The Jefferson Lab Q weak experiment determined the weak charge of the proton by measuring the parityviolating 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 ep 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 µA 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 helicitycorrelated beam properties that can induce false asymmetries. Møller 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• and 11.6• were bent in the toroidal magnetic field of a resistive copper-coil magnet. The electrons inside this acceptance were focused onto eight fused silicaČerenkov 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.

We report on the highest precision yet achieved in the measurement of the polarization of a lowenergy, O(1 GeV), continuous wave (CW) electron beam, accomplished using a new polarimeter based on electron-photon scattering, in Hall C at Jefferson Lab. A number of technical innovations were necessary, including a novel method for precise control of the laser polarization in a cavity and a novel diamond microstrip detector which was able to capture most of the spectrum of scattered electrons. The data analysis technique exploited track finding, the high granularity of the detector and its large acceptance. The polarization of the 180 µA, 1.16 GeV electron beam was measured with a statistical precision of < 1% per hour and a systematic uncertainty of 0.59%. This exceeds the level of precision required by the Q weak experiment, a measurement of the weak vector charge of the proton. Proposed future low-energy experiments require polarization uncertainty < 0.4%, and this result represents an important demonstration of that possibility. This measurement is the first use of diamond detectors for particle tracking in an experiment. It demonstrates the stable operation of a diamond based tracking detector in a high radiation environment, for two years.

We have performed a novel comparison between electron-beam polarimeters based on Møller and Compton scattering. A sequence of electron-beam polarization measurements were performed at low beam currents (< 5 µA) during the Q weak experiment in Hall C at Jefferson Lab. These low current measurements were bracketed by the regular high current (180 µA) operation of the Compton polarimeter. All measurements were found to be consistent within experimental uncertainties of 1% or less, demonstrating that electron polarization does not depend significantly on the beam current. This result lends confidence to the common practice of applying Møller measurements made at low beam currents to physics experiments performed at higher beam currents. The agreement between two polarimetry techniques based on independent physical processes sets an important benchmark for future precision asymmetry measurements that require sub-1% precision in polarimetry.

Experimental results are presented comparing the intensity of the bremsstrahlung produced by electrons with initial energies ranging from 10 to 20-keV incident on a thick Ag target, measured at forward angles in the range 0˚ to 55˚. When the data are corrected for attenuation due to photon absorption within the target, the results indicate that the detected radiation is distributed anisotropically only at photon energies, k, that are approximately equal to the initial energy of the incident electrons, E o . The results of our experiments suggest that as k /E o → 0, the detected radiation becomes essentially isotropic due primarily to the scattering of electrons within the target. A comparison to the theoretical work of Kissel et al. [6] suggests that the angular distribution of bremsstrahlung emitted by electrons incident on thick targets is similar to the angular distribution of bremsstrahlung emitted by electrons incident on free-atom targets only when k /E o ≈1. The experimental data are also in approximate agreement with the angular distribution predictions of the Monte Carlo program, PENELOPE.

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