By Dave Schleck, Daily Press
September 7, 2003
Some physicists get their kicks out of watching couples break up.
Lawrence Weinstein is one of them.
The Old Dominion University professor and his graduate student Rustam Niyazov recently finished work on an experiment studying pairs of subatomic particles at the Thomas Jefferson National Accelerator Facility in Newport News, also known as Jefferson Lab.
They found out that just like with some human couples, subatomic pairs tend to repel each other if they get too close, attract each other when they're at a comfortable distance and ignore each at great distances.
Thank goodness they sometimes attract each other. Otherwise, stable matter as we know it wouldn't exist, said Weinstein, who has presented his findings to scientists in the United States and Europe and submitted the paper to a science journal called Physical Review Letters.
You'll have to wait a while for a practical application to Weinstein's research, he says.
"Ask me in 20 years," Weinstein said. The nucleon-pair research joins several other experiments that may someday improve the quality of images produced by MRI — magnetic resonance imaging, a diagnostic technique that doctors use to see internal body tissues.
The particles in the nucleus of the atom are called nucleons, and they consist of positively charged protons and neutral neutrons.
Scientists have spent 30 years studying how these particles behave individually. But it is only in the past 10 years that they were able to experiment on pairs of nucleons.
The device that helped them along is a big atomic microscope at Jefferson Lab called the Continuous Electron Beam Accelerator Facility Large Acceptance Spectrometer, or CLAS for short.
CLAS is unique in that it can detect subatomic particles that fly off in any direction from the nucleus. The spectrometer is a 30-foot-wide sphere that surrounds the target while an electron beam is shot through it.
Weinstein's experiment took place in 1999, followed by four years of data crunching.
Here's how it worked:
Scientists shot electrons through the nucleus of Helium-3, a super-cooled liquid that has two protons and one neutron in its nucleus.
When the electron beam shot one of the nucleons out of the nucleus, it sometimes left behind a bonded pair — either two protons, or a proton and a neutron.
"By comparing the proton-neutron pairs and the proton-proton pairs, we noticed they behave the same way," Weinstein said. "In a surprising number of events, the two nucleons went out of the nucleus back-to-back."
In other words, they repelled each other. It was the "aha" moment of the data analysis.
Weinstein and Niyazov also discovered that nucleons are mostly solitary particles. Out of billions of collisions in the experiment, the scientists succeeded in capturing only about 3,000 pairs. Only about a quarter of nucleons exist in pairs at a time.
Particle pairs are brief affairs, only lasting a fraction of a second before breaking up.
They tend to be fast-paced, too, moving about 60 percent of the speed of light, compared to 20 percent the speed of light for solitary particles. That's 400 million mph compared to 100 million mph.
The research landed Niyazov a physicist job at Jefferson Lab — not a bad destination for a somewhat curvy road of research, Niyazov said.
"In other experiments people usually know what they are looking for," he said. "For us, we didn't know exactly where to dig or what to do. Every time we had to do something new and try different ways to analyze the data."
Weinstein hopes his research enables scientists to do more with the nucleus of the atom. Right now, the applications are fairly limited to things like nuclear energy and nuclear medicine.
"That would be the same as if all you can do with wood is burn it for energy," he said. "It's remarkably crude. Having a more sophisticated understanding of the nucleus, we'll be able to do more sophisticated things with it."
Just like humans learned how to transform wood into paper, maybe scientists can better understand how to transform the nucleus of the atom, Weinstein said.
Gerald Miller, a physics professor at the University of Washington not involved with the Jefferson Lab experiment, said scientists have been waiting for years for results from Weinstein's experiment.
"Theorists have long believed that pair correlations are responsible for holding the atomic nucleus together," Miller said.
"However, there have been almost no direct experimental measurements that confirm the fundamental assumptions that pair correlations exist."
Submitted: Sunday, September 7, 2003 - 12:00am