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

作为美国弗吉尼亚州托马斯·杰斐逊国家加速器设施（“杰斐逊实验室”）刚刚完成的12gev能量升级所需的新实验设备的一部分，超级大比特光谱仪（“SBS”）探测器设备正在实验大厅a中开发/建造。SBS设备主要准备用于一系列致力于精确提取高能弹性电子散射中核子形状因子的实验。特别值得一提的是，其中一个SBS实验将把质子的电形状因子的测量扩展到动量传递的最高值，从而有效地探测质子内部电荷的分布，直至探测到最小的分离距离。为了进行这种测量，高能电子（11 GeV）将从液氢靶中的质子中弹性散射，反冲质子将在“SBS”本身中被检测到——该装置也可以测量反冲质子的极化方向——而散射电子将在电磁量热计中被检测到。实验将在非常高的亮度下运行，从而创造一个非常高的背景辐射环境。为了便于在这种高背景情况下识别所需的弹性散射反应，在目标和电子量热计之间需要一个坐标探测器（“CDet”），以便更精确地测量垂直散射角（与量热计本身相比）。当结合在SBS中检测到的质子的角度/位置信息时，这种垂直坐标检测将允许选择共面电子-质子的最终状态。这种离线共平面性要求对于从高级背景中隔离和分离所需的弹性事件是必要的。在过去的5年里，圣玛丽小组与JLab的工作人员合作，协助开发了一种新的改进设计，该设计基于使用两个垂直堆叠的平面，薄（5毫米）闪烁条（4厘米厚）覆盖所需面积（294厘米x 104厘米）-需要2352条闪烁条（每个尺寸为5毫米x 4厘米x 52厘米）。新设计于2013年11月提交给SBS项目的能源部审查委员会，并在经过JLab内部技术审查后，于2014年4月正式批准为SBS项目管理计划的一部分。该设计依赖于588根杆的每个垂直堆叠的精确对齐，使每个杆的前表面保持垂直于从目标中心延伸的线；这种对齐要求将通过沿每个杆的52cm长度（顶部和底部）的精密加工来实现。该设备提案中的资源将允许进行和完成这种加工。

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.