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Measuring the Angular Distribution of Cosmic Ray Muons Using a Scintillator Telescope
Measuring the Angular Distribution of Cosmic Ray Muons Using a Scintillator Telescope
Brandon Bergerud and Max Veit
University of Minnesota
Methods of Experimental Physics Spring 2013
Introduction
Introduction
The angular distribution of cosmic ray muons was examined using scintillator panels and photomultiplier tubes. The angular distribution is expected to follow a relationship (see Theory page), where is the incident angle measured from the zenith. Furthermore, the interaction between charged cosmic rays and the earth’s magnetic field is expected to create an effect whereby more muons arrive from the west than the east.
The observed angular distribution was found to fall off faster than
for smaller angles (less than 30 degrees); the exponent in the generalized distribution was measured to be for this range of angles. In addition, a greater flux from the east was observed compared to the west. These discrepancies are due to shielding effects; the range of 1 <nop>GeV muons (the average energy at the surface is 4 <nop>GeV [Particle Data Group, 2012]) in concrete is 65.5 cm, so the concrete walls surrounding the laboratory could have easily cut off the lower-energy portion of the distribution and modified the observed shape. This observation sets limitations on measuring the properties of muons in buildings, but points to an interesting application of cosmic rays called passive muon radiography. This is an imaging technique where the scattering or absorption of cosmic-ray muons can be used to detect (or disprove the existence of) hidden chambers in large, solid structures (e.g. the Pyramids, see Alvarez 1970), non-invasively inspect cargo for smuggled nuclear materials (Hengartner), and map the density of portions of the Earth's crust (Takana 2007). See References page for more information.
Cosmic Rays
Cosmic Rays
Cosmic rays are a broad class of high energy particles (mostly protons and alpha particles) that enter the earth’s atmosphere, being formed in such processes as solar flares and supernovas (Ackermann, 2013). After entering earth’s atmosphere, the primary particle (cosmic ray) will interact with atmospheric nuclei and create lighter particles, such as pions and kaons. These daughter particles then interact with other nuclei and decay, creating a chain of interactions known as a cosmic ray shower (Jackson & Welker, 2001), as illustrated below:
(A schematic of a cosmic ray shower. When the primary particle interacts with atmospheric nuclei it creates secondary particles such as pions and kaons, which then decay into particles such as muons. Image from http://www.mpi-hd.mpg.de/hfm/CosmicRay/shower.png.)
The daughter particles eventually decay into lighter particles, such as photons, electrons, neutrinos, and, most importantly for this experiment, muons. Muons have a rest-frame lifetime of only 2.2 microseconds, which would be insufficient for the particles to reach the surface were it not for relativistic time dilation. Since these muons travel close to the speed of light and have appreciable Lorentz factors, their lifetimes (and decay lengths) are lengthened enough for many of them to reach the Earth's surface, where they can be measured using a variety of devices.
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