Polarimetry Report Sabine Riemann on behalf of the DESY/HUB group January 24, 2008 EUROTeV Annual Meeting, Frascati

24 Jan 2008, EUROTeV Annual Meetiing, Frascati Polarimetry Report 2 Outline ILC Positron Polarization: P e+ ≈ 30% (baseline design) P e+ ≈ 60% (update)  E166  Proof of principle  polarimetry at low energies  Status G4 with polarization  Polarimetry at low ILC energies (LEPOL)  Update of Bhabha polarimeter studies  Compton polarimetry after DR?  Polarized e+ source modeling  Summary

24 Jan 2008, EUROTeV Annual Meetiing, Frascati Polarimetry Report 3 SLAC Helical undulator: energy and polarization of photons Simulation of Conversion into e+ e- pairs in thin W target, polarization transfer to high energetic leptons 46.6 GeV e- beam

24 Jan 2008, EUROTeV Annual Meetiing, Frascati Polarimetry Report 4 e+ polarimetry at low energies Mott polarimeter  rotate longitudinal spin to transverse Bhabha (Moller) polarimetry ( E(e+) > 100 MeV ) Compton polarimeter  need small beam size E166:Compton transmission polarimeter (  and e+) –Reconversion of e+ to polarized photons (bremsstrahlung, annihilation) –Transmission of photons through magnetized iron (  and  ) Measurement of transmission with CsI calorimeter –Polarization dependent Compton cross section  asymmetry –Method suitable for E(e+) ≤ few tens MeV reconversion magnetized transmitted detector target iron photons e+

24 Jan 2008, EUROTeV Annual Meetiing, Frascati Polarimetry Report 5 E166: Compton transmission polarimetry Measurement of transmission in CsI calorimeter (3x3 crystals)  good signal-background separation »Asymmetry  for parallel / antiparallel polarized iron e+ polarization (P Fe = 7%) Analyzing power A e+ from simulation  Geant4 with polarized prozesses

24 Jan 2008, EUROTeV Annual Meetiing, Frascati Polarimetry Report 6 E166 results (preliminary) E166 publication coming soon... preliminary

24 Jan 2008, EUROTeV Annual Meetiing, Frascati Polarimetry Report 7 Geant4 with polarized processes Polarization is taken into account for –Pair-production –Bremsstrahlung –Compton scattering –Moller/Bhabha scattering –Positron annihilation into 2 photons Included since Geant4 release 8.2 (Dec 06) –Main focus: longitudinal (or circular) polarization (extension to transversal is foreseen) –Energy range of interest: MeV … ~5 GeV Applications: –Polarization transfer (target, E166) –Polarimetry (Bhabha/Moller, Compton transmission – e.g. E166) –ILC polarized positron source Cross checks with EGS, WHIZARD/O’mega Authors: A. Schälicke, K. Laihem, P. Starovoitov

24 Jan 2008, EUROTeV Annual Meetiing, Frascati Polarimetry Report 8

24 Jan 2008, EUROTeV Annual Meetiing, Frascati Polarimetry Report 9 Polarimetry at/near the e+ source(LEPOL) Requirements: –Suitable for low energy range –Suitable for large positron beam size –Fast –Non-destructive –Accuracy O(few %) Possibilities: –Bhabha polarimeter ( MeV) –Compton – after DR (beamsize!) –Compton transmission (~E166): after capture section but only for few bunches People: –DESY/HUB: R. Dollan, T. Lohse, S. Riemann, A. Schälicke, P. Schuler, A. Ushakov Minsk: P. Starovoitov, Tel Aviv U: G. Alexander

24 Jan 2008, EUROTeV Annual Meetiing, Frascati Polarimetry Report 10 Bhabha Polarimetry As Møller polarimeter already widely used Cross section: e+ and e- must be polarized maximal asymmetry at 90°(CMS) ~ 7/9 ≈ 78 % Example: P e+ = 80%, P e- = 7% A max ~ 4.4 % e+e+ Magnetized Iron Detector

