How SNS Works

Artists conception of SNS. (Click for larger view.)

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

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.)

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.)

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

The curved, rectangular object is the SNS target. Inside is liquid mercury, where spallation takes place. (Click for larger view.)

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.)

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.

Exterior of the first completed SNS instrument, the backscattering spectrometer. (Click for larger view.)

Scientific Research at SNS

	Atomic structures are obtained by letting the full neutron spectrum from the source scatter from a crystal into a position-sensitive multidetector.
	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.)

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.