253
See also: Cosmic rays 140 ■ Energy generation 182–83 ■ Gravitational
waves 328–31
NEW WINDOWS ON THE UNIVERSE
scientists thought, billions of
neutrinos should be passing through
every square centimeter of Earth’s
surface every second, but perhaps
only one in one hundred billion
might interact with atomic matter.
Davis thought that neutrinos
might be detectable through their
involvement in a type of radioactive
decay called beta decay. In theory,
an energetic neutrino should be
able to convert a neutron in an
atomic nucleus into a proton. In
his experiments, Davis found
that, on very rare occasions, a
neutrino passing through a tank
of a chlorine-containing substance
would interact with the nucleus of
a stable chlorine atom to produce
a nucleus of an unstable isotope
of argon called argon-87.
Homestake experiment
In 1964, in what was called the
Homestake experiment, Davis
began a trial using a large tank
of a chlorine-containing chemical
as a detector. An acquaintance of
Davis, astrophysicist John Bahcall
calculated the theoretical number
of neutrinos of different energies
that the sun should be producing
and, from this, the rate at which
argon-37 should be produced
in the tank. Davis began counting
the actual numbers of argon-37
atoms produced.
Although Davis’s experiment
showed conclusively that the
sun does produce neutrinos, only
about one-third of the number
of argon-37 atoms predicted
by Bahcall were detected. The
discrepancy between the number
of neutrino interactions predicted
and those detected became known
as “the solar neutrino problem.”
Building on his 1989 work,
in 1999, Masatoshi Koshiba
discovered the cause of the
discrepancy at Japan’s huge Super-
Kamiokande neutrino detector.
Neutrinos were found to oscillate
between three different types—
the electron neutrino, the muon
neutrino, and the tau neutrino—
while traveling through space.
Davis’s experiment had detected
only the electron neutrinos. ■Davis’s neutrino detector was
placed deep underground to protect
it from cosmic rays (another possible
source of neutrinos).Neutrino physics is largely
an art of learning a great
deal by observing nothing.
Haim Harari
Israeli physicist
Ray DavisRaymond Davis was born
in Washington, D.C., in 1914.
He earned a Ph.D. in physical
chemistry from Yale University
in 1943. Davis spent the later
years of World War II in Utah,
observing the results of
chemical weapons tests.
From 1946, he worked at a
laboratory in Ohio, carrying
out research on radioactive
chemical elements. In 1948,
he joined Brookhaven National
Laboratory, on Long Island,
which was dedicated to
finding peaceful uses for
nuclear power. He spent the
rest of his career studying
neutrinos. Davis retired from
Brookhaven in 1984, but
continued his involvement
in the Homestake experiment
until it ended in the late 1990s.
Davis met his wife, Anna,
at Brookhaven Laboratory
and together they had five
children. He shared the
Nobel Prize in Physics in
2002 with Masatoshi Koshiba
for pioneering contributions
to astrophysics. He died
in Blue Point, New York,
age 91 in 2006.Key work1964 Solar Neutrinos II,
Experimental