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


Outline

• Compton photon DAQ– Integrating method– FADC design

• Test run results• Problem areas


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


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)


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)


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


Outline

• Compton photon DAQ• Results

– FADC functionality– PMT performance– Accumulator signals– Asymmetries and polarization

measurement

• Problem areas


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


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


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

'


Sample Run – Accumulator Signals

Accumulator 2 (Window)Accumulator 0 (All) Accumulator 1 (Near)

Accumulator 3 (Far) Accumulator 4 (Stretched Window) Accumulator 5 (Stretched Far)


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


Dilution Factors

• Computed dilution factors for four production runs

• Small D means low S:N

Run 60325

Run 60327 Run 60328

Run 60326


Asymmetries!

• 3 accumulators, 4 production runs

Sign flip: Laser pol. change

Sign flip: IHWP change


Accumulator Combinations

• Suppose we combine two accumulators to compute asymmetries …

Left circularly polarized laser

Laser off – background only


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)


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


Outline

• Compton photon DAQ• Results• Problem areas

– Shape of energy spectrum– Signal size– Discrepancy between left/right

polarization states


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)


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


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


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


Thank you!

Diana Parno – July 22, 2008 January PREx Test Run: Compton Photon Analysis Diana Parno Carnegie...

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