Neutrinos are born when the nucleus of an atom changes in some way?—?for example, during fission, wherein a nucleus breaks apart; during fusion, when two nuclei join together; or during the decay of a radioactive element, when nuclei break down. A neutrino’s origin and subsequent travels affect its characteristics. It can come in one of three types (known as electron, muon or tau), and its energy level may correspond to that of the event that produced it.
• Earthly Sources
Nuclear bombs and power plants use fission as an energy source, so some of the earliest neutrino observations came from detectors set up beside these energy plants or near spots where a bomb had detonated. These projects revealed a plethora of electron neutrinos (so named because they often appear alongside electrons). Similarly, when radioactive elements break down deep within the earth, physicists pick up low-energy electron neutrinos created by this change. Detecting these particles can, for example, help people locate untapped sources of uranium.
• The Sun
In the 1960s, physicists detected an unusual collection of neutrinos using a detector in a gold mine in South Dakota. By studying the energy levels and fluctuation in the number of these neutrinos over time, they realized that these particles matched the predicted appearance of neutrinos produced by fusion reactions at the sun’s core. That discovery was a major breakthrough in solar research, as those events are otherwise impossible to investigate. Today, tracking the flux in solar neutrinos helps physicists study the scale and frequency of fiery nuclear events occurring more than 92 million miles away.
In fact, researchers suspect that the neutrino is necessary for those reactions to occur; without this particle, we would not enjoy the light and heat of our sun. We would not have oxygen and carbon. Stars would cease to burn.
• Cosmic Events
Another neutrino source: a supernova, or the collapse of a star. These light displays, which have been documented throughout human history, are the brightest events in the galaxy. They also offer a neutrino bonanza. “Light is only 1 percent of the energy emitted from a supernova, but 99 percent of the energy emitted is neutrinos,” says Stanford professor Giorgio Gratta. “Neutrinos keep the explosion going.” Physicists who detect neutrinos from these events can tell astronomers where to point their telescopes for the best view.
Supernovas create heavy elements, such as copper, silver and gold, which are used in fields ranging from science to art.
• Neighboring Galaxies
Researchers are just starting to explore one of the most exotic sources of neutrinos: outer space. Naoko Kurahashi-Neilson, a former graduate student of Gratta’s, participates in an experiment based at the South Pole that requires peering into a 1-cubic-kilometer block of ice to spot neutrinos way more energetic than the ones hailing from our sun.
In 2013, the collaboration announced that in two years of collecting data it had observed 28 such neutrinos. “We don’t yet know where they came from?—?perhaps very active black holes in the center of the galaxy. It’s a mystery,” says Kurahashi-Neilson, ’06, PhD ’10, now an assistant professor at Drexel University. “I have a feeling this is what I’ll spend the rest of my career on.”
• The Big Bang
Other neutrinos have survived as remnants of the nuclear reactions that occurred during the Big Bang. “One of my dreams would be to detect those neutrinos,” Gratta says. For now, there are no methods for doing so, but he’s optimistic. “We’re doing lots of work with theorists to develop techniques to be able to do that,” he says. “Call me back in 10 years!”
