Big bite is reborn
BigBite was originally used at NIKHEF, the National Institute for Nuclear Physics and High Energy Physics in the Netherlands. After the experimental area it was installed in closed, Hall A Leader Kees de Jager envisioned a productive future for BigBite at Jefferson Lab. He brought BigBite to the Lab in 1999 to enhance Hall A's research capabilities. However, since the spectrometer was designed for use at NIKHEF, the researchers had to refurbish it before it could be used here.
A group of University of Virginia (UVA) researchers applied for a National Science Foundation (NSF) Major Research Instrumentation (MRI) grant in 2002. NSF's MRI Program is designed to increase access to scientific and engineering equipment for research and research training in our Nation's organizations of higher education, research museums and non-profit research organizations.
Richard Lindgren, a UVA research professor and MRI principal investigator, explains that the researchers also acquired additional funding. "At that time, in order to get an MRI, you needed to have at least 30% matching funding from other facilities. And they could not be DOE-funded laboratories. So we had partners, such as the University of Glasgow and Tel Aviv, and they put up money to build part of the detector system," he says.
In October 2002, the team was awarded an MRI grant for $441,000 to upgrade BigBite. "The total amount of money of putting this package together is on the order of a million dollars, if you count everything," Lindgren says. The upgrade plan was crafted by Lindgren and Bogdan Wojtsekhowski, a Hall A Staff Scientist and a spokesperson for the neutron electric form factor experiment (GEn).
The MRI collaboration worked hard for over three years fine-tuning the design and constructing the new instrumentation for BigBite. The NSF funds were used to build a universal scattering chamber with a very large vertical opening to match BigBite's large acceptance and three Multi-Wire Drift Chambers (MWDC) with 15 planes and 2654 signal wires to provide a position resolution of 0.2 millimeters.
MRI Co-Principle Investigator and UVA Assistant Professor Nilanga Liyanage led a group of students from the Department of Physics at UVA in developing new infrastructure and completing the construction of the high-quality drift chambers. "This was an especially nice project for the undergraduates, because the physics was within their grasp; it was a nice match to the undergraduate curriculum," he says.
In all, Liyanage oversaw ten students, including two Ph.D. students and one Master's student. "I had two Ph.D. students, Brandon Craver and Mitra Shabestari. Also, I got a lot of help from UVA Research Associate Vladimir Nelyubin on this project," he says, "Basically, Vladimir and the students put [the BigBite wire chambers] together. " Lindgren notes that the preliminary plans for BigBite changed as the refurbishment progressed, "Initially we were going to build what are called proportional chambers. But as time went on, it was learned that [at least one of the experiments] was going to require more accurate measurements of the angles of the electrons scattered from the target. And in order to do that, we had to change the design from a proportional chamber to a drift chamber, which allows you to measure the particle angles more accurately. "
With the upgrade complete, BigBite is an essential tool for experiments that require large angular and momentum acceptances, and the upgraded spectrometer complements Hall A's smaller-acceptance High Resolution Spectrometers (HRS). "BigBite offers something that the High Resolution Spectrometers do not, and that's large acceptance," Lindgren comments. In addition, the new rate capability of the upgraded BigBite detector achieves the maximum luminosity obtained with polarized targets.
Dennis Kovar is the DOE Associate Director of the Office of Science's Office of Nuclear Physics, and Brad Keister is a Program Director in the NSF Directorate for Mathematical & Physical Sciences in the Division of Physics.
BigBite has been used in two Hall A experiments thus far. One was the short-range correlation experiment, which ran in the spring of 2005 (see the full story) and the other, which required the fully upgraded BigBite, was the neutron electric form factor experiment (GEn) completed in May 2006. GEn was the first test of the newly constructed wire chambers.
The spectrometer improved the typical Hall A count rate for polarized target experiments by a factor of 50, and the new detector package is allowing BigBite to operate at a luminosity that's 10,000 times higher than its limit at NIKHEF. "So a typical spectrometer wire chamber sees a couple of million particles per second. With BigBite, we see 20 million particles per second," Liyanage explains.
In addition to its recent successes, Wojtsekhowski also sees a bright future for BigBite. "The large acceptance and high luminosity features of BigBite have a great potential for future experiments at JLab, including experiments that will be proposed for the 12 GeV Upgrade," he says.
Hall A experiments scheduled to take advantage of BigBite's unique features account for 100 days of total approved beam time and include one of the original experiments put forth for upgrading BigBite, the electroproduction of pions at threshold experiment, E04-007. Other experiments which propose to use BigBite are: E05-009, E05-015, E06-010, E06-011, and E06-014.