The mysterious wraith of the subatomic world, the neutrino has a mass close to zero and no electric charge, making it notoriously difficult to detect. It's so stealthy that physicists have taken to calling it the "ghost particle."
But researchers working at Oak Ridge National Laboratory (ORNL) have made a breakthrough, reports Scientific American. They have detected neutrinos bumping into atomic nuclei, and they did it with a device about the same size as the ghost traps from the "Ghostbuster" films.
To get a grasp on just how impressive this feat is, consider the scales at play. Although a hundred trillion neutrinos pass through you every second, only about one per week actually grazes a particle in your body. That's because neutrinos are tiny, even when compared to atoms. The space surrounding atomic nuclei seems vast and endless to these diminutive specks, and they have little motivation to interact with other particles.
When they do interact, however, it is only through the so-called "weak force," the fundamental force that causes radioactive materials to decay. This compounds the problem of detection, because the weak force is aptly named — it only operates at subatomic distances. Getting a neutrino to strike an individual neutron or proton therefore requires throwing thousands of tons of atoms their way. And that's just to increase the odds of a neutrino striking a single proton or neutron.
Once a strike is made, that's only the beginning. Scientists then have to detect the strike, which is its own exercise in futility. Again, consider the scales at play.
“Imagine that you take a ping-pong ball and you throw it at a bowling ball,” explained Temple University physics professor Jim Napolitano, who was not involved in the study. “We know from conservation of momentum [that] a little bit of energy is imparted to the bowling ball. This [experiment] is detecting that bowling ball’s energy.”
The challenge for researchers was therefore to find a material with atomic nuclei large enough for neutrinos to hit easily, but also small enough that they would noticeably recoil on impact. (Basically, they needed to find the perfect-sized bowling ball to maximize the odds of detection.)
“That took me a lot of thinking—maybe 15 years,” claimed Juan Collar, one of the study’s lead authors.
Eventually, Collar stumbled upon sodium-doped cesium iodine as the ideal target for the ping pong-like neutrinos to bump into. Meanwhile, the source of the neutrinos would turn out to be the Spallation Neutron Source (SNS), a neutron-producing particle collider conveniently located at ORNL.
The materials are such a perfect match that researchers were able to detect the neutrinos bumping into the atoms with a device that could be held in their hands. The compact size of the device is what makes the detection such a breakthrough; previous experiments involved contraptions that were 40 feet tall.
Such small detectors will make neutrino-detection incredibly handy, allowing for the development of new ways of monitoring nuclear reactors, for instance. They could also revolutionize physics, making it possible to test whether the rates of neutrino detection in different materials match theoretical predictions. The device could even bolster our attempts to detect mysterious dark matter.
“This [study] is just the tip of the iceberg. There’s a whole lot more interesting stuff to come,” said University of Michigan physicist Josh Spitz.