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Sunken tanks could detect secret nukes
Scientists have come up with an unusual way of detecting secret atomic facilities - using massive tanks sitting at the bottom of the ocean.

The plan would allow officials such as the United Nations International Atomic Energy Agency (IAEA) to know if 'rogue' countries are operating clandestine nuclear reactors.

Dr Thierry Lasserre of the French Alternative Energies and Atomic Energy Commission and colleagues outline their idea in a paper submitted to the journal Physical Review C, and appearing on the pre-press website arXiv.org.

They say the key to finding nuclear reactors involves detecting antineutrinos generated by nuclear fission.

A gigawatt-sized reactor produces some sextillion (10 to the power of 21) antineutrinos every second, enough to make these reactors light up like beacons. But, antineutrinos interact weakly with ordinary matter, so 'seeing' them is difficult.

The standard technique for detecting antineutrinos involves filling a giant swimming pool with water, waiting for one to collide into a proton - generating a positron and a neutron - and measuring the Cherenkov radiation produced by the collisions using light detectors around the edge of the pool.

Since 2003, the IAEA has been exploring new technologies, including neutrino detectors, for monitoring nuclear reactors.

A detector can be placed a few metres from a reactor to determine if it contains fissile material and how much energy is being extracted from it.

Neutrino detectors could also be used to gain data from a reactor core at longer distances, hundreds of kilometres away. But because neutrinos can travel through the Earth, scientists need to determine where it originated.
Sunken detector

Lasserre and colleagues suggest converting a supertanker into an antineutrino detector by fitting it with photon detectors and a 97-metre long, 23-metre diameter cylinder filled with 138,000 tonnes of linearalkylbenzene (C13H30), which they call SNIF - Secret Neutrino Interactions Finder.

The plan is to sail the supertanker to the coast of a suspicious state and then drop the detector in up to 4 kilometres of water. It would remain on the sea floor for at least six months detecting antineutrinos.

The researcher believe they could calculate the distance a neutrino has travelled by measuring its oscillations.

Lasserre's team have even calculated what kind of background signal they're likely to see and what a suspicious signal would look like depending on its location.

Associate Professor Martin Sevior from the University of Melbourne's School of Physics is also studying methods for detecting secret nuclear reactors.

Sevior describes the supertanker idea as 'pie in the sky'.

"I think it's much more likely other, far cheaper, methods will be used," he says.

"[A better] way would be to look for the characteristic gamma ray spectrum, gamma rays with very specific energies characteristic of the fission products of a nuclear reactor."

Sevior says one example is gamma rays emitted by the noble gas xenon.

"There are no terrestrial sources for it, so if you find some that would be suspicious. And this would be radioactive xenon with a very specific gamma ray signature."

According to Sevior this would be far cheaper than converting supertankers.

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