COMPTON POLARIMETRY

• Collected data

• Cavity power

• Status on counting methods

• Systematic errors and hardware issues

Collected Data

Good Runs

Ebeam

(GeV)k’Max

(MeV)

<A>int

(%)AMax

(%)

4He ~450 2.76 129 2.3 4.8

LH2 ~450 3.18 171 2.7 5.5

Cavity Power• Laser power supply changed in June, tested OK: 1200W

• Down to 300W few days before the experiment

• Incident laser power still nominal• Laser beam waist measured in agreement with simulation• Alignment optimized• Back to 500W after optics cleaning

• Slow exponential decay of cavity power seen during part of the run• Cavity mirrors? Two finesse measurements so far, differ by a factor 2 to be done again…

Counting Analysis

650 m strips

PbW04

4.5 mm gap

Differential

Semi-integrated

Calibration e- Detector

Ee-

Position of the Compton edge calibrates the detector and determine the vertical gap Ydet.

Can use the rate or the asym spectra.Discrepancy taken as syst error.

Pe/Pe ~ 2 Y/Y

Y=200m Pe/Pe = 3.5% @ 3 GeV

Ee- Bdl, lever arm after D3 (~4.1m)

Calibration e- Detector

Compton edge position sensitiveto beam motion (coil pulsing)

Extracted Ydet is always underestimated

Comtpon+BackgroundBackgroundS/B

Calibration e- Detector

•Fit of the asymmetry spectra of the 4 planes is more robust because it uses information from all strips.

•Systematic error could be pinned down by simulation

•Stringent cross-check available on line…

Calibration cross-check•The response function of the detector is determined using the e- det as an energy tagger reference run.

•When fitting the photon spectrum, corrects for gain drift and error in the e- det calibration. expect =1 for the fit of the reference run itself

-1 = 10-4 !!

Had to correct for the fact that strip planes are 1 cm appart and use survey data for the lever arm after dipole 3 (4.11 m instead of 4.10)

Photon Analysis

No dependence on the software threshold over a wide rangeSystematics of the response function under control.

Comparison e- -

03/08/05 19/09/05

Comparison e- -

01/11/0515/10/05

Laser Polarization

Laser Polarization

Laser Polarizatione- and data are not compatibleorigin found to be electronic pickup: ADC pedestal changes by 4 channels depending on the cavity state!

Wasn’t there last year…

Could be due to logical signals of cavity ON/OFF,Left/Right present in several electronic crates?Easy to test.

Laser PolarizationAfter pedestal correction, all analysis are compatible at 1 level. Compton polarimetry is a tedious way to check pedestal correlations…

The correction doesn’t affect the mean value,but reduces the LeftRight discrepancy.

Left laser polarizationis ~0.6% lower than Right polarization?

Laser Polarization

Laser

/4 plates

CIP

Exit L

ine

Transfer Function

Monitoring of the laser polarization downstream the cavity

/4 Scans

x

x

y

y

Wollaston Prism

Integrating Sphere S1

Integrating Sphere S2

Incoming Polarization Ellipse

TE pol. state

TM pol. state

Rotatable/4 plate

Slow

9.88.8

0

0

1

1

2sin2sin2cos2cos2

132

2101 PPPPS

0

0

1

1

2sin2sin2cos2cos2

132

2102 PPPPS

Stokes Parameters: P0,P1,P2,P3

Laser Polarization

•30 /4 scans performed during helium run.

•We do find a 0.5% difference in laser polarization states but with opposite sign

Need to check the Left-Right definitions in both analysis…

Accumulated cavity power: Left state = Right state at few % level (~500W)

Conclusion

Significant improvements in the counting analysis Best agreement between the two methods ever achieved On track for 2% accuracy or better.

Hardware issues: compton is getting old…

• Should revise all cabling at some point• Need cross-talk study before GE

n starts• Numerous « end of run failed » remove 600Hz part of the aquisition?• Cavity power is low? How long before very low?

Systematic Errors

Source Last Year To Do Expect.

P 0.70% Open cavity?

0.7%-1%

Resp. Func. 1.25% Ana.+Simul. 1.1%

Dead Time 1.00% - ~0%

Pile up 1.00% Simulation 0.5%

Rad. Corr. 0.25% - ~0%

TOTAL 2.0% 1.4%-1.6%

Expect to reduce syst. error as well because of better calibration.(3.5% for now)

Counting photon

Counting electron