1. E166 - LEPOL - Low Energy Positron Polarimetry for the ILC
Ralph Dollan, HU Berlin

2. Outline E166 LEPOL scheme the helical undulator
transmission polarimetry
setup, analysis, simulation
LEPOL
purpose
options
simulation
03/12/18
R. Dollan

3. Outline E166 LEPOL scheme the helical undulator
transmission polarimetry
setup, analysis, simulation
LEPOL
purpose
options
simulation
Polarized GEANT4
03/12/18
R. Dollan

4. The International Linear Collider (ILC)
Center of mass energy: 500 GeV
Luminosity: L= 2·1034 cm-2s-1
Length: ~ 31 km
Polarized beams: P(e-) > 80%, P(e+) ~ 30%(60%†) †upgrade
Polarization of both beams is advantageous f. SM- and non-SM-physics
(eff. luminosity, signal/background in SM processes …)
03/12/18
R. Dollan

5. E166
Task: proof the possibility, to produce polarized positrons using a helical undulator !
1 m long helical undulator produces circular polarized photons
conversion of circularly polarized photons to longitudinally polarized positrons in thin W-target
measurement of polarization of photons and positrons by Photon transmission method
main parts: undulator, production target, spectrometer, e+/γ diagnostics
03/12/18
R. Dollan

6. The E166 Undulator wound left handed Design parameter: length 1m
period mm
aperture mm
on axis field 0.71 T
K 0.17
Eγ (1st harmonic) 7.8 Ebeam= 46.6 GeV
wound left handed
03/12/18
R. Dollan

7. Photon Energy and Polarization (K~0.17)
Ecutoff (1st harmonic) : 7.8 MeV
Undulator Photon energy spectrum
Undulator Photon degree of polarization
03/12/18
R. Dollan

8. Expected Positron Spectrum and Polarization
positron generation in a 0.5 X0 W-target for undulator design parameters:
03/12/18
R. Dollan

9. Compton Transmission Polarimetry
Detector
Magnetized
Iron Absorber
Reconversion
Target e+ γ
with
Transmission
Asymmetry
Photon Polarisation
Pe(Fe) = 6.9 ± 0.2 %
Analyzing Power
03/12/18
R. Dollan

10. The E166 Polarimeter Edep(-) - Edep(+) Measure Edep(-) and Edep(+)
Magnetized Iron
(-/+)
Edep(-) - Edep(+)
Measure Edep(-) and Edep(+)
-> Asymetry =
Edep(-) + Edep(+)
03/12/18
R. Dollan

11. E-166 in the FFTB beam energy Ebeam = 46.6 GeV
electrons/pulse Ne- ~1010
beam size σ = 40 µm
rep. rate 10 Hz
03/12/18
R. Dollan

12. Details of the setup
03/12/18
R. Dollan

13. The setup
03/12/18
R. Dollan

14. Positron data analysis
2 run periods (june and september 2005)
6 spectrometer settings (6 e+ energy points)
> 8 million triggers
~ 3000 cycles
Analysis steps:
background subtraction
normalization of the energy
deposition
cyclepairing
asymmetry determination Bg
Bg + signal
03/12/18
R. Dollan

15. Positron data analysis
2 run periods (june and september 2005)
6 spectrometer settings (6 e+ energy points)
> 8 million triggers
~ 3000 cycles
Analysis steps:
background subtraction
normalization of the energy
deposition
cyclepairing
asymmetry determination Bg
Bg + signal
03/12/18
R. Dollan

16. The asymmetries
preliminary
03/12/18
R. Dollan

17. The asymmetries preliminary Positron Polarization [%] ?
We have to transfer
the asymmetry into polarization
and the spectrometer current into
energy of the positrons/electrons
Positron energy [MeV] ?
03/12/18
R. Dollan

18. The asymmetries Detailed (polarized) simulation needed !!! preliminary
Positron Polarization [%] ?
Detailed (polarized) simulation needed !!!
We have to transfer
the asymmetry into polarization
and the spectrometer current into
energy of the positrons/electrons
Positron energy [MeV] ?
03/12/18
R. Dollan

