from outer space.
• When these primary cosmic rays hit
Earth's atmosphere, they ionize the
atmosphere forming a shower of
matter and anti-matter particles.
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Background Information (cont’d)
• This is where the muons come from: they are
the results of an interaction between a proton
(which are abundant in the universe) and the
atmosphere that produces a pion that decays
into a muon, among other particles.
• Primary cosmic rays are particles such as a
single proton (nuclei of hydrogen; about 90% of
all cosmic rays) traveling through the interstellar
medium. Most of these originate outside of the
solar system (i.e. from Supernovae), but some
of them come from the sun.
• When such a high-energy proton hits the
earth's atmosphere at around 30000m above
the surface, it will collide with a nuclei of the
atmospheric gas molecules. As a result of this
collision, many secondary particles are
produced, including lots of particles called
pions.
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Background Information (cont’d)
• A (charged) pion decays to a muon and two muon-neutrinos
(which is neutral therefore can not be seen) at about 10000m
(10 km) altitude.
• Some of these muons can make it through the earth's
atmosphere which can be detected and measured using a
suitable particle detector (such as a cloud chamber or a muon
detector) at the earth's surface. In cosmic ray showers, both
muons and anti-muons are produced.
• Although the muon at rest has a lifetime of only 2.2 µs, it should
have decayed after traveling a distance of only 660m. Thus
one would conclude that muons produced at this high altitude of
10000m from earth should not reach the ground.
• But muons can travel all the way down from a height of 10000m
(10 km) above the surface of the earth while traveling at 99.8%
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the speed of light.
Background Information (cont’d)
• The reason is that according to Einstein’s Special Theory of
Relativity, the muons age more slowly (in fact, about 16 times)
since they are traveling very fast at about 99.8% the speed of
light. This effect is called “time-dilation.”
• From the point of view of an observer on Earth the muon’s new
lifetime can be determined from Einstein’s Special Theory of
Relativity.
t
t0
v2
1 2
c
• c = speed of light, v = speed of the muon
which is 0.998c. (I.e. 99.8% the speed of light)
and t0 = lifetime of muon at rest which is
2.2 x 10-6 s.
• Thus this relativistic time dilation allows the
muon to travel about 16 times farther (10000m
instead of 657m) than would have been
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expected otherwise.
Background Information (cont’d)
• We hypothesized that a using a cloud chamber, the muon flux
count rate will be at least a factor of 10 or less since we are
using our naked eye to detect the muon tracks instead of the
sophisticated muon detector.
• The latitudes of Palo Alto (37) and Louisville (38) are very
close (within 1 of each other).
• Whereas Palo Alto’s elevation is about 262 ft., and Louisville’s
about 466 ft. The elevation of difference of about 200 ft should
not result in any significant difference in the muon flux count
rate except for the resolution of the two types of detectors.
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Background Information (cont’d)
• When a charged particle passes through a particular substance it can ionize
the surrounding particles and leave a trail.
• For example, in a cloud chamber, the air is cooled to the point that when an
atmospheric particle is ionized, it will cause the air to condense and thus
leaves a visible trail.
• The cloud chamber is essentially saturated with alcohol vapor. The dry ice
keeps the bottom very cold, while the top is at room temperature. The high
temperature at the top of the chamber means that the alcohol in the felt
produces a lot of vapor, which falls downwards. The low temperature at the
bottom means that once the vapor has fallen, it is supercooled. It is in a
vapor form, but at a temperature at which vapor normally can't exist. Since
the vapor is at a temperature where it normally can't exist, it will very easily
condense into liquid form.
• When an electrically charged cosmic ray comes along, it ionizes the vapor-that is, tears away the electrons in some of the gas atoms along its path.
This leaves these atoms positively charged (since it removed electrons,
which have negative charge). Other nearby atoms are attracted to this
ionized atom. This is enough to start the condensation process.
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Types of Cosmic Ray Muon Tracks
• The Figure on the left is an
example of a cosmic ray
muon track.
Example of a Muon Track
• The muon track can be seen
coming straight which then
"kinks" off to the left sharply
after knocking off an electron
from the atom in the material.
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Another Example of a Muon
Track
Yet Another Example of a
Muon Track
• Figure to the left is an
example of a very jagged
muon track. This is known
as "multiple scattering",
where a low-energy cosmic
ray bounces off of one
atom in the air to the next.
