A Detector for Muon Tomography

This report describes the conceptual design of a particle detector that could be deployed underground for tracking cosmic-ray muons to enable tomographic reconstruction of the overlying ground and structures. The concept of cosmic-ray muon tomography has been known for a long time but seldom used in practice since the first demonstration. Developments in technology and methods of particle detection during the intervening years make it appropriate to reconsider underground muon tomography as a practical tool for surveying very large structures. In the 1960’s, L.W. Alvarez and co-workers [1] invented muon tomography to study the internal structure of the Second Pyramid of Chephren. They used naturally occurring cosmic ray muons to “x-ray” the pyramid. Muon tracks were recorded in a particle detector system housed in a cavity— the Belzoni Chamber—within the pyramid’s base. The object of the study was to find undiscovered chambers that might have been built into the pyramid. The Alvarez group established the feasibility of muon tomography by observing the meter-scale limestone caps on the outside of the pyramid. Their principal scientific discovery was definitive, but negative: there are no additional chambers inside Chephren’s Second Pyramid. The basic idea behind muon tomography is to measure the rates of cosmic-ray muons detected underground at diverse angles and locations. Much like an x-ray image, differences in the numbers of muons detected