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Future Experiments
1
Experiment Energy (GeV) dA/A Compton
Edge (MeV)C-edge/0xing displacement
HAPPEX-II 3 1.2% 158 12mm/6.5mm
PREX-II 1 1% 35 6mm/3mm
CREX 2.2 3.3% 160 12mm/6.5mm
MOLLER 11 2% 3100 48mm / 28mm
SOLID PVDIS 11, 6.6 0.5% 1260 (32mm/
18mm)
Status Summary • PREX-I demonstrated 1% at 1 GeV from integrating photon analysis. Dominant
error: laser polarization measurement - should be improved PREX-II• CREX should be easier than PREX in photon detector, and e-det analysis will be
possible to improve precision• MOLLER/SOLID - collaboration working group evaluating critical upgrade plans to
meet very challenging ultimate goal of <0.5% accuracy
Upgrade
2
e- det: • Not necessary (but perhaps useful?) for PREX• Useful (though perhaps not truely necessary?) for CREX• Must be optimized for high precision for MOLLER/SoLID• uniform efficiency is highly important to assist calibration
γ det: • existing detector ideal for PREX and CREX• upgrade needed for high energy (MOLLER/SoLID)
Synch light field extensions:• Needed for 11 GeV - already planned for installation• Field mapping already planned
Counting DAQ:• No counting analysis at high flip rate without it. • Needed for PREX (only if using e-det), CREX (only for reliability)• Necessary for 11 GeV
Laser Polarimetry:• New (qweak) techniques require improved initial state control • New diagnostics should also be useful
Beam aperture:• Backgrounds are higher than residual gas - improved operability
should be expected at low E and especially high energy
Upgrade Components
3
Experiment Synch Shield e- det counting
DAQnew
photon Detlaser
polarimetrybeam
aperture
PREX-II n y y
CREX y y n y
MOLLER y y y y y y
SOLID PVDIS y y y y y y
PREX-I Results
4
Integrating Photon detectionHAPPEX+PVDIS+PREX experience(CMU, JLab, Syracuse, UVa)
Preliminary Results from Integrating Compton Photon Polarimetry in Hall A of Jefferson Lab. , Parno et al., J.Phys.Conf.Ser. 312 (2011) 052018.
Upgraded photon calorimeter with integrating readout for Hall A Compton Polarimeter at Jefferson Lab., Friend et al., Nucl.Instrum.Meth. A676 (2012) 96-105.
An LED pulser for measuring photomultiplier linearity., Friend et al., Nucl.Instrum.Meth. A676 (2012) 66-69.
Comparison of Modeled and Measured Performance of GSO Crystal as Gamma Detector, Parno et al., in preparation.
HAPPEX-II Results
5
e- det: 0xing-Compton Edge self-calibration
Analyzing Power calibration
fit rate for one Energy point
Fit 0xing for one energy point
Laser Polarization
6
Determining Laser PolarizationTransfer function translates measured transmitted polarization after cavity to the Compton Interaction Point
Do we know the polarization inside the cavity by monitoring the transmitted light?
Are there effects from ✓vacuum stress✓resonant depolarization✓power level (heating)✓alignment variations?✓model dependence of TF?
Current uncertainty: 0.35%-1%
Very High Precision will require significant improvements. Goal = 0.2%7
Vacuum / Assembly Stress Induced BirefringenceTransfer(Function(not(Constant(� Takes(days(and(hundreds(of(
careful(measurements(� Set(up(known(states(of(light(
in(cavity(and(measure(them(inside(and(in(the(exit(station(
� Fit(data(to(find(transfer(matrix(
� Automated(data(collection(saves(us(hours(
� The(TF(changed(when(we(tightened(the(bolts(on(the(vacuum(flanges(near(the(windows(and(when(we(pulled(vacuum.(
� How(accurate(is(our(TF(now?((
10(
QWP(Angle((deg)(
Circular(Polarization((%
)(
Circular(Polarization(vs(QWP(Angle((
760(Torr(
200(Torr(
10M6(Torr(
Qweak in Hall C
Measurement at exit changes with vacuum pressure. Is it a change on input? Output? Who knows?
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Optical Reversibility TheoremMaking'Use'of'Optical'Transport'Symmetry'
� Research'led'by'Mark'Dalton(UVA)'revealed'that'principles'of'optical'reversibility'allow'determination'of'cavity'DOCP'by'measuring'polarization'of'reflected'light'
� Reflected'circularly'polarized'light'is'blocked'by'the'isolator'and'is'dumped'while'residual'linear'polarization'is'transmitted'and'measured'by'the'photodiode'
� M��������������������� �������photodiode'maximizes'DOCP'at'cavity''
� Addition'of'a'HWP'allows'the'setup'of'any'arbitrary'polarization'state'so'that'we'can'produce'~100%'circularly'polarized'light'at'the'cavity.'
� Later'found'a'publication'detailing'the'use'of'this'technique'for'remote'control'of'laser'polarization.'
11'
Beam polarization is used for optical isolation: back-reflected circular light is opposite handedness, and is opposite to initial linear polarization after the QWP
This provides a technique to repeatably maximize circular polarization, even in the case of changing intermediary birefringent elements (vacuum or thermal stress, etc.)
This isolation fails, to the degree that light is not perfectly circular at the reflecting surface.
