Surveillance: Radiographic imaging with cosmic-ray muons
Natural background particles could be exploited to detect concealed nuclear materials.
Nature 422 , 277 (2003); doi:10.1038/422277a
Despite its enormous success, X-ray radiography 1has its limitations: an inability to penetrate dense objects, the need for multiple projections to resolve three-dimensional structure, and health risks from radiation. Here we show that natural background muons, which are generated by cosmic rays and are highly penetrating, can be used for radiographic imaging of medium-to-large, dense objects, without these limitations and with a reasonably short exposure time. This inexpensive and harmless technique may offer a useful alternative for detecting dense materials ? for example, a block of uranium concealed inside a truck full of sheep.
In X-ray radiography, the intensity of an image pixel is determined by the attenuation of the incident beam caused by absorption and scattering ? the maximum mean free path for photons is about 25 g cm -2 for all materials, corresponding to less than 2 cm of lead. For thicker objects, it is better to use a different type of radiography that is based on the interaction of charged particles with matter by multiple Coulomb scattering. The many small interactions add up to yield an angular deviation that roughly follows a gaussian distribution,
with the width, 0, related to the scattering material through its radiation length, L0, as follows: where pis the particle's momentum in MeV c -1 and cis its velocity 2. The radiation length decreases rapidly as the atomic number of a material increases, and 0increases accordingly: in a layer 10 cm thick, a 3-GeV muon will scatter with an angle of 2.3 milliradians in water, 11 milliradians in iron and 20 milliradians in lead. By tracking the scattering angles of individual particles, the scattering material can be mapped.
Our new technique relies on the scattering of atmospheric muons produced by primary cosmic rays. Muons are the most numerous cosmic-ray particles at sea level, moving at a rate of about 10,000 m -2 min -1 in horizontal detectors 3. These particles are highly penetrating: a typical cosmic-ray muon of energy 3 GeV will penetrate more than 1,000 g cm -2 (10 m of water, for example).
To demonstrate the concept of muon radiography, we developed a small-scale experimental system with four drift chamber detectors 4spaced 27 cm apart. Each detector has an active area of 60 60 cm 2and records particle tracks at two positions in each of two orthogonal coordinates. The upper pair of detectors records the tracks of incident muons, and the lower pair records the scattered tracks. A tungsten cylinder was used as a test object, supported by a plastic plate and steel support beams. The tungsten is clearly visible in the reconstructed image, and the steel support beams are also evident ( Fig. 1 , left).
We also developed a Monte Carlo simulation code that generated cosmic-ray muons and propagated them through a test volume. The reconstructed images are indistinguishable from those obtained experimentally ( Fig. 1 ), and the scatter angles of the simulated muons from the different materials (tungsten, lexan and steel) are consistent with the measured angles.
Simulation of larger, more complex objects demonstrates that we can reliably detect a 10 10 10 cm 3uranium object inside a large metal container full of sheep in 1 min of exposure. We conclude that cosmic-ray muons show promise as an inexpensive, harmless probe for radiography of medium-to-large objects, such as commercial trucks, passenger cars or sea containers. Our experimental results and simulations demonstrate the ability to reconstruct complex objects and to detect dense material of high atomic number hidden in a much larger volume of material of low atomic number, using only the natural flux of muons. This method is suitable for a range of practical applications in which radiography of dense objects with low radiation dose is required ? for example, in surveillance for cross-border transport of nuclear materials.
KONSTANTIN N. BOROZDIN 1, GARY E. HOGAN 1, CHRISTOPHER MORRIS 1, WILLIAM C. PRIEDHORSKY 1, ALEXANDER SAUNDERS 1, LARRY J. SCHULTZ 1& MARGARET E. TEASDALE 1
Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA |
