Jefferson Lab Experiment E03-012 has been approved to study neutron structure using the spectator tagging technique in Experimental Hall-B[ref- bonus proposal]. The collaboration is developing a Radial Time Projection Chamber (RTPC) detector for the spectator protons which will allow it to associate them with electron scattering vertices in a high pressure gas (7 atm.) target cell. By measuring the scattered electron in Hall-B's CEBAF Large Acceptance Spectrometer (CLAS) and the spectator proton in the Bonus RTPC, the experiment can completely determine the four-momentum of the recoiling neutrons. The concept is shown in the figure. Tracks in CLAS and in the RTPC will be connected on the basis of both timing and position information from the two detector systems.
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| A simplified cutaway view of CLAS showing a particle tracked from the target through the spectrometer. | Close-up view of the target
region showing a cross-section of the BoNuS detector, the track seen in
CLAS, and the associated spectator proton. |