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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Institut für Kernphysik
der Uni Mainz
Necessary, but not specific for PV-
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)
D measures R+-
Let x be a vector formed from the relevant parameters:
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!
Adjust to zero crossing &
Sketch of polarization optics
Result measuring I-HCA=(I+-I-)/(I++I-)
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.
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.
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)
Realistic only if second order effects (HC-Transmission changes) do not occur
Offset/(4b amplitude) while varying qk:
Offers to reduce
Problem by order(s)
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?
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.
LOW-E polarimetry provides some support for the experiment if it can be done convienently and fast!
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
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
at fixed scattering
(Mayer et al. Rev.Sci. Inst. (64,952(1993))