Only a few percent of the cargo containers and trucks that pass through national borders are thoroughly inspected for potential weapons of mass destruction. X-ray transmission imaging systems can identify lead shielding, false walls, and other smuggling methods, but these systems are unable to directly identify biochemical or nuclear agents. For that purpose, experts suggest additional sensors such as gamma ray detectors to catch smuggled nuclear material.
A new muon imaging system developed at Los Alamos National Laboratory (Los Alamos, NM) could allow border guards to both image structures inside containers and positively identify hidden high-atomic-number materials such as uranium and plutonium while alleviating many of the cost and health concerns that accompany high-power x-ray imaging systems. The group reported on the work at the SPIE Annual Meeting (San Diego, CA; 3-8 August; paper #5199A-39).
Muons are created when charged protons strike the Earth's atmosphere, creating pions. Pions quickly decay into muons that remain relatively stable through the atmosphere before striking the planet's surface. High-energy x-ray imaging systems for cargo inspection generate 9 MeV x-ray radiation capable of penetrating up to 12 inches of steel. However, naturally occurring, high-momentum muons with giga-electron-volt energies can easily pass through several feet of dense material. Muon absorption was used to find hidden chambers in the second pyramid at Giza, for example.
Figure 1. In an experimental run of 100,000 muons, the system imaged a 5.7 cm long, 5.5-cm radius tungsten cylinder on a 35 cm x 60 cm x 1 cm Lexan plate with two steel rails. The resultant 1 cm x 1 cm x 1 cm volumetric image clearly shows the tungsten in these horizontal slices of the image taken near the middle of the volume, moving from top to bottom. (LANL)
A group at Los Alamos led by Christopher Morris and William Priedhorsky has harnessed this naturally occurring radiation to reveal high-atomic-number materials by detecting the path of individual muons and analyzing the multiple Coulomb scattering caused by dense materials (see figure).
The group's first demonstration system uses four horizontal 60 x 60 cm2 muon drift detectors placed 27 cm apart: two above the object under test and two below. The first pair of detectors determines the initial path of individual particles as they pass through two detectors. The muons penetrate the object under test and pass through a second pair of drift detectors that measure the outgoing path of each particle after its interaction with the test object. A scintillator plate placed below the imaging system acts as a counter/trigger for the system, recording approximately 850 counts/min.
Analog signals from the detectors are amplified and separated by standard nuclear instrumentation module electronics, digitized by a fast encoding and readout analog/digital converter and fed to a standard PC using an acquisition software for analysis. The Los Alamos group has developed its own reconstruction algorithms based on the single-scattering approximation of multiple scattering interactions, resulting in successful detection of high-atomic-number materials in 3-D experimental image simulations.
"Several aspects of this work are very appealing," noted Patrick Doty of Sandia National Labs Engineered Materials Department and chairman of the program on hard x-rays at SPIE's 48th annual meeting. Using background radiation for security screening combines benefits of both active and passive techniques. As with active methods, materials that emit no radiation signature are detected, but like passive screening, the subject of interrogation receives no increased radiation dose. This "no-dose" radiography is good for public acceptance, as well as foiling smugglers' attempts to shield from or even detect the interrogation.
"It also enables new long-term monitoring scenarios in which humans can freely roam the scene without adverse health effects," Doty continues. For example, one can envision such systems installed in cargo holds, operating while freight is in transit. Integrating an image for the entire duration of a trip dramatically improves statistics relative to portal monitoring, reducing the minimum detectable mass, again with no penalty due to radiation dose.
Simulations on standard cargo containers have been done and corroborated by small-scale experiments. The next step will be to build a full-sized system of detectors capable of imaging a car, says Konstantin Borozdin of the Los Alamos team. In addition, the group continues to work on reconstruction algorithms and visualization to allow automatic detection of small quantities of nuclear materials.