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On Target (November 1995) | ||||
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First Experiment Begins
On November 15th it all came together. The teamwork, ingenuity, and dedication of countless CEBAF users and staff resulted in the successful "christening" of Hall C. With the start of the first experiment, the laboratory enters a new era of exploration never realized before. Experiment team leader Don Geesaman, a nuclear physicist from Argonne
National Laboratory in
Illinois, remarked that the experiment's magnitude demanded the expertise
of many.
"This is a group effort. No one person even knows 50 percent of the overall
project. The challenge is bringing the right combination of people
together. Everyone contributed," says Geesaman.
Over 100 scientists have contributed to the experiment's development and implementation. Many of the researchers previously conducted the experiment at Stanford Linear Accelerator Center. Geesaman says that data collected at CEBAF will be superior to earlier attempts of the experiment. This is due to the excellent properties of the laboratory's continuous beam and greater accuracy of CEBAF's spectrometers. Geesaman has also been impressed with the quality of CEBAF staff and equipment. "CEBAF has a large group of talented, dedicated people working on the experiment. I have been very pleased with the reliability of the accelerator. The beam has been excellent. The startup was slow, but that is the way things usually work. After startup, everything just seemed to click along," says Geesaman. Researchers analyze data as it is collected and will continue to do so in greater depth over the next year. Data collected may provide groundbreaking insights into the nature of nuclear matter. The results could seriously challenge previously held scientific models. To facilitate this, researchers are probing various target materials with a stream of electrons produced by CEBAF's accelerator. After several circuits through the race-track shaped accelerator, the
hair-width beam is released into Hall C where it collides into gold,
carbon, and iron foil targets. Some of the beam's electrons will interact
with the target's protons as beam passes through the foil causing target
protons to break away. Ninety-nine percent of the beam's electrons pass
through the target without any interaction. "When an electron does strike a
proton in the center of the nucleus, about 30 percent of the time protons
emerge without interacting with other protons or neutrons in the nucleus,"
says Geesaman.
Spectrometers measure parameters of the interactions like the speed,
mass,and direction of target protons displaced by the beam's electrons.
Using a simple ice-skating scenario the interaction can be visualized as follows: If a large skater collides with great force into a tightly packed group of skaters, members within the group will spin off due to collision forces exerted by the colliding skater. Skaters in the group would be displaced in varying degrees and distances based on their proximity to the point of collision and their individual weights and momentum. Many in the group though would not be displaced if distanced from the point of collision. After such an event, one could ascertain the weight, momentum, and previous placement of an individual skater within the group by examining the resting point and path of that individual skater. Another example of the process is an investigator's ability to ascertain the track, velocity ,and controllability of a vehicle after an accident by examining skid marks. Thus, project scientists are observing instances of interactions to gain greater insight and comprehension into the basic structure of matter. "We are not necessarily looking at individual events, though we will look at a few. We are interested in probabilities of interactions and will graph the momentum, energy, and charge," says Geesaman. The focus is to deduce particles' structure through an understanding of the trends in their behavior. This experiment is part of a group of experiments that Geesaman refers to as "first generation," meaning it will survey much of CEBAF's future research. "This [the experiment] will give us an overview and serve as a basis for probing the quark structure [the smallest unit of matter]." Geesaman admits he was cautious at the prospect of conducting CEBAF's first
experiment. "Getting started with physics at CEBAF is so important to the
Physics Community that I was willing to put up with problems of being the
first user. It [the work at CEBAF] is that important. Besides, working out
the bugs is part of the fun," said Geesaman.
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