College of Liberal Arts and Sciences at Iowa State University

Changing with the times

American Physical Society recognizes John Hill for his work in three different areas.
Not more than two years after John Hill first set foot on campus it looked like he would have to go elsewhere to continue his research.

But now almost 30 years later he is still on the faculty in the Department of Physics and Astronomy and has moved easily from one research field to another.

"People are often criticized for continuing to work for years only on topics related to their Ph.D. thesis," said Hill, now a professor of physics and astronomy. "I did that for a while, but then went on to do other things."

Back in 1975, Hill came to Iowa State to teach and study the decay of very unstable neutron-rich nuclei using the TRISTAN mass-separator system coupled to the Iowa State nuclear reactor.

At this time the first of three important phases of Hill's research occurred when he discovered several neutron-rich nuclei using TRISTAN and facilities at Los Alamos and the KFA laboratory in Julich, Germany.

Two years after his arrival at Iowa State, the nuclear reactor on campus was closed and he searched to find a new location for the TRISTAN system. He and physics colleague Fred Wohn moved TRISTAN to the HFBR reactor at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory on Long Island, N.Y.

"While we were in the process of moving the separator system to Brookhaven I started looking around for something else (a new research field) to do," Hill said.

Now it was time for thesecond important phase of Hill's research career.

"I found that some theorists predicted that purely electromagnetic interactions of very high energy heavy ions would have unusually large cross sections (probabilities)," he said.

He subsequently carried out a series of pioneering experiments on this process called electromagnetic dissociation, first at the Bevalac accelerator at Berkeley, next at the AGS accelerator at Brookhaven and finally at the SPS accelerator at Europe’s CERN Laboratory. In each step the new accelerator developed particle beams of heavier nuclei at higher energies thereby enhancing the electromagnetic dissociation effect.

Hill helped establish that large electromagnetic dissociation cross sections were major factors of the lifetime of heavy ion beams for the latest generation of heavy ion colliders like the Relativistic Heavy Ion Collider (RIHC) at Brookhaven.

That work led to the third and final important phase of his career.

In 1990 the HFBR reactor at Brookhaven was shut down and Hill was faced with a career crossroads.

"The decision was made by our group to go into something new," Hill said. "We decided to work with the RHIC experiment because we felt the potential for discovery was greater there than the other lab we were considering."

Hill has been rewarded by that decision. His leadership, along with that of his colleague Wohn, in the development of trigger systems of the AGS-E864 and PHENIX-RHIC experiments made possible recent searches for the quark-gluon plasma at Brookhaven. This is a form of hot dense matter that scientists believe existed only a few millionths of a second after the "Big Bang." Results from experiments at RHIC in 2002 and 2003 suggest that the quark-gluon plasma may have already been produced, but physicists are cautious and express the need for more data to confirm the discovery.

While Hill and students were building the trigger for the AGS-E864 experiments, Iowa State physics colleagues Wohn and John Lajoie built the first-level trigger for the $100 million PHENIX detector at RHIC.

Triggers help scientists select the few head-on collisions of Au nuclei most likely to produce quark-gluon plasma. Without the trigger, the PHENIX detector could not effectively select these events.

"There's always a risk to do other things by changing your research focus," Hill said. "The danger is not being an authority on anything. But at least for me the excitement of being involved in all these experiments was the great thing."

It has also recently led to Hill's election to Fellowship in the American Physical Society (APS). He was cited "for discovering several neutron-rich nuclei, measuring the large electromagnetic dissociation cross sections of relativistic heavy ions, and leadership in the development of trigger systems for the AGS-E864 and PHENIX-RHIC experiments."

Hill was one of two Iowa State physicists so honored this year. He joins colleague Michael Tringides whose area is condensed matter physics.

"These type of honors typically signify a significant contribution in your field over a period of years," Hill said. "My citation was not so much for one big thing, but three things that in a sense mirror what I've done in my career."