ITHACA, N.Y. -- Like a physician examining X-rays, Leslie Allee lends an expert eye to the film hanging on the light screen.
No broken bones here. Allee, a Cornell University doctoral student in entomology, is looking at film that shows a thin white line branching off in different directions. Around it are three or four tiny white marks, maybe 1/4-inch long -- the objects of her attention. These are living corn rootworms in soil, caught in the act of searching for the plant root.
"Here you can see where they were when we started. In this picture you can see where they were 24 hours later. The dark lines show the path of the rootworm," she says.
No ordinary pictures, these. Allee is making use of a Cornell resource to study, along with Paula Davis, assistant professor of entomology, the behavior of this major corn pest in the hope of developing a natural way to control it.
By using the 500-kilowatt nuclear reactor on campus, the entomologists can get sharp pictures on film of their corn roots and the rootworms in soil around it. The technique is called neutron radiography, and professors from art historians and agronomists to zoologists can use it to get detailed pictures that X-rays cannot produce. It is the first time neutron radiography has been used to study insects.
"X-rays pass through the rootworms without attenuation. Neutrons, however, are attenuated by the hydrogen in the rootworms," Davis said. "With a neutron beam, we can also see the corn roots and see the behavior of the insects as they are growing. And it's all totally non-invasive."
Here is how it works: The TRIGA Mark II nuclear reactor at Ward Laboratory produces a beam of neutrons that is focused to a plate holding a sample. Behind the sample is film. The neutrons pass through the film (unlike X-rays, without activating the emulsion) and land on a conversion screen (made of the rare-earth metal gadolinium) that absorbs them. The metal screen emits an electron for every neutron it absorbs, and that activates the emulsion and produces the photographic image on film.
The result looks like an X-ray, but it can reveal things X-rays cannot. An example: a toy plastic gun sealed in a 1-inch-thick lead box. The neutron radiograph shows details inside the gun, even its spring mechanism, but X-rays could not even penetrate the lead box.
"Neutron radiography is a new way of looking at things and has proved to be a very useful non-destructive evaluation tool," said Howard Aderhold, reactor supervisor who designed and built Cornell's neutron radiography facility a decade ago. "We think faculty from many different disciplines could benefit from this technology."
Indeed, its utility already has been shown by Cornell faculty in a variety of areas. W. S. Taft, professor of art, has used neutron induced autoradiography to peer beneath layers of pigments on a painting to see what lies beneath. As an experiment, he took a classical painting of a woman, painted an abstract modern painting atop that, and then painted a fabric drape over that.
When subjected to the neutron beam, each layer became visible. The goal: an understanding of the most common modern pigments so a more informed analysis of modern paintings could result. Taft used such case studies in an undergraduate course, "Art, Isotopes and Analysis."
Now he uses neutron induced autoradiography as a form of art itself. "It has always been used for analysis, but I was really interested in the images it produced," Taft said. So he started making radiographs of his paintings, which have been exhibited in California, Washington, D.C., and Ithaca.
"The painting is just a means of producing the radiographs; the radiographs are the final work," he said. "X-rays only make visible a very few number of elements. Using neutrons, we can see a broader range of elements. It's extremely useful." Now he is embarking on a new project, in collaboration with the Getty Conservation Institute in California, to try to resurrect an image from an old, faded photograph. The idea is to see the silver that's not visible to the human eye.
David Bouldin, professor of soil, crop and atmospheric sciences, and Michael Cahn, graduate student, showed water distribution, root growth rates and root distribution in soil using the technology. Richard Zobel, professor of plant breeding, and Victor Bushamuka, graduate student, later used neutron radiography to examine root growth and responses of tolerant plants to toxic aluminum and compacted soils.
The neutron radiography facility now has the capability for real-time observing. A video camera inside the high-flux beam configuration can show what happens to a sample, as it is happening. This beam is 40 times more intense than the beam used in high-resolution film radiography, such as to view corn rootworms. K. Bingham Cady, professor of nuclear engineering, is using it in collaboration with Jean-Yves Parlange, professor of agricultural and biological
engineering, and Tammo Steenhuis, associate professor of agricultural and biological engineering, to study oil and water penetration in sand. Such studies will be useful for determining optimum cleanup methods for oil spills.
"You can see the water and oil diffusing into the sand," Cady said. Neither X-rays nor neutrons can readily see the difference between water and oil, he explained. With neutron radiography, however, there is the potential for distinguishing between the oil and water by modifying the water phase in one of two ways: add gadolium nitrate to make the water more opaque to neutrons, or replace light water with heavy water to make the "water" more transparent to neutrons.
Cady also is proposing to use neutron radiography to measure vapor fractions in two-phase (boiling) flow. High vapor fractions are the precursor to sudden failure of heated surfaces in nuclear reactors. The vapor fraction measurements are in support of a two-phase flow model important in understanding the potential for severe accidents in nuclear reactors.