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100,000 HOURS OF OAK RIDGE ELECTRON LINEAR ACCELERATOR OPERATION
(This article also appears in the Oak Ridge National Laboratory
Review (Vol. 26, No. 1), a quarterly research and development
magazine. If you'd like more information about the research
discussed in the article or about the Review, or if you have any
helpful comments, drop us a line. Thanks for reading the Review.
When the Oak Ridge Electron Linear Accelerator (ORELA) went on line
in 1969, the U.S. nuclear power industry was enjoying its heyday,
the Soviet Union was supporting communist insurgencies in the Far
East and Central America, and Richard Nixon occupied the White
House. Nearly a quarter-century later, both the world and the field
of nuclear research have seen a lot of changes, but ORELA is still
going strong. In fact, on October 13, 1992, ORELA logged its
100,000th hour of on-line operation--an indication of the enduring
vitality and value of the research done at the facility.
The facility's accelerator, target room, and several of the test
stations are located under a group of grass-covered mounds not far
from the Laboratory's main entrance. The more distant test
chambers, located up to 200 meters (650 feet) from the target room,
can be seen poking up out of adjacent parking lots.
At the heart of ORELA is a 180-MeV electron linear accelerator that
uses electromagnetic waves to fire bursts of electrons pulsed from
a hot-cathode gun down a 23-meter (75-foot) acceleration tube at
nearly the speed of light. At the end of the tube this joyride
comes to an abrupt end as the electrons slam into a water-cooled
tantalum target, where collisions with the target and the
surrounding water produce neutrons with a wide range of energies.
This spray of neutrons is then channeled to one or more of the test
chambers. Seven stations, located from 10 to 200 meters (33 to 650
feet) from the target, are used because neutrons of varying
energies and intensities are needed for different experiments--and
these qualities vary with the distance the neutron travels from the
target.
ORELA'S ROOTS
Jack Harvey, director of ORELA and a research physicist since 1950,
recalls that in 1969 the facility represented a quantum leap
forward in the technology used to obtain neutron cross sections for
various materials.
Cross sections are measurements of the likelihood that neutrons of
a given energy will interact with the nucleus of a material, rather
than passing through it. Because most nuclear reactions produce a
lot of neutrons, nuclear researchers at the time of ORELA's
construction were interested in learning more about neutron cross
sections to help them develop more effective shielding materials
for use in production reactors or nuclear research facilities.
"Thirty or forty years ago," says Harvey, "investigating neutron
cross sections became an area of great interest both for people
building reactors and those studying basic nuclear physics." Most
early research facilities measured neutron cross sections using a
"fast chopper" in conjunction with a reactor. "The chopper," says
Harvey, "was a rotating cylinder with slits in the sides, which
provided microsecond-long bursts of neutrons. This was adequate for
measuring cross sections in low-energy regions, up to a few
thousand electron volts, but the resolution was not good enough to
work at higher energies."
In 1965 plans were announced to build plutonium fast breeder
reactors, including Oak Ridge's Clinch River Breeder Reactor
project, which was ultimately canceled in 1983. Breeder reactors
rely on neutrons given off during the process of nuclear fission by
reactor fuel, such as plutonium-239, to transform a blanket of
non-fissionable material, like uranium-238, into plutonium-239.
This approach to reactor design harnesses the fission process for
the production of both fuel and power. Because plutonium breeds
fissionable material best at high energies, researchers needed
high-resolution information on nuclear interactions that occurred
at energies between a few thousand electron volts and a few hundred
thousand electron volts. "As a result," says Harvey, "funding was
approved for a facility capable of measuring the cross sections for
reactions in these regions. That's why ORELA was originally built."
ORELA was designed primarily to measure neutron cross sections of
materials thousands of times more precisely than could be done
using fast choppers. At the time of its construction, the facility
produced neutron bursts that were 10 times as intense and 5 times
narrower than those of comparable facilities, filling a gap between
experimental results provided by low-energy fast choppers and
high-energy cross sections provided by Van de Graaff accelerators
then operating at ORNL and other laboratories.
STRETCHING THE NEUTRON
Measuring neutron cross sections of various materials isn't the
only activity going on at ORELA, however. Because the facility's
pulsed neutron source is so intense compared to its background
"noise," it has recently been used to probe the structure of the
neutron itself.
The neutron is believed to be held together by the so-called
"strong" force, one of the four physical forces in the universe, a
group that also includes gravity and electromagnetism. Previous
experiments have suggested that neutrons are made up of three
subatomic particles, known as quarks. One quark has two-thirds of
a positive charge, and the other two have one-third of a negative
charge each, making the total charge of the neutron zero. To
measure the force holding these quarks together, researchers had to
find a way to stretch the neutron, slightly separating the positive
quark from the negative quarks.
To accomplish this feat, a group of Austrian researchers, along
with Harvey and Nat Hill, a retired researcher from the
Instrumentation and Controls Division, fired beams of low-energy
neutrons at a target of lead-208, an isotope with an extremely
strong electric field around its nucleus. The interaction between
this electrical field and the neutron's charged quarks causes the
quarks to separate slightly.
