Muon tomography: looking inside dangerous places.

Abstract

Muons In 1936, during a study of cosmic radiation at Caltech, Carl D. Anderson and Seth Neddermeyer detected negatively charged particles whose trajectories in a magnetic field curved less acutely than those of electrons but more acutely than for protons (1). On the assumption that its charge was the same as that on an electron, it was concluded that this new particle was heavier than an electron but lighter than a proton. new particle was initially named as a mesotron, based on the Greek word meso-, meaning mid-. Re-dubbed as a muon, the particle's existence was confirmed in 1937 by J.C. Street and E.C. Stevenson in a cloud chamber experiment (2). Muons are formed when high energy protons from cosmic rays strike the nuclei of light elements in the Earth's upper atmosphere. process initially produces pions (and other short-lived particles, e.g. kaons), which decay over a distance of a few meters into muons, with accompanying muon neutrinos. muon tends to continue in the motional direction of the proton that created it, and travels at a near-light velocity. muon decays on a microsecond timescale, and in the absence of relativistic effects would only travel a (half-survival) distance of 456 m (2). However, according to the theory of special relativity, the effect of time dilation enables the muons to live long enough to reach the Earth's surface. effect may be viewed alternatively in terms of length contraction which, in the inertial frame of the muon, means that the critical distance that the particle must travel is shortened. Either way, it is the relativistic velocity of the muons which preserves them such that they can not only reach the Earth's surface, but penetrate hundreds of metres into the ground. It is this highly penetrating property of muons, due to their very high momentum (typically 3-4 GeV/c), that allows them to be used for imaging much thicker samples than can be accessed using X-rays. muon flux at the Earth's surface is of the order of 10,000 particles per square metre, per minute, meaning that every second one muon passes through an area about the size of a human hand (3). Muon transmission imaging (muon radiography) Cosmic ray muons have been used for imaging purposes since the 1950s, the pioneer being E.R George who employed them to determine the depth of the ice burden above the Guthega-Munyang tunnel in Australia (4,5). In the next decade, Luis Alvarez, who won the Nobel Prize for Physics in 1968, for his work on the hydrogen bubble chamber, and also postulated correctly that the dinosaurs had become extinct due to an asteroid impact (not a massive volcanic eruption), used muon transmission imaging to look for hidden chambers in the Pyramid of Chephren in Giza (6). It is sometimes said that he found nothing but, more accurately, he was able to demonstrate that no such chambers were present, which is a definite result and of considerable significance. Indeed, the technique can be used to screen pyramids, in order that only those with apparently interesting internal features are chosen for more detailed physical exploration by archaeologists, the others being left alone (7). A more recent investigation is of The Pyramid of the at Teotihuacan--The City of the Gods--near modern day Mexico City, discovered by the Aztecs in the fourteenth century (and many centuries after it was constructed). By volume, this is the third largest pyramid known on earth, and is 74 m in height, set on a square base with 225 m sides. Its exterior is covered with 3 million tonnes of volcanic rock (8), while the interior is a mound of earth. Muon detectors were placed under the centre of the pyramid, inside a tunnel that runs under its base. While the muons are deflected when they hit more dense materials, if a cavity is present, more of them will pass through to the other side of the pyramid. Thus a relative two-dimensional density map is created, from which it was concluded that, in contrast to the smaller and neighbouring Pyramid of the Moon, the Sun Pyramid contains no hidden chambers. …