1. COMPTON POLARIMETRY Analysis status Scaling laws Measurement @ 3 GeV Toward 850 MeV

2. Overview Compton Int. Point  detector e - detector

3. Detection 650  m strips PbW04 Hall A e - only  only Coincidences 7mm gap

4. “e - Only” Differential method, Running-time  Main systematic error = calibration Typical 2.5% relat. error @4.5 GeV Installed online

5. Response Function Electron detector   energy tagger 1 strip selected Response func. over e - det. Range Semi-integration method Optimized software threshold Cross-check of syst.

6. Error Budget Source of errorRelat. Error (%) A exp Statistical0.8 Position & angle0.3 Background0.05 Dead Time0.1 PP 0.5 Analyzing Power Response function0.45 Calibration0.6 Pile up0.45 Radiative corr.0.26 TOTAL1.4 % E beam =4.5 GeV I beam = 40 mA 40 min run = 5.8% Running conditions:

7. A Max  k x E Scaling Laws  tot ~ cst Run-time  (  x A 2 )  (k 2 x E 2 ) 0.85 GeV 3.0 GeV 4.5 GeV Error in previous estimates…

8. Measurement @ 3 GeV Reduce gap from 7 to 5mm Covers more than 75% of E range 1% stat. in 1h Goal: 2% syst.

9. Toward 850 MeV Kinematics with IR laser: E (GeV)4.50.85 A Max (%)7.71.5 (%)3.850.75 (%)1.530.32  tot (barn) 0.6170.650 k’ Max (MeV)33512.7 Vert. gap (mm)153

10. e - Detection Differential method  -strips of 50  m for good calibration I.R. laser Green laser -Need to detect e- between 2.5 and 3mm of the primary beam -Could be done with remote position control of the e- detector. -1% stat in 20h -Compton edge 6mm from the primary beam -1% stat in 5h (assuming 1.5kW laser) Bkg? Beam position stability? -Light upgrade of electron det. -Distances from beam currently achieved -Major hardware change

11. A Max  detection Integration method with very low threshold (few% of Compton edge) -No syst. from resolution and calibration -Need to know the det. Efficiency -1% stat. in 4.5 days Requirements with IR laser: E  = [120 keV-12 MeV] 100 kHz High efficiency

12.  Detection  Bkg level: can be tested in April during GDH.  Monitoring of efficiency: few % level? radioactive sources,  expected flat above. 2 layers detector: PbWO4, pure CsI: 1  10 MeVLSO: 0.1  1 MeV Huge light yield, 420nm Dense: 7.4g/cm3 Fast: 42ns Size of 4x4x4 cm available 20 to 200  e per  Transparent to 420 nm 6 to 60  e per   PMT Main systematics:

13. CONCLUSION With electron detector 2mm lower: -OK for HAPPEx2 -Error budget tight for 4 He Upgrades: -Green laser: 175 k$, 2 man/year Would make e- detect. Work -New photon detector: Can be tested in spring 2003 with current setup