New Radiation Detector Technology Will Help Secure U.S. Cities
(Page 2 of 2)
Locating the neutron source
Meet Peter Vanier
When Peter Vanier was growing up in St. Kitts in the West Indies, his father was both a studious lawyer and an industrious beekeeper who taught his three sons how to use a variety of woodworking and metalworking tools in their basement workshop. More...
Peter Vanier’s team of researchers from the Nonproliferation and National Security Department wants to know the origin of the neutrons they encounter, rather than their precise spectroscopy, since they occur infrequently in the natural environment. Cosmic rays generate a few neutrons that travel in random directions.
“We’ve developed systems that tell the direction they are coming from,” Vanier said. “If you detect 20 neutrons coming from the same point, you have to be suspicious that it’s a man-made source.”
The team has been exploring two main detection methods: a helium-3 camera for thermal or “slow” neutrons, and a “fast” neutron double scatter camera. In the first method, a helium-3 pressure chamber containing arrays of wires functions as a film that detects a pattern of arriving neutrons. An enclosure lined with cadmium acts like a pinhole camera, comparable to what you might use to image the sun during a solar eclipse. Neutrons that have slowed down until they are in thermal equilibrium pass through a specially designed pattern of apertures in the cadmium, creating a shadow of the pattern. The overlapping shadows can be mathematically converted into a picture that can reveal the source using the chosen properties of the coded apertures. Brookhaven scientists built the first thermal neutron coded aperture camera and have extensively tested it in collaboration with other DOE laboratories.
“We hope to construct a much bigger detector that can build up the picture more rapidly,” Vanier said. “A bigger detector is more likely to find a source in a given time frame and at a given distance. The more quickly it’s traveling, the harder it is to find.”
To locate the origin of “fast” neutrons, Vanier and his team use an entirely different method — a “double scatter” camera that works like an analysis of billiard shots. When a neutron hits a proton in a plastic scintillator, it makes a particle track creating light that can be detected. Two planes of detectors give two sets of timing information for these scattering events. By comparing the energies of the scattered neutrons, the angle of the incoming neutron is calculated and projected in the form of cones. The direction where the cones intersect is the most likely source of the neutrons.
“We can’t get an exact location from one neutron, but we can if there are many,” Vanier said. “The double scatter camera gives two types of information: that there is a point source, and where that point source is. If they are all coming from one direction, you know you need to investigate.”
