Cosmic-ray Muon Tomography and Its Application to the Detection of High-z Materials

Each minute, about 10000 muons rain down on every square meter of Earth. These charged elementary particles are produced by cosmic rays striking the upper atmosphere. Millions of highly penetrative muons pass through our bodies, cars and houses daily. Penetrating the objects, muons interact with atoms of different materials, mainly electromagnetically. They are more strongly deflected, or scattered, by high-Z materials, including nuclear materials, like uranium and plutonium, and gamma-ray shielding materials, like lead, tungsten or gold. This allowed us to develop a technique which uses multiple scattering of cosmic-ray muons to detect shielded packages of nuclear materials in a background of normal cargo. The advantages of this technique are that it is passive, does not deliver any radiation dose above background, selective to high-z dense materials, and is suitable for large amount of shielding. Physical basis of the technique, current status of our research and its possible perspectives will be discussed. INTRODUCTION The detonation of a nuclear device in a major city is widely recognized as the most frightening threat posed by the modern international terrorism. Reduction of this threat requires enhanced detection capability of nuclear materials at transportation checkpoints, in particular, border crossings. US customs has begun using a set of radiation detectors and x-ray scanners for this purpose. Unfortunately, it has been demonstrated that existing methods are not always effective in detecting compact, well-shielded packages of strategic nuclear materials (SNM). Both SNM and most effective gamma-ray shielding materials have high densities and atomic numbers (Z). A new technique capable of passively detecting such high-Z materials have been recently developed at Los Alamos [1, 2]. The technique is based on the detection of the increased scattering of cosmic-ray muons by the high-Z objects. COSMIC RAYS Primary cosmic rays are energetic particles and nuclei, mostly protons, incident at the top of the terrestrial atmosphere. Cosmic rays interact with atmosphere producing showers of secondary particles. By the time the shower of “atmospheric” cosmic rays reaches the Earth’s surface, it is comprised mostly of neutrinos and muons. Muons are the most numerous charged particles at sea level. The mean energy of the muons at the ground is about 4 GeV. The integral