Do neutrinos streaming from the sun have mass or not? It's one of the
puzzles of physics. Confirming that neutrinos have mass could help astrophysicists
account for part of the missing matter in the universe, possibly providing
insights into the fate of our rapidly expanding cosmos.
A neutrino is
an electrically neutral subatomic particle that penetrates steel, concrete,
our bodies, and the earth itself. Neutrinos are produced by stars, including
the sun, by the decay of radioactive isotopes, and by nuclear reactors.
In June 1998 an international team of scientists
at the Super-Kamiokande detector in Japan—a specially constructed
underground lake of highly purified water surrounded by a huge array
of light detectors—reported evidence that neutrinos have mass.
It is important that scientists at other neutrino facilities verify
this finding and accurately measure neutrino properties.
A proposal for a detector facility that could bring
scientists even closer to a definitive answer to the question of whether
neutrinos have mass will be made to DOE by mid-2000. The approximately
$50-million facility, called the Oak Ridge Laboratory for Neutrino Detectors
(ORLaND), would be built near the Spallation Neutron
Source (SNS). The ORLaND proposal is being prepared by a group of
nearly 100 scientists from universities and national laboratories in
the United States and abroad. Frank Avigonne, professor of physics at
the University of South Carolina, is the spokesman of the ORLaND collaboration.
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Artist’s
conception of ORLaND, which could be located near the SNS. |
The SNS will be the source of the world's
most intense, pulsed beams of neutrons for use in neutron scattering
research. The SNS facility will include a liquid-mercury target for
bombardment by beams of protons from the SNS accelerators and beam lines
to carry neutrons produced in the target to experiment stations. It
is the target stage of the SNS that interests physicists like Yuri Efremenko,
a University of Tennessee research professor of physics based in ORNL's
Physics Division. He, along with Frank Plasil, an ORNL corporate fellow
in the division; Glenn Young, a Physics Division section head; and Ken
Carter from the Oak Ridge Institute for Science and Education, are helping
draft the neutrino proposal and leading the ORNL effort to construct
the facility.
"When the protons interact with the mercury
atoms in the target," Efremenko says, "not only neutrons, but also unstable
subatomic particles called pions are formed. After 26 nanoseconds, each
pion decays into a neutrino and a negatively charged particle called
a muon. Each muon is unstable, and after 2.2 microseconds, it decays
into one electron and two neutrinos. So, for every positively charged
pion produced from the mercury target, we get three neutrinos."
The
advantage of the SNS as a source of neutrinos is that it does not produce
a steady stream of neutrinos as do the sun and reactors. The neutrinos
(like the neutrons) will be produced in pulses whose on and off times
are known. Thanks to computers, the arrival times of the neutrinos at
the ORLaND detector can be predicted accurately. Therefore, scientists
will know exactly when to expect a neutrino signal in the detector.
If they see weak signals in between the pulses, they will attribute
them to background radiation such as cosmic rays hurtling through the
atmosphere. Thus, they will be able to separate real events from spurious
ones.
The main detector of the ORLaND facility will be a cylindrical
tank placed in a 33-m- (110-ft-) deep bunker near the SNS target building.
The detector itself will be about 15 m (50 ft) deep and 15 m in diameter.
On top of it will be 4.5 m (15 ft) of large steel and concrete blocks
to reduce the intensity of cosmic rays coming through. The facility
will provide space for outside groups of researchers to assemble their
own special detectors as part of an international research effort on
neutrino measurements.
"ORNL's 2000-ton liquid scintillation detector
will be 25 times smaller than the Japanese detector," Efremenko says.
"It will contain mineral oil mixed with an additive that emits light
when struck by charged particles. The emitted light will be detected
by an array of thousands of photomultipliers, each about 20 cm (8 in.)
in diameter." Despite its small size, the proposed ORLaND detector will
still be the world's most effective neutrino facility because of the
large neutrino flux produced by the SNS during routine operation.
When
a neutrino strikes a proton in the hydrogen- and carbon-containing mineral
oil, a positively charged electron (positron) and a neutron are produced.
The neutron then interacts with another proton, thereby emitting a gamma
ray with a total energy of 2.2 million electron volts. The gamma-ray
photons, in turn, collide with electrons, imparting to them a recoil
energy. It is these scattered electrons that finally produce light in
the oil-additive liquid scintillator, resulting in the indirect detection
of the neutrino.
"Our main goal," Plasil says, "is to look
for neutrino oscillations, in which one type of neutrino changes to
another type. If we detect neutrino oscillations, then we will know
that neutrinos have mass. If they have mass, the universe will be heavier
than we thought, and our current understanding of subatomic particles
embodied in the so-called Standard Model will have to be revised. We
also plan to conduct experiments to determine the probability with which
neutrinos interact with different types of nuclei. This research will
be of great interest to the astrophysics community."
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