Diana Parno – 18 April 2009 Compton Photon Detector Upgrade Diana Parno 18 April 2009 HAPPEX Collaboration Meeting.
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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!
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