If the electron neutrino is changing flavors, part of the trick to watching it disappear is to position the detector in the trough of its probability wave. The smaller the mass difference, the longer the oscillation's wavelength is going to be. On the other hand, you don't want to put the detector much beyond the wavelength, because the total neutrino flux falls off with the square of the distance from the source. The farther away you get, the bigger and more expensive your detector has to be. Early results from Kamiokande had shown a slight surplus of electron neutrinos as well as the famous muon-neutrino deficit, suggesting that a small percentage of muon neutrinos were going to electron neutrinos and that a site one kilometer or so from the neutrino source would be a good distance from which to watch this happen. Therefore, the Palo Verde detector was built 890 meters from Reactors 1 and 3, and 750 meters from Reactor 2. Says Petr Vogel, senior research associate in physics and the house theorist for the project, "This is 10 times farther away from the reactor than any previous such experiment, which means that our flux is 100 times less. We have to really push the detector technology in order to see any neutrinos at all."

Boehm and Vogel have been chasing neutrinos for 20 years, starting with an experiment in Grenoble, France, in 1979. (In fact, they wrote the book on massive-neutrino physics.) The field was launched that year at Caltech by Murray Gell-Mann, then Millikan Professor of Theoretical Physics, and postdocs Harald Fritzsch and Peter Minkowski, who did the first calculations of neutrino oscillations. Says Vogel, "Mixing is the hottest issue in particle physics today. Since 1992, four or five other experiments have confirmed that the muon deficit exists. Nobody doubts that neutrinos have mass any more, so the question now is what the mass is and what the mixing angle is. That will be the program for the next decade, to explore this parameter space."

The Palo Verde project is about five years old. It took three years for grad students Brian Cook (MS '93, PhD '96), now at JPL, and Mark Chen (PhD '94); Humboldt Fellows Ralf Hertenberger and Andreas Piepke; postdocs Nick Mascarenhas and Vladimir Novikov; and staff engineer John Hanson to design, develop, and test the detector elements, while member of the professional staff Herb Henrikson, who got his BS in mechanical engineering at Caltech in 1953 and has been a project engineer here ever since, did the nuts-and-bolts design. At the same time, Boehm had to find a site for the project, line up money and collaborators, bid out the construction contracts, and so forth. A year's worth of ground was lost to a competing experiment, subsidized by the French nuclear-power industry, when the initial plan to use the San Onofre reactor, about an hour's drive south of Caltech, fell throughÜendangered gnatcatchers were nesting on the proposed excavation site. Assembling the detector apparatus and building the underground chamber that houses it took another year, followed by a six-month shakedown period. The detector has been fully operational and taking data since October 1998 under postdoc K. B. Lee and colleagues from Caltech, Stanford, and the University of Alabama.

Detecting something that has built a career out of not interacting with matter in any form is, shall we say, a bit of a challenge. You have to rely on indirect evidence: in this case, the flashes of light produced when a neutrino hits a proton, creating a positron (or anti-electron) and a neutronÜas mentioned earlier, the neutron-decay reaction run backward. To maximize the collision rate, the detector contains 12 tons of proton-rich mineral oil, whose average molecular formula is C22H46. The oil is heavily laced with pseudocumene, abenzene derivative that has half a dozen easily excitable electrons per molecule. The positron jangles these electrons as it screams by with an average kinetic energy of three million electron volts (MeV). In a process called scintillation, the excited electrons emit flashes of blue light that are recorded by photomultipliersÜlight detectors capable of sensing a single photonÜand the energy measurement of each flash is sent to a computer. The positron travels about two centimeters, losing energy with every electron it twangs. But to slow down is to dieÜeventually (within about 30 billionths of a second, that is) it no longer has enough zip to get by its mortal enemy. The last electron it runs into annihilates it, producing two gamma rays at 0.5 MeV, which is the energy equivalent of the mass of an electron or positron. These gamma rays also jangle the pseudocumene's electrons, causing two more pulses of light.