The watcher in the water

“TRUST, but verify.” That was Ronald Reagan’s mantra for nuclear agreements, though the proverb itself is Russian. But verifying that a country is not cheating on one important matter of nuclear diplomacy, the manufacture of plutonium, is hard. At the moment, it can be done only by visiting the site of any and every nuclear reactor which could be employed for the task—even then, one might be hidden away. But if a project now being undertaken in America works well, hiding reactors will become much more difficult.

The Water Cherenkov Monitor for Antineutrinos, or WATCHMAN, brainchild of the energy department and the National Nuclear Security Administration, should be able to spot a suspicious reactor up to 1,000km away. A network of such devices, set up within range of someone who might not be playing by the rules, should indeed verify whether he can be trusted.

The WATCHMAN is a neutrino detector—or, to be precise, an antineutrino detector. Neutrinos and their antimatter equivalents are particles that have little mass and no electric charge. They are produced in huge quantities by stars such as the sun, by the explosion of supernovae and by nuclear reactors on Earth, but they interact with other forms of matter so weakly that a piece of lead a light-year thick (around 9 trillion kilometres) would block only half of those passing through.

It is this penetrative power which may make these particles useful for diplomats. No amount of shielding can stop them escaping from a reactor. If it were possible to tell both where the particles were coming from, and whether that source was natural or artificial, then it would be impossible to hide a reactor. The WATCHMAN’s designers think they can do that. Though most of the neutrinos and antineutrinos which pass through the 3,500 tonnes of gadolinium-doped water a WATCHMAN will contain will remain unnoticed, a minuscule minority will interact with those contents. These interactions should be enough to give the game away.

Flashes of inspiration

Determining where a neutrino or antineutrino has come from is reasonably easy. Several neutrino “telescopes” around the world can manage the trick, and are thus able to tell, for example, which neutrinos come from the sun. They do it by watching for flashes of light, called Cherenkov radiation, which are made when something electrically charged is moving faster than the speed of light in the medium in which it is travelling. (Though nothing travels faster than light in a vacuum, light’s velocity falls when it passes through matter.) In the case of neutrinos or antineutrinos, the light-generating particle is an electron knocked at speed out of a water molecule by a neutrino’s impact. The direction of the Cherenkov light, which is measured by sensitive photodetectors in the wall of the vessel that contains the water, shows the direction of the original particle’s source.

This so-called “recoil” method cannot, however, distinguish between neutrinos and antineutrinos. That matters, because the nuclear reaction which turns the non-fissile form of uranium into fissile plutonium, from which bombs can be built, makes antineutrinos. By contrast, the fusion that powers the sun and other stars makes neutrinos. To tell a reactor from sources like those, you must be able to tell between neutrinos and antineutrinos. And this is what the gadolinium is for.

Antineutrinos sometimes do something that neutrinos do not: they merge with a proton to create a neutron and a positron (the antimatter version of an electron). The positron then shoots off so fast that it creates a flash of Cherenkov radiation. The neutron, meanwhile, blunders around until it runs into a gadolinium atom. Gadolinium nuclei have an appetite for neutrons, and the process of absorption generates a second flash of light. The two flashes in quick succession show that the apparatus has captured an antineutrino.

You now have both parts of the puzzle. Double flashes show the size of the antineutrino flux. Recoil flashes show where that flux is coming from. At least, that is the theory. The practice will be tested shortly. A prototype WATCHMAN is under construction in an old salt mine (to shield it from cosmic rays and other sources of interference) in Painesville, Ohio. This is 13km from a nuclear power station at North Perry, on Lake Erie. Though the Perry reactor is built for electricity generation rather than plutonium production, all reactors create some plutonium as a by-product, so its proximity will be a good test for the WATCHMAN system.

If that system works, and the decision is taken to deploy it, then there will still be the question of where and how. The predicted 1,000km range means quite a bit of diplomatic arm-wrestling may be involved, for the detectors would be of little use if built on American soil. But if, say, a country like Turkey could be persuaded to house one, the nuclear activities of a neighbour such as Iran might thus be monitored without inspectors having to set foot on the soil of the country in question. If that can be done, the WATCHMAN may help make the world a safer place.