20/05/06 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

20/05/06 Outline The problem HC-intensity asymmetry Sources of other HC-fluctuations Low energy polarimetry

20/05/06 Polarized source tasks 1.) Reliable beam production at desired intensity level 2.) Provide desired spin orientation 3.) High Polarization (>80%) Necessary, but not specific for PV- Experiment. I.) Polarization meas.  A/A ~  P 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.

20/05/06 Scattering experiments (simplified) SA T D measures R +- +  Let x be a vector formed from the relevant parameters:  measure P accurately! (I)

20/05/06 What if ? 2.) Relative sensitivities have to be determined and are only known with limited accuracy. Higher order coefficients usually not well known.  (x i + -x i - ) should be “small” (i.e. sufficiently close to zero). 1.) The (average) values of x i + -x i - have to be measured with good accuracy.  good stability of x i +  high spin flip frequency desirable Goal: Error of HCA should be small against other error contributions. (  P,  stat ) { :=HCA

20/05/06

Source Set-up

20/05/06 HC-Control schematics (PVA4) (Almost) no active HC-compensation, except by stabilization!

20/05/06 Important example: Intensity-HCA (I-HCA) Sketch of polarization optics Result measuring I-HCA=(I + -I - )/(I + +I - ) Adjust to zero crossing & Observe stability!

20/05/06 Modelling the I-HCA Description with 4X4 polarization transfer matrices: For ‘thin’ cathodes: I +- ~ S +- 0 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)

20/05/06 A ISR =Analysing power of cathode, with polarizer axis oriented at 2  k,  Measured for several high P cathodes: A ISR =  a = f + +f - /  : Normalized asymmetric phase shift of pockels cell  (forced zero crossing!),  a =0.03 (typ.)  3 =circular stokes component of light at input of Pockels cell,  Not measured, est. to <0.003  = diagonal polarisation component at input of Pockels cell,  c = deviation of half wave plate from 180 degree retardation  0.01 (quote by company) D,  =function of birefringence of optical elements between PC and cathode.  (measurable, D~0.01) Expand Matrix elements to first order in the imperfections:  Predicted I-HCA as function of compensator rotation angle 

20/05/06 1.) Stability does not depend on the symmetric phase shift error (f + -f - )-  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. First consequences

20/05/06 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

20/05/06 Compensating the offset term Offset/(4  amplitude) while varying  k : Offers to reduce Problem by order(s) of magnitude….But {

20/05/06 Two questions A ISR 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?

20/05/06 ‘ideal’ Experiment Pulser He/Ne Laser Lock- in Sw. PC Detektor with low analysing power Experiment results in I-HCA of ppm (no lock in needed!)  Luck! The signal is so large that it´s easy to find a reason….

20/05/06 Prism Screen Backreflexions for the different helicity states.

20/05/06 Hypothesis 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

20/05/06 Intensity asymmetry in laser beam Pulser diode-laser Lock- in Sw. PC Movable Detektor with pinhole  Helicity correlated movement of centroid is 1  m.

20/05/06 Causes for HC-fluctuations ParameterHCA at sourceHCA at targetdominating Cause intensity0-4000ppm ISR position100nm~10nmInterference angle radPhase space transformation energy-Few eVPhase space transf.

20/05/06 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!

20/05/06 Moderately ambitious approach: Mott polarimeter at 3.5 MeV Goal 1: fast relative measurement at full current with good reproducibility Goal 2: accuracy < 2%

20/05/ MeV Mottpolarimeter Measurement time < stat.  A Beam installation time req: (40min) will be reduced to <15min.

20/05/06 Asymmetry vs. Spin rotator angle (164 Grad)

20/05/06 8 hour measurement of asymmetry

20/05/06 Analyzing power calculation Theo: Low energy: Fink et al.: Phys Rev A (38,12), 6055 (1988) ‚High‘ energy: Uginicius et al.:Nucl Phys A (1970) Exp: Low energy: Gray et al.: Rev. Sci. Instrum. 55,88 (1984) High energy: Sromicki et al. Phys. Rev. Lett. 82,1, 57 (1999) Z=79 Analyzing power can be calculated with less than 1% accuracy

20/05/06 Double scattering effects Energy variation at fixed scattering angle 

20/05/06 Very ambitious approach Low energy may be very accurate (  P/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.

20/05/06 Summary 1.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 3.light optical effects are rather complicated but ‘treatable’ 4.Better understanding + technology offers potential to keep situation acceptable also for future exp.