Supernovas: Making Astronomical History

Neutrinos

The sun is a mass of incandescent gas

A gigantic nuclear furnace

Where hydrogen is built into helium

At a temperature of millions of degrees. . . .

--They Might Be Giants

The Super-Kamiokande detector, courtesy Kamioka Observatory(Replaced), ICRR (Institute for Cosmic Ray Research), The University of Tokyo

The Neutrino is a subatomic particle famous for its ability to slip through matter without interacting. Neutrinos have none of the "handles" by which most other particles affect one another: no electric charge, almost zero mass. They are so elusive that a light-year of lead, nine and one-half trillion kilometres (six trillion miles) would only stop half of the neutrinos flying through it. The only hope for detecting them is to put a large quantity of matter in one place and hope the occasional neutrino will, by dumb luck, strike an atom somewhere and interact with it. Because so many other radiation sources are releasing energy throughout the Universe, any detector trying to spot neutrinos has to deal with backgroud noise. Picking the signal out of this noise can be a challenge. To make the problem easier, neutrino detectors are built underground, often within deep mineshafts. The rock around the detector blocks any radiation not powerful enough to penetrate beneath the Earth; because neutrinos are so "slippery", they can pass through the rock and reach the detector device.

Neutrinos are valuable to astronomers precisely because they are so evasive. Since even large thicknesses of matter don't have much effect, neutrinos can flow right through things which distort or block other types of radiation. For example, our Sun is a ball of hot gases, 1,392,000 kilometres (870,000 miles) in diameter. Nuclear fusion reactions at the Sun's core heat these gases, producing vast quantities of energy. We would like to know the details of what's going on inside the Sun's core, but the gaseous layers in the way block our view. The gas atoms scatter light so well that a single photon, the basic particle of light, takes roughly fifty thousand years to reach the Sun's surface. Photons leave the core, hit nearby atoms, bounce off them, hit other atoms, and spend centuries doing more and more of the same, until they manage to leak out in the thinner regions near the surface. All that scattering and jostling obscures the details of the interior, just like a bright city skyline looks vague and indistinct when observed through a thick fog. Neutrinos avoid this problem, because they don't like to interact with the Sun's atoms. Once nuclear reactions in the core produce neutrinos, they can radiate away and rapidly escape the Sun. Neutrino detectors, then, can tell us what happens deep within the solar core, because they bring us information directly from the source. In the city analogy's terms, they zip through the fog and reveal the metropolis behind it.

The neutrino entered physics as the brainchild of Wolfgang Pauli (1900-1958). Pauli was trying to explain a puzzling feature of beta decay, a type of nuclear reaction that frequently occurs in unstable heavy elements. In beta decay, a neutron within the atomic nucleus breaks down and turns into a proton, releasing an electron which flies away from the atom. Measurements showed that the electron's energy varied: sometimes it barely crept out of the nucleus's pull, and sometimes it shot away at high speed. Physicists could explain the high-energy case fairly easily: the electron simply carried the maximum energy the reaction could produce. What about the lower-energy cases, Pauli wondered.

A basic principle, the Conservation of Energy, says that energy cannot vanish from existence. In cases where that appears to happen, it is in fact being transformed into a less obvious form. (To a physicist, watching energy vanish from a situation is like how many people feel watching money disappear from their bank account.) For example, when you throw a broken computer out the window, the Earth's gravity gives it a certain amount of energy, which shows up as the computer's speed. The higher it begins, the more energy and hence speed it gains by the time it hits the sidewalk below. When it impacts the ground, though, where does all that energy go? Answer: the kinetic energy the falling computer had from its motion went into thermal energy (both computer and sidewalk are a little warmer than before) and acoustic energy (the crash makes a sound). Also, some goes into distorting the shape of the computer, which (among other things) puts potential energy into bending pieces of metal and plastic.

What kind of phenomenon could carry away the energy the electron didn't use? Pauli dreamt up a new particle, an entity which would pick up the slack, so to speak. It would have to be hard to detect, elusive enough to explain why no one had seen it before. Pauli decided the particle would have no electric charge, and that it would be very light, either totally massless or almost so. Enrico Fermi (1901-1954) named this particle the neutrino, from an Italian word meaning "little neutral one".

