About Neutron Diffraction

Physicists have created a new form of water, one that stays liquid at hundreds of degrees C below zero. This novel state of H₂O occurs when water molecules are trapped inside a nanotube (see drawing), a recently-discovered form of carbon that is a close relative of the Buckyball (see the Physics in Action feature Tubular Peas).

Side view of nanotube cylinder showing network of interlocked carbon atoms; the nanotube is a graphite sheet, rolled up into a cylinder (Copyright V. H. Crespi. Distributed under the Open Content License)

The molecular structure of liquid water, left, and ice, right. The fixed arrangement of hydrogen bonds between hydrogen and oxygen atoms opens up the structure of ice. (image courtesy of MathMol, New York University)

Water is a polar molecule, so the electrons are unevenly distributed, with an excess of negative charge on the oxygen atom and positive charge on the two hydrogens. The resulting electrostatic forces between neighboring water molecules form hydrogen bonds between the oxygen atom of one and a hydrogen of another (a total of four bonds per molecule), which produces the crystal structure of ice. The open atomic arrangement in ice (see drawings) is responsible for the decrease in the density of H₂O upon freezing. In liquid water, the molecules also form hydrogen bonds, although they break and reform rapidly.

The structure of nanotube water is studied by neutron diffraction. You’ve no doubt seen the colored spectra produced when white light strikes a compact disk. This effect is an example of diffraction, which is a fundamental property of waves that can be used to investigate the nature of materials. Diffraction from an object becomes significant when the wavelength is comparable to the characteristic spacing—for the light scattering from the CD, when the wavelength of light is comparable to the distance between the grooves.

Visible light won’t work to investigate materials, since the wavelength of light is thousands of atoms long. To get down to typical atomic spacings, material scientists use x-rays. X-rays are scattered by the electrons in atoms, in proportion to the number of electrons. Although x-rays work well for investigating most crystals, they can’t be used to locate hydrogen atoms, because hydrogen has only one electron, and it scatters x-rays only weakly.

That’s where the neutrons come in. Neutrons, like other atomic and subatomic particles, have properties of both particles and waves. In fact, the relationship between momentum and wavelength—they are inversely proportional—is the same for both matter waves and for photons (the particles of light).

Since neutrons are uncharged, they are highly penetrating. Moreover, they scatter strongly from the atomic nuclei, and they clearly reveal the position of hydrogen atoms in the presence of heavier elements—just what is needed to investigate ice. Also, a related method, neutron inelastic scattering, shows how the energy is absorbed from the neutron beam, providing information about how the atoms are moving.

Argonne National Lab’s Intense Pulsed Neutron Source (IPNS) experimental area (photo courtesy of Argonne National Laboratory

A disadvantage of using neutrons is that it takes a big, expensive facility to produce them. Neutron scattering experiments originally were performed at nuclear reactors, but more recent facilities, such as Argonne National Lab’s Intense Pulsed Neutron Source (IPNS), produce neutrons from a pulsed beam of accelerated protons (see photo). The pulsed proton beam slams into dense metal targets (uranium at IPNS), setting off nuclear reactions that yield a hail of neutrons. The resulting pulsed neutron beam is then intensified by reflection from blocks of beryllium. The IPNS produced the beam of neutrons that probed nanotube water.