The Effects of Atmospheric Attenuation on Cosmic Ray Muons: How is Surface Level Cosmic Ray Muon Flux Affected by Atmospheric Attenuation?

(1) Interlake High School, Bellevue, Washington, (2) Independent Researcher, Redmond, Washington

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Cosmic rays are high-energy astronomical particles originating from various sources across the universe. After undergoing nuclear reactions in the upper atmosphere, the primary form they take on the Earth’s surface is in the form of muons. Here, we sought to understand how surface-level cosmic-ray muon flux is affected by atmospheric attenuation by measuring the variation in relative muon-flux rate relative to zenith angle, testing the hypothesis that muons follow an exponential attenuation model. Using a QuarkNet cosmic ray muon detector (CRMD), we collected relative muon-flux data while varying the rotational orientation of the CRMD with respect to the magnetic east-west axis. We then calculated a sensitivity function for the detector with respect to muon entry angle. We parameterized a relative muon angular-density function based on the model of exponential attenuation through the Earth’s atmosphere. Then, we convolved these functions and fitted parameters to experimental data using least-squares regression. The attenuation model predicts an attenuation length of 6.3 km. This result implies that only a maximum of 24% of muons can reach the Earth’s surface, due to both decay and atmospheric interactions. The agreement between data and model (χ2=0.351) provides evidence of exponential muon attenuation through the atmosphere.

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cosmic rays muons atmospheric attenuation quarknet