Neutrinos are subatomic particles that are typically generated by high energy environments such as inside stars. Trillions of these particles pass through everything and have been observed in underground ultrapure water reservoirs to increase the probability of detecting their rare interactions. Contributions by Dr. Francis Halzen created new methods of detection using deep Antarctic ice, warranting a Nobel Prize in Physics.
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Astro Brief is a podcast collaboration between KSMU, the Missouri Space Grant, and MSU's Department of Physics, Astronomy and Materials Science. Hosted by Dr. Mike Reed, Astro Brief focuses on astronomical events, the field of astronomy, and astronomy-related guests. It airs Thursdays at 9:45 am on KSMU.
Transcript
This episode is going to focus on the recent Nobel Prize in Physics given to Dr. Francis Halzen for his work on cosmic neutrinos.
A good place to start would be with defining what a neutrino is.
It is very small. It is a subatomic particle, smaller than an atom, and typically in physics it is treated as a point particle with no internal structure or volume. Particles like protons and neutrons are made of even smaller particles called quarks, but particles like electrons and our friend the neutrino are not. They themselves are the smallest particles with our current knowledge.
As their name implies, neutrinos have no electric charge, they are neutral. They also do not interact with the strong nuclear force, but only with gravity and the weak nuclear force. Because of their immunity to electricity, magnetism, and the strong nuclear force, they are exceedingly non-interactive. They pass right through the Earth without even noticing. 100 trillion neutrinos pass through our bodies every second, and yet over our entire lifetime, there's only a 25% chance that a single neutrino will interact with an atom within us.
They are produced in nuclear reactions within stars, including our Sun, by exploding stars called supernova, and by cosmic rays which are extremely energetic charged particles like protons. We make them in nuclear reactors in particle accelerators, and fun fact, even bananas make them when a potassium particle decays.
They are quite tricky little particles.
They come in three varieties called flavors — Electron neutrinos, Muon neutrinos, and Tau neutrinos. Our Sun and all stars produce huge amounts of electron neutrinos, but what makes them so tricky is that as they travel, they change flavors. As I tell my students, we thought neutrinos were vanilla, but it turns out they're neapolitan. Imagine you buy a vanilla ice cream cone and as you walk, it becomes chocolate and then strawberry and then back to vanilla again. This discovery called neutrino oscillations won two Canadian physicists the Nobel Prize in 2015.
So they are made by the trillions upon trillions, change flavors as they travel, and they pass right through us and even the Earth with rarely a notice. And that's where Dr. Halzen's contribution comes into play.
In the rare instance when a neutrino interacts with a water particle, liquid, or frozen, it produces a flash of blue light called Cherenkov radiation. In a 1988 symposium paper, Dr. Halzen, with John Learned of the University of Hawaii at Manoa, outlined how glacial ice in Antarctica could be used as a giant neutrino detector, and that was the genesis of the Ice Cube Observatory.
But that idea required funding and development.
It wasn't until 2011 that 86 holes had been drilled kilometers down into the Antarctic ice and strings of photo detectors were installed. The array of detectors covers a cubic kilometer of ice between one and a half and two and a half kilometers beneath the surface. This huge volume is required since neutrinos are so hard to catch and the IceCube Observatory now detects several hundred blue Cherenkov flashes each year caused by neutrinos and it is expanding with the IceCube-Gen2 facility.
That facility will cover eight cubic kilometers including 18,000 optical and radio sensors with seven detector arrays. There will be four in the ice, two on the surface, and one radio array. The updated facility is expected to detect a million neutrinos per year and is capable of detecting neutrinos from the most energetic engines nature can create like supernova and supermassive black hole collisions down to the lowest energy neutrinos created with the birth of our universe.
It's worth noting that neutrinos elusiveness is also one of their best benefits.
For example, the light from our sun leaves from just its outer 1%. If we want to see the nuclear engine in its core, or the cores of all stars, then neutrinos is the way to do it. When we look at the cosmic microwave background, we are seeing light that was emitted when our universe was about 380,000 years old, but not before. Neutrinos were likely released within one second of the beginning of our universe. Neutrinos are also produced when galactic cosmic rays interact with another particle and so the study of neutrinos allows astronomers and physicists to study all sorts of phenomena that regular light cannot penetrate.
Well worth a Nobel Prize.