Though far fewer in number than the neutrinos generated by the sun's proton-proton reaction (they represent only a small fraction, a mere 0.01 percent, of the total flux of neutrinos coming out of the sun), these boron-reaction neutrinos are easier to detect. In 1946 Bruno Pontecorvo, a student of Fermi's, first suggested how such high-energy neutrinos might be captured. "His idea," says Wilkerson, "was to take an atom of chlorine-37, which is a stable nucleus with 17 protons and 20 neutrons, and add a neutrino to it. That turns the chlorine into argon-37, which is radioactive and can be watched for its decay." More than 20 years went by, though, before physicists could actually embark on such an ambitious endeavor.

UNDERGROUND TELESCOPES

The world's first neutrino observatory was finally established in America's heartland in 1967, and it has been gathering vital clues on the quirky nature of the neutrino ever since. The observatory's "telescope" is a huge tank of chlorine-rich cleaning fluid, 100,000 gallons of perchloroethylene, set in the Homestake gold mine situated nearly a mile beneath the Black Hills of South Dakota. Such a depth is required to keep the measurements free from disruptive cosmic rays. Additional shielding was added to eliminate interference from natural sources of radioactivity within the deep chamber. It took 20 railroad tanker cars to fill the tank, roughly the amount of stain remover American consumers use in a day (see Figure 1.6).

With this gargantuan apparatus, University of Pennsylvania radio chemist Raymond Davis, the founding father of neutrino astronomy, has been catching a few electron neutrinos out of the legions that are continually spewed into the solar system as the sun burns its nuclear fuel. For more than two decades now, the chlorine atoms in his cleaning fluid have been occasionally stopping some of the cagy particles. Following Pontecorvo's original scheme, the electron neutrino gives itself away by turning an atom of chlorine into traceable radioactive argon. The argon atoms, suspended within the perchloroethylene, are extracted after 2 months of exposure by bubbling helium gas through the cleaning fluid. Once the gas is collected, it is sent through a cold trap, where the argon freezes out. The extracted argon is then placed in low-background proportional counters, which record any radioactive decays of the argon-37 atoms.

But the results over the years suggest that the chlorine is capturing neutrinos (and subsequently transforming into argon) at an unexpectedly



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