Neutron cameras “can detect, unambiguously, at a greater distance, and through more shielding,” said Jim Lund, manager of the Rad/Nuc Detection Systems group at Sandia National Laboratories in Alameda, Calif.
The neutron scatter camera has an advantage over traditional neutron detection because it can differentiate low energy neutrons from high energy neutrons.
While some gamma rays can be blocked from detectors, neutrons are much more difficult to conceal. In a lab test, the camera easily detected and imaged a source placed across the hallway, through several walls and cabinets. The camera has the potential to reduce false alarm rates — a critical issue for in-transit radiation detection.
If brought within a few feet of nuclear material, current gamma-ray detectors can see through shielding, but there are too many ships and containers to scan them all up close.
A neutron scatter camera works by arraying two orthogonal detectors so that any incident neutrons can be traced to a specific trajectory in three-dimensional space–thus identifying the direction from which they came, as well as their energy level. Low-energy background neutrons, from cosmic rays and elsewhere, are ignored, while the high-energy neutrons typical of radioactive materials are imaged, albeit not in real time. Once an image is processed–taking several minutes–it reveals all the nuclear hot-spots within its field of view and at a distance (the exact distance is classified).
The extreme sensitivity of a neutron-scatter camera is exacted at a price, however, since the liquid detectors used by the neutron camera are too bulky to be handheld, are inflammable, are hazardous to humans, and require special handling and disposal procedures. A neutron-scatter camera could, however, be mounted on the back of a truck for mobile duty at a seaport, or from the deck of a Coast Guard vessel that scans incoming cargo ships.
The detectors are housed in a proton-rich liquid-filled scintillator, which fluoresces when struck by neutrons. The protons serve as the bumpers off which the neutrons bounce, scattering about (thus, “neutron scatter”) like billiard balls. The impact nudges the protons to a higher energy level, but when they fall back to normal they shed a photon to get rid of the extra energy. Photomultiplier tubes are coupled to the scintillator to detect the visible light photons. Software analyzes the output from the photomultiplier and constructs a visual image that identifies the nuclear hot spots.
Next, the research group is going to calibrate its detector by locating several in the normal environment in which they will be used–one is already at sea, and several more will be located around New Mexico and California. The normal background neutrons scattered in these locations will enable the units to be calibrated to prevent future false alarms. After calibration, they will be tested with real concealed nuclear materials.
The researchers also claim that solid scintillator materials are possible to engineer, but the effort to do so will have to follow on successful completion of the current calibration and testing regimes.
Neutron-scatter camera and a gamma-ray detector need to be used together.
Current and future port security