COMPTON POLARIMETRY

•Analysis status

•Scaling laws

•Measurement @ 3 GeV

•Toward 850 MeV

Overview

Compton Int. Point

detector

e- detector

Detection

650 m strips

PbW04

Hall A

• e- only• only•Coincidences

7mm gap

“e- Only”

•Differential method, Running-time <A2>

•Main systematic error = calibration

•Typical 2.5% relat. error @4.5 GeV

•Installed online

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.

Error Budget

Source of error

Relat. Error (%)

Aexp

Statistical 0.8

Position & angle 0.3

Background 0.05

Dead Time 0.1

P 0.5

Analyzing Power

Response function

0.45

Calibration 0.6

Pile up 0.45

Radiative corr. 0.26

TOTAL 1.4 %

•Ebeam =4.5 GeV

•Ibeam = 40 mA

•40 min run

•<Ana. Power> = 5.8%

Running conditions:

<A>

AMax

<A2>

k x E

Scaling Laws

tot ~ cst

Run-time ( x A2) (k2 x E2)

0.85 GeV

3.0 GeV4.5 GeV

Error in previous estimates…

Measurement @ 3 GeV

•Reduce gap from 7 to 5mm

•Covers more than 75% of E range

•1% stat. in 1h

•Goal: 2% syst.

Toward 850 MeV

Kinematics with IR laser:

E (GeV) 4.5 0.85

AMax (%) 7.7 1.5

<A2> (%) 3.85 0.75

<A> (%) 1.53 0.32

tot (barn) 0.617 0.650

k’Max (MeV) 335 12.7

Vert. gap (mm)

15 3

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

<A>

AMax

<A2>

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

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: 110 MeVLSO: 0.11 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:

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