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Just as people behave differently as couples than as individuals, protons and neutrons (also known as nucleons) inside the nucleus of the atom behave differently in pairs. Scientists at Old Dominion University and the Thomas Jefferson National Accelerator Facility (Jefferson Lab), led by ODU physics professor Larry Weinstein, have just completed one of the first clear measurements of nucleon pairs in nuclei. Their findings are reported in a paper submitted to the journal Physical Review Letters, "Two-Nucleon Momentum Distributions Measured in 3HE9e,e'pp)n."

The recent Old Dominion/Jefferson Lab experiment aimed a beam of two billion-electron-volt electrons at a target of Helium-3 using the Continuous Electron Beam Accelerator Facility (CEBAF) Large Acceptance Spectrometer to detect the electrons that bounce off and the protons that are knocked out. The scientists then reconstructed what was happening prior to the subatomic collision.

"Nuclear Physicists have spent the last 30 years measuring the behavior of single protons in nuclei," said Weinstein. "Thanks to the capabilities of the CLAS spectrometer (and a bit of luck), we have now taken an important step toward measuring the behavior of protons in pairs."

While most people pair up, only about a quarter of nucleons exist in pairs at a time. Human relationships can endure for decades, but nucleon pairs last only a fraction of a second. However, like some people, at great distances nucleons ignore each other, at medium distances they attract each other and when they get too close, they violently repel each other.

Weinstein's experiment measured the behavior of very close nucleon pairs. He, along with Rustam Niyazov, a graduate student in the Nuclear and Particle Physics Group and their collaborators reconstructed billions of collisions to find the 3,000 events where one of the three nucleons of Helium 3 was knocked out cleanly, leaving behind an almost undisturbed nucleon pair. Unpaired nucleons move relatively slowly, with a speed that rarely exceeds 20 percent of the speed of light. The paired nucleons were measured to have speeds up to 60 percent of the speed of light, providing clear experimental evidence of the violent coupling between two nucleons.

CEBAF at Jefferson Lab emits high-energy beams of electrons that are used to study the nucleus of the atom. In this recent experiment, particles from the collision were detected in the CEBAF Large Acceptance Spectrometer (CLAS). The CLAS is designed to detect almost all of the charged elementary particles (electrons, protons and pions etc.) that emerge from an electron-nucleus collision. The spectrometer, a 30-foot diameter, multimillion-dollar particle detector, has six layers of detectors arranged around a toroidal superconducting magnet. Old Dominion, in partnership with Jefferson Lab, built one of the six CLAS detector layers.

Jefferson Lab is a basic research nuclear physics user facility managed by the U.S. Department of Energy and operated by the Southeastern Universities Research Association, a consortium of 61 universities. Old Dominion is a state-assisted university located in Norfolk, Va. The university's nuclear physics program includes eleven tenured internationally known faculty. Nuclear physics research at Old Dominion is supported in part by grants from the Department of Energy.

For more information, contact Weinstein at (757) 683-5803 or weinstei@physics.odu.edu. You can see an animation of a collision at http://www.physics.odu.edu/~weinstei/eventloop.gif.

This article was posted on: August 29, 2003

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