19. Polarization dependent processes in GEANT4
Conversion Target
G4ePolarizedBremsstrahlung
Polarized Photons
Undulator
G4ePolarizedIonisation
(G4PolarizedMollerBhabhaModel)
G4PolarizedGammaConversion e+
With the use of:
G4StokesVector
G4PolarizationHelper
G4PolarizationManager
G4PolarizationMessenger
G4PolarizedCompton
Vacuum chamber e+
Analyzing Magnet
ReConversion
Target e+
CsI e+ e+
G4eplusPolarizedAnnihilation
03/12/18
R. Dollan

20. Spectrometer calibration
Calculated and
measured field map
were the input for the
G4 Simulation
03/12/18
R. Dollan

21. Positron production Target
Input: undulator spectrum
Polarization Transfer:
G4PolarizedGammaConversion
High energetic positrons
carry high degree of polarization e- e- e+ e- e+
0.25 X0 W
long. pol. e+e- pairs
circ. pol. photons
03/12/18
R. Dollan

22. Simulation of the Polarimeter Analyzing Power
N e+ = 104
E e+ = 7 MeV
P e+ = ±100%
±100%
03/12/18
R. Dollan

23. The asymmetries preliminary Positron Polarization [%] ?
Positron energy [MeV] ?
03/12/18
R. Dollan

24. The asymmetries
preliminary
preliminary
03/12/18
R. Dollan

25. The asymmetries preliminary preliminary Still ongoing:
Understanding of systematics
Detailed simulation studies
-> Polarization values
03/12/18
R. Dollan

26. The asymmetries preliminary preliminary Still ongoing:
Measured also photon asymmetries in the expected range:
exp. Results expected
from simulation
Photon Calorimeter : 3.67 % ± % %
Aerogel Counter : 3.31 % ± 0.12 % %
preliminary
preliminary
Still ongoing:
Understanding of systematics
Detailed simulation studies
-> Polarization values
03/12/18
R. Dollan

27. E166 Summary
E-166 produced data with good quality and has shown, that the helical undulator works – polarized positrons have been measured
The E166 asymmetries are in the expected range
The E166 simulation made polarized processes in GEANT4 necessary – they have been implemented
Interpretation of the data and publication in progress
03/12/18
R. Dollan

28. Low Energy Positron Polarimeter
Measurement of positron polarization at the source
-> Optimization of the positron beam polarization/intensity
-> Control of polarization transport
Beam Parameters
e+ / bunch Ne+ 2·1010
bunches / pulse 2820
Rep. Rate R 5 Hz
Energy E 30 – 5000 MeV
Energy spread ΔE/E 10 %
Normalized emittance ε* ~ 3.6 cm rad
Beam size σx,y ~ 1 cm
Desired: non-destructive method with accuracy at percent level
~125 MeV
~400 MeV
03/12/18
R. Dollan

29. Polarization measurement -> measure Asymmetries !
The challenges
Polarization measurement -> measure Asymmetries !
Find for the low energy range a process with
sensitivity to longitudinal polarization of positrons (electrons)
good signal/background ratio
significant asymmetry
Desired:
non-destructive
good reliability
easy to handle
fast (short measuring time)
~125 MeV
~400 MeV
03/12/18
R. Dollan

30. Available Processes Laser Compton Scattering (ex.: SLC, HERA)
High intensity Laser on low emittance beam
Only after Damping Rings (Intensity, Energy)
High precision
Bhabha/Møller (ex: SLAC, JLAB, VEPP-3)
Thin magnetized Target
Suitable for desired energy range
Compton Transmission (ex.: E166, KEK-ATF Pol. Experiment)
Beam absorbed in thick target
Very low energy ( < 100 MeV)
Mott
Transverse polarized positrons, high background
Synchrotron radiation (ex.: VEPP-4 storage ring)
Transverse polarization
Near/in damping ring ?
Low signal – Asymmetry < 10-3
03/12/18
R. Dollan

