Neutrino Detection For Fun And Profit

An infinite number of neutrinos shoot across the earth's surface every second, radiated by distant stars, and yet it has taken up until the mid 20th century for anyone to realize it. Finding neutrinos in your own backyard is easy. All you need is a ten-ton vat of pure water, 13000 photomultiplier tubes, and $11 million dollars in research funding. More on that later, but first, it would help to know a little more about what you're hunting.

The neutrino is an elusive beast possessing neither mass nor charge. The only proof of its existence comes in measuring its recoil effect. In the cold heart of subatomic physics, beyond the protons and neutrons that until recently served as the basic building block of atoms, a wellspring of subnuclear particles have been discovered in recent years. This much-theorized-upon neutrino is one of these particles. Research has discovered three types of neutrinos: the electron neutrino, the tau neutrino, and the muon neutrino. An anti-neutrino particle has also been discovered. All are created as the result of particle decay.

Neutrinos, born of decay, are given off as one particle deteriorates into a more stable state. Neutrinos are emitted in positron (another type of subnuclear particle) beta decay while the anti-neutrino is emitted from electron beta decay. As a pion decays into a muon, the muon neutrino emerges along side the muon. When a pion decays, a neutral particle must be emitted in the direction opposite that of the muon in order to conserve momentum. The original assumption was that this particle was the neutrino that conserves momentum in beta decay. In 1962, however, researchers proved that the neutrino accompanying pion decay is different. At this point, little is known about the tau neutrino.

Neutrinos come from a variety of sources, nuclear reactions like those caused by nuclear warheads and our own sun create neutrinos. Super-novas, that is, exploding distant stars, create neutrinos. Neutrinos also come from the earth's own atmosphere, as cosmic rays bombard atmospheric particles to create new particles, some of them the unstable pions that deteriorate into muons, that further deteriorate into electrons. At each of these deteriorations, neutrinos are given off.

Clever scientists first caught wind of the idea for the neutrino by a perplexing flaw in their formerly-solid physics equationing. Understanding neutrinos are radiated by nuclear reactions and the sun itself is a "giant atom-smashing machine" powered by nuclear reactivity, the sun then is a source for neutrinos. However, the math doesn't work out. The predicted number of neutrinos is off, scientists can only observe only 50% of the neutrinos that "should be" there. Why? This is the "solar neutrino problem" being explored by scientists at the Super-Kamiokande detector, but first one should look at the past before diving into the future.

A History of the Neutrino

Nobel Prize winner Wolfgang Pauli proposed in 1930 that the missing energy in nuclear beta decays was carried by a neutral particle. Three years later, Nobel-Prize-Winning nuclear physicist Enrico Fermi named the particle "neutrino," meaning "little neutral one," to distinguish it from the much larger neutron and developed a theory calculating that a neutrino and electron were both emitted in decay. However, both scientists were just blowing smoke at this point as no one had yet detected the theoretical neutrino. The problem was that little could stop a neutrino long enough to detect it and it plowed through light-years worth of matter making containment seemingly impossible.

In 1956, UC Irvine physicists Frederick Reines and Clyde Cowan stepped up to the challenge of detecting the elusive neutrino and succeeded. After changing locations to avoid the interference of cosmic rays, the two settled near the Savannah River nuclear reactor. With their detector made up of two tanks of water sandwiched between layers of photomultiplier tubes (very sensitive light detectors), they dissolved CdCl2 and watched history being made. The water slowed the reaction and the dissolved cadmium captured the neutron microseconds after the positron escaped. Gamma rays emitted and the photomultiplier tubes picked up on the light emitted as proof of the neutrino. Reines won the 1995 Nobel Prize for his work (no mention of what Cowan got, however).

A great moment in experimental neutrino physics came in the 1987 explosion of Supernova SN1987A in a satellite galaxy of the Milky Way. A super nova, that is, a star collapsed on itself and sends out a shockwave, heating the outer layers of the star. This supernova is, all at once, brighter than an entire galaxy, but in the core, electrons and protons bond together into neutrons and as the proton changes into a neutron, massive amounts of neutrinos are released. So many, in fact, that scientists on Earth, 170,000 light years away could detect them.

Today in Neutrinos

Reine's first neutrino detector was only about two meters in length and contained 200 liters of water. Today, a cooperative effort between American and Japanese scientists maintain a detector, the Super-Kamiokande, that is forty meters by forty meters and contains 50,000 tons of water. Still, the two have much in common. Reines moved his detector from the Hanford nuclear reactor to Savannah to reduce background interference from natural radioactivity and cosmic rays; the Super-K reactor is shielded from cosmic rays 2700 deep in the Kamioka Mozumi mine.

In purpose as well as operation, the two detectors are identical in every way except scale. The Super-K still detects neutrinos the old fashioned (and only way), though the light given off in their reactions as seen through the sensitive photomultiplier tubes.

What problems is the Super-K trying to address? Why recreate Reine's experiment on a massive scale? The solar neutrino problem (which is not limited to solar neutrinos, but all neutrino detection) is that the predicted number of neutrinos produced, using all known physical laws, is always drastically off. The answer cutting edge nuclear physics has to offer is neutrino oscillations.

Neutrinos come in three varieties in nature, electron-, muon-, and tau-neutrinos. They also have a negligible mass and more likely, no mass at all. However, to assume momentarily that neutrinos do have mass, there is a possibility of 'mixing' between the types. In practice this would suggest that an electron-neutrino on its way from the sun to the Earth could transform into a muon-neutrino and upset the predicted number found in the equation. With the greater numbers of photomultipliers and this knowledge in mind, Super-K engineers can look specifically for this transformation.

What's the point?

The cynical non-scientist grumpily looks at the balance sheet and demands to know, what's the point? Why invest so much in a particle with no mass, no charge and, seemingly, no practical applications? Will neutrino technology make life better for the population at large-- make our teeth whiter and cereal flakes stay crunchy in milk -- or is this just science for science's own sake. What's the payback for looking for neutrinos?

"Bah!" quips Super-K technician R.J. Wilkes, when asked if neutrino research is a big waste of time. He cites that the neutrino discoveries have opened new doors in astrophysics. Understanding neutrinos means a better understanding of nuclear reactions of stars, as in the explosion of SN1987A.

Works Cited:

"Detection of the Free Neutrino: A Confirmation", C.L. Cowan, Jr.,F. Reines, F.B. Harrison, H.W. Kruses and A.D. McGuire, Science 124, 103 (1956).

"40 Years of Neutrino Physics", Frederick Reines, Progress of Particle and Nuclear Physics, Vol. 32, 1 (1994).

Websites

The University of Washington's Super Kariokande Info Page

UC Irvine Physics Department

and the assistance of Professor R.J. Wilkes of the University of Washington

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