1. Diana Parno – July 22, 2008 January PREx Test Run: Compton Photon Analysis Diana Parno Carnegie Mellon University HAPPEX Collaboration Meeting

2. Diana Parno – July 22, 2008 Outline Compton photon DAQ – Integrating method – FADC design Test run results Problem areas

3. Diana Parno – July 22, 2008 Photon DAQ: Integrating Method 3 ways to measure a Compton-scattering asymmetry: –Differential: scattered photon count as a function of energy –Integrated: scattered photon count without energy information –Energy-weighted: total energy deposited (no photon counts) New DAQ allows us to use energy-weighted method: –At low energies, detector response function becomes complicated and thus harder to know with precision –Energy-weighted integrated method is less sensitive to the precision of the detector response function

4. Diana Parno – July 22, 2008 FADC Design As specified, the FADC (from Struck DE) samples the data at 200 or 250 MHz (programmable) Six accumulators sum over a time interval (30 ms). Each signal sample contributes to at least two accumulators, depending on which criteria it meets. Data can be read out in sampling mode (first ~50000 sample words are included along with accumulator words) or integrating mode (accumulator values only)

5. Diana Parno – July 22, 2008 FADC Design: Accumulators Thresholds, and degree of stretching, are programmable Ideally, Compton events fall within the “window” – so we can compute asymmetries using Accumulators 0, 2, and 4 Pedestal Near Threshold Far Threshold Accumulator 1 (Near) Accumulator 0 (All) Accumulator 3 (Far) Accumulator 2 (Window) Accumulator 4 (Stretched Window) Accumulator 5 (Stretched Far)

6. Diana Parno – July 22, 2008 Test Run Goals: DAQ See Compton events with new FADC Test signal splitting (to use original DAQ and new DAQ simultaneously) Compute asymmetries and beam polarization using only energy-weighted integrated data Study software, hardware behavior and systematics

7. Diana Parno – July 22, 2008 Outline Compton photon DAQ Results –FADC functionality –PMT performance –Accumulator signals –Asymmetries and polarization measurement Problem areas

8. Diana Parno – July 22, 2008 Studying Pulses: Two Modes Integration mode: Read accumulators only; no deadtime –Several pulses in each 30-ms “event” Sampling mode: Read individual 5-ns samples. Detect peaks with a software threshold –Detailed study of individual pulses (e.g. snapshots) –Precise location of pedestal

9. Diana Parno – July 22, 2008 Saturation Plotting pulse area vs. pulse amplitude shows clear saturation during the test run Compton edge PMT (12-stage, - 2500 V) major contributor Compton events are in relatively linear region

10. Diana Parno – July 22, 2008 Accumulator Physics Signals The raw accumulator values can be used to extract the total physics signal: We can apply a deadtime correction to the physics signals from the window accumulators (2 and 4): Number of samples Average pedestal value Average signal

11. Diana Parno – July 22, 2008 Sample Run – Accumulator Signals Accumulator 2 (Window)Accumulator 0 (All)Accumulator 1 (Near) Accumulator 3 (Far)Accumulator 4 (Stretched Window)Accumulator 5 (Stretched Far)

12. Diana Parno – July 22, 2008 Dilution Factors S includes Compton signal C and background signal B The measured asymmetry in S differs slightly from the asymmetry in C We can correct for this by dividing the asymmetry in S by a dilution factor D:

13. Diana Parno – July 22, 2008 Dilution Factors Computed dilution factors for four production runs Small D means low S:N Run 60325 Run 60327Run 60328 Run 60326

14. Diana Parno – July 22, 2008 Asymmetries! 3 accumulators, 4 production runs Sign flip: Laser pol. change Sign flip: IHWP change

15. Diana Parno – July 22, 2008 Accumulator Combinations Suppose we combine two accumulators to compute asymmetries … Left circularly polarized laser Laser off – background only

16. Diana Parno – July 22, 2008 Sensitivity to Pedestal A small mistake in finding the pedestal has a large effect on the computed asymmetries Correct pedestal value (2017.2) Incorrect pedestal value (2016)

17. Diana Parno – July 22, 2008 Electron Beam Polarization Megan Friend’s Geant simulation of analyzing power: 0.02316 for test run –Still to be included: PMT, PMT nonlinearity We have everything we need to compute beam polarization:

18. Diana Parno – July 22, 2008 Outline Compton photon DAQ Results Problem areas –Shape of energy spectrum –Signal size –Discrepancy between left/right polarization states

19. Diana Parno – July 22, 2008 Problem: Shape of Energy Spectrum Compton cross section σ(ρ) is a parabola. In the past, this shape has been echoed in the detected photon energy spectrum. Baylac et al., 2002 In January, the measured energy spectrum did not look remotely like a parabola. Why not? January test run (central Saclay crystal)

20. Diana Parno – July 22, 2008 Signal Size Mystery During the test run, signal from the new detector seemed smaller than expected Afterward, cosmic ray measurements at CMU (same detector) showed a much bigger response Photon source Approx. deposited energy Pulse area response Pulse amplitude response Compton130.9 MeV44 RAU-S/MeV7.3 RAU/MeV Cosmics20.4 MeV138 RAU-S/MeV39 RAU/MeV

21. Diana Parno – July 22, 2008 Discrepancy: Left and Right Large discrepancy between asymmetries (and thus polarizations) measured when laser left- circularly polarized vs. right-circularly polarized Always in the same direction – even when IHWP flips electron beam helicity

22. Diana Parno – July 22, 2008 Future Work With GEANT, investigate energy spectrum shape Incorporate PMT into analyzing power model Investigate L/R discrepancy Improve analysis code; incorporate coincidence data Improve detectors and mounts in beamline

23. Diana Parno – July 22, 2008 Thank you!