1. Beijing, Feb 3 rd, 2007 LEPOL 1 Low Energy Positron Polarimetry for the ILC Sabine Riemann (DESY) On behalf of the LEPOL Collaboration

2. Beijing, Feb 3 rd. 2007 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

3. Beijing, Feb 3 rd. 2007 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

4. Beijing, Feb 3 rd. 2007 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 parameters e + / bunch N e+ 2·10 10 bunches / pulse2820 Rep. Rate R5 Hz Energy E30 – 5000 MeV Energy spread ΔE/E10 % Normalized emittance ε*~ 3.6 cm rad Beam size σ x,y ~ 1 cm

5. Beijing, Feb 3 rd. 2007 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 –  Mott ~E -4  Moller ~E -2  Bhabha ~E -2  high background at relevant energies

6. Beijing, Feb 3 rd. 2007 LEPOL 6 Considered Processes Compton Transmission (ex.: E166, ATF) –Reconversion of e+ to  in target –Polarization dependent transmission of  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)

7. Beijing, Feb 3 rd. 2007 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

8. Beijing, Feb 3 rd. 2007 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

9. Beijing, Feb 3 rd. 2007 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: P e+ = 80%, P e- = 7% A max ~ 4.4 %

10. Beijing, Feb 3 rd. 2007 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

11. Beijing, Feb 3 rd. 2007 LEPOL 11 Studies for a Bhabha Polarimeter E beam = 200 MeV  E beam = ±10% 2·10 8 positrons ang. spread =±0.5 o 80  m Fe target P Fe = 100% P e+ = 100% electrons(+) electrons(-) positrons photons after Bhabha scattering

12. Beijing, Feb 3 rd. 2007 LEPOL 12 Regions of maximum asymmetry

13. Beijing, Feb 3 rd. 2007 LEPOL 13 Bhabha Polarimeter 50MeV < E < 150 MeV 20MeV < E < 150 MeV 0.04 <  < 0.120 0.04 <  < 0.120

14. Beijing, Feb 3 rd. 2007 LEPOL 14 Bhabha Polarimeter

15. Beijing, Feb 3 rd. 2007 LEPOL 15 Bhabha Polarimeter Significance - Energy range: 50 – 150 MeV 20 – 150 MeV cosθ: 0.04 – 0.12 rad 0.04 – 0.21 rad

16. Beijing, Feb 3 rd. 2007 LEPOL 16 Photon distributions cosθ: 0.08 – 0.4 rad no energy cut

17. Beijing, Feb 3 rd. 2007 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

18. Beijing, Feb 3 rd. 2007 LEPOL 18 ~5m e+ ~2.5m e+ Angular range large enough  no bend needed to kick out the scattered e-,e+,  Detector size: O(40*60 cm2) signal rate: O(10 9 ) per second for 80  m Fe foil detector mask, shielding exit window (?) spectrometer higher energy, lower  lower energy, higher  electrons positrons photons +background Side view top view

19. Beijing, Feb 3 rd. 2007 LEPOL 19 Magnetized Iron Target Heating of the target -> Magnetization decreases –Simulation for 30 µm –Cooling by radiation –T C (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% Target temperature vs. time Magnetisation vs. Temperature

20. Beijing, Feb 3 rd. 2007 LEPOL 20 Compton Transmission Method ? Destructive ! Working point: E e + < 100 MeV ideal after capture section O(~30 MeV)  Dimensions O(1m)  Experiences from E166, ATF  Thick Target (1 to 3 X 0 ), with high energy deposition O(~kW)  Small asymmetries O(<1%) at higher energies Example: E beam 30 MeV, P e- =7.92%, Target: 2X 0 W, Absorber 15cm Fe A(P e+ =30%) ~ 0.4% A(P e+ =60%) ~ 0.8%

21. Beijing, Feb 3 rd. 2007 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