24 Jan 2008, EUROTeV Annual Meetiing, Frascati Polarimetry Report 11 Bhabha Polarimetry Centre-of-mass system  Lab system Position for Bhabha polarimeter: –After pre-acceleration and beam separation: 400 MeV –Studies done for E e+ = 400 MeV –Target foil heating (<30  m) is in acceptable limit (simulation)

24 Jan 2008, EUROTeV Annual Meetiing, Frascati Polarimetry Report 12 Example: e - distribution and e - asymmetry angular range of interest: 0.03 – 0.1 rad  Asymmetry in the angular range: A e- ~50 % (A e+ ~5%, A γ ~-15%) P z (Target) = -100% P z (Target) = +100% 30 μm magnetized Fe foil E beam : 400 MeV (10 % spread) Angular spread: 0.5º 100% e+ and e- polarisation assumed e- asymmetry (analyzing power ) e - distribution

24 Jan 2008, EUROTeV Annual Meetiing, Frascati Polarimetry Report 13 scattered e- in detector plane (beam: 1*10 9 e +, E: 400 MeV (10 % spread), ang spread: 0.5º, int. B-gield 0.1 Tm asymmetry (analyzing power) e - distribution - run + run Distance analyzing magnet – detector ~ 5m Optimization is ongoing

24 Jan 2008, EUROTeV Annual Meetiing, Frascati Polarimetry Report 14 Compton Polarimetry after DR Large e+ beam size  no Compton polarimetry before the DR After DR:  small beam size, 5GeV,  transverse polarization (DR…Turnaround)  ~longitudinal polarization (spin rotation after turnaround) taking into account earth curvature

24 Jan 2008, EUROTeV Annual Meetiing, Frascati Polarimetry Report 15 Compton Polarimetry after DR Possible option: –Compton polarimetry after turnaround (5GeV) First considerations for (5 GeV): –see G. Alexander, P. Starovoitov, LC-M Use laser of laser wire Laser wire:  /2 crossing angle between e and  e beam size larger than laser beam waist –Compton: ~  crossing angle between e and  small e beam size for high rate –Discussions with ‘instrumentation’ WP leader of former EDR started….

24 Jan 2008, EUROTeV Annual Meetiing, Frascati Polarimetry Report 16 Polarized positron source modeling Activation and radiation damage studies ( A. Ushakov et al., EPAC06: EUROTEV-REPORT , ILC07: arXiv: ) Production and tracking of polarized e+ at the source: –Use G4 (spin tracking) and FLUKA, ASTRA (acc.) –PAC07: PAC07-THPMN0 –Optimization of capture efficiency and polarization (see next slide)

24 Jan 2008, EUROTeV Annual Meetiing, Frascati Polarimetry Report 17 Polarized positron source modelling

24 Jan 2008, EUROTeV Annual Meetiing, Frascati Polarimetry Report 18 Polarized positron source modeling Recent study of beam displacement  with photon collimator polarization is ‘robust’ up to now `ideal’ undulator spectrum as input used Fast helicity reversal –RDR design not sufficient, –need helicity flip with frequency as for electrons

24 Jan 2008, EUROTeV Annual Meetiing, Frascati Polarimetry Report 19 Summary & Conclusion Lessons from E166 for low energy polarimetry –Detailed simulations are essential – but time consuming Tools for design/performance studies: –G4 with polarization extension: Ongoing: validation, spin tracking, transverse pol. e+ polarimeter: –Compton transmission (few bunches, E < 50 MeV) –Bhabha (~400 MeV) Ongoing simulation studies for polarimeter layout –Compton (5GeV, longitudinal polarization) Further studies only in cooperation with machine and Lw people –Report on LEPOL will be delivered end of 2008 –No prototype Source modeling – status report will be delivered end of 2008 Fast helicity flip to match the experimental precision for physics –Design exists (seeSLAC-TN ), work on alternatives is planned