The Elusive Muon
Cosmic rays are a series of sub-atomic particles that originate from extraterrestrial sources, traditionally supernovas or solar flares (1). Made up of electrons, protons, and atomic nuclei stripped of all their electrons, these rays constantly bombard the earth’s atmosphere. When they reach Earth, the cosmic rays finish their several million light year long journey in a spectacular way, by slamming head on into atomic nuclei high up in the atmosphere, at an altitude of about 15 kilometers (2). The average rate of impacts is 1000 collisions per square meter per second (1). Scattering and diffusing, the resulting stew of subatomic particles rain down on the earth below. Making up a good portion of this fallout are tiny muon particles.
Muons are the embodiment of the kinetic energy transported by the cosmic rays. A member of the lepton family of sub-atomic particles, the muon is a distant cousin to the more common electron. Like electrons, muons are negatively charged, but have much more mass, clocking in at 1.88*10-34 kilograms resting mass, which is 207 times larger than an electron (1). As a result and in conjunction with the high energy they obtain upon creation and their much larger mass, muons usually pass unmolested through the earth’s atmosphere, avoiding almost all of the nearby atomic charges on their way to the ground below. Due to this small cross-section, muons tend to be some of the only particles created through cosmic rays interactions that reach sea level. Although their small mass allows them to easily bypass all other matter, muons only have a lifetime of about 2.20 microseconds before decaying into a single electron and two neutrinos (2).
Down here on the ground, muons are the only cosmic ray particles we interact with on a daily basis. Muons make up a good chunk of extraterrestrial radiation that we receive, with an average human receiving the equivalent of ten chest X-rays worth of radiation annually (3). Like all other forms of radiation, muons have the chance to cause a mutation in the DNA of an organism, which can either lead to evolutionary advantages or debilitating diseases such as cancer. Understanding how muons originate and interact with other matter may help us to better understand these processes and track radiation exposure.
For this experiment, we hypothesized that the muon flux would increase with the altitude, as there would be less chance of muons decaying before they reached the detector. Additionally, we predicted that there would be a sharp decline in the muon flux past the altitude of 15 kilometers, as this would be past the point where the cosmic rays collide with atomic nuclei to produce muons in the first place.
Sources:
(1.) Cosmic Ray Muons and the Muon Lifetime. (2013, October 14). Retrieved July 17, 2014, from http://www.phys.ufl.edu/courses/phy4803L/group_I/muon/muon.pdf
(2.) Liu, L., & Solis, P. (2007, November 18). The Speed and Lifetime of Cosmic Ray Muons. Retrieved July 17, 2014, from http://web.mit.edu/lululiu/Public/pixx/not-pixx/muons.pdf
(3.) Schirber, M. (2009, August 27). Death Rays From Space: How Bad Are They? Retrieved July 17, 2014, from http://www.space.com/7193-death-rays-space-bad.html
Muon decay graphic courtesy of Cobb, J. H., & Achenbach, P. (2001, May 14). The Airborne Detector for low Energy cosmic Rays. . Retrieved July 19, 2014, from http://www.physics.ox.ac.uk/neutrino/HAMF/ADLER%20Home%20Page.htm
Header photo courtesy of Simon Swordy (U. Chicago), NASA.