Local Researchers Speak About Pioneering Experimentby Jeremy Whitlock
The Canadian Nuclear Society presented three authors of a breakthrough paper on the subject, published last November in the science journal Nature: Bhaskar Sur of AECL, and John Katsaras and Ron Rogge, both of the NRC s Neutron Program for Materials Research, located at Chalk River Laboratories. (Two other authors of the paper were V. Anghel of AECL and R. Hammond of the NRC group.)
The subject of Tuesday s talk was "Life Without a Crystal: An Overview of Neutron Holography and its Potential Applications .
Holography, from the Greek for "whole message , is a technique for imaging objects in three-dimensions. Used most commonly on credit cards, assorted gimmickry, and as an art form in its own right, it is also a powerful tool in science, engineering and medicine.
Hungarian scientist Dennis Gábor invented holography in 1948 but had to wait until 1962, and the invention of the laser, to see it demonstrated. Regular, non-laser, light waves are out of synch (like a crowd of people walking randomly), but holography works best with waves moving in unison (like soldiers marching in step, and like laser light).
The interference between a beam of laser light and the same light bounced off a nearby object records the depth-of-field information not found in normal two-dimensional photography of the same object.
Imagine a "magic photo of people standing in a room, conveying along with each person s 2-D image, their thoughts as to where everyone else in the room was standing relative to themselves. Not only could you reconstruct a three-dimensional model of the scene with this information, but you could also do it starting with any torn-off piece of the photo (as long as it contained at least one person).
That s how holograms work.
In theory holograms can be generated with any energy transmitted as waves, from visible light to x-rays, to ultrasonic waves, and to thermal (slow) neutrons. The latter are made abundantly in AECL s NRU reactor, which at 45 years old can now add "holography to its long list of applications.
The three speakers on Tuesday took turns covering coupled parts of the story.
John Katsaras began with a background of crystallography, the branch of science that looks at the structure of molecules, usually in an ordered (crystallized) pattern. The most common inspection tool is x-ray diffraction, developed 90 years ago, and extended with neutron diffraction, developed 55 years ago.
However, both techniques are analogous to the two-dimensional photo of people standing in a room: they lack depth information. Crystallographers wishing to piece together the 3-D puzzle must use a variety of inference techniques, becoming increasingly difficult with more complex molecular structures.
Enter x-ray, and now, neutron, holography.
Bhaskar Sur explained, without a single mathematical equation, how holograms work, and performed a fascinating demonstration using 2-D patterns and an overhead projector.
Ron Rogge stepped to the plate and batted the story home, describing the experiment at Chalk River Labs and its significance.
Neutron holography, until recently thought to be unattainable as a useful tool, works similarly to the well-established technique of x-ray holography, but with different and complementary applications.
Both techniques utilize an "emitter within the molecule itself to produce the soldiers marching in step: the synchronized energy waves. The best emitter for neutron holography is the hydrogen atom, which happens to be abundant in biological material. This, along with a few other properties, gives neutron holography a useful niche alongside its x-ray counterpart.
Best of all, holography does not require crystals (ordered structures) of the material to be grown, since enough information is contained in the signals generated by individual portions of the material - much like the magic photo conveying people s thoughts.
The Chalk River experiment used a relatively simple mineral called Simpsonite (no relation to Bart, Rogge points out). The sample was irradiated in a beam of NRU neutrons, enticing hydrogen atoms within to re-radiate marching soldiers in all directions, and a magic photo was taken that contained the thoughts of all atoms regarding their location in the scheme of things.
It was as simple as that.
Of course, the magic photo itself took two weeks to acquire, but this was simply a function of available instrumentation. In theory the shutter time can be reduced to two minutes.
Then there was the extensive computer generation of the three-dimensional image, involving much data filtering and signal massaging.
The result is a sort of CAT-scan (or in this case, a Kat-scan?) of the molecular structure of Simpsonite, slices of which graced the pages of Nature, and flashed around the world s scientific circles to enthusiastic attention over the last four months.
The three scientists pointed out that this is only the tip of the iceberg, as improvements have yet to be made to all aspects of the new technology. The sense is clearly one of standing at the entrance to a whole new field of scientific discovery, pioneered in our own backyard. Again.