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The purple star marks the preferred
mixing angle and mass difference derived from Super
Kamiokande’s results, within the area of experimental
uncertainty enclosed by the purple line. However,
the Palo Verde results exclude the area lying above
and to the right of the red line, so only the blue
area remained to be explored. (As E&S went
to press, the accumulation of additional data had
excluded this area as well.) The mixing angle is plotted
horizontally, with 0 being no mixing, and 1 being
the maximum possible mixing, i.e., a 45° mixing
angle. The mass difference is plotted vertically in
logarithmic units of electron volts squared.
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But electron
neutrinos must mix, because of another long-standing conundrum called
the solar neutrino problem. For decades, people have been measuring the
electron-neutrino flux from the most powerful nuclear reactor in our neighborhood—our
friend, Mr. Sun. These measurements are only coming up with about half
as many neutrinos as the solar physicists say should be produced. Either
we don't understand the nuclear reactions going on inside the sun as well
as we think we do, which is highly unlikely, or else electron neutrinos
are disappearing en route to Earth. With a flight path of 93 million miles,
even a very tiny mass and minuscule mixing angle would show an effect.
The Palo
Verde collaboration will continue to run the experiment through the end
of 1999 in order to refine the statistical accuracy of their numbers tenfold—down
to the residual uncertainty left in the calculations of the reactor's
flux and detector's efficiency. Then the detector gets dismantled. "It's
expensive to run," says Boehm. "We have to pay rent to the utility. We
have to keep somebody on site to maintain the complex electronics, do
all the calibrations, change computer disks, and so on. That person also
has to reset all the detectors whenever there's a thunderstorm—we
get power outages all the time." You'd think that, being on the premises
of a nuclear power plant, they'd have an uninterruptible source of electricity,
but no—all their amps come by wire from Phoenix.The
next step, says Vogel, is to explore longer wavelengths. The Caltech group
is collaborating on a proposal to build a new detector, called Kamland,
down in that Japanese zinc mine. The mine is located near the city of
Kamioka, which lies some 40 kilometers north of Osaka near the center
of the main island of Honshu. Japan gets about one-third of its electricity
from nuclear power, and Kamland will use the 16 nuclear plants on the
island as its neutrino source. (If calibrations with three reactors at
Palo Verde were tough, calibrating this detector is going to be a real
bear!) The plants lie from 100 to 300 kilometers away from the detector,
which will contain 1,000 tons of scintillator oil. But even with a detector
that size, the collaboration expects to see only about a thousand neutrinos
a year, because of the distances involved. Still, this very long baseline
will make the experiment sensitive to mass differences 1,000 times smaller
than either Super Kamiokande or Palo Verde could see.
The Palo
Verde project has been very fruitful, says Boehm. "We have clearly shown
that, unlike atmospheric (muon) neutrinos, reactor (electron) neutrinos
do not oscillate at these wavelengths. We explored a promising set of
wavelengths, and answered a challenging question in neutrino physics while
advancing the state of the art in scintillator technology." And they did
so for a bargain-basement price: the whole shebang only cost about $2.5
million to build, which is peanuts as particle physics goes. Although
the Department of Energy and the collaborating institutions have helped
finance the project, Caltech put up a substantial contribution out of
the provost's discretionary funds, says Boehm. "Both Jennings and Koonin
felt this was an important opportunity, and have been very supportive.
We certainly appreciate it."
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