Precision Polarimetry at JLab, 6 GeV Era G. B. Franklin Carnegie Mellon University

Transcription

1 Precision Polarimetry at JLab, 6 GeV Era G. B. Franklin Carnegie Mellon University Hall A Compton Upgrade Team: M. Friend, D. Parno, F. Benmokhtar, A. Camsonne, G.B. Franklin, R. Michaels, S. Nanda, K. Paschke, B. Quinn, P. Souder Compton Scattering as a Polarimetry Tool General Considerations Complications and Systematic Errors JLab Hall A Compton Photon Calorimeter GSO Performance (Simulations and benchmarks) Integrating DAQ Polarimeter Performance Results

2 Compton Polarimetery Electron beam passes through polarized photon beam Spin-dependence of Compton scattering -> analyzing power Electron beam Electron detector Unscattered electrons Fabry-Perot cavity Magnetic dipoles Photon calorimeter

3 Unpolarized Cross Section Very forward peaked (GeV electrons on ev photons) Example: Beam E = 3.48 GeV Cavity Photons ω = ev Compton Edge ω max = 24 MeV Max photon energy dσ/dρ (barns) dσ/dρ (barns) Compton Edge ρ Klein-Nishina Formula

4 Compton Analyzing Power Peak analyzing power 1% to 2% Strong dependence on scattered photon energy λ=164nm, ω =1.165 ev 8GeV 4GeV 1GeV ρ

5 Compton Analyzing Power Peak analyzing power 1% to 2% Strong dependence on scattered photon energy 8GeV λ=164nm, ω =1.165 ev λ=532nm, ω =2.33 ev 4GeV 1GeV 8GeV 4GeV 1GeV Doubling laser -> energy doubles analyzing power ρ

6 Negligible theoretical error Higher order diagrams Denner & Dittmaier Nucl. Phys. B54 (1999) ~.3% correction to A l at 3.5 GeV beam energy Increases with energy

7 Experimental Challenges Electron beam Electron detector Unscattered electrons Fabry-Perot cavity Magnetic dipoles Photon calorimeter Thresholds and non-linearities a problem (Strong energy dependence of A l ) Large dynamic range of photon energies (Compton edge varies from ~2 MeV to ~5 MeV) Electrons or photons can be detected Scattered electrons near beam halo at low beam energies Photon detection requires knowledge of resolution function

8 Hall A Compton Photon Calorimeter Single GSO crystal manufactured by Hitachi Chemical.5% Ce-doped Gd 2 SiO 5 6 cm diam x 15 cm length Flash ADC integrates Compton signal Customized Struck SIS332 FADC No threshold, Dead-timeless 1 Primary Data word for each 1/3 sec helicity period Auxiliary monitoring info

9 Crystal Properties GSO PbWO4 BGO CeF 3 BriLanCe 38 PreLude 42 Density (g/cm 3 ) Rad Length (cm) Moliere Radius (cm) Decay time (ns) Light output (% NaI) photoelectrons (# / MeV) ~ ?? ~ %.4% 9% 6.6% 165% 84%

10 Energy Weighted Asymmetry: S E A Exp E dσ = LT ε ( E) E ( E) ( 1± Pe Pγ Al ( E) ) + E E = + E + E ± max = E dσ LT s( E) ( E) ( 1± Pe Pγ Al ( E) ) ± max Longitudinal Compton Asymmetry Actual Asymmetry Weighted by Detector Signal Average detector signal for photon energy E A Exp = S S S S = P P e γ γ E max E dσ Al ( E) s( E) ( E) dσ s( E) ( E) max = P P e γ A ls

11 Energy Weighted Asymmetry: S E A Exp E dσ = LT ε ( E) E ( E) ( 1± Pe Pγ Al ( E) ) + E E = + E + E ± max = E dσ LT s( E) ( E) ( 1± Pe Pγ Al ( E) ) ± max Longitudinal Compton Asymmetry average response function required Actual Asymmetry Weighted by Detector Signal Average detector signal for photon energy E A Exp = S S S S = P P e γ γ E max E dσ Al ( E) s( E) ( E) dσ s( E) ( E) max = P P e γ A ls

12 Statistical Considerations Energy Weighted Statistics σ E 2 ESum 2 Esum = E E m m E 2 dn E dn 2 = 1 1 m m 2 dρρ dn dρ dρρ dn dρ 2 σ Esum E Esum =1.2 1 N (Using Compton shape for dn/dρ) 1 khz counting rate è Statistical accuracy in a few hours

13 Systematic Considerations Dominate E dσ ± S = LT max s( E ) ( E )(1 ± Pe Pγ Al ( E ) ) Function of detector and electronics response Detector Response: GEANT4 Simulations Performed Vahe Mamyan & Megan Friend Shower Generation 2 MeV Photon Event Optical Photon Tracking 3 MeV Photon Event

14 GEANT produces GSO average response, s(e) A ls = γ E max A l (E)s(E) dσ (E) E max s(e) dσ (E) Light at PMT/ Photon Energy (au)

15 Verification of Detector Response Simulations Tests at Duke s HIGS facility Monoenergetic photons 2, 22, 25, 3, & 4 MeV See D. Parno et. al. NIM A (213) DOI 1.116

16 Systematic Considerations PMT Linearity Mapped with 2-LED Pulser System Simulated GSO pulse Measures PMT/Base linearity Monitors gain shifts Amplitude (au) Slope (au) Time (au) Poor Linearity Pulse Size (au) Slope (au) Good Linearity Pulse Size (au)

17 Systematic Considerations Time-Dependent Systematics & Background Electron Beam Helicity Flipped at ~3 Hz (pseudo-random) Fabry-Perot Cavity Laser Cycle: 6 sec Locked on Right Circular Polarization 3 sec Unlocked (used for background subtraction) 6 sec Locked on Left Circular Polarization 3 sec Unlocked Significant background Synchrotron Radiation and Beam-Halo Bremsstrahlung Synchrotron Radiation ~ E 4 potential problem for 12 GeV running S + - S - S + + S -

18 Systematic Considerations Geometry and Alignment Photon radius (mm) Collimator Cutoff (if centered) ρ = Normalized photon energy If misaligned, collimators can distort energy spectrum at low end 1mm tungsten radiators/ scintillators Used for horizontal and vertical scans

19 Systematic Considerations Verification For each helicity period, FADC Data-stream includes: Signal Sum (Main analysis) Prescaled Integrated Triggered GSO Pulses Random Sampled FADC sample periods Triggered Compton GSO data Data compared to Monte Carlo

20 Prescaled triggered data can be used to measure polarization

21 Right-Circular Laser Left-Circular Laser Ph.D. Thesis of M. Friend

22 From: M. Friend et al., Upgraded photon calorimeter Systematic Errors Laser Polarization.8% Signal Analyzing Power: Nonlinearity.3% Energy Uncertainty.1% Collimator Position.5% Analyzing Power Total Uncertainty Gain Shift:.33% Background Uncertainty.31% Pedestal on Gain Shift.2% Gain Shift Total Uncertainty.37% Total Uncertainty.94%

23 Conclusion Accuracy of 1% has been achieved Significant improvements possible Improved determination of photon polarization Reduction in Synchrotron Radiation (Particularly for high electron beam energy) Careful monitoring of gain shifts (Cavity on vs. Cavity off)

24 Example Compton Edge and Analyzing Powers ωk = 1.165eV (IR) kω = 2.33eV ev (green) E e a kω mmax a x A mmax a x a kω mmax a x A mmax a x (MeV) (MeV) (MeV) 1, , , ,.817 1, ,11.32