Low Energy Positron Polarimetry for the ILC

1. Low Energy Positron Polarimetry for the ILC Sabine Riemann (DESY) On behalf of the LEPOL Collaboration LEPOL

2. Outline • Low Energy Polarimeter (LEPOL) • Task within EUROTeV WP4 (Polarized Positron Source) • Why & where do we need it ? • - Available processes  Polarimeter options • - Our suggestion: Bhabha polarimeter • - Backup: Compton Transmission Polarimeter • - Summary LEPOL

3. Measurement of positron polarization at the source  Control of polarization transport  Optimization of positron beam polarization  Commissioning Desired: non-destructive method with accuracy at few percent level LEPOL

4. e+ Polarisation Measurement near the source Polarization measurement  measure asymmetries ! Find a process with • sensitivity to longitudinal polarization of positrons (electrons) • good signal/background ratio • significant asymmetry In the energy range 30 MeV … 5000 MeV Desired: non-destructive easy to handle fast (short measuring time) e+ beam parameterse+ / 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 LEPOL

5. Considered Processes • Compton Scattering (ex.: SLC, HERA) • Laser backscattering on beam • Preferred polarimeter option at IP • Not an option for the LEPOL (very small rates due to large beam size) • after damping rings beam size smaller  this option is under study by Tel Aviv U (G. Alexander) • But: very far from source • Mott • Transverse polarized positrons • sMott~E-4 sMoller~E-2 sBhabha~E-2  high background at relevant energies LEPOL

6. Considered Processes • Compton Transmission (ex.: E166, ATF) • Reconversion of e+ to g in target • Polarization dependent transmission of g through magnetized Fe • Small, simple setup • Can deal with poor beam quality • Destructive  Beam absorbed in relatively thick target • Less efficient with increasing beam energy (E < 100 MeV) LEPOL

7. Considered Processes • Synchrotron radiation (ex.: VEPP-4 storage ring) (S.A. Belomestnykh et al., NIM A 227 (1984) 173 • Transverse polarization needed • Angular asymmetries of synchrotron radiation in damping ring • Relative simple setup • Non-destructive, non intrusive • Very small signal: Asymmetry < 10-3 • position far from source LEPOL

8. Considered Processes • Laser Compton Scattering • Compton Transmission Experiment • Mott Scattering • Synchrotron radiation • Bhabha/Møller magnetized iron target; e- polarization in Fe: ~7%, angular distribution of scattered particles corresponds to e+ polarisation LEPOL

9. Bhabha Polarimetry • As Møller polarimeter already widely used (SLAC, VEPP) • Cross section: • maximal asymmetry at 90°(CMS) ~ 7/9 ≈ 78 % • e+ and e- must be polarized Example: Pe+=80%, Pe-=7% Amax ~ 4.4 % LEPOL

10. Bhabha Polarimetry Working point: • After pre-acceleration 125 MeV – 400 MeV • First design studies done for 200 MeV Used for simulations: Polarized GEANT4, release 8.2 contact: A. Schälicke and collaborateurs, http://www-zeuthen.desy.de/~dreas/geant4 LEPOL

11. Studies for a Bhabha Polarimeter Ebeam = 200 MeV DEbeam = ±10% 2·108 positrons ang. spread =±0.5o 80 mm Fe target PFe = 100% Pe+ = 100% electrons(+) electrons(-) positrons photons after Bhabha scattering LEPOL

12. Regions of maximum asymmetry LEPOL

13. Bhabha Polarimeter 50MeV < E < 150 MeV 20MeV < E < 150 MeV 0.04 < q < 0.120 0.04 < q < 0.120 LEPOL

14. Bhabha Polarimeter LEPOL

15. Bhabha Polarimeter Significance - Energy range: 50 – 150 MeV 20 – 150 MeV cosθ: 0.04 – 0.12 rad 0.04 – 0.21 rad LEPOL

16. Photon distributions cosθ: 0.08 – 0.4 rad no energy cut LEPOL

17. Bhabha Polarimetry • Best significance using asymmetries of scattered Bhabha electrons • Working point: • After pre-acceleration and separation of e+ beam: 120 MeV – 400 MeV • Asymmetries: • Detection of scattered Bhabha electrons is sufficient • Detection of scattered Bhabha positrons for checks • Use of photons (annihilation in flight) Conclusion for layout • Separation of energy range  spectrometer • Separation of e+ and e-  magnetic field (spectrometer • Separation of angular range  masks • Target LEPOL

18. spectrometer mask, shielding exit window (?) detector Side view e+ e+ ~2.5m ~5m Angular range large enough  no bend needed to kick out the scattered e-,e+,g Detector size: O(40*60 cm2) signal rate: O(109) per second for 80 mm Fe foil top view higher energy, lower q electrons positrons photons +background lower energy, higher q LEPOL

19. Magnetized Iron Target Target temperature vs. time 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 ΔAsy • Magnetization (monitoring, tilted target?) • Cooling in real Multiple scattering  additional angular spread of ≤4% Magnetisation vs. Temperature LEPOL

20. Compton Transmission Method ? • Destructive ! • Working point: Ee+ < 100 MeV ideal after capture section O(~30 MeV)  Dimensions O(1m)  Experiences from E166, ATF  Thick Target (1 to 3 X0), with high energy deposition O(~kW)  Small asymmetries O(<1%) at higher energies Example: Ebeam 30 MeV, Pe-=7.92%, Target: 2X0 W, Absorber 15cm Fe A(Pe+=30%) ~ 0.4% A(Pe+=60%) ~ 0.8% LEPOL

21. Summary Recommended for polarimetry near the source – but not yet tested: Bhabha polarimeter at ~400 MeV • In principle, the Bhabha polarimeter could work during ILC operation • Backup possibility: Compton Transmission • After DR: Compton polarimeter To be discussed: - where will we need to check the e+ pol? - when? Commissioning/ operation  type of polarimeter Our plan: Finalize the design study, think about target tests, suggest a polarimeter design LEPOL Collaboration: DESY: K. Laihem, S. Riemann, A. Schälicke, P. Schüler HU Berlin: R. Dollan, T. Lohse NC PHEP Minsk: P. Starovoitov Tel Aviv U: G. Alexander LEPOL