A neutron is one of the fundamental particles that make up matter. This uncharged particle, identified in 1932 in England by James Chadwick, exists in the nucleus of a typical atom along with its positively charged counterpart, the proton. Protons and neutrons each have about the same mass, and both can exist as free particles away from the nucleus.
In the universe, neutrons are abundant, making up more than half of all visible matter. But, for research on physical and biological materials, neutrons of the right brightness are in short supply. Just as we prefer a bright light to a dim one to read the fine print in a book, researchers prefer a source of brighter neutrons like the SNS, which will give more detailed snapshots of material structure and, with the help of computer programs, make "movies" of molecules in motion.
The neutron has excellent properties for probing matter. The neutron is electrically neutral, able to penetrate materials a few centimeters, sensitive to light atoms in the presence of heavier ones, sensitive to magnetic interactions, and able to cover a range of energies or wavelengths, enabling researchers to probe distances between atomic layers in lattices, magnetic excitations, and slow dynamical processes in polymers and proteins.
Neutrons are especially sensitive to light atoms, such as hydrogen and carbon found in life-giving, organic molecules, and oxygen found in high-temperature superconducting oxides. But neutrons also reveal the presence of heavier elements. Neutrons, which interact with atomic nuclei, complement X rays, which interact with electrons and are, thus, most sensitive to heavier, electron-rich elements.
Each SNS 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 at a slower velocity, 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 the neutrons travel 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.
With their range of wavelengths, neutrons cover all the length scales of interest to most scientists studying structure, from the size of an atom to that of a metallic crystalline grain or a macromolecule such as a nanocluster, polymer, or protein. Also, neutrons enable researchers to determine the motions, or dynamics, of nanoscale building blocks over a wide range of time scales ranging from ultrafast structural relaxation to vibrations to slow liquid diffusion and protein folding processes—a remarkable time scale that spans 10 orders of magnitude.
Instruments at the SNS include diffractometers and reflectometers, for determining structure by neutron diffraction; and spectrometers, for determining dynamics by measuring vibrations and other motions of atoms and molecules in samples.
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