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Antineutrino Device Tackles Nuclear Proliferation
KentuckyFC writes "One of the biggest problems in nuclear proliferation is verifying that countries are not secretly transferring fissile material by taking it out of reactors and selling it. Now a group of US scientists say they've developed a machine that can remotely detect whether a reactor has been switched on and off by detecting the antineutrinos produced by nuclear reactions. The detector is about the size of a car engine and is designed to be left near a reactor to record data. The group has been testing a prototype at the San Onofre Nuclear Generating Station in Southern California and says it works well (abstract). Now it's up to the International Atomic Energy Authority in Vienna to decide whether to deploy the new machine."
They are extremely difficult to detect, but not impossible. Moreover, nuclear reactors produce quite a lot of them.
As a recent example, the KamLAND neutrino experiment (http://kamland.lbl.gov/) used a 1000 ton detector in Japan to study the flux of neutrinos emitted from dozens of reactors in Japan and Korea, some hundreds of miles away. KamLAND performed precision studies of the propagation of neutrinos over distance, and was also able to detect the rising and falling neutrino fluxes as various reactors powered up and down.
The detection device described in the article is much smaller, but it's located much closer to the reactor. I've heard talks on this, and it seems quite reasonable.
However the proposal is to place the device rather near to a nuclear reactor. A reactor generates enough flux that big detectors can measure the neutrino flux from miles away. So a small (and probably less shielded) detector that is much closer (<100 m) will receive enough flux to get statistically significant data.
The preprint says: For example, cubic meter scale hydrogenous scintillator detectors, containing 10^28 target protons Np, will register thousands of interactions per day at standoff distances of 10-50 meters from typical commercial nuclear reactors. The proposal thus requires the detector to be installed on-site at the nuclear reactor. You may wonder "Is this actually useful for monitoring, then? If someone wants to lie about their reactor usage and burn rate, they will just falsify the detector records, too." Well, the idea is that this local detector is completely independent from the reactor (it doesn't rely on sensors hooked into the reactor, for instance). Thus it can be locked and sealed off completely; installed and managed by a completely independent oversight organization.
First, it's very very heavy. You need a thick lead shield to block cosmic rays, but that's not the biggest problem.
Neutrino (antineutrinos work the same, but give off a different signal) collisions are very rare. The device needs to be close to the source, so that many neutrinos pass through the device. The density of neutrinos fall off as the square of the range (inverse square law). If you double the range, you get one quarter the number of collisions. Even close to the reactor, they are rare enough that it's hard to tell collisions from the local reactor versus background noise (the sun and other reactors are probably the biggest sources). This why it takes them hours to figure out when the reactor has turned off. The whole thing relies on the statistics of a very low probability event. From space, the background noise would easily swamp the signal from a specific reactor.
You might be able to tell which part of the earth is releasing more neutrinos if your orbit was low enough, but it would be very tough to pinpoint a source to a specific country. It would be very hard to get both decent spacial resolution and time resolution.
Unlike this detector, then neutrino detector at South Pole, Amanda, can tell which direction a neutrino came from. It uses the Earth for shielding and many kilometers of ice as the collision target. I assume they detect neutrinos from reactors, but probably not often enough to tell exactly when they are on (just a guess).
The problem is one of mass, not of interference. It takes a long time (days) to start a reactor back up after shutting it down, so a satellite in a typical 90-minute polar orbit could check every reactor on Earth twice a day, at a distance close enough to tell it from all other reactors. Since the atmosphere provides almost no shielding against high-energy cosmic rays, both a ground-based detector and an orbital detector require some other way of filtering those out.
However, such a satellite would be too massive to launch. A detector with a 50-meter range weighs about 1000kg, and neutrinos obey the inverse-square law. A satellite at an altitude of 500km would weigh over 1,000,000,000 kg.
"They redundantly repeated themselves over and over again incessantly without end ad infinitum" -- ibid.
One of the things that makes reactor operation so hard is that they don't like being run at low power. If you run a reactor at 50% power for more than a few minutes, the buildup of fission byproducts will cause the reactor to shut down, requiring a very expensive restart process.
"They redundantly repeated themselves over and over again incessantly without end ad infinitum" -- ibid.
Disclaimer: I am NOT a particle physicist (but I did stay at a Holiday Inn Express last night). We seem to be drawing an overall conclusion that it is antineutrinos that are being sought by these detectors. However, since the neutrino and antineutrino are both neutral (read: they have no charge) particles, is it not possible that they are the same particle, or perhaps simply two variants of a core type of particle?
If I recall correctly, the only differing property between the neutrino and antineutrino is helicity; the neutrino has left-handed helicity, the antineutrino right-handed. Perhaps someone can clarify this a bit better?
That isn't a problem because the difficulty is to detect when the reactor is off thou they claim it isn't. During normal operations a reactor will run close to full power for up to a year at a time, but if you want to produce nuclear weapons you must keep shutting it down to refuel since leaving the plutonium in the reactor for too long severely degrades its suitability in a bomb. Thus all the detector needs to do in order to blow the whistle is to show that the reactor operated in a very unusual pattern rather than the continuous high power mode that is more common.
Furthermore, for baseload plants like large nuclear power stations you can't just change the power output as you like because the type of turbines used do not spin up and down very easily. Special load-leveling plants ( typically natural gas or hydroelectric ) are used for this purpose, so if somebody is running a large baseload nuclear plant at 50% power it would almost certainly attract suspicion.
People usually focus on charge reversal for particle-anti particle opposition; charge reversal is a consequence of anti-particle formation, not a cause. If a given particle has, say, 3 specific quarks, its anti-particle is the one with the 3 anti-quarks of them. If the sum of the quarks yield a charge, then by nature, the anti-particle will have the opposite charge, but if the total charge is 0, then the anti-particle is also neutral - but not equivalent.