Since 9/11, radiation detection has taken on a new immediacy as a means of preventing a nuclear weapon attack within the US. Gamma-ray and neutron detectors are being deployed at border crossings and ports, with the goal of enabling interdiction of a nuclear weapon or material before it enters the country.
A neutron scatter camera being developed at Sandia/California offers a new way to detect radiation. The instrument is able to count neutrons from a source of special nuclear material (SNM) and localize it — meaning it doesn’t just tell you there is radiation, but where it is emanating from and, under some circumstances, how much there is.
The results so far are encouraging, says principal investigator Nick Mascarenhas (8132). The neutron scatter camera potentially could detect SNM in quantities of interest in national and homeland security and from distances competitive with or beyond other capabilities. Nick says the goal is to reach much greater standoff detection ability.
Distance is a significant benchmark because it also means the neutron scatter camera can detect through heavy shielding, a concern at any border crossing or point of entry.
“This instrument can pinpoint a hot spot in another room through walls, something not typically possible with gamma-ray detectors,” he says. “It’s beating the older technologies, performance-wise, but we want to push the limits of what this instrument can do — to increase sensitivity and detection distance.”
Jim Lund, manager of Rad/Nuc Detection Systems Dept. 8132, thinks the neutron scatter camera might be the best answer to the problem of seeking out smuggled SNM.
“It’s more penetrating and can detect unambiguously at a greater distance and through more shielding,” he says.
The project is supported by NNSA’s Office of Nonproliferation R&D (NA-22). After successful initial development, the technology is being transitioned to both the Defense Threat Reduction Agency (DTRA) and Domestic Nuclear Detection Office (DNDO) to support specific applications.
DNDO impressed by device
Recently, representatives from DNDO sat in on a presentation by Nick to NNSA. They were so impressed that they asked him how quickly he could have it ready to ship to Hawaii as part of George Lasche’s (6418) in-transit radiation characterization project (Lab News, Aug. 17, 2007).
The neutron scatter camera will make three round-trips to Hawaii — the first departed from the Port of Oakland in early September. George says the camera has the potential to reduce false alarm rates — a critical issue for in-transit radiation detection.
“Our other instruments have told us a lot about the nature of nuclear radiation at sea, but not where it is coming from. The neutron scatter camera can tell us where the radiation is coming from and the size of the object. This information is very helpful in deciding if we have a serious threat on our hands and can lead to fewer false alarms and a better chance of not missing the real thing,” he says.
DTRA is funding a separate project to use the neutron scatter camera to measure and characterize background neutrons at Sandia/California, Sandia/New Mexico, and in Alameda, Calif.
“There are neutrons all over the place from cosmic radiation, even when you are sitting indoors,” says Nick. “Our instrument can measure the energies, rates, and angular variation. This is important in understanding standard operating conditions. You can’t really detect anomalies until you understand what’s normal. This data can also be used to improve instruments to better suppress the standard operating conditions.”
The neutron scatter camera has an advantage over traditional neutron detection because it can differentiate low-energy neutrons from high-energy neutrons.
Device only sees high-energy neutrons
“It doesn’t have to worry about the low-energy nuisance neutrons that are always all around us because it can only see high-energy neutrons, and the high-energy neutrons carry almost all of the imaging information,” says George.
Another advantage is shielding. While gamma rays can be blocked from detectors quite easily, 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.
Jim notes that the neutron scatter camera is limited in terms of size and time, compared with gamma-ray detectors.
“Ideally, we’d use both systems,” he says. “The neutron scatter camera isn’t practical as a handheld detector with immediate feedback.”
Nick and his team — Kevin Krenz, Peter Marleau, Stan Mrowka (all 8132), and Jim Brennan (8321) — took a slow, careful approach to developing the neutron scatter camera, which has paid off. They started with just two elements and worked to understand everything about how the instrument worked on a simpler scale before moving on. The result is a scalable instrument.
The camera consists of elements containing proton-rich liquid
scintillators in two planes. As neutrons travel through the scintillator, they bounce off protons like billiard balls. This is where “scatter” comes into play — with interactions in each plane of detector elements, the instrument can determine the direction of the radioactive source from which the neutron came.
The neutron eventually flies off, but not before energizing the protons with which it has interacted. The proton will lose its energy in the scintillator. As that energy is lost, it is converted into light. Photomultiplier tubes coupled to the scintillator detect the light.
Computers record data from the neutron scatter camera, and using kinematics, determine the energy of the incoming neutron and its direction. Pulse shape discrimination is employed to distinguish between neutrons and gamma rays.
The biggest obstacle to the camera becoming widely adopted is the liquid scintillator, which is flammable, hazardous, and requires special handling. According to Nick, materials exist that could be used as a solid scintillator, but they need to be mass-produced and made readily available in the US for this purpose. Solid scintillator material, he says, is not in the scope of the current project but is a logical next step.
The current version of the neutron scatter camera has four elements on one side and seven on the other. To improve sensitivity and direction, all that is required is to add more elements.
Nick describes scaling up as an engineering challenge rather than a scientific limit. Bigger means more places where things can break down, but this isn’t a physics issue, he says.