Diana Parno – July 22, 2008 January PREx Test Run: Compton Photon Analysis Diana Parno Carnegie...
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Transcript of Diana Parno – July 22, 2008 January PREx Test Run: Compton Photon Analysis Diana Parno Carnegie...
Diana Parno – July 22, 2008
January PREx Test Run:January PREx Test Run:
Compton Photon AnalysisCompton Photon Analysis
Diana ParnoDiana Parno
Carnegie Mellon Carnegie Mellon UniversityUniversity
HAPPEX Collaboration HAPPEX Collaboration MeetingMeeting
Diana Parno – July 22, 2008
Outline
• Compton photon DAQ– Integrating method– FADC design
• Test run results• Problem areas
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
NN
NNAexp
EE
EEAexp
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)
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)
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
Diana Parno – July 22, 2008
Outline
• Compton photon DAQ• Results
– FADC functionality– PMT performance– Accumulator signals– Asymmetries and polarization
measurement
• Problem areas
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
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
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):
SPNAcc nn
Number of samples
Average pedestal value
Average signal
nnnn AccPNSNS
All
FarWindow N
N Far
Window
WindowWindow S
SS
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)
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:
BCS
BCC
CC
SS
SSAdiluted 2
BC
BBCD
Diana Parno – July 22, 2008
Dilution Factors
• Computed dilution factors for four production runs
• Small D means low S:N
Run 60325
Run 60327 Run 60328
Run 60326
Diana Parno – July 22, 2008
Asymmetries!
• 3 accumulators, 4 production runs
Sign flip: Laser pol. change
Sign flip: IHWP change
Diana Parno – July 22, 2008
Accumulator Combinations
• Suppose we combine two accumulators to compute asymmetries …
Left circularly polarized laser
Laser off – background only
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)
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: le APPA exp
Diana Parno – July 22, 2008
Outline
• Compton photon DAQ• Results• Problem areas
– Shape of energy spectrum– Signal size– Discrepancy between left/right
polarization states
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)
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
Compton
130.9 MeV 44 RAU-S/MeV 7.3 RAU/MeV
Cosmics 20.4 MeV 138 RAU-S/MeV
39 RAU/MeV
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
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
Diana Parno – July 22, 2008
Thank you!