Q_weak实验装置
The Q_weak Experimental Apparatus
摘要：
Jefferson实验室的Q_weak实验通过测量非极化液氢靶在小动量转移时纵向极化电子的宇称违反弹性散射不对称性来确定质子的弱电荷。为该实验设计了定制装置，以满足所测量的最小和最精确的${\vec{e}}$ p不对称性所带来的技术挑战。杰斐逊实验室在目标功率、束流、束螺旋度反转率、偏振测量、探测率和螺旋度相关束流特性控制方面取得了技术里程碑。该实验使用180微安的89%纵向极化电子，其螺旋度每秒反转960次。电子被加速到1.16 GeV，并被引导到具有广泛仪器的束线，以测量螺旋度相关的束特性，这可能会引起虚假的不对称性。采用Moller和Compton偏振仪测量电子束偏振度，使其优于1%。电子束入射到34.4cm的液氢靶上。在通过三重准直器系统之后，在5.8度和11.6度之间的散射电子在电阻铜线圈磁体的环形磁场中弯曲。接受器内部的电子被聚焦到围绕光束轴对称排列的八个熔融石英切伦科夫探测器上。约7 GHz的总散射电子速率入射到检测器阵列上。探测器通过定制的低噪声前置放大器和18位采样ADC模块以积分模式读出。动量转移Q^2 = 0.025 GeV^2是通过专门的低电流（~100 pA）测量确定的，在环形磁体之前有一组漂移室（在环形磁体之后有一组漂移室和触发闪烁计数器）。
英文摘要：
The Jefferson Lab Q_weak experiment determined the weak charge of the proton by measuring the parity-violating elastic scattering asymmetry of longitudinally polarized electrons from an unpolarized liquid hydrogen target at small momentum transfer. A custom apparatus was designed for this experiment to meet the technical challenges presented by the smallest and most precise ${\vec{e}}$p asymmetry ever measured. Technical milestones were achieved at Jefferson Lab in target power, beam current, beam helicity reversal rate, polarimetry, detected rates, and control of helicity-correlated beam properties. The experiment employed 180 microA of 89% longitudinally polarized electrons whose helicity was reversed 960 times per second. The electrons were accelerated to 1.16 GeV and directed to a beamline with extensive instrumentation to measure helicity-correlated beam properties that can induce false asymmetries. Moller and Compton polarimetry were used to measure the electron beam polarization to better than 1%. The electron beam was incident on a 34.4 cm liquid hydrogen target. After passing through a triple collimator system, scattered electrons between 5.8 degrees and 11.6 degrees were bent in the toroidal magnetic field of a resistive copper-coil magnet. The electrons inside this acceptance were focused onto eight fused silica Cerenkov detectors arrayed symmetrically around the beam axis. A total scattered electron rate of about 7 GHz was incident on the detector array. The detectors were read out in integrating mode by custom-built low-noise pre-amplifiers and 18-bit sampling ADC modules. The momentum transfer Q^2 = 0.025 GeV^2 was determined using dedicated low-current (~100 pA) measurements with a set of drift chambers before (and a set of drift chambers and trigger scintillation counters after) the toroidal magnet.
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