Solar Imaging: Neutrino

Contents

Introduction

In 1926, English physicist Sir Arthur Eddington offered astronomers a new explanation for how stars shine. Hydrogen is fused into helium, giving rise to enough energy to light the stars for billions of years. Then in 1939, German physicist Hans Bethe finished working out all of the important details for how 'thermonuclear fusion' would actually work inside stars. Since that time, the assumption that thermonuclear fusion occurs in the cores of all stars has been the fundamental cornerstone to nearly all of modern astronomy.

Meanwhile, in 1930, physicist Wolfgang Pauli predicted that a new type of particle, later called the neutrino by Enrico Fermi, should be produced by many kinds of nuclear reactions, including a crucial step in the process that lights our own sun and all other stars. Neutrinos are not electromagnetic radiation (light, radio waves, X-rays etc) but actual particles that carry almost no mass at all, and travel at essentially the speed of light. They do not interact with matter very easily, so can penetrate MILLIONS of kilometers of matter without being absorbed.

Neutrino Measuring Instruments

Combining these two ideas, it was only a matter of time before physicists had built instruments capable of actually detecting neutrinos from the sun. First there was the huge detector built in the late 1960's by Dr's. Raymond Davis and John Bahcall at the Homestake Gold Mine in South Dakota. For decades the monitored the solar neutrinos but could only find about half the number predicted by the most advanced models of solar nuclear fusion. The 'Solar Neutrino Problem' loomed large in the minds of most astronomers for decades, and could not be made to go away.

In 1998 Japanese physicist Dr. Masatoshi Koshiba and his team used another abandoned mine to build the Super Kamiokande neutrino 'telescope', an enormous tank of water watched by over 11,000 sensitive photodetectors in total darkness. Occassionally, a neutrino slices through the tank, creating a burst of light as the neutrino interacts with a single atom.

By using both the Sudbury Neutrino Detector in Canada, and Kamiokande neutrino data, physicists were able to solve the Solar Neutrino Problem by making a radical discovery. Enroute to Earth, enough time elapses that some of the neutrinos can change to lower-mass neutrinos. The Sudbury Detector could count the low-mass electron neutrinos but not the heavier ones that Kamiokande could. Each detector individually 'saw' a problem but by combining their results they accounted for nearly exactly the predicted numbers after all!

In 2002, astronomer Dr. Robert Svoboda at Louisiana State University plotted the directions of 500 days worth of these occasional neutrino interactions in the Super Kamiokande detector, and tracked the neutrinos backwards to a source in the sky: The Sun.

Sunlight is created in the sun's core and takes several thousand years to escape the core and reach the surface. From there, it is only a short 8.5-minute trip to Earth. Neutrinos, however, take a much faster trip. Literally trillions of neutrinos that pass through our bodies each day while the sun is overhead, but by night neutrinos from the sun also pass through the entire Earth and again pass through your body as you sleep! Once a neutrino is created in the sun's core, it immediately travels at the speed of light, unimpeded by its interaction with the sun's matter, and reaches Earth just under 9 minutes later…not thousands of years! Neutrinos carry energy out of the sun's core, but this has almost no effect on the way the sun behaves or is destined to evolve in time. For more massive stars, however, the energy lost from a massive stars core can equal or exceed by many times the energy lost by electromagnetic radiation. Properly speaking, these should be called 'neutrino stars' because they shine by neutrino 'light' not ordinary light. But it gets worse.

The loss of so much energy from the core cannot be maintained for long. The core begins to implode. Once it becomes so dense that even neutrinos can't escape, they deposit their enormous load of energy in the infalling material. Within seconds, implosion is followed by explosion. The star detonates as a supernova as a blast wave tears through the star from the inside out. Luckily our own sun will not evolve this away. The color figure to the right shows a computer-generated map of where the neutrinos came from and spans 1/8 of the entire sky. The bright spot in the center is much bigger than the disk of the sun because the Super-K detector has very poor resolution. In fact, with a better instrument, the most intense neutrino 'light' should come from a region at the center of the sun less than 1/3 the diameter of the actual solar disk in the sky!

External Links

• Brookhaven Neutrino Laboratory - Brookhaven National Lab, Chemistry Dept.
• "History of the Neutrinos" - Laboratoire d'Annecy-le-vieux de Physique de Particules.
• "Neutrino Experiments" - Laboratoire d'Annecy-le-vieux de Physique de Particules.
• "Neutrinos from the Sun" - John Bahcall , 1969.
• Raymond Davis Biography - Department of Energy (DOE) Research and Development (R&D) Accomplishments.
• "Solar neutrino problem solved" - SpaceFlight Now, Jeff Foust. June 20, 2001.
• Sudbury Neutrino Observatory - The Sudbury Neutrino Observatory Institute (SNOI).
• "Where did all the Neutrinos go?" - Technology Through Time - Issue #59. NASA.
  

Preview Image

The core of the sun fuses hydrogen to helium. A by-product of this is a slippery particle called the neutrino, so why can't scientists find them? Nuclear reactions that produce four different energies of neutrinos in the sun. They are predicted to have energies of 0.86, 1.44 and 14 million electron volts (MeV). By comparison, photons in the visible spectrum carry only about 1 electron volt of energy. (Source: NASA.)