Diana Parno – 18 April

2009

Compton Photon Compton Photon Detector UpgradeDetector Upgrade

Diana Parno18 April 2009

HAPPEX Collaboration Meeting

Diana Parno – 18 April

2009

Outline

• Photon detector crystal upgrade– GEANT simulations– Tests at HIγS– Compton spectra

• Photon detector DAQ upgrade

• Preliminary results from d2n run

Diana Parno – 18 April

2009

GSO Crystal• GSO crystal (6cm

diameter, 15cm length) installed in Hall A in December

• GEANT simulations suggest high efficiency

• How much can we trust those simulations?

Diana Parno – 18 April

2009

HIγS Tests• Visited High-Intensity γ Source (HIγS) • Tested GSO response to monochromatic

photon beams at several energies

HIγS data

Simulation

• Results agree well with GEANT4 simulations

• Limited by uncertainty in HIγS beam energy spread

Diana Parno – 18 April

2009

Compton Spectra

• Plotted as a function of photon energy ρ, the Compton cross section σ(ρ) is a parabola

Baylac et al., 2002

PbWO4 crystal (Saclay array)

GSO Crystal

Diana Parno – 18 April

2009

Outline

• Photon detector crystal upgrade• Photon detector DAQ upgrade

– Triggered mode– Accumulator mode

• Preliminary results from d2n run

Diana Parno – 18 April

2009

Triggered Mode

• SIS 3320 FADC samples signal at 200 MHz

• Write 512 preceding samples for every trigger– Quick collection of high statistics– Availability of individual pulses for study– Straightforward interface with electron detector

• Trigger logic and .crl files developed at CMU and tested in Hall A– Thanks due to Fatiha Benmokhtar and Gregg

Franklin

Diana Parno – 18 April

2009

Accumulator Mode Basics

• FADC integrates signal for each helicity window

• Signal size determines which samples contribute to each of six accumulators

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 – 18 April

2009

Accumulator Mode (continued)

• From the raw accumulator data, we can extract a physics signal

• Dilution-corrected asymmetry in physics signal tracks electron beam polarization

SPNAcc nn

Number of samples

Average pedestal value

Average signal

nnnn AccPNSNS

elnn

nnn PPA

SS

SS

DA

1

Diana Parno – 18 April

2009

Outline

• Photon detector crystal upgrade• Photon detector DAQ upgrade

• Preliminary results from d2n run

– Extracting asymmetries– Cross-calibration with Møller

measurements– Rates and linearity checks

Diana Parno – 18 April

2009

FADC Asymmetries: Consistency Check

Diana Parno – 18 April

2009

Towards a Beam Polarization

• The Compton asymmetry Aexp is proportional to the electron beam polarization

• We can cross-check Aexp with Møller results

• Proportionality constant will be different for the two beam energies

Diana Parno – 18 April

2009

Rates

• Compton photon rates of 3-5 kHz measured in scaler during d2

n running (15 μA)

• Agrees with rates measured in original DAQ

• Rates well separated for cavity on/off states

Diana Parno – 18 April

2009

Linearity Check

61.035000

23000

Edge at 35,000

Edge at 23,000

Edge at 11,000

Edge at 7,40068.0

11000

7400

Diana Parno – 18 April

2009

Future Work• Photon detector crystal

– Detector response function– PMT linearity studies (Megan Friend’s talk)– Further tests at HIγS?

• DAQ – Use multiplex buffers in FADC memory to reduce

deadtime • First tests by Fatiha Benmokhtar and Wouter

Deconinck• Preliminary results: DAQ rate > 10 kHz at CMU

– Commission Accumulator + Triggered mode• Analysis

– Refine asymmetry calculations– Fold in detector response function– Report beam polarizations!

Diana Parno – 18 April

2009

Thank you!