William & Mary

Faster Than A Speeding Bullet, And Even Harder To See

Particle Physicists Utilize Innovations in Neutrino Detection


Physicist Jeff Nelson, at the College of William and Mary, explains a device that
    tests the fluorescent plastic used in neutrino detection experiments. Photo  by Eric Dale.

             Every second, a number of neutrinos equal to fifty thousand times the number of stars in the Milky Way pass right through your body at nearly the speed of light. These infinitesimally tiny particles are the most rugged travelers in the Universe: after their birth in exploding stars and nuclear reactions at the core of our sun, neutrinos rocket out in every direction, stopping at nothing. But despite their overwhelming ubiquity, neutrinos are nearly impossible to detect.

            Physicists often refer to neutrinos as “ghost particles,” because they elude detection by zipping right through any obstacle like a knife through butter. We consider our bodies to be solid, but to neutrinos, your atoms are separated by gaping holes, and a collision with any of them is highly unlikely. In order to detect neutrinos, particle physicists must construct instruments that facilitate neutrino impacts. This challenging task requires extraordinary measures—immense neutrino detectors, international scientific collaboration, and abandoned iron mines.

            As with many areas of physics, the existence of neutrinos was first hypothesized by scientists trying to balance an equation. When atoms decay from one form into another, they appear to lose energy, violating a fundamental law of physics. Scientists hypothesized and subsequently proved that the seemingly missing energy is carried away by the neutrinos released in the radioactive decay.

            This type of radioactivity is an integral part of the way nature functions, so understanding neutrinos is an important step towards creating a unified theory of physics that explains the machinations of the Universe. Neutrinos may also help us resolve some persistent mysteries about what happened just after the Big Bang. However, solving the problem of effective neutrino research has been a more recent endeavor, with several successes. Particle physicists have worked out some clever ways to nab speeding neutrinos.

            Deep underground in the outskirts of Chicago is a neutrino gun of sorts—Fermilab’s “Neutrinos at the Main Injector” beam. It fires short bursts of man-made neutrinos northwest into an old iron mine in Soudan, Minnesota, where a gigantic neutrino detector resides. The detector consists of many layers of plastic rods that light up during neutrino impacts. Fiber optic cables attached to the rods transmit these tiny flashes of light to a computer that records each neutrino. The whole endeavor is known as MINOS, the Main Injector Neutrino Oscillation Search.

            Most of the neutrinos pass straight through the detector, but the few daily impacts are carefully documented. The time it takes them to arrive from Chicago allows physicists to hone in on the velocity of neutrinos (which was briefly thought to be faster than the speed of light due to a loose plug in an Italian experiment), as well as their mass, which we have yet to precisely determine.

            But something strange happens when you look for neutrinos fired from a beam 735 kilometers away: a few of them disappear. The Soudan detector wasn’t catching nearly as many neutrinos as expected, and this strange phenomenon led to the confirmation of different “flavors” (yes, that’s the word scientists use) of neutrinos. Neutrinos seem to undergo periodic oscillations between three different types, which led to the profound results in Soudan.

            Physicists working on the Super-K project in Japan have taken an approach to neutrino detection that may be more cost effective. Instead of glowing plastic rods, they use a tank full of fifty thousand tons of pure water surrounded by hyper-sensitive light detectors. They’ve been able to plot the sun’s path across the sky by measuring the tiny flashes of light resulting from neutrino impacts with atoms of water in the tank.

           While using hundreds of tiny flashes of blue light to chart the sun’s location on the inside of a giant underground sphere sounds strangely beautiful, there may be an even more eye-catching use for neutrinos. They travel so fast and are so hard to stop that when an aging star explodes and releases massive amounts of radiation, the neutrinos generated by the blast escape and start speeding away before any photons do. This means that the light from a supernova does not reach the Earth until after its neutrinos do. Sure enough, when the explosion of a star in the Large Magellanic Cloud was observed in 1987, neutrino scientists reported a corresponding spike in their readings. Neutrino detectors across the globe are now networked together, waiting to inform astronomers of some future supernova in the Milky Way galaxy.

            In a mere six decades, physicists have progressed from ignorance about even the existence of the neutrino to the ability to make their own, to detect them in large quantities, and to distinguish different flavors. As our beams become more intense and our detectors become larger and larger, the mysteries of neutrinos and of the Universe may reveal themselves. And while the waves of these tiny particles will go on whether we understand them or not, studying them will never go out of flavor.

 * This story was written as part of a course in the Environmental Science & Policy program at the College of William & Mary, ENSP 249: Science Communication. For more information on the course, please email the course instructor, Dr. Ibes at dcibes@wm.edu.



 Bionta, R.M. et al. 1897. Observation of a neutrino burst in coincidence with Supernova 1987A in the Large Magellanic Cloud: Physical Review Letters. v. 58, p. 1494–6.

 Cravens, J.P. et al. 2008. Solar neutrino measurements in Super-Kamiokande-II: Phsyical Review D. v. 78, p. 032002-1–11.

 Goodman, M.C. 1995. The atmospheric neutrino anomaly in Soudan 2: Nuclear Physics B (Proceedings Supplement). v. 38, p. 337–42.

 Jones, T. 2005. Nelson prepares to capture neutrinos a half-mile underground: W&M News. web.wm.edu/news/archive/index.php?id=4390 (accessed February 2014).