Ghostly particles

Don’t look now, but the ghostly particles known as neutrinos are right behind you. And in front of you. And going through you. First proposed in 1930, these ghost particles are still puzzling researchers today. Alan Poon, deputy director, Nuclear Science Division and senior scientist, is one of the researchers who is trying to hunt these ghosts to better understand their place in the universe.

Q: What are neutrinos?

A: Neutrinos are tiny particles, very much like electrons. Neutrinos and neutrons have no charge, but electrons do. Electron-type neutrinos (there are three types) are the lighter cousins of the electrons. They are everywhere and like a ghost, they go through everything. Hold up your hand and take a look. Neutrinos are passing through it right now.

Q: If neutrinos are so ghost-like, how do we know they exist?

A: Wolfgang Pauli in 1930 proposed the existence of neutrinos to explain beta decay, which is when a nucleus decays into something else. When they measured the energy of the electron they saw something that would violate the law of conservation of energy so Pauli proposed there was something else carrying away the energy. Enrico Fermi called it neutrino from Italian for little neutral one at a conference in the 1930s and the name stuck. In the 1950s the signature of the neutrino was finally seen at an experiment to look for antineutrinos from a nuclear reactor. They still couldn't directly "see" an actual neutrino, but could observe the results, a neutron and a positron, after neutrinos interacted with a detector medium. The work resulted in a Nobel Prize in Physics in 1995.

With our current equipment, we can see the neutrino in action. We can see them bouncing off electrons or interacting with the material we use to detect neutrinos.

Q: So can we see that neutrino?

A: Well a lot of researchers are working on that. Since the neutrinos are everywhere we have to go far away to isolate them so we head underground. We went down as far as 6,800 feet in a tank filled with heavy water, and even then we had only three high-energy background events in an hour. The high-energy events are cosmic rays that can penetrate that deep and they confuse the sighting of neutrinos. We tried to count 30 neutrinos a day. Once in a while, we would see them interact with the detector material. It’s like a batter in baseball. He might come up to bat three times in a game and feel good if he gets one hit. That’s how researchers feel when they saw a neutrino. I am currently working on experiments to look for decays in nuclei to figure out how neutrinos get their mass, how heavy they are, and whether they are their own antiparticles. We can try to figure out how heavy they are by doing some arithmetic to see how much energy is taken away from the electrons in beta decays.

And just to add to our ghosts, we now know they come in three flavors. There are electron, muon, or tau neutrinos, and they can change between the three options. And there are antineutrinos and researchers are working to understand how we can use them to monitor nuclear reactors.