A new study says elusive neutrinos were spotted for the first time using a detector. Neutrinos are particles that travel through the universe at almost the speed of light. Physicists calculate that over hundred trillion neutrinos pass through us every second, but only one per week grazes a particle in our bodies.
These particles are particularly difficult to study because their interactions with matter are extremely rare. However, an international team of scientists just succeeded in catching a neutrino, using a detector about the size of a fire extinguisher.
The study, which involved more than fifty authors, was published Thursday in the journal Science.
Researchers detected tiny nuclear recoils caused when neutrinos bump atom’s core
The researchers, all part of an experimental collaboration dubbed COHERENT, studied a phenomenon called CEvNS (pronounced “seven”), also known as coherent elastic neutrino-nucleus scattering. Coherent elastic neutrino-nucleus scattering occurs when a neutrino bumps off the nucleus of an atom.
In 1974, a physicist named Daniel Freedman predicted the way for neutrinos to interact with matter. The researchers of the new study just confirmed his theory, by observing the scattering process using the detector.
“Why did it take 43 years to observe this interaction?” Juan Collar, a professor of physics at the University of Chicago and co-author of the study, told Phys.org. “What takes place is very subtle.”
When a neutrino bumps into the nucleus of an atom, a small, measurable recoil is created. The researchers noted that making a detector out of heavy elements cesium, iodine, or xenon considerably increases the probability of a neutrino interaction.
However, they explained that the tiny nuclear recoils are harder to detect as the nucleus becomes heavier.
“Imagine your neutrinos are ping-pong balls striking a bowling ball. They are going to impart only a tiny extra momentum to this bowling ball,” said Collar.
To detect those tiny nuclear recoils, the researchers decided that a cesium iodide crystal doped with sodium was the best material to build the detector. Also, as the nucleus needed to be lighter to catch the recoils, they chose to build a small detector rather than the large, huge detectors common in neutrino research.
Researchers are also building detectors to see WIMPs
The neutrino detector weighs only 32 pounds (14.5 kilograms). In contrast, the world’s most famous neutrino observation labs are equipped with thousands of tons of detector material.
“You don’t have to build a gigantic laboratory around it,” said Bjorn Scholz, University of Chicago doctoral student and co-author of the study. “We can now think about building other small detectors that can then be used, for example, to monitor the neutrino flux in nuclear power plants. You just put a nice little detector on the outside, and you can measure it in situ.”
Neutrino physicists, however, are mostly interested in using the technology to understand the properties of neutrinos better. Collar said neutrinos are one of the most mysterious particles in the universe, and scientists ignore many things about them. They know they have mass, but don’t know how mass exactly, said Collar.
The findings also have implications for the research of Weakly Interacting Massive Particles, or WIMPs. Experts believe WIMPs are candidate particles for dark matter – invisible material with an unknown composition that accounts for about 85 percent of the mass of the universe.
Collar said they are also building a WIMP detector and they expect to observe the same results as they did with the neutrinos detector.
The Spallation Neutron Source generated neutrinos used in the experiment
The COHERENT Collaboration had 90 scientists from 18 institutions working to achieve the coherent elastic neutrino-nucleus scattering. The experiments were carried at the Spallation Neutron Source at Oak Ridge National Laboratory in Tennessee.
The scientists installed detectors in a basement corridor that is now known as “neutrino alley.” The corridor is shielded by concrete and iron from the radioactive neutron beam target area, located about 20 meters away from “neutrino alley.”
The neutrino alley also helped the scientists solve a major problem for neutrino detection, as it screened out nearly all neutrons generated by the Spallation Neutron Source, but neutrinos were still able to reach the detectors. That allowed the team to see neutrino interactions in their experiments better.
The Spallation Neutron Source is known for being the best generator of intensely pulsed neutron beams in the world. The beams are used for scientific research and industrial development. While it generates neutrons, the SNS also produces neutrinos, although in smaller quantities.
“You could use a more sophisticated type of neutrino detector, but not the right kind of neutrino source, and you wouldn’t see this process,” said Collar. “It was the marriage of ideal source and ideal detector that made the experiment work.”