COMPTON POLARIMETRY

1
COMPTON POLARIMETRY

• Analysis status
• Scaling laws
• Measurement _at_ 3 GeV
• Toward 850 MeV

2
Overview
e- detector
g detector
Compton Int. Point
3
Detection
650 mm strips
Hall A
7mm gap
PbW04

• e- only
• g only
• Coincidences

4
e- Only

• Differential method, Running-time a ltA2gt
• Main systematic error calibration
• Typical 2.5 relat. error _at_4.5 GeV
• Installed online

5
Response Function
Electron detector o g 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 error Relat. Error ()
Aexp
Statistical 0.8
Position angle 0.3
Background 0.05
Dead Time 0.1
Pg 0.5
Analyzing Power
Response function 0.45
Calibration 0.6
Pile up 0.45
Radiative corr. 0.26
TOTAL 1.4
Running conditions

• Ebeam 4.5 GeV
• Ibeam 40 mA
• 40 min run
• ltAna. Powergt 5.8

7
Scaling Laws
Error in previous estimates
0.85 GeV
3.0 GeV
4.5 GeV
stot cst
Run-time a (s x A2) a (k2 x E2)
8
Measurement _at_ 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) 4.5 0.85
AMax () 7.7 1.5
ltA2gt () 3.85 0.75
ltAgt () 1.53 0.32
stot (barn) 0.617 0.650
kMax (MeV) 335 12.7
Vert. gap (mm) 15 3
10
e- Detection

• Differential method
• m-strips of 50 mm for good calibration

Green laser
I.R. 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)
-Light upgrade of electron det. -Distances from
beam currently achieved -Major hardware change
Bkg? Beam position stability?
11
g 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

• Eg 120 keV-12 MeV
• 100 kHz
• High efficiency

12
g Detection
2 layers detector
PbWO4, pure CsI 1?10 MeV
LSO 0.1?1 MeV

• Huge light yield, 420nm
• Dense 7.4g/cm3
• Fast 42ns
• Size of 4x4x4 cm available
• 20 to 200 ge per g

• Transparent to 420 nm
• 6 to 60 ge per g

Main systematics

• Bkg level can be tested in April during GDH.
• Monitoring of efficiency few level?
• radioactive sources, e expected flat above.

13
CONCLUSION
With electron detector 2mm lower -OK for
HAPPEx2 -Error budget tight for 4He
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