Each minute, about 10,000 muons rain down on every square meter of Earth. These charged subatomic particles are produced when cosmic rays strike air molecules in the upper atmosphere. The cosmic rays themselves are mostly energetic protons produced by the sun, our galaxy, and probably supernova explosions throughout the universe. Thousands of muons pass through us every minute, but they deposit little energy in our bodies and thus make up only a few percent of our natural radiation exposure. [figure: cosmic-ray muons]

A team of Los Alamos scientists—Konstantin Borozdin, Gary Hogan, Chris Morris, Bill Priedhorsky, Andy Saunders, Larry Schultz, Margaret Teasdale, John Gomez, and Val Armijo—has found a promising way to use this natural source of radiation to detect terrorist attempts to smuggle uranium or plutonium into the country. Either nuclear material could be used to make an atomic bomb (see sidebar). The technique also detects lead and tungsten, which could be used to shield the gamma rays emitted by nuclear materials—or other radioactive materials—in order to elude detection.

The new technique uses the fact that muons are more strongly deflected, or scattered, by nuclear or gamma-ray-shielding materials than they are by materials such as plastic, glass, and aluminum. This enhanced deflection occurs mainly because the atomic nuclei of nuclear and gamma-ray-shielding materials contain large numbers of protons, which exert large electrostatic forces on muons passing nearby. Since the number of protons is given by the atomic number Z, such materials are called "high-Z" materials.

The deflection is also determined by how many nuclei a muon encounters while passing through the material, which is proportional to the number of nuclei per unit volume—the number density. The number density equals the material's density divided by the mass of its nuclei. The materials that most strongly deflect muons have high atomic numbers and high number densities. Several low- and high-Z materials along with their deflections of cosmic-ray muons are listed in the table (shown below).

In muon detection, particle detectors above and below a vehicle or container record each muon's path before and after the muon passes through the cargo. A change in a muon's trajectory means the muon has been scattered by the cargo. Using the path information and muon scattering theory, a computer program then constructs a three-dimensional image of the cargo's dense, high-Z objects. [figure: muon deflections]

Los Alamos simulations, validated with small-scale experiments, show that cosmic-ray muons can penetrate the 3-millimeter-thick steel walls of a freight truck to detect a block of nuclear or gamma-ray-shielding material 10 centimeters (4 inches) on a side hidden among other cargo, such as livestock or auto parts. The muon scan takes about a minute. People who stay in a vehicle during a scan will receive no more radiation than if they had stayed home in bed. Thus, muon radiography poses no health hazard. [figure: small-scale experiment]

The Los Alamos team can discriminate between different materials even more precisely by measuring muon energies as well as deflections. In computer simulations, this improved technique easily distinguishes between tungsten and steel, for example. Experiments to confirm this discrimination capability are planned for the near future.

Next Step, Border Inspection?
Border inspectors now use gamma-ray radiography to detect nuclear materials. Because of their high atomic numbers and number densities, nuclear materials strongly absorb gamma rays as well as strongly deflect muons. Thus, nuclear materials are fairly opaque to gamma rays and cast dark shadows in radiographs of vehicles and freight containers.

A gamma-ray scanner uses a radioactive pellet a few millimeters in diameter to produce gamma rays that are energetic and intense enough to scan a large vehicle or container in as little as a few minutes. Although a gamma-ray scan would expose a vehicle's occupants to a negligible dose of radiation—less than a hundredth that of a dental x-ray—occupants are usually removed before scanning, or only the volume of the trailer rig is scanned.

Muon scans will have several advantages over gamma-ray scans. First, the gamma-ray scanner's radioactive pellet must be properly handled and its emissions properly controlled. The pellet must also be replaced after a time about equal to its half-life—the time required for the pellet's radioactivity to decrease to one-half its initial value. Cobalt-60, the most penetrating and hence preferred gamma-ray source, has a half-life of five years. By contrast, cosmic-ray muons do not require radioactive sources that must be replaced. The muons are already there, continuously, wherever the inspection site.

Second, because gamma-ray radiography produces two-dimensional images of the cargo, it can be hard to find a block of nuclear material surrounded by, say, a load of steel auto parts. The superimposed gamma-ray shadows of many objects can prove confusing and create a problem called "clutter." Muon radiography's three-dimensional views overcome this problem.

Third, cosmic-ray muons are also far more penetrating than the gamma rays emitted by even cobalt-60. With an energy of 1 million electronvolts, the gamma rays penetrate about 1 centimeter of lead. With an average energy of 3 billion electronvolts (at sea level), muons penetrate nearly 2 meters of lead. Greater penetration depth means the muons can detect nuclear materials surrounded by greater amounts of high-Z shielding material or clutter.

Nuclear Threats
Two materials that can be used to make an atomic bomb are plutonium-239 and highly enriched uranium, which contains at least 20 percent of uranium-235. Since both materials have high atomic numbers and number densities, both can be detected by muon or gamma-ray radiography.

Although plutonium can be detected with either neutron detectors or gamma-ray radiography—both techniques are now used for border inspection—uranium-235 presents greater detection problems. It has no significant neutron emission, and its natural gamma-ray emissions can be shielded—usually with a layer of high-Z material such as lead or tungsten. In addition, there is much more highly enriched uranium in the world than there is plutonium-239; thus uranium-235 is more available to terrorists (see sidebar).

Although both muon and gamma-ray radiography can detect highly enriched uranium and its gamma-ray-shielding materials, muon radiography's greater penetration depth and more precise materials discrimination promise enhanced detection capabilities. For this reason, work is now underway to utilize the ubiquitous and benign cosmic-ray muons for detecting nuclear contraband.