Vol. 6 •Issue 1 • Page 52
Super Bright X-Rays
Serves Scientific Investigations
The home to the most intense X-ray beams available on the planet is in Illinois. The Advanced Photon Source (APS) is a synchrotron X-ray facility being tested at Argonne National Laboratories, Argonne, Ill. The APS has been designed, from the ground up, as a facility to support guest researchers in any one of 35 laboratories. The staff at Argonne will be able to design a beam to the research group’s specifications (e.g., bandwidth, frequency) for experiments, testing hypotheses at the atomic level.
The APS passed an important milestone in August 1995, producing high brilliance X-ray beams for the first time. These beams are 100 million times brighter than those used in medical X-ray equipment, according to David Moncton, associate laboratory director for the APS. The APS is slated to be fully operational by next month. It will offer resolution of detail as small as 0.01 nanometers, according to Russell Huebner, manager of policy and planning for the APS.
The APS has several analytical modes.
• The first is direct imaging, as in a medical radiograph, but with a much smaller subject, such as a structure within a single cell.
• The second is diffraction imaging, from which molecular structures can be calculated. Most of what we know about the structure of DNA and viruses is the result of X-ray diffraction studies, said Huebner.
• The third is spectroscopy, analyzing the kinds of atoms, even trace impurities in a sample.
• And the fourth is timing, which could be used to watch the relaxation of a crystal following laser excitation, or observing a chemical reaction in progress.
Due to the high intensity photon beam, the APS will permit imaging using smaller sample sizes, faster chemical reactions, and in greater structural detail than current X-ray diffraction systems.
The beam is generated by bending a high energy (7 GeV) stream of electrons or positrons with permanent magnets. Each bend in the beam produces photons. The energy and spectrum of the photons can be controlled by changing the number and spatial frequency of the bends. These arrays of permanent magnets are referred to as insertion devices. Insertion devices at the APS come in two basic types: undulators and wigglers. Undulators produce discrete spectra from relatively small beam deviations. Wigglers produce broad spectra from relatively large beam deviations. Circularly polarized X-rays can be produced by spiraling the beam. The laser-like beams are about 0.1 mm in diameter, and don’t diverge much over long distances, according to Huebner. Images are produced on a state-of-the-art charge-coupled device detector, not film.
Here are some examples of direct imaging with such an intense beam.
• Very small crystals: only very small quantities of a substance are needed to conduct investigational imaging studies.
• Quality control: the manufacturing quality of microdevices can be assessed.
• Microtomography: protons of a cell can be imaged, allowing computer tomography of the mitochondria and other structures.
The APS covers an area of 80 acres, so chances for widespread replication at university medical schools are slim. Yet other groups are investigating clinical applications of synchrotron radiation. The Synchrotron Radiation Center at Stanford University has produced mammalian cardiac images, but it may be several years before clinical investigations are conducted at the APS, said Huebner. He also said some efforts are in progress to miniaturize synchrotron sources to make them more readily available to hospitals. Ed Westbrook, director of structural biology at Argonne, and has produced images of cholera toxin.
It’s a good thing the APS can make images as quickly as it can, since one byproduct of such intense light is intense heat. Special precautions will have to be taken by researchers to avoid melting their instrumentation. Huebner suggested that water, liquid gallium or liquid nitrogen could be used to control the heat.
Huebner described the organizational structure of the APS in an article published in the Journal of the American Medical Association (JAMA). “Users who intend to publish research results will not have to pay for machine time, but they will have to pay for experimental equipment and supplies. Those doing proprietary work will be able to retain patent rights from any resulting invention, but they will have to pay for beam access at a full cost recovery rate,” he explained in JAMA.1
Each of the 35 laboratories at the APS will be managed by members of a Collaborative Access Team (CAT). Individuals and organizations with an interest in using the APS must join a CAT to do so.
The APS exist primarily to make photon beams for users, but is also engaged in some ancillary research, according to Huebner. Designs for detectors, undulators, wigglers, energy selection devices and data handling are being pursued by the APS staff.
About 15 groups have been approved to begin equipment installation. A consortium consisting of Dow Chemical Company, E.I. DuPont de Nemours & Co. and Northwest-ern University plans to begin research in the spring of 1996.
1. Skolnick AA (Sept. 21, 1995). Brightest X-rays Ever Give Researchers Brand New View of Biological Molecules. JAMA, 272, 11.