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Relative Flux of Positive and Negative Cosmic Ray Muons at the Surface of
the Earth
Alan J. Gelman
Glenbrook South High School
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Table of Contents
Abstract1
Safety Sheet2
Title Page3
Table of Contents4
Acknowledgements5
Purpose and Hypothesis5
Background Research6
Materials and Methods7
Results and Discussion8
Data8
Data Analysis and Discussion10
Error Analysis11
Conclusion11
References13
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Acknowledgements
First and foremost, I would like to thank Ms. Mary Ann Ericksen, for supervising and
sponsoring this experiment, and for providing valuable help. I would also like to thank Mr.
Jeffrey Rylander for providing me with all the necessary equipment to make this experiment
possible, and the Glenbrook South Science Department for providing me a room in which to
perform this experiment. In addition, I would also like to thank my fellow members of the
Glenbrook South Particle Physics Research Group, Ann Isaacs, Keyur Patel, Dennis
Cherepanov, Matthew Moran, Nicholas Ermolov, and George Gikas for their input and
assistance with the experimental procedure.
Purpose and Hypothesis
The purpose of this experiment is to find whether or not positive muons or negative
muons are more abundant at the surface of the Earth, and to determine by how much they are
more abundant. I hypothesized that positive muons will be the most abundant as other
experiments have shown that positive muons are more abundant in the Earth’s upper atmosphere,
and I expect these results to be consistent at the Earth’s surface.
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Background Research
Cosmic rays are beams of subatomic particles such as protons and neutrons that move
through the interstellar matrix. When a cosmic ray enters the Earth’s atmosphere, it collides with
molecules that exist in the upper atmosphere, and a shower of secondary subatomic particles is
created (Rylander 1996). One of these secondary particles is called a “muon.” While the majority
of the other subatomic particles, such as neutrinos, pass through the Earth’s surface undetectably,
muons are actually able to reach the Earth’s surface, and are detectable with the use of muon
detectors. However, they might also further decay into an electron, a neutrino, and a muon
neutrino, none of which are detectable with a muon detector (Dorigo 2010). Because of the fact
that muons are charged particles, they are affected by the Earth’s magnetic field in the sense that
their trajectories toward the Earth’s surface are bent by a certain amount. Positive muons tend to
be deflected from the east toward the west, and negative muons tend to be deflected from the
west to the east.
Studies have shown that in the Earth’s upper atmosphere, positive and negative muon
flux can vary, and such studies have also measured that at any point in the Earth’s upper
atmosphere, positive muon flux is larger than negative muon flux (Relative Abundances of
Positive and Negative Muons in the Atmosphere n.d.).
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Materials and Methods
This experiment was performed in three stages. In the first stage, the control stage, a pair
of muon detectors were placed in stacked orientation facing directly upward. This was performed
as a control, and ideally, equal amounts of positive and negative muons would be detected. After
the detectors were placed in this configuration, flux data were recorded for a four day period,
from Tuesday to Friday, and saved onto a computer. For the first period of data collection, the
detectors were angled eastward at 120 ± 1.50 (Fig. 1), with a meter of space in between them to
ensure that data was collected accurately and with as few stray muons as possible. Flux data
were then recorded for another four day period. During the second period, the detectors were
angled westward at 120 ± 1.50 (Fig. 2) with the same meter of space between them, and flux data
were recorded for yet another four day period. When all the data was collected and stored on the
computer, flux graphs were made for each period based on the data.
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Fig. 5
Period 1 (East)
Day Tuesday Wednesday Thursday Friday
Events 233 1,796 1,534 1,576
Period 2 (West)
Day Tuesday Wednesday Thursday Friday
Events 12 100 146 93
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Data Analysis and Discussion
The results from the three periods of data collection are shown in figures 3, 4, and 5
above, as well as the two tables above, labeled for the period of data collection that they
represent. Figure 3 shows the results from the control trial, and figures 4 and 5 show the results
from periods one and two of data collection, respectively. There is a significantly greater amount
of muon flux shown for period one of the study (fig. 4) than for period two (fig. 5). This shows
that a significantly greater amount of positive muons exist at the surface of the Earth than
negative muons. The results of this experiment, that positive muons are much more abundant
than negative muons at the surface of the Earth, are consistent with the upper atmospheric muon
experiment (Relative Abundances of Positive and Negative Muons in the Atmosphere n.d.),
which concludes that at any point in Earth’s upper atmosphere, positive muons are more
abundant than negative muons.
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Error Analysis
The presence of some outliers for the second period of data collection (fig. 5) may be the
result of noise, unwanted particles, being detected by the muon detectors. In addition, all the data
was captured in three singletrial, four day runs. Had a few more trials been performed and had
each trial taken more days, then the final results would have probably been much more precise,
and with comparatively fewer outliers.
Conclusion
The data recorded in this experiment shows that at the surface of the Earth, positive
muons are significantly more abundant than negative muons, which proves my hypothesis due to
the overwhelmingly higher amount of flux of positive muons as opposed to negative muons. The
data in figures 4 and 5, as well as that in the tables for the two periods of data collection
demonstrate this difference in showing that there is over than ten times as many positive muons
at the surface of the Earth than negative muons. One explanation for this phenomenon could be
that the cosmic rays from which these muons originate could be a subatomic particle which
decays into specifically positive muons, and that that subatomic particle is much more abundant
in the universe than its symmetric equivalent which decays into negative muons. Another
explanation could be that most negative muons might, for whatever reason, decay into electrons
and neutrinos more readily higher in the atmosphere than positive muons. These inferences may
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be explored in further experiments. Finally, the results that were obtained in this experiment are
validated by the fact that they coincide with the results collected by the upper atmospheric muon
flux experiment (Relative Abundances of Positive and Negative Muons in the Atmosphere n.d.),
which stated that positive muons are more abundant in any part of the upper atmosphere than
negative muons.
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References
Relative Abundances of Positive and Negative Muons in the Atmosphere. (n.d.). Retrieved
November 21, 2015.
Muon basics. (n.d.). Retrieved December 28, 2015, from
http://www2.fisica.unlp.edu.ar/~veiga/experiments.html
Understanding Muon Decay. (2010, March 4). Retrieved December 28, 2015, from
http://www.science20.com/quantum_diaries_survivor/understanding_muon_decay
Rylander, J. W. (1996). Muon mean lifetime measurement in a high school classroom
(Unpublished master's thesis).
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