Compton Polarimeter based on Hall-A/C experience for the EIC Alexandre Camsonne Jefferson Laboratory EIC Users Meeting June 27th 2014 ERHIC 6/27/2014 A. Camsonne EIC Users meeting • 5 GeV to 21.2 GeV • 9.4 MHz Repetition rate • 50 mA with “Gatling gun” design up to 24 sources • Need to measure each sources polarization • 80 % min polarization • Similar recirculation to CEBAF 2 MEIC Warm large booster (up to 25 GeV/c) Three Figure-8 rings stacked vertically SRF linac Pre-booster Ion source Cold 25-100 GeV/c proton collider ring Warm 3-12 GeV electron collider ring • Storage ring – Ring ring • 748.5 MHz = 1.33 ns bunch structure • 3 A at 3 GeV and 180 mA at 11 GeV • 2 macrobunch with one polarization 2.3 us • Every electron bunch crosses every ion bunch • Measure polarization average of the two macrobunch Medium-energy IPs with horizontal beam crossing Injector 12 GeV CEBAF 6/27/2014 A. Camsonne EIC Users meeting 3 Compton asymmetry s( e + g 6/27/2014 s( e + g e’ + g’ ) A. Camsonne EIC Users meeting e’ + g’ ) 4 Hall A Compton chicane •2.2 GeV to 11 GeV •120 uA max •Repetition rate 499 MHz with lower rates available •Polarized beam up to 85% •Fast helicity flip Photon detector on movable table 6/27/2014 A. Camsonne EIC Users meeting Vertical motion of electron detector to move detector close to the beam ( up to 5 mm ) 5 Cavity power • Green laser using IR seed laser and PPLN frequency doubling • Around 5 kW power • 10 kW reachable • Laser polarization flip • Abdurahim Rakhman (2011) PhD Thesis Syracuse 6/27/2014 A. Camsonne EIC Users meeting 6 Hall A Photon detector • FADC readout SIS3320 250 MHz FADC • Digital integration with 240 Hz helicity flip • Record all the signal for a given helicity • Compute integrated asymmetry for a pair 6/27/2014 A. Camsonne EIC Users meeting 7 Happex III results Friend Nucl.Instrum.Meth. A676 (2012) 96-105 Friend PhD Thesis CMU 2012 6/27/2014 A. Camsonne EIC Users meeting Pe =89.41% 8 Hall C Compton Electron Detector Diamond microstrips used to detect scattered electrons Radiation hard Four 21mm x 21mm planes each with 96 horizontal 200 μm wide micro-strips. Rough-tracking based/coincidence trigger suppresses backgrounds (D. Dutta Missipi State University) Compton Electron Detector Measurements Polarization analysis: Yield for each electron helicity state measured in each strip Background yields measured by “turning off” (unlocking) the laser Asymmetry constructed in each strip Strip number corresponds to scattered electron energy Endpoint and zero-crossing of asymmetry provide kinematic scale 2-parameter fit to beam polarization and Compton endpoint (D. Dutta) 6/27/2014 A. Camsonne EIC Users meeting 10 Polarization Measurements Measured polarization (%) Q-Weak Run 2 – November 2011 to May 2012 Photocathode re-activation 90 87.5 85 Preliminary Moller polarimeter 82.5 Compton polarimeter - electron detector 22500 23000 23500 24000 24500 25000 25500 Compton run number PMoller +/- stat (inner) +/- point-to-point systematic (0.54%) 0.64% normalization unc. not shown PCompton +/- stat +/- preliminary systematic (0.6%) 6/27/2014 A. Camsonne EIC Users meeting 11 Preliminary Systematic Uncertainties Systematic Uncertainty Uncertainty ΔP/P (%) Laser Polarization 0.1% 0.1 Dipole field strength (0.0011 T) 0.02 Beam energy 1 MeV 0.09 Detector Longitudinal Position 1 mm 0.03 Detector Rotation (pitch) 1 degree 0.04 Asymmetry time averaging 0.15% 0.15% Asymmetry fit 0.3% 0.3% DAQ – dead time, eff. Under study ?? Systematic uncertainties still under investigation, but final precision expected to be better than 1% DA- related systematics likely the most significant remaining issue to study 6/27/2014 A. Camsonne EIC Users meeting 12 Simulation background • 1 kW green laser • Halo ( electrons around beam envelope) contribution modeled on Babar PEP II at SLAC • Background significant in photon detector • Compton signal rate at MHz level : 1% statistical error in s Photon detector signal Halo (Dave Gaskell) 6/27/2014 A. Camsonne EIC Users meeting Bremstrahlung Electron detector signal 13 Simulation background 2 cm cavity aperture Photon detector signal 1 cm cavity aperture Halo Bremstrahlung Halo Electron detector Bremstrahlung signal • signal to noise ratio improves at high energy for electron detector •Background is worse for photon detector •Need to pay attention to apertures which can generate background from halo •Laser choice depends on background contribution : need more simulation 6/27/2014 A. Camsonne EIC Users meeting 14 Possible implementation in low Q2 for MEIC Exploring Compton polarimeter in low-Q2 chicane Same polarization as at the IP due to zero net bend Non-invasive continuous polarization monitoring Polarization measurement accuracy of ~1% expected No interference with quasi real photon tagging detectors Laser + Fabry Perot cavity Photon calorimeter c Quasi-real high-energy photon tagger Quasi-real low-energy photon tagger Electron tracking detector 6/27/2014 A. Camsonne EIC Users meeting e- beam 15 eRHIC lepton polarimeter: Location? Polarimeter Laser Compton photon detector laser polarization needs to be monitored p e • Option to measure at IP with empty hadron bunch •Measure after dipole in machine ? •Dedicated chicane ? •Constraint on detector technology for the Gatling gun design : detectors signal less than 100 ns ( E. Aschenauer ) 6/27/2014 A. Camsonne EIC Users meeting 16 Perfect test bench for EIC •2.2 GeV to 11 GeV •120 uA max •Repetition rate 499 MHz with lower rates available •Polarized beam up to 85% •Fast helicity flip Photon detector on movable table 6/27/2014 A. Camsonne EIC Users meeting Vertical motion of electron detector to move detector close to the beam ( up to 5 mm ) 17 Conclusion • Compton polarimetry at 1% level achieved at Jefferson Laboratory and aiming at 0.5 % for 12 GeV parity program : will benefit EIC • Jefferson Lab ideal ground for Compton testing for EIC since Compton is non invasive – Photon detector testing straight forward – Electron detector testing doable with planning because of vacuum. Looking into Roman pot option for ease of detector swapping 6/27/2014 A. Camsonne EIC Users meeting 18
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