• Figure to the left is the third
example of a muon track. It
shows a muon decaying
into an electron and two
neutrinos (actually one
electron-neutrino and one
anti-electron neutrino)
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Procedure
• The cloud chamber was constructed to
consistently produce an environment
where muon tracks could be detected.
• Felt pads were saturated with 91% isopropyl
alcohol inside of the chamber (only on the
top and bottom sides) with the goal of
creating a super-saturated atmosphere of
alcohol within the chamber.
• The chamber was then set on a block of dry
ice and was cooled to create the required
environment for muon detection.
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Procedure (cont’d)
• To build the cloud chamber the following
materials were needed • A basketball display box, a conducting
sheet of metal, silicon cement, razor blades,
weather-strip, black felt, four push-pins,
Windex, 91% pure isopropyl alcohol and
paper towels.
• First, we replaced the bottom of the display
box with the thin sheet of conducting metal.
We used silicon cement to ensure firm
placement of the metal at the bottom.
• Next, we took the same silicon cement
and ran it around the inside and outside
edges of the display box to make sure that
no air will be able to get into the box when
the experiment is going on.
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Pasteur Hall
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Procedure (cont’d)
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Procedure (cont’d)
• We placed the cloud chamber on top of the dry ice block and
then turned off the lights and shone a high intensity light
through the center/side of the chamber.
• We waited for the fog to form at the bottom and begin to time
the twenty minutes.
• At every minute mark, we tallied the number of cosmic rays
observed within the twenty minutes and repeated the
experiment five times.
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CLOUD CHAMBER RESULTS
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Results
The cosmic ray muons detected in each 20
minute five trial run varied from 0 – 17.
This figure shows the same results, charted in
a bar graph.
This figure shows the average number of muons
per minute.
This figure shows the average number of muons
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per minute in each trial.
Results (cont’d)
These are the actual pictures of
muons tracks that were detected
inside the cloud chamber we built.
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Conclusion
• The experimental data supported the goal of this research project,
which was to measure the muon flux in Louisville.
• During a 100-minute time frame in 5 different trials, a total of 593
muons were observed or and average of about 119 muons per 20
minutes. Therefore we detected an average of 6 muons per minute.
• The limitation of the experimental setup was the effective area of
observation was about one-ninth of the size of the box, due to the
location of the light source that was directed at the cloud chamber.
• The chamber measured 30 cm (L) x 30cm (W), which equals an area
of 900cm2, whereas the effective dimension of focus was about
10 cm (L) x 10cm (W) which gives the effective area of focus of about
100 cm2.
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Conclusion (cont’d)
SLAC Cosmic
Ray Detector Data
(Muon flux count
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rate)
Conclusion (cont’d)
• The SLAC data shows that the muon flux in Palo Alto was anywhere
from 0.3 - 0.9 muons/min/cm2 or an average muon flux of
0.6 muons/min/cm2 of during that time which corresponds to
60 muons/min/100cm2, to match with our effective area of
observation which was about 100cm2.
• Our hypothesis predicted that with a cloud chamber the expected
resolution would be about ten times less (10% or less).
• Our cloud chamber experiment detected a mean flux of
6 muons/min/100cm2 (or 0.06 events/minute/cm2) which was
consistent with the hypothesis regarding the resolution of this
experimental setup.
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Awards and Recognition
• March 7, 2008:
– duPont Manual
Science and
Engineering Fair
• 2nd Place Team in
Physical Science
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Awards and Recognition
• March 29, 2008:
– (KYSEF) Kentucky Science
and Engineering Fair
• Certificate for 1st Place
Team
• Trophy for Best of Fair
high school Team Project
• University of Kentucky
Presidential Scholarship
for 1st place prize in State
Competition
• University of Louisville
Trustee Scholarship for 1st
place prize in State
Competition
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Awards and Recognition
• April 14, 2008:
– Certificate of Recognition
for duPont Manual High
School Kentucky Science
and Engineering Fair
Winner presented by
Jefferson County Board of
Education
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Awards and Recognition
• April 19, 2008
– Kentucky Junior
Academy of Science
(KJAS) 1st Place
Winner
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Awards and Recognition
• May 11-16 2008:
– Finalist at the INTEL
International Science
and Engineering Fair
• Certificate Presented by
Agilent Technologies
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