Hall A Compton Calorimeter G. B. Franklin Carnegie Mellon University 1. Compton Scattering Polarimetry General Considerations Complications and Systematic Errors 2. CMU Integrating DAQ Content of Data Integrating Accumulator Analysis Systematic Errors Triggered event analysis 3. Recent DAQ/Analysis Upgrades 4. DVCS running 5. Upcoming DAQ/Analysis Upgrades 1
1. Compton Scattering 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 2
Unpolarized Cross Section Laser: ω 0 = 2.33 ev (green) Compton Edge dσ/dρ (barns) dσ/dρ (barns) E e (GeV) ω max (MeV) 1.00 25.00 1.20 34.42 2.00 49.23 4.48 616.9 11.00 3098.0 3
Compton Analyzing Power Peak analyzing power 1% to 35% Strong dependence on scattered photon energy 8GeV λ=1064nm, ω 0 =1.165 ev λ=532nm, ω 0 =2.330 ev 4GeV 1GeV 8GeV 4GeV 1GeV Increases ~linearly with beam energy and photon energy. ρ 4
Compton scatted photons forward-peaked Geometry and Alignment Radius (mm) 5 mm radius collimator If misaligned, collimators can distort energy spectrum at low end 1mm tungsten radiators/ scintillators Used for horizontal and vertical scans 5
Hall A Compton Photon Calorimeter Few GeV Running Single GSO crystal (Hitachi Chemical) 0.5% Ce-doped Gd 2 SiO 5 6 cm diameter x 15 cm length Preparation Higher Energy Running 4-element PbWO4 array 6 cm x 6 cm x 20 cm Flash ADC integrates Compton signal Customized Struck SIS3320 FADC No threshold, Dead-timeless 1 Data word per 1/30 sec helicity window Auxiliary monitoring info 6
3) Integrating DAQ Based on customized Struck FADC Data for each helicity window includes 1. 1 FADC Accumilator 0 word (integrated with no thresholds) 2. Additional Accumulator words (integrated above thresholds) 3. Sample of Compton events (PMT charge per event) 4. Time stamp of each Compton event 5. 1 sampled Compton snapshot waveform (5 ns binning) 6. Occasional sampled random time samples within FADC 7. Aux. info. (Beam position, charge, etc.) Buffered data allows readout during following helicity window Snapshot for HAPPEX III 7
Energy Weighted Asymmetry Avoids Thresholds S E A Exp E dσ = LT ε ( E) E ( E) ( 1± Pe Pγ Al ( E) ) 0 + E E = + E + E ± max = E dσ LT s( E) ( E) ( 1± Pe Pγ Al ( E) ) 0 ± 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 0 E max 0 E dσ Al ( E) s( E) ( E) dσ s( E) ( E) max = P P e γ A ls 8
Energy Weighted Asymmetry Avoids Thresholds S E A Exp E dσ = LT ε ( E) E ( E) ( 1± Pe Pγ Al ( E) ) 0 + E E = + E + E ± max = E dσ LT s( E) ( E) ( 1± Pe Pγ Al ( E) ) 0 ± max average response Longitudinal Compton Asymmetry 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 0 E max 0 E dσ Al ( E) s( E) ( E) dσ s( E) ( E) max = P P e γ A ls 9
Systematic Considerations Dominate E dσ ± S = LT 0 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 200 MeV Photon Event Optical Photon Tracking 30 MeV Photon Event 10
PREx-I Compton Accumulator 0 Polarimeter Results 11
Estimated Systematic Errors HAPPEX-III Systematic Errors Laser Polarization 0.80% Signal Analyzing Power: Nonlinearity 0.30% Energy 0.10% Collimator Position 0.05% Analyzing Power Total 0.33% Gain Shift: Background 0.31% Pedestal on Gain Shift 0.20% Gain Shift Total 0.37% Total Uncertainty 0.94% PREx-I Systematic Errors Laser Polarization 0.7% Nonlinearity 0.3% Collimator Position 0.02% Gain Shift: 0.9% Total Uncertainty 1.2% From: PREx-II Proposal From: M. Friend et al., Upgraded photon calorimeter 12
Estimated Systematic Errors HAPPEX-III Systematic Errors Laser Polarization 0.80% Signal Analyzing Power: Nonlinearity 0.30% Energy 0.10% Collimator Position 0.05% Analyzing Power Total 0.33% PREx-I Systematic Errors Solved by new polarization optimization? Laser Polarization 0.7% Nonlinearity 0.3% Collimator Position 0.02% Gain Shift: 0.9% Total Uncertainty 1.2% Gain Shift: Background 0.31% From: PREx-II Proposal Pedestal on Gain Shift 0.20% Gain Shift Total 0.37% Total Uncertainty 0.94% From: M. Friend et al., Upgraded photon calorimeter 13
