The beam from the linac must be sharpened more than 1,000 times to produce the short, sharp “bunch” of neutrons needed for optimal neutron-scattering research. For this stage of the process, the ions then pass through a stripper foil that removes electrons, reducing the ion to a single proton. The protons pass into a ring where they accumulate in bunches. Approximately 1,200 turns are accumulated, and then all these protons are released at once, producing, at full power, a 1.4 megawatt pulse lasting less than 1 millionth of a second. These pulses strike a container of a heavy-metal target and spall it. SNS uses liquid mercury, as it is not damaged by radiation like some solids (other neutron sources use tungsten or tantalum), its high atomic number makes it a reliable source of neutrons (120 to each atom), and as a liquid, mercury can absorb the heat and the shock of the proton pulses. For every one proton that strikes the target, 20 to 30 neutrons are directly kicked out and scatter in a spherical pattern. Some of the neutrons distribute into the 18 neutron beam lines that extend radially from the center and these neutrons stream toward the neutron scattering instruments located at the end of the beam lines. The shutters, and their safety catchers, ring the neutron-generation area and are located on each of these beam lines. When the shutters are open, the neutrons exit and strike the sample inside the instrument (analogous to exposing film when the shutter is open).
Once spalled, the corresponding pulses of neutrons are slowed down in a moderator and guided through beam lines to areas containing special instruments such as neutron detectors. To be suitable for research, neutrons coming out of the target must be low-energy, i.e., room temperature or colder. To be at room temperature, passing through cells filled with water slows the neutrons emerging from the target. To produce even colder neutrons (useful for research on polymers and proteins), the pulse is passed through containers of liquid hydrogen at a temperature of 20 K. These moderators are located above and below the target.
When a beam of neutrons strikes a target, many will pass through the material. Enough, however, will interact directly with the target’s nuclei and ricochet at an angle in a behavior called neutron diffraction, or neutron scattering. Under bombardment, the nuclei also heat up, causing some of its neutrons to eject. Using detectors, scientists count neutrons scattered by the SNS, measure their energies and the angles at which they scatter, and map their final position. Used in the study of the arrangement, motion, and interaction of atoms in materials, neutron scattering has multiple applications and provides information not obtainable by other techniques. A neutron acts like a tiny bar magnet that points like a compass needle; the size and direction of this magnetization is called a magnetic moment. Beams of “polarized neutrons” whose moments all point in the same direction can be created. Such beams allow scientists to probe properties of magnetic materials and measure fluctuations in magnetic fields penetrating and produced by superconductors.
Neutrons scattered from hydrogen in water can locate microscopic cracking in fighter jet wings that is an early warning of corrosion and signal the wing should be replaced. Neutron-scattering research has led to advances in materials research, as well as medicine, high-temperature superconductors, and superconducting ceramics.
Neutrons can locate other light atoms among heavy atoms. This capability of neutrons allowed scientists to determine the critical positions of light oxygen atoms in yttrium-barium-copper oxide, a high-temperature, superconducting ceramic. Since the energies of thermal neutrons almost match the energies of atoms in motion, neutrons can be used to track molecular vibrations, movements of atoms during catalytic reactions, and changes in the behavior of materials subjected to rising temperature, pressure, magnetic fields, or other outside forces. Via this process, researchers can make movies of atoms in action.