Special requirements for photosources operating at pv electron scattering exp
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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. 

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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

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 ?



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).

Source Set-up

HC-Control schematics (PVA4)

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

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

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

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

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


Offset/(4b amplitude) while varying qk:

Offers to reduce

Problem by order(s)

of magnitude….But

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





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






Movable Detektor

with pinhole


  • Helicity correlated movement of centroid is 1mm.

Causes for HC-fluctuations

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

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

  • Goal 2: accuracy < 2%

3.5 MeV Mottpolarimeter

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

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

Asymmetry vs. Spin rotator angle (164 Grad)

8 hour measurement of asymmetry

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

Energy variation

at fixed scattering



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.

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