The Qweak experimental apparatus

• Published in: Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment Vol. 781; no. C; pp. 105 - 133
• Main Authors: Allison, T., Anderson, M., Androić, D., Armstrong, D.S., Averett, T., Averill, R., Balewski, J., Beaufait, J., Benesch, J., Benmokhtar, F., Bessuille, J., Birchall, J., Bonnell, E., Bowman, J.D., Brindza, P., Brown, D.B., Carlini, R.D., Cates, G.D., Cavness, B., Dusa, S. Covrig, Davis, C.A., Dean, D.C., Deconinck, W., Diefenbach, J., Dow, K., Dowd, J.F., Duvall, W.S., Echols, J.R., Elaasar, M., Falk, W.R., Finelli, K.D., Finn, J.M., Gaskell, D., Gericke, M.T.W., Grames, J., Gray, V.M., Grimm, K., Guo, F., Hansknecht, J., Harrison, D.J., Henderson, E., Hoskins, J.R., Ihloff, E., Johnston, K., Jones, D., Jones, M., Jones, R., Kargiantoulakis, M., Kelsey, J., King, P.M., Korkmaz, E., Kowalski, S., Kubera, A., Leacock, J., Leckey, J.P., Lee, A.R., Lee, L., Liang, Y., MacEwan, S., Mack, D., Magee, J.A., Mahurin, R., Mammei, J., Martin, J.W., McCreary, A., McDonald, M.H., McHugh, M.J., Meekins, D., Mei, J., Michaels, R., Morgan, N., Musson, J., Mesick, K.E., Nelyubin, V., Nuruzzaman, Page, S.A., Paschke, K.D., Phillips, S.K., Pitt, M.L., Poelker, M., Roberts, W.R., Roche, J., Rose, P.W., Sawatzky, B., Shabestari, M.H., Simicevic, N., Smith, G.R., Sobczynski, S., Solvignon, P., Spayde, D.T., Stokes, B., Subedi, A., Subedi, R., Suleiman, R., Tvaskis, V., Urban, E., Waidyawansa, B., Wang, P., Wood, S.A., Zielinski, R.B.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.05.2015; Elsevier
• Subjects: Electron scattering; High luminosity; Liquid hydrogen target; Parity violation; Particle detectors; Parity violation; High luminosity; Particle detectors; Electron scattering; Liquid hydrogen target
• ISSN: 0168-9002; 1872-9576
• DOI: 10.1016/j.nima.2015.01.023

The Jefferson Lab Qweak 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 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 180μA of 89% longitudinally polarized electrons whose helicity was reversed 960 times per second. The electrons were accelerated to 1.16GeV and directed to a beamline with extensive instrumentation to measure helicity-correlated 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 34.4cm 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 silica Cherenkov detectors arrayed symmetrically around the beam axis. A total scattered electron rate of about 7GHz 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 Q2=0.025GeV2 was determined using dedicated low-current (~100pA) measurements with a set of drift chambers before (and a set of drift chambers and trigger scintillation counters after) the toroidal magnet.