Jefferson's accelerator tunnel lies 25 feet below the Earth's surface on an old sea bed. About 25,000 cubic yards of concrete were used to build the tunnel. The electron beam travels around the underground tunnel five times in 21 millionths of a second. At that speed, the electron beam could circle the Earth seven and one-half times in one second!
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Probing the Quirks of Quarks
Although the lab is fiendishly complicated, the basic idea is straightforward. A beam of electrons is accelerated in the underground tunnel by microwave radiation in a series of superconducting cavities. After a few laps, when the electrons are traveling at the desired speed, the beam is split up and they are directed to three house-sized spectrometers located in three different experimental halls. Rensselaer built a key component of the Jefferson Lab's Hall B spectrometer.
The Hall B spectrometer contains a gas target such as liquid hydrogen that offers its nuclei as targets for the electrons. When an electron strikes a target nucleus it bounces off while creating and knocking out other particles. A powerful torroidal magnetic field generated by six superconducting coils directs these particles through layers of surrounding detectors. The angle of deflection and the momentum of each ricocheting electron and the other particles are detected and recorded. To the physicist, that information yields tremendous insights into the nucleus that was struck. Scientists sift through reams of such data, analyzing the results to prove or disprove their hypotheses. In fact, says Stoler, Hall B is expected to generate a terabyte of data a day (1,000 gigabytes), which makes finding the important data a bit like finding the proverbial needle in a haystack.
"Lots of things happen in these atomic interactions, but most of them aren't interesting," says Charles Sinclair '60, senior scientist in charge of the Injector Group and the 100-person Operations Group at Jefferson Lab. The Injector Group is charged with preparing the electron beam, arguably the most important task at the lab. "Most of the interactions are known by now and we're looking for new things. A very useful aspect of this lab, though, is that because we have a continuous beam, many experiments can be conducted simultaneously and continuously. Then, once the data is collected, it can be sorted by class or by the type of experiment."
A diagram of the Jefferson Lab.
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An Agitated Feeling
The Jefferson Lab began as a belief in the scientific community that the available research tools weren't adequate. "In the 1970s there was some agitation toward the goal of giving physicists a greater ability to see what was going on at the nuclear level," says Stoler. "High energy levels were important, but what was also needed was a fantastic ability to resolve tiny nuclear structures that could not be resolved by then-current equipment."
Stoler ran a series of workshops at Rensselaer in 1980 at which he and fellow concerned scientists generated ideas and laid out what kind of work could be done with a continuous electron beam accelerator facility (called "CEBAF"). The proceedings of those meetings were compiled at Rensselaer into an influential volume that became known as the Blue Book. It was instrumental in convincing the government to support the ideas that were generated, and the U.S. Department of Energy eventually held a competition to build the lab. It was decided in the early 1980s to build CEBAF in Newport News. Construction began in 1983 and the facility was commissioned in 1996.
Collaborations of physicists from universities and national laboratories nationwide and worldwide have been lining up since the late 1980s to conduct experiments at the Jefferson Lab. Proposed experiments are judged by a panel of internationally prominent physicists, which then recommends how to allot precious "beam time" among accepted proposals. Recently, the first Rensselaer-led experiment at the lab was completed. The quality and quantity of the data were far greater than previously possible, and the results already are settling some important controversies about how quarks behave within the nucleons.