Word: neutrinos
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This is how existing neutrino detectors work in Japan, Italy, Russia and the U.S. What makes S.N.O. different is its exclusive use of heavy water, abundantly available in Canada because it is stockpiled for use in a type of nuclear reactor Canadians favor. Says Barry Robertson, S.N.O.'s associate director: "It's the heavy water that makes this project worth the trouble." An extra neutron in the nucleus doesn't make the water's appearance, chemistry or taste any different from ordinary water used in other detectors. It does, however, change its nuclear structure enough to make this observatory sensitive...
PARTICLE SLEUTHS When Wolfgang Pauli first proposed the existence of the neutrino in 1930, he labeled his hypothetical particle "a frightful thing." The neutrino would neatly explain a tiny energy imbalance in certain nuclear reactions, but it would also be so ethereal that the average neutrino could zip through a trillion-mile-thick chunk of lead without hitting a single atom. Since the particles would presumably sail undetected through any measuring device, Pauli lamented, his clever idea could never be proved correct...
Rather than try to detect one neutrino at a time, Reines and Cowan used a nuclear reactor that spewed out trillions of neutrinos every second. With so many particles, they reasoned, observed over a long enough period of time (it ended up being years), they should be able to measure at least a few neutrino impacts in their detector, a tank of chemical-tinged water...
...discovery of what the Nobel citation called one of "nature's most remarkable subatomic particles" tied up an important theoretical loose end and spawned a new field of neutrino physics. But by the early 1970s, it was also clear that the neutrino was only one element in an elegant organizational scheme, now known as the Standard Model, by which nature groups the subatomic particles...
...ordinary matter, physicists had learned, was made of four basic particles: electrons, neutrinos and two kinds of quarks. But there was another family of particles, plentiful in the early universe but now found almost exclusively in nuclear accelerators, that seemed to be divided into the same four types: the muon (a sort of heavy electron), the muon neutrino and two more quarks. And in 1976, Stanford University physicist Martin Perl announced he had found a third, even heavier electron, which he dubbed the tau--a discovery that earned him the other half of this year's physics Nobel. Perl...