Using a new detector and other special electronics provided by the
Austrian research team, it was determined that, as a result of this
separation, more of the neutrons were scattered by the lead-208
sample, rather than passing through. By comparing the results of
this experiment to those obtained when no target was present,
researchers determined the change in the "size" of the neutron
resulting from its interaction with the electrical field of the
lead-208 nucleus, and from that they determined the magnitude of
the force holding the neutron's charged quarks together.
"Twenty earlier attempts at measuring this force at other research
centers have resulted in values with margins of error that were
greater than the values obtained from the measurements," said
Harvey. "The value obtained in the work with the Austrians was four
times larger than the margin of error."
PHONONS AND SUPERCONDUCTIVITY
ORELA also provides researchers with an opportunity to study the
role of phonons in superconductivity. Superconductivity occurs when
electrons in a metal are attracted to each other, but because
electrons are all negatively charged, they usually repel each
other. Under certain conditions, however, this repulsion can be
overcome.
For example, a fast-moving electron traveling through an array of
positively charged, slow-moving metal ions attracts the ions,
causing them to move toward it as it passes and leaving a
concentration of positive charges in its wake. These positive
charges may then attract other electrons, causing an indirect
attraction between electrons.
The units of vibrational energy that cause the metal ions to move
toward the electron in this example are called phonons. Studies of
the vibrational spectra, or phonon spectra, of materials that
conduct electricity normally in one temperature range and are
superconductors in another have shown systematic differences
between the two states, but until recently, the hypothesized change
in the phonon spectrum at the transition point between the two
states had not been observed for the new, high-temperature
superconductors.
To get a good look at phonon behavior at the superconducting
transition point, Herb Mook and other researchers at ORELA used a
technique called neutron resonance absorption spectroscopy (NRAS)
to measure the vibrational spectra of the high-temperature
superconductor modes of bismuth, strontium, calcium, and copper
(BSCCO). The NRAS technique has several advantages, including being
able to study the vibrations from each element in a sample
separately, obviously a plus when dealing with molecules as complex
as BSCCO. Another advantage of NRAS is that it enables researchers
to study the structure of non-uniform material by varying its
orientation in the neutron beam.
Using this technique, scientists at ORELA were able to observe a
change in the phonon spectrum of the BSCCO's copper component at
the transition point. This finding shows that there is a close
relationship between phonons and high-temperature superconductivity
and suggests that any explanation of the high transition
temperatures of high-temperature superconductors should take the
role of phonons into account.
NEW TOOLS
The latest addition to ORELA is an intense "slow" positron-
generating facility developed by Lester Hulett of ORNL's Analytical
Chemistry Division. Positrons are electron-sized particles that
have positive charges equal to the electron's negative charge. The
new facility takes advantage of gamma photons that are scattered
beyond ORELA's primary tantalum target by placing a secondary
tungsten target in their path. When the photons strike the
tungsten, they are converted to slow, or low-energy, positrons and
channeled by a magnetic field into an adjacent experiment room.
If this gamma radiation weren't a cost-free by-product of ORELA's
neutron-producing activities, it would cost about $300,000 annually
to generate it. Reseachers are taking full advantage of this
windfall by using the resulting positrons to conduct a range of
slow-positron spectroscopy studies, such as those planned for a
cooperative research and development agreement between ORNL and
AT&T Bell Laboratories.
This joint effort will employ slow-positron spectroscopy to develop
more effective semiconductor devices. Inefficiencies in
semiconductor performance are often the result of defects, such as
missing atoms, which trap positrons. The number and distribution of
these defects determine how the device's performance will be
affected. Once snared by a defect, a positron's lifetime is
extended, changing the energies of the gamma rays produced when it
finally interacts with an electron and is annihilated. Measuring
these gamma emissions enables researchers to determine if defects
are present in a semiconductor and whether or not they occur in
areas critical to the device's function.
The positron facility is also being used to try to get a
fundamental understanding of how organic molecules like benzene are
ionized--how electrons are removed--by comparing spectra produced
by positrons with those produced by electrons and photons. This
line of study may eventually lead to better ways of doing
analytical mass spectrometry.
Understanding the ionization process in medium-sized organic
molecules, like benzene, may also result in a better understanding
of how larger organic molecules, such as DNA, are ionized.
Preliminary studies using bacteria have found that extremely low
doses of positrons may have mutagenic effects.
WHAT'S NEXT?
Over the years, thousands of publications, reports, and
presentations have documented research results from ORELA, but
researchers are not content to rest on their laurels. Plans for the
future include continuing the research initiatives mentioned
previously, more collaborations with the Austrians--this time
hoping to measure the charge distribution of the neutron at lower
energies, and further forays into the realm of astrophysics to
determine the properties of heavy elements formed in and around
stars as a result of lighter elements capturing neutrons. ORELA is
also involved in detector development for DOE's multibillion-dollar
Superconducting Super Collider project, as well as other radiation
damage and characterization studies.
"Normally, accelerators have less than a 20-year life span, but
we're 23 and we're still going strong," says Harvey, who shows no
signs of slowing down either. "We really have an excellent facility
here, and it's running well. We've done a lot with it, and I think
there's still a lot of good work to be done."
Jim Pearce
(keywords: nuclear research, linear accelerators, neutrons)
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Date Posted: 1/26/94 (ktb)