Big Bite does its stuff

Spectrometer helps pick pairs out of a crowd

The neutron detector was positioned vertically and sat several yards behind the proton detector. Components for BigBite’s detector packages for this experiment were from Tel Aviv University, Glasgow University, Indiana University and Hampton University. The scattering chamber was funded through a “major research initiative” grant by the University of Virginia.

Jefferson Lab's core mission is to study the heart of ordinary matter: the nucleus of the atom. Now Hall A has a new magnet and detector system designed to help physicists look at the nucleus in a whole new light. "BigBite" has debuted in its first experiment at Jefferson Lab.

BigBite is a spectrometer that was originally built in 1995 for nuclear physics experiments at NIKHEF, the National Institute for Nuclear Physics and High Energy Physics in the Netherlands. Kees de Jager, Hall A Leader, led several research programs at NIKHEF before coming to Jefferson Lab. He was the program leader of Internal Target Physics at NIKHEF, which built and used BigBite to explore the electric charge distribution in the neutron and the shape of the deuteron (a form of the hydrogen atom containing one proton and one neutron in its nucleus). "BigBite was used as a spectrometer for the internal target, which was inside the storage ring at NIKHEF," de Jager notes.

After he came to Jefferson Lab, de Jager arranged for BigBite to be shipped to JLab once its planned experiments at NIKHEF were complete. Here, BigBite is used to complement Hall A's two High Resolution Spectrometers (HRS).

According to Doug Higinbotham, one of the NIKHEF Ph.D. students that originally used BigBite and the Hall A staff scientist responsible for BigBite's current experimental program, BigBite is very useful for what physicists call triple-coincidence measurements. In these experiments, physicists need to measure three particles. Two particles are detected by Hall A's resident High Resolution Spectrometers, while BigBite detects the third.

"It's a giant microscope. So with the HRS, you see things really, really well, but you can only see some tiny part of the slide," he explains, "BigBite can see the whole slide, but it can't zoom in." However, what BigBite lacks in precision, it makes up with acceptance -- the size of the area where it can detect particles. Physicists sometimes refer to a detector's acceptance as its bite.

"The HRS have a very small bite, on the order of a degree; BigBite has a huge bite -- almost 10 degrees," Higinbotham explains, "So for some experiments, you'd like to have high resolution detectors [like the HRS] detecting interacting particles and another large bite or large acceptance spectrometer [BigBite] detecting recoil particles."

JLab's first experiment using BigBite -- Experiment E01-015 -- was inspired by an original measurement at Brookhaven National Lab. Scientists there were trying to measure Short Range Correlations (SRCs). The nucleus is built of one or more nucleons (protons and neutrons). Studying how nucleons interact tells physicists something about how they're glued together. A SRC occurs when two nucleons interact very strongly with one another via the strong force, the force that glues them together in the nucleus.

Think of the strong force as being like a spring. It keeps nucleons at just the right distance from one another. Should one stray a little far, stretching the strong force spring out, the force works to pull the nucleons back together. At the same time, when nucleons push a little too close together, squeezing the strong force spring in, it pushes the particles apart again. Hall A's E01-015 aims to spot SRCs by catching the strong force in the act of pushing nucleons apart.

According to Steve Wood, a spokesperson on the experiment, "You really have to understand every part of the force between nucleons. And part of that force is what's happening at a short distance. The force between nucleons gets very repulsive at short distances." So in that moment, the nucleons each have extra momentum, or energy, pushing them in opposite directions.

If an electron from CEBAF's beam strikes one of the nucleons as they're being pushed apart by the strong force, the scientists can calculate that extra momentum. The researchers are specifically looking to hit protons that are interacting with other protons or neutrons in SRCs. So the HRSs were set to detect the electron from CEBAF and the proton it struck. Meanwhile, BigBite and an accompanying neutron detector were set to detect the proton or neutron that the struck proton was interacting with. "We detect a high energy electron and a high energy proton, and we use the standard Hall A spectrometers for that. Then we look with BigBite and the neutron detector at the place where we expect a recoil from the reaction from the two particles we hit," Wood says.

Post-experimental analysis of the data from the High Resolution Spectrometers, BigBite and the neutron detector will reveal what momentum the two nucleons had at the moment the electron struck the proton. From that, the researchers can get a more complete picture of SRCs, including perhaps a better idea of how nucleons interact via the strong force and how often SRCs occur. Experiment E01- 015 looked at SRCs inside a carbon nucleus. Data-taking wrapped up in mid-April, and analysis is underway.

To date, seven experiments have been approved for BigBite. The next experiment is scheduled to begin installation in December.