A later discovery, flavor change, complicates this issue somewhat. From the mid-1960s, when the solar neutrino flux was first measured, up until about 2002, the "solar neutrino problem" caused much debate. All detectors confirmed a puzzling result: the Sun was only emitting one-third to one-half of the neutrinos we expected. Large amounts of cleverness have gone into solving this problem, and developments in the past few years have been very exciting. The idea of flavors not only helped solve the issue of lower numbers than expected coming from solar neutrinos, but also pioneered another whole branch of research in the field of neutrinos.

How We See Them

It must first be established that the neutrinos come in their three flavors, electron, muon, and tau, these all have their own characteristics and react differently with different things. Most detectors are primarily sensitive to electron neutrinos, as their main reaction is fairly easy to spot, however there is a secondary reaction that can occur with any of the neutrinos, but this reaction in turn gives us no information as to which neutrino is actually being seen.

The way that neutrinos are actually detected is a bit of a round about way of doing it. Rather than detect the actual neutrinos, we detect the things that the neutrinos interact with, which are often given large bursts of energy out of seemingly nothing. From there it can be infered that a neutrino is the only explanation for such an event. There are two primary ways that the neutrinos interact, the first, much more prominent way is for an electron neutrino to smash into an atom, and react with either a neutron or a proton, which after a reaction causes either an electron or a positron (a positively charged electron) to shoot off in a semi-random direction and hit the detectors that are plastered across the sides. Not much is able to be said about the actual direction of the neutrino from this reaction however, as its momentum only has about a 10% influence on the direction of the electron, the rest of the energy comes from the quantum "jiggling" of the neutron/proton that it collides with, giving it a basically random trajectory.

A "picture" of the sun made from looking at the direction of solar neutrinos. Courtesy of Nasa's APOD

The second way that these neutrinos can be seen is a little more intuitive, as it is essentially just a neutrino coming close enough to an electron to interact with it, then from there they bounce off of eachother in a collision that greatly increases the momentum of the electron. This turns out to gives us quite a bit of information that the first reaction does not, mainly, its direction. This is possible by looking at the direction and momentum of the electron, then doing a bit of physics to figure out where the neutrino must have come from in order to fire the electron off in that direction. The problem with this way of seeing them is not only can you not tell what type of neutrino hits the electron, but it also is much less frequent, as the cross sectional "traget" that it has to hit for this secondary collision is much smaller than the primary collision. The actual direction of this neutrino is still only somewhat accurate as well, as there are many factors that can play into the electrons final momentum that give it a general uncertainty.

Using this info we can get a general idea of where a burst of neutrinos is coming from, which is how that "image" of the Sun was made, as it is less a picture and more of a showing of where the majority of the neutrinos were pointing too, with the brighter areas being spots of higher concentration. That picture however did take qutie a bit of time, about 500 days worth of data went into making that, and even with all that data, the actual exact location of the sun would still be unclear (luckily we have other ways of pinpointing it, such as seeing what part of the sky hurts our eyes to look at). To give you an idea, that image is what it would look like if you stretched that data across the entire sky, with each of the pixels being about a degree in size. This is why in the situation of a supernova, less powerful telescopes can be quite valuable, as they have a much wider few of the sky, which makes it easier to spot an event like a supernova with a fairly large potential direction.

References and Further Reading

On neutrinos in general:

The 2002 Nobel Physics Prize was awarded for discoveries about cosmic radiation. Half of the Prize went to Raymond Davis, Jr. and Masatoshi Koshiba for "pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos".

The 2015 Nobel Physics Prize awarded to Takaaki Kajita and Arthur B. McDonald for their work in discoveries around neutrino oscillation and using that to prove that neutrinos must have a mass.

The following articles, part of assigned class readings at MIT, may be useful to those with some experience in quantum mechanics.

Researched, written and maintained by Blake Stacey.