31. Compton Transmission Method
Destructive !
Polarized positrons reconverted into polarized gammas
Polarization dependent transmission due to Compton scattering in magnetized Iron
Working point: Ee+ < 100 MeV
ideal after capture section O(~30 MeV)
Dimensions O(1m)
Experiences from E166
Thick Target (1 to 3 X0)
with high energy deposition O(~KW)
Small asymmetries O(<1%)
Example: Ebeam 30 MeV, Pe-=7.92%, Target: 2X0 W, Absorber 15cm Fe -> A(Pe+=30%) ~ 0.4%
A(Pe+=60%) ~ 0.8%
Detector
Magnetized
Iron Absorber
Reconversion
Target e+ γ
03/12/18
R. Dollan

32. Bhabha Polarimetry Example: Pe+= 80%, Pe-= 7% Amax ~ 4.4 %
As Møller Polarimeter already used (SLAC, VEPP-3)
Non-destructive !
Working point:
After pre-acceleration MeV – 400 MeV
First design studies done for MeV / 400 MeV
Cross section:
Theor. maximal asymmetry at 90°(CMS) ~ 7/9 ≈ 78 %
Example: Pe+= 80%, Pe-= 7% Amax ~ 4.4 %
03/12/18
R. Dollan

33. Bhabha Polarimeter e+ Polarized GEANT4
Measures Asymmetry of scattered particles (e+,e-,(γ)) of two magnetization states of the target
Mask or shielding selects angular range with max. asymmetry
Spectrometer -> particle separation, energy selection e+
Magnetized
Iron
Detector
6 m
Polarized GEANT4
03/12/18
R. Dollan

34. Bhabha Polarimeter: Target
Magnetized thin IronTarget
Heating of the target -> Magnetization decreases
Simulation for 30 µm
Cooling by radiation
TC (Fe) = 1039 K; melting point 1808 K
Ongoing considerations on target layout
ΔT -> ΔM -> ΔP -> ΔA
Magnetic field (tilted or not)
Cooling in real
Monitoring of magnetization
Target temperature vs. time
Magnetisation vs. Temperature
03/12/18
R. Dollan

35. Example: e- distribution and e- asymmetry
30 μm magnetized Fe-Foil
Ebeam : 400 MeV (10 % spread)
Ang. Spread : 0.5º
Pz(Target) = -100%
e+,e-,γ e+
asymmetry (analyzing power)
Pz(Target) = +100%
ang. range of interest: – 0.1 rad
--> Asymmetry in the ang. range : ~50 %
03/12/18
R. Dollan

36. Geometry Detector plane Hom. B-field: (req.: BdL ~ 0.1 Tm) mask O(~5m)
30 μm Fe Target,
rtarget= 2.5 cm
O(~5m)
ang. range
0.03 – 0.1 rad
room for 15 cm beampipe
+ gap
03/12/18
R. Dollan

37. Towards a Geometry for a LEPOL
Mask
Detector plane
Mag. field
Target
Questions to answer:
detector area with best significance ?
detector type ?
mask dimensions, material ?
magnet dimensions ?
03/12/18
R. Dollan

38. Detector plane, int. B-field 0.1 Tm
e- distribution
e- distribution
asymmetry (analyzing power)
(beam: 1*109 e+, E: 400 MeV (10 % spread), ang spread: 0.5º
03/12/18
R. Dollan

39. LEPOL Summary Present ongoing work:
Detailed design studies for Bhabha polarimeter:
Layout
Target
Implementation into beam line
Compton transmission performance studies
Simulation studies of Laser Compton Method (Minsk, TelAviv)
Polarimetry at the positron source is
possible
necessary
but not easy
The LEPOL Collaboration:
Karim Laihem, Sabine Riemann, Andreas Schälicke, Peter Schüler, Andriy Ushakov, DESY
R.D., Thomas Lohse, HU Berlin
Pavel Starovoitov, Minsk
Gideon Alexander, TelAviv
03/12/18
R. Dollan

40. Summary of the summaries
There is progress on both fields,
E166 and LEPOL
In addition -> “polarized” GEANT4 - not only as side effect
Thank you for your attention !
03/12/18
R. Dollan