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Importance of Neutron Science
What
Is a neutron? Why Use Them?
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Data produced data for a model that shows peptides (cylinders) inserting themselves in holes they form in a cell membrane.
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A neutron is one
of the fundamental particles that make up matter. This uncharged particle,
identified in 1932, 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 brighter source of neutrons that will give more detailed
snapshots of material structure and make "movies" of
molecules in motion. SNS will provide these
brighter neutrons. Like a flashing strobe light
providing high-speed illumination of an object,
SNS will produce pulses of neutrons every
17 milliseconds, with more than 10 times more
neutrons than are produced at the most powerful
pulsed neutron sources currently available.
Like water spraying from a rock washed by a
garden hose, neutrons from a beam will "scatter" from a target material in a way that reveals
its structure and properties.
What
are some properties of neutrons?
Because of their
unique sensitivity to hydrogen atoms, neutrons can be used to precisely
locate hydrogen atoms, enabling a more accurate determination of molecular
structure, which is important for the design of new therapeutic drugs.
Because large biological molecules contain numerous hydrogen atoms, the
best way to see part of a biomolecule is through isotope substitution-replacing
hydrogens with heavy hydrogen (deuterium) atoms. Deuterium atoms and hydrogen
atoms scatter neutrons differently. Thus, in a technique called contrast
variation, scientists can highlight different types of molecules, such
as a nucleic acid or a protein in a chromosome, and glean independent
structural information on each component within the macromolecular complex.
Neutrons scattered from hydrogen in water can locate bits of moisture
in fighter jet wings-signs of microscopic cracking and early corrosion
that pinpoint the part of the wing that should be replaced.
Besides hydrogen,
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 (YBCO), a promising
high-temperature, superconducting ceramic. YBCO wires may someday be used
to increase the energy efficiency of electric motors, generators, transmission
lines, transformers, and magnet-containing devices, such as particle accelerators
for research, medical diagnostic machines, and levitated, high-speed trains.
Atomic
model of yttrium-barium-copper oxide, a superconducting ceramic
whose oxygen positions were determined by neutron scattering. |
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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 (like those on your credit card or in compact
discs) and measure fluctuations in magnetic fields penetrating and
produced by superconductors.
Because 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 outside forces, such as
rising temperature, pressure, or magnetic field strength. In this
way, researchers can make movies of atoms in action.
Properties of Neutrons
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Nobel Laureate Clifford Shull was among the ORNL researchers who pioneered neutron scattering by using neutrons from the Laboratory's Graphite Reactor. |
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What is spallation?
When a fast particle, such as a high-energy proton, bombards a heavy atomic nucleus, some neutrons are "spalled," or knocked out, in a nuclear reaction called spallation. Other neutrons are "boiled off" as the bombarded nucleus heats up. It's something like throwing a baseball at a bucket of balls, resulting in a few being immediately ejected and many more bouncing around and falling out. For every proton striking the nucleus, 20 to 30 neutrons are expelled.
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When a high-energy proton bombards a heavy atomic nucleus, causing it to become excited, 20 to 30 neutrons are expelled. |
Why is neutron scattering useful to researchers?
Neutron scattering is a useful source of information about the positions, motions, and magnetic properties of solids. When a beam of neutrons is aimed at a sample, many neutrons will pass through the material. But some will interact directly with atomic nuclei and "bounce" away at an angle, like colliding balls in a game of pool. This behavior is called neutron diffraction, or neutron scattering.
Using detectors, scientists can count scattered neutrons, measure their energies and the angles at which they scatter, and map their final position (shown as a diffraction pattern of dots with varying intensities). In this way, scientists can glean details about the nature of materials ranging from liquid crystals to superconducting ceramics, from proteins to plastics, and from metals to micelles to metallic glass magnets.
The importance of neutron scattering to the scientific community was recognized by the awarding of the 1994 Nobel Prize for Physics to Clifford Shull and Bertram Brockhouse. Shull pioneered the use of neutron scattering at Oak Ridge to decipher the structure of materials, and Brockhouse found ways to use it in his Canadian laboratory to learn about the motions of atoms in materials.
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