Scintillator Machining for the new Coordinate Detector in Jefferson Lab's High Momentum-Transfer Proton Electric Form Factor Experiment "GEp-5"

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As part of the new experimental equipment required for use in conjunction with the just-completed 12 GeV energy upgrade at the Thomas Jefferson National Accelerator Facility ("Jefferson Lab") in Virginia, USA, the Super Bigbite Spectrometer ("SBS") detector facility is being developed/constructed in experimental Hall A. The SBS facility is being prepared primarily for use in a series of experiments dedicated to precision extraction of nucleon form-factors in high-energy elastic electron scattering. In particular, one of these SBS experiments will extend measurements of the proton’s electric form-factor to the highest values of momentum-transfer ever achieved - which effectively probes the distribution of electric charge inside the proton to the very smallest separation distances ever probed. To make this measurement, high-energy electrons (11 GeV) will elastically scatter from protons in a liquid hydrogen target, with recoiling protons being detected in the "SBS" itself - outfitted to also measure the polarization direction of the recoil protons - and the scattered electrons being detected in an electromagnetic calorimeter. The experiment will run at very high luminosity, and thus create a very high background radiation environment. In order to facilitate identification of the desired elastic-scattering reactions in this high-background situation, a Coordinate Detector ("CDet") is needed between the target and the electron calorimeter for a more precise measurement of the vertical scattering angle (compared to what will be possible with the calorimeter itself). This vertical coordinate detection will allow for selection of co-planar electron-proton final states when combined with the angle/position information of the proton detected in the SBS. This off-line co-planarity requirement is necessary to isolate and separate the desired elastic events from the high-level background. The Saint Mary’s group has worked with JLab staff for the past 5 years to assist in the development of a new, improved design for the CDet based on using two planes of vertically-stacked, thin (5 mm) scintillating bars (4 cm thick) to cover the needed area (294 cm x 104 cm) - requiring 2352 scintillating bars (each measuring 5 mm x 4 cm x 52 cm). The new design was presented to the SBS program’s Department of Energy review committee in Nov. 2013, and formally approved as part of the SBS Program Management Plan in April 2014 after undergoing a internal JLab technical review. The design relies on accurate alignment of each vertical stack of 588 bars to have a slight progressive tilt to allow the front face of each bar to remain perpendicular to a line extending from target centre; this alignment requirement will be achieved by precision machining along the 52cm length (top and bottom) of each bar. The resources from this equipment proposal will allow for this machining to be undertaken and accomplished.