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Stanford Solar Physicist Finds New Way to Acquire Data from Deep Solar Interior

Workers maintain the Super-Kamiokande neutrino detector in Hida, Japan. (Image credit: Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo)

Neutrinos from the sun carry information about its fiery core but they are extremely hard to detect. Now, Stanford researchers may have found a much easier and less expensive way to study these elusive particles.

In 2009, applied physicist Peter Sturrock visited the National Solar Observatory in Tucson, Arizona, and the observatory’s deputy director told him to read a controversial article about radioactive decay. While the subject was beyond Sturrock’s field, he was so inspired with that idea that he immediately contacted the study author Ephraim Fischbach, a physicist at Purdue University, to propose a new collaboration. Fischbach replied, “We were about to phone you.”

Now, over seven years later, that collaboration could lead to a low-cost tabletop device that may not only detect elusive neutrinos more efficiently and inexpensively than currently possible, but may also simplify researchers’ ability to explore the inner workings of the sun. The results of the study have been reported in the Nov. 7 issue of Solar Physics.

If we’re correct, it means that neutrinos are far easier to detect than people have thought. Everyone thought that it would be necessary to have huge experiments, with thousands of tons of water or other material, that may involve huge consortia and huge expense, and you might get a few thousand counts a year. But we may get similar or even better data from an experiment involving only micrograms of radioactive material.

Peter Sturrock, Professor, Stanford University

Why, How We Study Neutrinos

For two decades, Sturrock along with his colleague Jeff Scargle, astrophysicist and data scientist at NASA Ames Research Center, have explored subatomic particles called neutrinos that have nearly zero mass and no electric charge. These particles can be used to get a better understanding of the inner workings of the sun.

Neutrinos are produced when nuclear reactions occur in the center of the sun. Neutrinos rarely interact with other particles and can easily escape the sun, providing information about the deep interior of the sun. The sun continues to be a mysterious object. Direct information about the sun’s core can be obtained by studying neutrinos, which can also provide information about the creation of the universe, supernovas, and much more.

On Earth, 65 billion neutrinos can pass through an area the size of a fingernail, each second. However, only one or two neutrinos in an entire lifetime will stop in our bodies. In order to study neutrinos, massive equipment and expenses are required to trap sufficient amount of the elusive particles for analysis.

Currently, Japan’s $100 million observatory - Super-Kamiokande - is the gold standard for neutrino detection. In use since 1996, the observatory lies 1,000 meters under the ground and contains a tank filled with 50,000 tons of ultra-pure water, covered by approximately 13,000 photo-multiplier tubes.

If a neutrino enters the water and interacts with nuclei or electrons, a charged particle is produced that travels faster than the speed of light in water. This produces an optical shock wave, a cone of light known as Cherenkov radiation. This light is then projected onto the tank wall and eventually recorded by the photomultiplier tubes.

Past Challenges in Detection

Masatoshi Koshiba of Super-Kamiokande and Raymond Davis Jr. of Homestake Neutrino Observatory received the 2002 Nobel Prize in Physics for the development of neutrino detectors and “for the detection of cosmic neutrinos.” One puzzling aspect of this study was that, with their revolutionary detection methods, the scientists were able to detect only one-third to one-half as many neutrinos as predicted, a problem called the “solar neutrino problem.”

Initially, this shortfall was believed to be caused by experimental problems; however, the deficit was accepted as real after it was confirmed by Super-Kamiokande.

However, the year before the Nobel, researchers proposed a solution to the solar neutrino problem. It was found that neutrinos oscillate among three forms such as electron, tau and muon, and detectors were only sensitive to electron neutrinos. For discovering these oscillations, Takaaki Kajita of Super-Kamiokande and Arthur B. MacDonald of the Sudbury Neutrino Observatory received the 2015 Nobel Prize in Physics.

However, despite these Nobel Prize-worthy developments in equipment and research at their disposal, researchers were able to detect only a few thousand neutrino events per year.

A New Option for Research

The study that Sturrock learned about in Tucson concerned fluctuations in the decay rate of radioactive elements. At that time, the fluctuations were highly controversial because it had been assumed that the rate of decay of any radioactive element was constant. Sturrock decided to explore these experimental results using analytical methods that he and Scargle had devised for the neutrino study.

While studying the fluctuations in radioactive decay, the team discovered that those fluctuations matched with the patterns they had observed in Super-Kamiokande neutrino data, with each showing a one-month oscillation attributed to solar rotation. One potential conclusion is that beta-decays are being affected by neutrinos from the sun.

While other researchers have also theorized this relationship dating back 25 years, the Sturrock-Fischbach-Scargle analysis provides the best proof yet. If this connection holds, then a revolution in neutrino research may well be underway.

It means there’s another way to study neutrinos that is much simpler and much less expensive than current methods. Some data, some information, you won’t get from beta-decays, but only from experiments like Super-Kamiokande. However, the study of beta-decay variability indicates there is another way to detect neutrinos, one that gives you a different view of neutrinos and of the sun.

Peter Sturrock, Professor, Stanford University

According to Sturrock, this could set the beginning of a new field in solar physics and neutrino research. He and Fischbach envisage the possibility of advanced bench-top detectors that may cost thousands of dollars instead of millions.

The next step will be to collect better and more data and to work toward a hypothesis that can elucidate the connection between these physical processes.

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