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How SNS Works
Artists conception of SNS. (Click for larger view.)
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Negatively charged hydrogen ions are produced by an ion source. Each ion consists of a proton orbited by two electrons. The ions are injected into a linear accelerator, which accelerates them to very high energies. The ions are passed through a foil, which strips off each ion's two electrons, converting it to a proton. The protons pass into a ring where they accumulate in bunches. Each bunch of protons is released from the ring as a pulse. The high-energy proton pulses strike a heavy-metal target, which is a container of liquid mercury. Corresponding pulses of neutrons freed by the spallation process will be slowed down in a moderator and guided through beam lines to areas containing special instruments such as neutron detectors. Once there, neutrons of different energies can be used in a wide variety of experiments.
The baseline design calls for an
accelerator system consisting of an ion source,
full-energy linear accelerator (linac), and an accumulator
ring that combine to produce short, powerful pulses
of protons. These proton pulses impinge onto a mercury
target to produce neutrons through the spallation
nuclear reaction process. At full power, SNS will
deliver 1.4 million watts (1.4 MW) of beam power
onto the target, and it has been designed with the
flexibility to provide additional scientific output
in the future. This approach is intended to provide
a facility that will meet the neutron intensity needs
of the science community well into the next century.
Ion Source
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The
1,000-foot SNS linear accelerator
is made up of three different types
of accelerators. It is the first
of its kind used to generate a
pulsed energy beam. (Click for
larger view.)
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The SNS
front-end system includes an ion source,
beam formation and control hardware, and
low-energy beam transport and acceleration
systems. The ion source produces negative
hydrogen (H- ) ions—hydrogen with an additional
electron attached—that are formed into
a pulsed beam and accelerated to an energy
of 2.5 million electron volts (MeV). This
beam is delivered to a large linear accelerator
(linac).
Linac
The linac accelerates the
H- beam from 2.5 to 1000 MeV, or 1 GeV.
The linac is a superposition of normal
conducting and superconducting radio-frequency
cavities that accelerate the beam and
a magnetic lattice that provides focusing
and steering. Three different types of
accelerators are used. The first two,
the drift-tube linac and the coupled-cavity
linac, are made of copper, operate at room
temperature, and accelerate the beam to
about 200 MeV. The remainder of the acceleration
is accomplished by superconducting niobium
cavities. These cavities are cooled
with liquid helium to an operating temperature
of 2 K. Diagnostic elements provide information
about the beam current, shape, and timing,
as well as other information necessary
to ensure that the beam is suitable for
injection into the accumulator ring and
to allow the high-power beam to be controlled
safely.
Accumulator Ring
The SNS ring intensifies the high-speed ion beam and shoots it at the mercury target 60 times a second. (Click for larger view.)
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The accumulator ring structure bunches
and intensifies the ion beam for delivery onto
the mercury target to produce the pulsed neutron
beams. The intense H- beam from the linac
must be sharpened more than 1000 times to
produce the extremely short, sharp bunch
of neutrons needed for optimal neutron-scattering
research. To accomplish this goal, the
H- pulse from the linac is wrapped into
the ring through a stripper foil that
strips the electrons from the negatively
charged hydrogen ions to produce the protons
(H+ ) that circulate in the ring. Approximately
1200 turns are accumulated, and then all
these protons are kicked out at once,
producing a pulse less than 1 millionth
of a second (10-6seconds) in
duration that is delivered to the target.
In this way, short, intense proton pulses
are produced, stored, and extracted at
a rate of 60 times a second to bombard
the target.
Target
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The curved, rectangular object is the SNS target. Inside is liquid mercury, where
spallation takes place. (Click for larger view.)
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Because of the enormous amount of energy
that the short, powerful pulses of the incoming
1-GeV proton beam will deposit in the spallation
target, it was decided to use a liquid mercury
target rather than a solid target such as tantalum
or tungsten. SNS will be the first scientific
facility to use pure mercury as a target for
a proton beam.
