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

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

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

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 …) http://www.ippp.dur.ac.uk/~gudrid/source/ 03/12/18 R. Dollan

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

The E166 Undulator wound left handed Design parameter: length 1m period 2.54 mm aperture 0.889 mm on axis field 0.71 T K 0.17 Eγ (1st harmonic) 7.8 MeV @ Ebeam= 46.6 GeV wound left handed 03/12/18 R. Dollan

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

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

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

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

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

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

The setup 03/12/18 R. Dollan

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

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

The asymmetries preliminary 03/12/18 R. Dollan

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

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

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

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

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

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

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

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

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

The asymmetries preliminary preliminary Still ongoing: Measured also photon asymmetries in the expected range: exp. Results expected from simulation Photon Calorimeter : 3.67 % ± 0.071 % 3.22 % Aerogel Counter : 3.31 % ± 0.12 % 3.54 % preliminary preliminary Still ongoing: Understanding of systematics Detailed simulation studies -> Polarization values 03/12/18 R. Dollan

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

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

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

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

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

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 125 MeV – 400 MeV First design studies done for 200 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

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

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

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: 0.03 – 0.1 rad --> Asymmetry in the ang. range : ~50 % 03/12/18 R. Dollan

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

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

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

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

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