A group of Vanderbilt engineers--the largest of its kind in the United States--is playing a major role in developing technology that protects the integrated electric circuits in communication satellites from powerful energetic particles that exist in space.
The Radiation Effects Group at Vanderbilt, composed of eight engineering professors, is studying the effects of radiation on the systems and exploring the development of materials that can stand up to the high-energy bombardment. The research also has implications for the earthbound: as computer components get smaller and more densely packed, the microelectronic devices become increasingly sensitive to radiation at ground level, said Ronald D. Schrimpf, professor of electrical and computer engineering. "We can see that our work will eventually make computers work better and longer, whether they're hovering overhead in a satellite or sitting on our desks at home," he said.
The Radiation Effects Group at Vanderbilt is involved in a number of research projects. It has received support for research into space radiation effects from a variety of sources, including the U.S. Navy, the Air Force Office of Scientific Research, the Defense Special Weapons Agency, NASA and three private firms, Mission Research Corp. of the United States and Aerospatiale and Alcatel, both from France. In addition to Schrimpf, the research group draws on the collective talents of Engineering Dean Kenneth F. Galloway and engineering professors David Kerns, Sherra E. Kerns, Bharat Bhuva, Jim Davidson, Robert Weller and Lloyd Massengill.
Without the protection and strengthening of radiation-hardening techniques, a satellite's integrated circuitry and microprocessor are vulnerable to the effects of space radiation that can range from gradual degradation of a system to catastrophic failure. The damage can be done within months or even weeks of a launch into orbit. For example, Massengill said the guidance system of the Hubble Telescope has to shut down, dump its memory and then reload every time it passes through the Van Allen belts, which consist of high energy protons and electrons which encircle the earth and are trapped by the earth's magnetic field.
"This radiation creates problems in the Hubble guidance computer of such a magnitude that it cannot operate through, or even recover from, the scrambled instructions created by the radiation," Massengill said. "It must be reset and reloaded."
Radiation also caused a very large number of errors on NASA's tracking and data relay system (TDRS), which is a system of four satellites used for high-speed data communication to other space systems such as the Hubble Telescope and space shuttles. The first of the four satellites was launched in 1983. "The errors did not cause failure because they were caught and corrected by error detecting and correcting software," Massengill said. "However, the number of errors were so high and worrisome to NASA that subsequent satellite flight models made use of specially-designed and radiation-hardened chips."
The Vanderbilt research team is focused on designing improved, lower-cost electronic devices, determining how well new hardening techniques will work and predicting the lifetime of electronics in space. During the Cold War, much of the research in the field was aimed at producing what is known as radiation hard or "rad-hard" integrated circuits so that missiles could function effectively in the event of nuclear war. With the easing of global tensions, the field has branched out to include such commercial ventures as the communications satellite business.
With approximately 1,000 satellites to be launched by the United States alone in the next five years, the field is "exploding," Schrimpf said. "As the demand for commercial satellite communications systems has grown, so has the need for less expensive, more effective ways to protect equipment from radiation," Schrimpf said. "Making 'rad-hard' devices for use in space electronics is a complicated and expensive process."
The two primary effects of radiation on electronics in space are known as "total dose" and "single-event upset." Total dose effects stem from the accumulation of ionizing radiation over a period of time and result in permanent degradation of the device's performance. A single heavy ion passing through an integrated circuit results in a single event upset. Current Vanderbilt projects include:
ß Simulating radiation effects and reliability: Technology Computer-Aided Design (TCAD) systems are used to predict the effectiveness of new radiation-hardening technique and designs. The researchers have developed software to project a design's resistance to both total dose and single-event radiation effects.
ß Heavy ion (single event) effects: The Vanderbilt group is designing software tools and analysis capabilities for predicting electronic device performance amid bombardment of heavy ions, both in space and on the ground. The software can predict the rate of errors satellite circuits would experience each day.
ß Space radiation effects on bipolar circuits: The Vanderbilt group is developing testing and predictive procedures on bipolar transistors, which are used in the control systems that keep satellites in orbit.
ß Total ionizing dose radiation testing: The Vanderbilt group has an ARACOR semi-conductor x-ray irradiation machine that simulates the natural space environment and is used for testing advanced technologies.
Galloway, who remains active in the field, headed radiation effects research at the University of Arizona prior to being named engineering dean at Vanderbilt. He recruited Schrimpf, with whom he worked at Arizona, to join him at Vanderbilt. Massengill, already at Vanderbilt, works closely with Schrimpf and Galloway
"Ron is really an expert in semiconductor devices and device physics and Lloyd is more an expert in circuits," Galloway said. "They really complement one another. We really have a terrific group of colleagues with radiation effects interests at Vanderbilt."
Proof of Vanderbilt's preeminence in the field is its leadership in the Nuclear and Radiation Effects Conference held by the Institute of Electrical and Electronic Engineers each year. Massengill served as technical program chairman of this year's conference, which attracted about 600 participants from all over the world. Schrimpf will serve as overall chairman of the conference next year. Galloway is a past chairman of the conference and Sherra Kerns is a past technical chairman.
Vanderbilt has about 14 graduate students and three post-doctoral students working in the radiation effects lab, which is equipped with 11 Sun work stations and two silicon graphics work stations plus equipment for electrically characterizing semiconductor devices.
Vanderbilt physics professors Socrates Pantelides, Len Feldman and Norman Tolk have also researched the effects of space radiation. Interdisciplinary research between the two Vanderbilt groups is already underway with more planned in the future.
Materials provided by Vanderbilt University. Note: Content may be edited for style and length.
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