High-energy light for high-intensity research
Brookhaven, N.Y. — ``When you accelerate an electron by changing its velocity or direction, it gives off light,'' says Katrina Smith, a high school senior participating in Brookhaven's summer program. The electrons in the National Synchrotron Light Source (NSLS) are moving so fast that they give off light of extremely high energy. Their vast numbers contribute to the light's high intensity. The electrons are first fired from an ``electron gun'' into a booster ring that accelerates them to near the speed of light. Then they are vented into either the ultraviolet (UV) storage ring or the larger X-ray storage ring, where they cycle for hours, giving off tremendous amounts of light at every turn.
``It's not really a circle - it's actually a straight section, a bend, a straight section, a bend,'' says Nick Brookes, who operates an experiment at the NSLS.
Each time an electron goes around the ring, it goes through an area called the ``RF cavity'' that ``keeps pumping life back into it,'' or replacing energy that was lost in the form of synchrotron light, says Mike de Temple, a student from West Virginia.
At one point along the UV ring is a special device called an ``undulator,'' which guides the beam past rows of magnets with alternating polarity. As the beam moves past, the electrons wiggle in the magnetic field and give off even more ultraviolet light - up to a hundred times as much light, Dr. Brookes says.
One experiment the high school students performed involved calculating the distance between the undulator's magnets and the electron beam by measuring the color of the light given off. Brookes himself is using the high-intensity light to look at the arrangements of electrons in thin films of metals such as iron, nickel, and cobalt.
X-rays from the synchrotron ``can be used to find crystal structures,'' says Sunita Bhatia of Delaware. ``You hit [the atoms] with the rays and they start vibrating. Of course, the next atom over starts vibrating, too.'' By examining the interference pattern created by the two waves, Sunita says, ``you can tell how the crystal is structured.''
``Synchrotron light has revolutionized the field of protein crystallography,'' says Gregory Petsko, a professor of chemistry at the Massachusetts Institute of Technology. ``It has made it possible to look at much larger structures with much smaller crystals.''
``It has also opened up the possibility of [observing the structure of] biological molecules as they function,'' Dr. Petsko says. ``The intensity of the beam is such that you can collect data in a very short time.''