Why Build SNS?

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?

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.