Neutron Holography

Neutron holography with atomic-scale resolution has been performed, for the first time, with an "inside-detector" approach.

Holography generally includes a source of illuminating waves, an object to be imaged, and a detector or film in which waves direct from the source interfere with waves scattered from parts of the object. The interference pattern, stored in the detector medium, is later read out (and a 3D image of the object viewed) by sending waves into the detector.

Holograms with visible light are common enough: they adorn most credit cards. Holograms using electrons (considered in their "wave" manifestation, not as particles) provide sharp pictures, but because the electrons cannot penetrate far into a solid sample, the imaging process is usually restricted to surface regions.

Holograms using x rays go can penetrate much farther, but their limitation consists of the fact that the penetration depth improves as the square of the atomic number. Therefore x-holography is not very good for materials with light elements.

Holograms with neutrons are different; rather than scattering from the electrons in the atoms of the sample, neutrons scatter only from nuclei, which are 100,000 times smaller than the atoms in which they reside. This is important when it comes time to reconstruct an image of the interior of a crystal lattice.

In an experiment carried out with a beam of neutrons from a reactor at the Institute Laue-Langevin in Grenoble, a group of scientists has produced, for the first time, an atomic-scale map of a crystal, in particular a sample of lead atoms, using a technique in which the "detector," a trace amount of atoms (cadmium-113) whose nuclei readily absorb neutrons, are embedded inside the sample itself.

The holographic process unfolds as follows: neutron waves can strike a Cd nucleus directly (reference beam) or by first scattering from a Pb nucleus. In either case, the absorption of a neutron stimulates a Cd nucleus to emit a high-energy photon observable in a nearby detector. The overall interference pattern for these two processes (absorbing scattered or direct neutron waves) is monitored as the profile of the sample to the beam is stepped through various angles.

The result: a crisp picture of a unit cell of 12 lead atoms (see figure). This process should be great for spotting foreign atoms in a solid (dopants if the atoms are desired, impurties if they're not).

Since the neutron has a magnetic moment, n-holography might also contribute to an understanding of the magnetic nature of the sample atoms, in addition to imaging their whereabouts. (Cser et al., Physical Review Letters, 21 October 2002; contact Laszlo Cser, Central Research Institute for Physics, Budapest, cser@sunserv.kfhi.hu, 36-1-392-2222 extension 1526.)