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Why Build SNS?
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Evolution
of the performance of reactors and pulsed
spallat ion sources. In recent years, dramatic
improvements in accelerator technology
have made it possible to design and construct
a source to produce very intense neutron
pulses (updated from Neutron Scattering,
K. Skold and D. L. Price, eds., Academic
Press, 1986). |
Neutron scattering is used by a variety of scientific disciplines to study the arrangement, motion, and interaction of atoms in materials. Neutron scattering is important because it provides valuable information that often cannot be obtained using other techniques, such as optical spectroscopies, electron microscopy, and X-ray diffraction. Scientists need all these techniques to provide the maximum amount of information on materials.
Why a new spallation source?
The neutron science community has long recognized
the need for both reactor-based (steady-state
or continuous) and accelerator-based (pulsed)
neutron sources. For many research problems of
interest, having neutrons available in a series
of pulses is just as good or better than having
a continuous neutron source. Neutron pulses can
be produced with a much higher intensity than
that available from continuous sources. In recent
years, dramatic improvements in accelerator technology
have made it possible to design and construct
a source to produce intense neutron pulses. SNS
will produce pulses that each contain a neutron
intensity 50 to 100 times higher than that obtainable
from the best continuous source. Intense, short-pulse
neutron beams from accelerator-based sources
make it possible to perform time-of-flight analysis
of the scattered neutrons and to study a wide
range of scientific problems and perform real-time
analysis.
What are the advantages of
a pulsed spallation source?
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Neutrons are separated in energy after traveling over a fixed path (L), permitting neutrons of many different energies and wavelengths to be used for experiments. |
Each pulse contains neutrons of a range of wavelengths and energies; the highest-energy neutrons have the shortest wavelengths, and the lowest-energy neutrons have the longest wavelengths. Because thermal neutrons move slowly enough, their progress can be timed accurately over short distances. Each pulse contains neutrons of all thermal energies, so neutrons of different energies can be separated by letting them travel down a short path of a few meters. The high-energy neutrons reach the sample ahead of the medium-energy neutrons, and the lowest-energy neutrons take the longest to arrive at the sample. Because the neutron energies are spread out in time, the energy of an individual neutron is easily determined by its "time of flight" to the sample. Because thermal neutrons of all energies are available for use in scattering experiments, the time-of-flight technique enables the collection of many data points for each source pulse reaching a sample. Furthermore, unlike the usual situation at a continuous neutron source, it is not required that the neutron detectors move during an experiment, making it easy to arrange large detector arrays or multidetectors around the sample.
How will SNS bring new opportunities to the
neutron science community?
Although the United States pioneered the development
and use of early neutron sources, Europeans
and the Japanese have capitalized on this early
experience and developed newer sources that
have been the best in the world for the past
15 to 20 years. Even these sources, however,
are quite old. Because SNS will be the most
advanced and powerful pulsed neutron source
in the world, it will provide research opportunities
unavailable elsewhere. Hence, this unique facility
will attract scientists and researchers in
a variety of disciplines from all over the
globe. Studies conducted at SNS will go
beyond basic research and development and will
lead to technological and industrial breakthroughs
that will ultimately benefit not only the scientific
community but also the business and industrial
communities.
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