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Pocket-sized physics detector does big science



Compact detector on display in CEBAF Center lobby.
Click here for a high resolution photograph.

How do quarks and gluons, the elementary constituents of all matter, combine to form the protons and neutrons in the nucleus of the atom? This is a fundamental unsolved question in nuclear physics that researchers at Jefferson Lab are working to answer. The internal structure of the proton has been studied for several decades, and scientists have learned a great deal. However, much less is known about the structure of the neutron.

To study the nucleus of the atom and its building blocks at JLab, nuclear physicists start with a stream of electrons from CEBAF, JLab's accelerator. The electrons strike a target, and as they pass through, some collide with the particles that make up the target. Physicists learn about these particles by using detectors placed around the target to record information about the electrons that scattered and about the particles that are knocked out.

Hydrogen is a good target material for studying the proton this way. It's nucleus is a single proton, so studying it is synonymous with studying a "free proton." However, there are is similar material for studying neutrons. The only neutrons that can be studied with accelerators are found bound together with protons inside nuclei. The nucleus of deuterium is composed of one proton and one neutron and is the simplest nucleus containing neutrons. The difficulty with using deuterium as a target to learn about neutrons, however, is that the structure of a neutron is altered when it is bound up inside a deuterium nucleus.

To overcome this problem, the BONUS (Barely Off-shell NUcleon Structure, or BOund NUcleon Structure) experiment at JLab used a novel technique in which electrons collide with neutrons in the deuterium nucleus at a moment when they are only loosely bound inside the nucleus, appearing as if they are "nearly free."

To pick out and measure these particular weakly-bound neutrons, scientists needed to detect protons that were moving very slowly relative to the neutrons inside the nucleus. These protons are difficult to detect, because they don't travel very far before losing all their energy and coming to a halt. With the original experimental equipment at JLab, the protons would have stopped before reaching the detectors. So physicists built the BONUS detector. This detector could be placed close enough to the target to capture these very important protons (or VIPs, as the BONUS scientists call them) as they straggled away from the nucleus.

The data from this experiment, which was staged in Hall B, October through December 2005, will allow physicists to unambiguously examine the structure of a nearly-free neutron, providing a much more comprehensive picture of these building blocks of matter.

The (functional spare) BONUS detector a Radial Time Projection Chamber is currently on display by the main stairway in the CEBAF Center lobby. The spare was installed in CLAS for the June 2005 test run but a different (identical) one was used in the late-2005 production run. That detector functioned throughout the experiment's run, so the spare was not called upon. The Radial Time Projection Chamber, which weighs just about 5 pounds and measures less than 10 inches long, was placed inside Region 1, the innermost part of the CLAS detector.

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