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Special requirements for Photosources operating at PV electron scattering exp. . International Workshop PAVI 2006 Milos Island 20/05/2006 by Kurt Aulenbacher Institut für Kernphysik der Uni Mainz B2/A4 collaboration. Outline. The problem HC-intensity asymmetry

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Special requirements for photosources operating at pv electron scattering exp
Special requirements for Photosources operating at PV electron scattering exp. 

International Workshop

PAVI 2006

Milos Island



Kurt Aulenbacher

Institut für Kernphysik

der Uni Mainz

B2/A4 collaboration


  • The problem

  • HC-intensity asymmetry

  • Sources of other HC-fluctuations

  • Low energy polarimetry

Polarized source tasks

Necessary, but not specific for PV-


Polarized source tasks

1.) Reliable beam production at desired intensity level

2.) Provide desired spin orientation

3.) High Polarization (>80%)

I.) Polarization meas. DA/A ~DP small 

limiting factor in several PV-exp.

II.) HC-control A always important

limiting especially when A<10-6

Source team can provide support for point (I),

(II) is more important.

Scattering experiments (simplified)




  • measure P accurately! (I)

D measures R+-



Let x be a vector formed from the relevant parameters:

What if
What if ?



Goal: Error of HCA should be small against other error contributions. (DP, Dstat)

1.) The (average) values of xi+-xi- have to be measured with good accuracy.

 good stability of xi +  high spin flip frequency desirable

2.) Relative sensitivities have to be determined and are only known with limited accuracy.

Higher order coefficients usually not well known.

 (xi+-xi-) should be “small” (i.e. sufficiently close to zero).

HC-Control schematics (PVA4)

(Almost) no active HC-compensation, except by stabilization!

Important example intensity hca i hca
Important example: Intensity-HCA (I-HCA)

Adjust to zero crossing &

Observe stability!

Sketch of polarization optics

Result measuring I-HCA=(I+-I-)/(I++I-)

Modelling the i hca
Modelling the I-HCA

Assuming analysing power of

Photocathode, imperfections in the alignment and in the phase shifts (birefringences) of the optical

Elements (similar to Humensky et al. NIM A 521 (2004) 261)

Description with 4X4 polarization transfer matrices:

For ‘thin’ cathodes: I+- ~ S+-0

Expand Matrix elements to first order in the imperfections:

Predicted I-HCA as function of compensator rotation angle b

AISR=Analysing power of cathode, with polarizer axis oriented at 2qk,

 Measured for several high P cathodes: AISR=0.02-0.05

fa= f++f-/p: Normalized asymmetric phase shift of pockels cell

(forced zero crossing!),fa=0.03 (typ.)

e3=circular stokes component of light at input of Pockels cell,

 Not measured, est. to <0.003

s~0.999 = diagonal polarisation component at input of Pockels cell,

fc= deviation of half wave plate from 180 degree retardation

0.01 (quote by company)

D,c=function of birefringence of optical elements between PC and cathode.

(measurable, D~0.01)

First consequences
First consequences

1.) Stability does not depend on the symmetric phase shift error (f+-f-)-p

2.) Parameters extracted from fit in agreement with reasonable values

of optical imperfections

3.) Introducing an additional half wave plate

(General sign changer) will also change I-HCA.

Compensation prediction of thermal stability
Compensation: Prediction of thermal stability

1.) Absolute value of phase shift does not contribute to IHCA (in first order)

2.) Asymmetric phase shift + compensator temperature dependence!

3.) Sensitivity depends on steepness of zero crossing

4.) Reduction of sensitivity due to stabilization! (1/G~2-10)

From fit-curve:

Realistic only if second order effects (HC-Transmission changes) do not occur

Compensating the offset term
Compensating the offset term


Offset/(4b amplitude) while varying qk:

Offers to reduce

Problem by order(s)

of magnitude….But

Two questions
Two questions

AISR is the analyzing power of the photocathode which will depend on the photocathode

type (composition, thickness,,,) typically: superlattice 2%, strained layers 4%, GaAs:<0.2%.

1.) Why did PV-experiments before 1990 observe large

asymmetries and position fluctuations

with very small analyzing power of the photocathode?

2.) What is the origin of the HC-fluctuation

of other parameters like position, angle, energy?

Ideal experiment
‘ideal’ Experiment





He/Ne Laser

Detektor with

low analysing power


Experiment results in I-HCA of 10000 ppm

(no lock in needed!)

 Luck! The signal is so large that it´s easy to find a reason….



Backreflexions for the different helicity states.


  • For scattering centers at different positions the ability to interfere (at an image point) is changed by switching the helicity.

  • The interference pattern on the photocathode is therefore also helicity dependent, especcially in the ‘halo’ of the laser beam

Intensity asymmetry in laser beam
Intensity asymmetry in laser beam






Movable Detektor

with pinhole


  • Helicity correlated movement of centroid is 1mm.

Can polarimetry at low energy help a high energy experiment
Can Polarimetry at low energy help a high energy experiment?

LOW-E polarimetry provides some support for the experiment if it can be done convienently and fast!

Moderately ambitious approach mott polarimeter at 3 5 mev
Moderately ambitious approach: Mott polarimeter at 3.5 MeV

  • Goal 1: fast relative measurement at full current with good reproducibility

  • Goal 2: accuracy < 2%

3 5 mev mottpolarimeter
3.5 MeV Mottpolarimeter

Measurement time < 2min @1% stat. Acc. @20 mA

Beam installation time req: (40min) will be reduced to <15min.

Analyzing power calculation
Analyzing power calculation


Low energy: Fink et al.: Phys Rev A (38,12), 6055 (1988)

‚High‘ energy: Uginicius et al.:Nucl Phys A 158 418 (1970)


Low energy: Gray et al.: Rev. Sci. Instrum. 55,88 (1984)

High energy: Sromicki et al. Phys. Rev. Lett. 82,1, 57 (1999)


Analyzing power can be calculated with less than 1% accuracy

Double scattering effects
Double scattering effects

Energy variation

at fixed scattering



Very ambitious approach
Very ambitious approach

  • Low energy may be very accurate (DP/P < 1%)

    (Mayer et al. Rev.Sci. Inst. (64,952(1993))

  • Always possible to achieve low set-up time

  • Spin losses under control <<1%

  • Spin orientation can be calculated to <1 deg.

  • Measurement at full exp.current possible and fast.

  • Calibration check may be handled as accelerator ‚service‘ good calibration tracking.


  • HC-effects do contribute to, but do not dominate

  • the error budget (at PVA4).

  • 2. Stable operating conditions have to be achieved, if necessary

  • extensive stabilization systems have to be used

  • light optical effects are rather complicated but ‘treatable’

  • Better understanding + technology offers potential

  • to keep situation acceptable also for future exp.