Estimated Systematic Errors HAPPEX-III Systematic Errors Laser Polarization 0.80% Signal Analyzing Power: Nonlinearity 0.30% Energy 0.10% Collimator Position 0.05% Analyzing Power Total 0.33% Gain Shift: Background 0.31% Pedestal on Gain Shift 0.20% Gain Shift Total 0.37% Total Uncertainty 0.94% From: M. Friend et al., Upgraded photon calorimeter PREx-I Systematic Errors Solved by new polarization optimization? Laser Polarization 0.7% Nonlinearity 0.3% Collimator Position 0.02% Gain Shift: 0.9% Total Uncertainty 1.2% From: PREx-II Proposal Reduce using more sophisticated MiniMegan LED Pulser? (Work initiated at CMU) 14
Alternate Analysis (prescaled scattering event triggers) Verifies model of calorimeter response For each helicity period, FADC Data-stream includes: Signal Sum (Main analysis) Prescaled Integrated Triggered GSO Pulses Random Sampled FADC Sample Periods HAPPEX-III data Triggered Compton GSO data Data compared to Monte Carlo Photon Energy (srau) 15
Prescaled triggered data can be used to measure polarization Photon Energy (srau) Photon Energy (srau) 16
3) Recent improvements in Integrating DAQ DAQ Significant clean up Improvements in parameter file ( flags file ) Insertion of parameters into data stream All Hall A DAQs enter run number info into EPICS Logbook auto entries (start_run and stop_run entries) Analysis Major rewrite/clean up of analysis code structure Designed for on-line monitoring (replaces French DAQ) Also a framework for analysis code 17
Synchrotron Radiation Background Solved? Synchrotron radiation a problem Exit of D2 and entrance of D3 Electron Beam D1 D4 D2 Resonant Cavity D3 Pb Shield Photon Detector Problem: Analyzing Power ~E Signal ~E 2 Synch Radiation ~E 4 (for constant bend radius) Solution: Dipoles modified to extend fringe field Reduce bend radius at D2 exit & D3 entrance Synch radiation reduced orders of magnitude J. Benesch, G.B. Franklin, B.P. Quinn, and K.D. Paschke, Simple modification of Compton polarimeter to redirect synchrotron radiation PhysRevSTAB.18.112401 (2015) 18
4) Recent DVCS Running Short commissioning Spring 2015 DVCS Spring 2016: 4.5 GeV and higher PbWO4 4-element crystal (single PMT readout) Beam through Compton 2/13/2016 19
Helicity-Bit Correlated Pedestal Shift (Problem with prompt helicity reporting) Run S + -S - (rau) Spring 2015: Data Run 0.0200 ±0.0010 Run 1288 Revamped DAQ V1: Pulser 0.0489 ±0.0003 VME NIM/ECL Revamped DAQ V2: Pulser 0.0140 ±0.0003 VME ECL only Revamped DAQ V3: Pulser 0.0015 ±0.0002 Pseudo delayed Helicity reporting 2015 Run Delayed Hel. Analysis 0.0007 ±0.0004 Simulated full delayed reporting Projected False Asymmetry (compare to 12% analyzing power at 11 GeV) Run S + +S - (rau) A_false Spring 2015: Data Run 0.20 10% Assuming Pseudo-Delayed Reporting 0.20 0.75% Expected Increased Signal Amplitude 0.40 0.38% Increased counting rate??? 20
Spring 2016 (now) 4.5 GeV and higher Beam through Compton 2/13 o First vertical scan saw no events o 1 cm diameter Pb collimator changed to 2 cm diameter o Second scan found Compton Events o Later research -> Compton table ~5 mm high o Horizontal and vertical fingers are important ~1 week of running at 4.5 GeV o Problems with background, beam lock, etc. o 4-pass starts today? 21
Spring 2016 (now) Analysis Status o Learned to dislike PbWO4 o Compton edge at ~55 mv out of PMT o Accumulator 0: poor signal/noise o Should be saved by high analyzing power o Work in progress 22
5) Upcoming Improvements DAQ Move to Linux-based ROC (this summer?) Move to JLAB 250 MHz FADC 1. Use a JLab-standard module 2. Improve gated FADC accumulator algorithm (Variable length summing gates was a bad idea) 3. Need to replicate STRUCK multiplexed buffers (No deadtime readout during next helicity window) 4. Implement triggered pulse summing within FADC (Should be able to record full trigger rate) Analysis Develop turn-key Compton monitoring software Bob Michaels developing Counting DAQ 23
Conclusion No major issues (But PbWO4 a problem at 2-pass) Need to reduce laser-on to laser-off gain-shift uncertainty Continue to automate preliminary data analysis pass Expect ~1% polarimetery 24