Mercury was chosen for the target for several reasons: (1) it is not damaged by radiation, as are solids; (2) it has a high atomic number, making it a source of numerous neutrons (the average mercury nucleus has 120 neutrons and 80 protons); and (3), because it is liquid at room temperature, it is better able than a solid target to dissipate the large, rapid rise in temperature and withstand the shock effects arising from the rapid high-energy pulses.
The neutrons coming out
of the target must be turned into low-energy neutrons
suitable for research—that is, they must be moderated
to room temperature or colder. The neutrons emerging
from the target are slowed down by passing them
through cells filled with water (to produce room-temperature
neutrons) or through containers of liquid hydrogen
at a temperature of 20 K (to produce cold neutrons).
These moderators are located above and below
the target. Cold neutrons are especially useful
for research on polymers and proteins.
SNS is an inherently
safe way to produce neutrons because the neutron
production stops when the proton beam is turned
off. It also produces few hazardous materials.
To maximize the safety of the facility, the SNS
is designed to have many levels of containment
to keep potentially hazardous material from getting
into the environment.
Instrumentation and Experiment Facilities
Schematic
SNS instrument suite for showing the 18
currently planned beam lines. Final instrumentation
will be determined by the user community through
the SNS Instrument Oversight Committee. (Click
to see a detailed view of the instruments for
each beam line and the scientific areas where
each instrument will be applicable.)
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SNS will initially have one target station
operating at a frequency of 60 Hertz (Hz). Two "thermal" moderators and two "cold" moderators will be used to service 18 beam lines, and a variety of instruments will be constructed on these beam lines. For the experiment facilities, the SNS expects 1000 to 2000 users each year from all walks of science and industry. Because not all these users will be experts in neutron scattering, the SNS will provide scientists and technicians to maintain and operate the instruments and work closely with the user community.
The broad user community
has been and will continue to be involved in the
selection, design, construction, and operation
of the instruments. The user community recommended
and prioritized a suite of ten instruments for
initial installation at the 60-Hz target station.
Eight beam lines have been be reserved for
cooperative research teams to develop and install
additional instruments.
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Exterior
of the first completed SNS instrument,
the backscattering spectrometer. (Click
for larger view.)
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Scientific Research at SNS
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Atomic structures
are obtained by letting the full neutron spectrum
from the source scatter from a crystal into a
position-sensitive multidetector. |
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Pattern obtained
from the multidetector in time and space for
a crystal of the high-temperature superconductor
YBa2Cu307.
The position of the atoms can be obtained from
the pattern. (Click for larger view.)
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The instruments at the
SNS, such as neutron spectrometers, will be used to determine the positions, or
arrangements, of atoms in crystals, ceramics, superconductors, and proteins. How
does a neutron spectrometer work? A pulse of neutrons generated by the spallation
source follows a flight path to the sample. Because the neutrons have varying
energies and wavelengths, they spread out in time, presenting a continuous spectra
to the sample. When the distance between atoms in a crystal matches the wavelength
of an incident neutron, that neutron is scattered into a multidetector that records
the position (scattering angle) and time of arrival of the scattered neutron.
The result is a pattern of peaks showing the different positions and arrival times
of various numbers of neutrons reaching each point in the multidetector. This
pattern tells scientists how different atoms are arranged in the crystal.
Instrumentation based on
the same principles can be used to determine the atomic structure of glasses and
complex fluids or the residual stresses in industrial parts. Instruments to measure
inelastic scattering will require measurement of the time of neutron travel over
paths leading to and from the sample. In this way, instruments can determine the
excitation spectra of materials of importance and thus the nature of the forces
that hold the atoms in place. The time-of-flight technique makes it possible to
collect a large number of data points for each neutron pulse. The efficiency of
instruments that measure neutron time of flight and the ability of accelerator-based
spallation neutron sources to produce pulsed beams of increasing intensity promise
to provide continuously improved neutron sources in the future.
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