Image reconstruction and material Z discrimination via cosmic ray muon radiography

Highly penetrating cosmic ray muons shower the Earth at the rate of 10,000 m-2 min-1 at sea level. In our previous work (Nature 422 (2003) 277; Rev. Sci. Instr. 74(10) (2003) 4294; Cosmic Ray Muon Radiography for Contraband Detection, in: Proceedings of AccApp'03, San Diego, CA, June 2003), we presented a novel muon radiography technique which exploits the multiple Coulomb scattering of these particles for nondestructive inspection without the use of artificial radiation. In this paper, we describe the concept of and theory behind cosmic ray muon radiography. We discuss the information carried by the scattered muons and our approaches for exploiting that information with image reconstruction algorithms. We discuss preliminary and advanced reconstruction algorithms, which take advantage of the scattering angle, scattering location, and locations where strongly scattered muons cross paths. Our algorithms are validated with both experimental demonstrations and Monte Carlo simulations. Based upon the results from both the experiment and simulations, we conclude that scattering muon radiography can be useful for both material discrimination and fast (minute order) detection of compact high-Z objects. Our ray-crossing algorithm, which highlights locations where strongly scattered muons cross paths, is effective even in the presence of a medium-Z background matrix