New Superconducting Magnet To Probe Proton Structure

Funded by the National Science Foundation, the $2.75 million magnet was designed for an upcoming experiment at the Thomas Jefferson National Accelerator Facility in Newport News, Va. The experiment, called G0 (pronounced “Gee Zero”), involves about 100 scientists from many institutions. Steve Williamson, a UI physicist, is the experiment coordinator.

Over the next few months, researchers will meticulously inspect the magnet, cool it to liquid-helium temperatures and turn it on for the first time. As the power is gradually increased, a robotic test rig will precisely monitor the growing magnetic-field strength in three-dimensional space, and alert the researchers to potential problems.

Late this year, with testing complete, the magnet is scheduled to be shipped to the Jefferson facility. There it will serve as the centerpiece of the G0 experiment – a major effort to closely examine the role that the strange quark plays in generating proton structure and nuclear magnetism.

“We know that the proton’s structure – in particular, its magnetic moment – comes from the up, down and strange quarks inside the proton,” said UI physicist Doug Beck, spokesman for the experiment. “But exactly how it is put together is what we are trying to find out.”

In the experiment, an intense beam of polarized electrons will scatter off liquid hydrogen and deuterium targets located in the magnet’s core. Detectors, mounted around the perimeter of the magnet, will record the number and position of the scattered particles.

The new magnet will provide a much broader view of the small-scale structure of the proton, compared to earlier “snapshots” obtained with other experiments, such as the SAMPLE apparatus at the MIT/Bates Linear Accelerator Center, Beck said.

In SAMPLE experiments conducted during the summer of 1999, and reported in the Dec. 15, 2000, issue of the journal Science, researchers used the weak magnetic force to deduce the presence of a surprisingly large parity-violating electromagnetic effect known as the proton’s anapole moment. The proton’s anapole moment had long been predicted, but never measured.

“The new magnet should allow measurement of the anapole moment and other aspects of the proton structure with much greater precision over a wide range of momentum transfers,” Beck said. “For example, instead of seeing the proton’s overall magnetic moment, we will be able to vary the size of our probe to study small structures within the proton.”

The 80,000-pound magnet was constructed by BWXT in Lynchburg, Va., and required three years to build. It was moved to the UI in mid-December. A photograph of the magnet being readied for testing can be found at http://www.news.uiuc.edu/scitips/01/01magnet.html.