Nuclear ghosts

By Jesse Amspak, the article is published with the permission of Scientific American Israel and the Ort Israel Network 09.07.2017

The year is 2030. After many years of bickering, the North Korean leadership agrees to stop producing plutoniumAt the enrichment level of nuclear weapons and destroy its nuclear plutonium stockpiles. Officials in the North Korean government invite inspectors to watch the loading of The nuclear fuel this into a reactor to convert it into a material that cannot be used to make bombs. However, the North Koreans secretly remove some of the plutonium and in its place, fill the reactor with low-enriched uranium. Uranium emits radiation, including particles Neutrino and particles antimatter their equivalents, Antineutrino: massless subatomic particles that are almost harmless, penetrating like ghosts even through lead or rock. The international authorities suspect that this is a ruse, and decide to monitor what is happening using a facility the size of an SUV placed near the North Korean reactor. Within a few months it becomes clear that it is indeed a scam, as revealed by the pattern of the emission of antineutrino particles coming in a stream from the reactor.

This scenario may become a reality in the coming years, when tools used in particle physics research will be used to combat illegal nuclear programs. A recent proposal, the details of which have recently appeared in the online database of scientific papers in phase Pre-publication, arXiv.org, describes the construction method of an anti-neutrino particle detector by means of which it is possible to determine, after monitoring for a few months, whether fuel enriched to the level of nuclear weapons is indeed being used in the reactor.

The need for such monitoring and detection methods has become even more urgent recently. As North Korea refines its missile technology and Iran develops its ability to advance its own nuclear weapons program, verification becomes a key factor. In March 2017, US Secretary of State Rex Tillerson called for a "different approach" to thwarting North Korea's nuclear ambitions, noting that diplomatic pressure alone did not achieve its goal.

Antineutrinos are a byproduct of fission A nuclear reactor, where the atomic nucleus of a radioactive element, such as plutonium, is split into lighter particles. in a certain kind of Radioactivity, the machine Beta radiation (or beta decay), a positron and a neutrino or an electron and an anti-neutrino are emitted from the atomic nucleus. The anti-neutrino is the signal indicating the nature of the activity in the reactor, since only the radioactive elements in nuclear fuel emit a stream of anti-neutrino particles at a constant rate.

Supervision of nuclear programs based on the emission of antineutrino particles is the basis of the WATCHMAN project, which is led by the USA. device WATCHMAN consists of a tank containing thousands of tons of water in which the element is dissolved gadolinium, and in its ability to detect, theoretically, the emission of antineutrino particles from an illegal reactor located up to 1,000 kilometers from the facility. It is difficult to diplomatically approach a foreign country that is wary of revealing its secrets and ask that it allow inspectors to build from a huge water tank near sensitive, well-secured facilities, and therefore the possibility of such remote monitoring and detection is extremely important.

When an anti-neutrino hits aproton - A hydrogen nucleus in a water molecule in the giant tank - the proton splits into a neutron and a positron. The positron moves at such a high speed that it emits so-called radiation Cherenkov radiation, the optical equivalent of Sonic boom, emitted when a charged particle moves faster thanspeed of light in some way. No body moves in a vacuum faster than the speed of light, but in another medium, such as water, glass or air, light moves slower and other bodies can move faster than light. And so, a positron originating from an antineutrino hitting a proton creates a flash of light in the WATCHMAN tank. Meanwhile, gadolinium atoms in the water "absorb" the neutron, a process that creates another flash. This typical double flash reveals the existence of a nuclear reactor and the direction in which it is located.

The WATCHMAN facility can detect if a nuclear reactor is active and where it is located, but not the exact mix of the fuel: for example, plutonium or highly enriched uranium. Patrick Jafka, a postdoctoral student engaged in research bUS National Laboratory Los Alamos and co-author of the proposal which was recently raised to build a detector of antineutrino particles, מציע A scaled-down version of the detector that can be placed near a reactor to determine the type of nuclear fuel in it based on the analysis of the activity of the anti-neutrino particles. The detector model offered by Jafka is supposed to measure thespectrum and the shape of the initial flash of Cherenkov radiation and based on the analysis of the energy of the positrons, determine what the energy of the original antineutrino is. According to the energy distribution diagram of the identified positrons, an inspector monitoring what is happening in the reactor will be able to estimate what part of the flux of antineutrino particles emitted from the reactor comes from a certain type of fuel in the reactor core.

Instead of water, Jafka suggests using plastic orhydrocarbon Another proton-rich one to increase the chances of collisions of antineutrino particles and reduce the dimensions of the facility by several orders of magnitude. Such a detector would be possible to place several tens of meters away from a nuclear reactor.

And if such a detector will be smaller, it will still be necessary to deal with the problem of background noise. Cosmic radiation, for example, can create neutrons that look similar to those originating from the activity of neutrino particles. Burying the anti-neutrino particle detector at a depth of five to ten meters underground, quite close to the reactor, may solve the problem, says Stephen Daisley fromThe American Lawrence Livermore National Laboratory, who in 2016 led an analysis of the background noise problem at the WATCHMAN facility. Additional shielding against radiation can also be helpful.

Other ideas for facilities that need little or no shielding have also been suggested. Several teams around the world engaged in developing technologies to detect neutrinos and anti-neutrinos for physical research purposes may also help in finding solutions.

"For a long time [scientists] have been looking for a practical use for antineutrinos," says Jafka. "This is one of the coolest applications" of them, using them to discover nuclear fuel whose level of enrichment is high enough to be used as a weapon. And we hope that nuclear fuel will never be found at such an enrichment level.