Mark Dalton
This technique appears in the literature as well, for similar configurations (“Remote control of polarization”)
mirror bounces, vacuum windows
9
Direct Test of Optimizing Circular Polarization
10
Return power (through isolator)
Measurements while scanning over initial polarization set by QWP and HWP.
DoCP in (open) cavity
Excellent agreement
If minimizing return power, maximizing
DoCP at 99.9%+*
Direct Test of Optimizing Circular Polarization
10
Return power (through isolator)
Measurements while scanning over initial polarization set by QWP and HWP.
DoCP in (open) cavity
Excellent agreement
If minimizing return power, maximizing
DoCP at 99.9%+*
Fitting Entrance Function
11
Measurements while scanning over initial polarization set by QWP and HWP.
DoCP in (open) cavity
Return power, then fit to (simple) optical model
FitMeasured
relates to DoCP
Fitting Entrance Function
12
Measurements while scanning over initial polarization set by QWP and HWP.
DoCP in (open) cavity
DoCP from fit to (simple) optical model
Fit DoLPFit DoCP
Measurement at 0.1% level in DoCP from external measurements
Fitting Entrance Function
12
Measurements while scanning over initial polarization set by QWP and HWP.
DoCP in (open) cavity
DoCP from fit to (simple) optical model
Fit DoLPFit DoCP Residuals:
measured vs. fit
Measurement at 0.1% level in DoCP from external measurements
Electron beam aperture
13
Existing Compton Interaction RegionCollimators protect optics at small crossing angles... but at the cost of larger backgrounds?
Typical “good” brem rate: ~ 100 Hz/uAResidual gas should be about 10x less
How much larger will the halo and tail be, due to synchrotron blowup and the small CEBAF magnetic apertures?
~3.6 degrees puts aperture at size of beampipe, Laser luminosity drops by a factor of 3, but with
9kW this should still be sufficient. Which gives better accuracy?
UPTIME and PRECISION will go up if we use larger apertures (and therefore larger crossing angles)
14
15
Distance from primary beam [mm]0 1 2 3 4 5 6
Anal
yzin
g Po
wer
[%]
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
Analyzing Power, 1.00 GeV and 532 nm
Distance from primary beam [mm]0 1 2 3 4 5 6
Anal
yzin
g Po
wer
[%]
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
PREX
16
Distance from primary beam [mm]0 2 4 6 8 10 12
Anal
yzin
g Po
wer
[%]
-2
0
2
4
6
8
Analyzing Power, 2.20 GeV and 532 nm
Distance from primary beam [mm]0 2 4 6 8 10 12
Anal
yzin
g Po
wer
[%]
-2
0
2
4
6
8
CREX
17
HAPPEX
18Distance from primary beam [mm]0 2 4 6 8 10 12
Anal
yzin
g Po
wer
[%]
-1
0
1
2
3
4
5
Analyzing Power, 3.00 GeV and 532 nm
Distance from primary beam [mm]0 2 4 6 8 10 12
Anal
yzin
g Po
wer
[%]
-1
0
1
2
3
4
5
1024
Distance from primary beam [mm]0 5 10 15 20 25 30
Anal
yzin
g Po
wer
[%]
-5
0
5
10
15
20
Analyzing Power, 6.60 GeV and 532 nm
Distance from primary beam [mm]0 5 10 15 20 25 30
Anal
yzin
g Po
wer
[%]
-5
0
5
10
15
20
19
Distance from primary beam [mm]0 10 20 30 40
Anal
yzin
g Po
wer
[%]
-5
0
5
10
15
20
25
30
Analyzing Power, 11.00 GeV and 532 nm
Distance from primary beam [mm]0 10 20 30 40
Anal
yzin
g Po
wer
[%]
-5
0
5
10
15
20
25
30
20
High Precision Goals
correlated
uncorrelated
Participants from UVa, Syracuse, JLab, CMU, ANL, Miss. St., W&M
Independent detection of photons and electrons provides two (nearly) independent polarization measurements;
each should be better than 0.5%
Rela%ve Error (%) electron photonPosi%on Asymmetries -‐ -‐Ebeam and λlaser 0.03 0.03Radia%ve Correc%ons 0.05 0.05Laser Polariza%on 0.20 0.20Background/Dead%me/Pileup 0.20 0.20
Analyzing Power Calibra%on / Detector Linearity 0.25 0.35
Total 0.38 0.45
What’s been achieved: ~1% (HAPPEX-3, PREX, Qweak)
Primary Challenges:• Laser Polarization• Synchrotron Light• Signal / Background
21
GSO Photon Detector
(&C#$134%/5#D04*57%'*-&'+75/%'*-4#*>#5/1201#.134%/542 3MNO#(0) '-.'=0-%#,9*%*-4P!HO#,9*%*#050.%1*-Q#R0) =0,*4'%0=
S0%%01#%*#740#.90/,01T#@'2201#.134%/5U##:0/=V25/44U
G"5?(+�7*'#/�'&7�(+&8%A
G"5?(+�(+&8%A
=H*;%*&8�E�>#�7*'#/�H�,C�>#
G"5?(+�7*'#/
Existing detector: GSO scintillating crystal,
15cm long, 6cm diameter~60ns, ~150 photoelectron/MeV
Something larger needed to contain showers at high energy, (maybe 6”x6”x15”)
Lead tungstate? Other scintillating or Cerenkov detector? Options exist: simulation and tests needed.
Large GSO detector would be $$$
22