[Paper Review] The Physics Behind the CosmicWatch Desktop Muon Detectors

Spencer Axani|arXiv (Cornell University)|Jul 31, 2019

Astrophysics and Cosmic Phenomena40 references4 citations

TL;DR

This paper details the physics underlying the CosmicWatch Desktop Muon Detector, a low-power, portable undergraduate research tool that measures cosmic-ray muons and radioactive decay. It demonstrates how the detector enables measurements of muon rate variations with altitude and latitude, validates Poissonian statistics in radioactive decay, and implements a cosmic-ray-based true random number generator using SiPM and scintillator technology.

ABSTRACT

The CosmicWatch Desktop Muon Detector is a Massachusetts Institute of Technology (MIT) and Polish National Centre for Nuclear Research (NCBJ) based undergraduate-level physics project that incorporates various aspects of electronics-shop technical development. The detector was designed to be low-power and extremely portable, which opens up a wide range of physics for students to explore. This document describes the physics behind the Desktop Muon Detectors and explores possible measurements that can be made with the detectors. In particular, we explore various physical phenomena associated with the geomagnetic field, atmospheric conditions, cosmic ray shower composition, attenuation of particles in matter, radioactivity, and statistical properties of Poisson processes.

Motivation & Objective

• To develop a low-cost, portable, and low-power desktop muon detector for undergraduate and high school physics education.
• To investigate the physical phenomena influencing cosmic-ray muon detection, including geomagnetic effects, atmospheric attenuation, and particle interactions.
• To demonstrate measurable physical phenomena such as muon rate dependence on altitude and latitude, and the Poissonian nature of radioactive decay.
• To implement and validate a true random number generator using cosmic-ray muon arrival times as a source of entropy.
• To provide a comprehensive educational resource for building, operating, and analyzing data from the detector in diverse experimental settings.

Proposed method

• Utilizes silicon photomultipliers (SiPMs) and plastic scintillators to detect ionizing radiation from cosmic-ray muons and background radioactivity.
• Employs a microcontroller (Arduino) to record trigger times and timestamps, enabling time-resolved data acquisition and coincidence detection.
• Applies the Heitler model to describe electromagnetic showers in the atmosphere and estimate muon flux at different altitudes.
• Uses a toggle flip-flop circuit synchronized to a 1 kHz clock to convert random trigger timestamps into binary digits (0s and 1s) for random number generation.
• Applies the Poisson distribution to model the statistical behavior of muon and radioactive decay events over time intervals.
• Analyzes data in master and coincidence modes to measure angular distributions, rate variations, and background suppression.

Experimental results

Research questions

• RQ1 How does the cosmic-ray muon rate vary with altitude, as measured during a commercial flight at 33,000 ft and a high-altitude balloon flight at 107,000 ft?
• RQ2 To what extent does the geomagnetic field influence the directional and rate distribution of cosmic-ray muons at different latitudes?
• RQ3 Can the random arrival times of cosmic-ray muons be used to generate a statistically valid true random number sequence?
• RQ4 How well does the observed distribution of muon counts per time bin match the theoretical Poisson distribution?
• RQ5 What are the effects of atmospheric pressure on the measured muon rate, and can this correlation be quantitatively extracted from flight data?

Key findings

• The cosmic-ray muon rate decreased by approximately a factor of 2.5 between sea level and 33,000 ft altitude, consistent with atmospheric attenuation models.
• At 107,000 ft, the muon rate was measured to be about 10 times higher than at sea level, confirming the strong dependence of muon flux on atmospheric overburden.
• The distribution of muon counts per time bin closely followed the Poisson distribution, with a reduced χ² p-value indicating good agreement with statistical expectations.
• The random number generator based on cosmic-ray muon timestamps produced a uniform distribution across 0–255 values, with a p-value from χ² test supporting the null hypothesis of uniformity.
• The muon rate showed a measurable negative correlation with atmospheric pressure, with a correlation coefficient of approximately -0.35 observed in flight data.
• Coincidence measurements between two detectors confirmed the directional and temporal correlation of muon events, with increasing solid angle leading to higher coincidence rates.