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THE NEW FOCAL PLANE POLARIMETER IN HALL C
HALL C USERS MEETING, January 18, 2008
Mehdi MEZIANE, the College of William and Mary
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OUTLINE

• INTRODUCTION

• ROSENBLUTH SEPARATION

• POLARIZATION TRANSFER METHOD

• HMS TRIGGER

• FOCAL PLANE POLARIMETER (FPP)

• FPP DRIFT MAPS

• FPP EFFICIENCY

• CONCLUSION

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INTRODUCTION
Two methods to obtain the form factors of the
proton

• Rosenbluth separation of cross section data
  which gives

and
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ROTHENBLUTH / TRANSFERT POLARIZATION DATA
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ROSENBLUTH SEPARATION
Cross section
with
Jlab experiment 01-001 I.A. Quattan et al.,
Phys. Rev. 94, 142301 (2005).
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POLARIZATION TRANSFER METHOD
The transferred polarization is
with
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ASYMMETRIES

• Measure asymmetry distribution after
  rescattering in the analyser

• For 2 helicities of beam (1) with polarization
  Pe , relative asymmetry in
• the polarimeter is

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ASYMMETRIES
The physical asymmetries are calculated from the
difference distributions
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HMS TRIGGER
NEW HMS trigger
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S0 DETECTOR

• 4 PMTs
• 2 scintillators
• 2 wave shifters

Upper part S0X1 (part of
inelastic) Lower part S0X2 (elastic)
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HMS TRIGGER
NEW HMS trigger
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Focal Plane Polarimeter (FPP) in Hall C

• 4 Chambers with 3 planes each

12 planes of detection
Active area 134166 (cm2)

• 2 split CH2 analysers
• 145 111 60 (cm3)

• Gas mixture Argon50/Ethane50

• Operating HV 2350V (field and
• cathode wires)

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Focal Plane Polarimeter (FPP) in Hall C
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Focal Plane Polarimeter (FPP) in Hall C
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FPP Read Out System
twisted pair ribbon cables (1 for 2 cards)

• Amplifier/read out cards
• 1 card for 8 sense wires
• ECL logic signals
• 37 cards per chamber TOTAL of 148 cards

VME based F1-TDC
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FPP Drift Maps
Background window
Mapping this distribution into a flat distance
distribution
Time/distance correlation
Drift Time
Subtracting background
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FPP efficiency Straight Through

• At the beginning of each kinematic 1 hour of
  straight through data
• (WITHOUT analyser) are taken.

FPP 1, 2 refers to chamber pair AB and CD
respectively
e 86 for FPP1
e 85 for FPP2
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HMS/FPP 2D correlation
cm
cm
cm
Chamber A
Chamber C
cm
Predicted drift distance based on HMS and FPP
wire hit information VS Calculated FPP drift
distance
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FPP efficiency N5hits / N6hits

• The efficiency e per plane is given by

gt 98.6
with the ratio
(6 planes of detection)
92

• The efficiency per pair of chambers is then

gt 99.7
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FPP 6 planes efficiency evolution
1) Back to the nominal HMS threshold
2) HMS/FPP gas problem fixed
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FPP1,2 ASYMMETRIES, Q2 2.5 GeV2
Q2 2.5 GeV2
Ebeam 3.539 GeV
?e 32.9
24 M triggers
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GEP 2? ERRORS
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CONCLUSION

• New HMS trigger with a new S0 detector

• Good drift maps computation

• FPP Straight through efficiency of 85

• 5 OR 6 plane FPP efficiency above 99.7