Hungry magnet, detector package will feed on subatomic particles at Jefferson Lab

July 03, 2002

Anything over eight feet tall, six feet wide and weighing over 20 tons might be expected to have a healthy appetite. But no traditional foods are ingested by this behemoth. For the BigBite magnet, the nourishment of choice is subatomic particles, and lots of them. The BigBite spectrometer, which consists of the magnet along with its detectors, will be able to discern scattered particles over a range of energies and angles far greater than can be obtained with the other spectrometers used in Jefferson Lab's Hall A.

BigBite is the latest addition to the Department of Energy's Jefferson Lab family of particle detectors. It comes via the Netherlands' National Institute for Nuclear and High Energy Physics, NIKHEF, in Amsterdam which commissioned the magnet's construction by Russian scientists in 1994. When the NIKHEF accelerator ceased operations in 1999, the institute sold the magnet to Jefferson Lab. The magnet was stored until, with the approval of a trio of Hall A experiments, researchers began refurbishing the magnet and building the associated particle detectors.

"BigBite will be able to work with the Hall A high resolution spectrometers or stand alone," says Douglas Higinbotham, the Hall A staff scientist who is coordinating the BigBite project. "There are three upcoming experiments that will definitely put BigBite through its paces. Four other experiments, proposed but not yet approved by the Lab's program advisory committee, also wish to use BigBite."

Unlike the other Hall A spectrometers, BigBite has no focusing properties. While this allows BigBite to easily detect particles over a large range of angles and energies, the lack of focusing means BigBite will not be able to determine these quantities as precisely as the high resolution, small-acceptance Hall A spectrometers. For the approved BigBite experiments, large angular and energy coverage with moderate resolution is exactly what is required.

The precise fit of the BigBite in Hall A will be tight, since researchers require that the magnet be located one meter from the Hall's scattering chamber. The scattering chamber is where the Lab's electron beam collides with targets and the out going particles are produced. The placement of BigBite will require the construction of a special platform and cantilevered arm so that the spectrometer can be maneuvered into position for operation. Also, the observational window in the Hall's scattering chamber will need to be enlarged to accommodate BigBite's large angular view.

"The project," says Higinbotham, "wouldn't be possible without the ongoing and substantial support from institutions and universities worldwide that are contributing equipment and personnel." The Massachusetts Institute of Technology has a graduate student and a post-doctoral research scientist stationed at Jefferson Lab working full time on the project. Tel Aviv and Glasgow Universities are building the particle detectors needed for the first experiment and the University of Virginia is working on the more precise detectors required for the subsequent experiments. The University of Virginia is collaborating with Florida International and California State Universities to develop the new scattering chamber and target systems.

"Without user support this project couldn't be done," Higinbotham asserts. "It's very much an international effort. And as more experiments are proposed, the project has been gaining collaborators willing to help with the construction effort. The number of new proposals has been very encouraging. The project is feeding on itself."

BigBite is scheduled for installation in Hall A by late fall 2002. Testing and commissioning will follow. If everything progresses as planned, the first of the three approved BigBite experiments should commence in fall 2003.

DOE/Thomas Jefferson National Accelerator Facility

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