It’s been a common trope in films since the 1950s; a madman with an atomic bomb holds a city for ransom while the authorities race to find it in time. If such a thing ever does come about, Sandia National Laboratories is working on taking the suspense out of the situation with its Mobile Imager of Neutrons for Emergency Responders (MINER) – a nuclear device detector capable of narrowing a search to within a city block without door-to-door sweeps.
MINER is a portable neutron scatter camera consisting of 16 proton-rich liquid scintillator cells installed inside a large, shielded cylinder standing 3 ft (91 cm) high and weighing 90 lb (40.8 kg), and has its own power source. This makes it smaller, lighter, capable of being assembled in 10 minutes, and easier to use than previous versions, as well as having the ability to detect fast neutrons at long distances and through shielding.
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In operation, the shielding keeps out extraneous radiation while allowing neutrons through. These interact with the liquid in the scintillator cell, where they are detected. By having a number of such sells set in a pattern, MINER can not only detect the neutrons, but also provides a bearing on the source.
What’s particularly clever about MINER is that it doesn't just detect neutrons; it can also discriminate between different kinds in the neutron spectrum. This allows the detector to separate the bomb or other target’s signature from the background neutrons and measure its intensity.
"Simple neutron counters are unable to distinguish a threat source from an elevated neutron background. However, an imager such as MINER can do this by observing a ‘hot spot’ against the neutron background," says Sandia physicist John Goldsmith. "In addition, MINER’s ability to measure the neutron spectrum enables it to distinguish plutonium, a threat source, from AmBe [americium-beryllium, the most common commercial source of neutrons], which is not a threat source. Among imaging approaches, this capability is unique to MINER."
MINER also has a certain degree of versatility. When its shield is opened, it can act as a gamma-ray detector, so when its near its target, it can gather further information to aid disposal teams.
MINER recently completed a field test in Chicago using a shielded container of laboratory radioactive material as a stand-in for a bomb with a similar radioactive signature. According to Sandia, MINER was able to accurately locate the source inside of 30 minutes. Had it been shielded, the time would have increased to a couple of hours.
The test included three other neutron imagers, but Sandia stresses that the mix was intended to spot the particular strengths of the detectors rather than as a competition.
"Other imaging detectors have a very fixed field of view, so they look at a specific spot," says Sandia physicist Mark Gerling. "MINER images all the way around and up and down, or a full 4π steradians. We imaged part of one side of an entire high-rise building at once and narrowed the search to a specific room. It’s extremely effective in this situation."
Sandia is currently working on improving MINER’s measurement abilities and will take part in a sea search.