Nuke Stoppers: "Hidden" Detectors?
One of the biggest homeland security nightmares is a nuke, smuggled aboard a shipping container. Today, port authorities "scan containers for illicit radioactive materials ashore," New Scientist notes. But "to avoid delaying shipments... detectors generally have no longer than 1 minute to do their work, which is not always long enough."
One possible solution, from MIT's Richard Lanza: hide radiation detectors "inside ordinary shipping containers and sent [them] around the world with other cargo. These covert detectors would spot high-energy gamma rays given off by plutonium or HEU, which cannot easily be shielded."
Lanza proposes using detectors consisting of inorganic crystal scintillators that emit photons when hit by gamma rays. Each emitted photon has a different energy level depending on the isotope the gamma rays come from, allowing the isotope to be identified.
Lanza has made a detector with an array of scintillators behind a mask pierced with holes. Gamma rays passing through a hole would excite one of the scintillators, causing it to emit a photon. He has shown that this can be used to generate an image of a radiation source, allowing the source to be located.
"The technology certainly has merit," one radiation detection specialist, working for the government, tells Defense Tech. And "the Coast Guard, [along] with Customs and Border Patrol, has been considering the use of 'sticky pagers': small boxes that would clamp on a container out of, say, Antwerp, and would take a continuous 1-week reading of the contents of the container as it's shipped across the ocean."
Obviously, you'd be able to get a very good reading of the half of the container nearest the detector, but the minimum detectable activity might be pretty bad near the far side.
I don't know of any specific "sticky pager" development programs going on within DHS [Department of Homeland Security] (including the Coast Guard) right now, but just because I don't know about it doesn't mean it isn't happening. There is interest, though -- there were a few presentations on this type of thing (mostly out of LANL [Los Alamos]) at the winter meeting of the American Nuclear Society.
Our expert does have a small, geeky quibble with the New Scientist story, however. The article keeps talking about "U-232" and how its radiation would "penetrate 22 metres of cargo on average." First of all, U-232 isn't really used in nuclear weapons -- that'd be another isotope, U-235. And U-232's penetration? More like 22 centimeters. Plus, New Scientist: note the spelling of "meters," ok? That's an American-built Internet you're publishing to. We expect things to be spelled our way.
Sigh. The strong signature of U-232 comes from one of its eventual daughters, Thallium 208. Unfortunately, Thallium 208 is also a daughter of naturally occuring (and very common in welding rods, lantern mantles, and now valve seats in car engines) Thorium 232.
So, if you see the 2614 gamma, you can't be sure if you're looking at U-232 (unlikely) or Tl-208 (likely). There is other signal processing that has to occur to distinguish the two, and that relies on much lower energy gammas.
Posted by: enders at January 12, 2007 8:13 AM