Spallation Neutron Source

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April 2005 aerial photo of the SNS site, Oak Ridge, Tennessee, USA. Photo courtesy of Oak Ridge National Laboratory.
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April 2005 aerial photo of the SNS site, Oak Ridge, Tennessee, USA. Photo courtesy of Oak Ridge National Laboratory.

The Spallation Neutron Source (SNS) is an accelerator-based neutron source being built in Oak Ridge, Tennessee, USA, by the U.S. Department of Energy (DOE). SNS is being designed and constructed by a unique partnership of six DOE national laboratories: Argonne, Lawrence Berkeley, Brookhaven, Jefferson, Los Alamos, and Oak Ridge.

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[edit] Facility

Construction was completed for SNS on time and under budget in 2006. SNS now provides the most intense pulsed neutron beams in the world for scientific research and industrial development. SNS is operating as a user facility that enables researchers from all over the world to study the science of materials that forms the basis for new technologies in energy, telecommunications, manufacturing, transportation, information technology, biotechnology, and health. A planned upgrade to heavy water (deuterium oxide) as central cooling water will improve the neutron output. SNS is managed by Oak Ridge National Laboratory for the DOE, Office of science.

The Central Laboratory & Office Building (CLO) accommodates presently 400 to 500 outside research visitors a year, who explore the structural and dynamic behavior of materials and their interfaces by neutron bombardment. A separate building connected to the CLO houses the Center for Nanophase Material Sciences, separately funded by the Department of Energy. The SNS facility is designed so that a second target facility building can be constructed to double the overall experimental capacity.

The SNS target will accommodate up to 24 neutron beam instruments. Currently, 17 of these beam positions have been allocated and will include world-class diffractometers, spectrometers, and reflectometers designed specifically for the unique capabilities of SNS. One beam line will be devoted to the study of fundamental neutron physics. Each instrument is optimized for a particular area of science. As a whole, the instruments will support a wide variety of research into the structure and dynamics of materials at the atomic and molecular levels. Use of the instruments will range from highly applied topics such as the study of localized strains around welds to the study of magnetic phenomena in layered materials to experiments on functionality in biological materials. The first three of the instruments will become operational in 2006, and other instruments will be completed at the rate of one to four per year through at least 2011.

On April 28, 2006, the SNS achieved beam on target for the first time at 2:04:13PM EST and the first significant amount of spalled neutrons were detected at 2:08PM EST.

[edit] What is spallation?

When a high-energy proton bombards a heavy atomic nucleus, some neutrons are "spalled," or knocked out, in a nuclear reaction process called spallation. Other neutrons are "boiled off" as the bombarded nucleus heats up. For every proton striking the nucleus, 20 to 30 neutrons are expelled. Meson production limits spallation efficiency above 140 MeV. At the 1 GeV proton energy level, the Spallation Neutron Source will require 30 MeV per neutron produced. Neutron scattering is used by a variety of scientific disciplines to study the arrangement, motion, and interaction of atoms in materials. It's 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.

The SNS process is, briefly:

  1. Negative hydrogen ions (a proton with two electrons) are first generated in pulses;
  2. accelerated to 1 GeV (almost 90 percent of the speed of light) by a linear accelerator using both standard and superconducting techniques;
  3. stripped of electrons and concentrated into a 2 MW proton beam of less than 1 μs pulses at 60 Hz in an accumulator ring;
  4. directed at a liquid mercury target (chosen for mercury's large nucleus containing many neutrons and its liquid form at ambient conditions capable of absorbing rapid temperature rise and intense bombardment shock) in the target building, which ejects 20 to 30 neutrons per mercury nucleus hit by a proton (spalling in all directions);
  5. which are slowed down by moderators to useful energies;
  6. and applied through 18 surrounding beam lines to various materials and interfaces;
  7. where up to 24 instruments chosen by users record the results for interpretation. Examples of the neutron scattering instruments to be used are a backscattering spectrometer for high resolution spectroscopy, and magnetism and liquid reflectometers for studies of surfaces and interfaces.


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