1. Status of Moeller Polarimeter Peter Otte April 7, 2008 11th Collaboration Meeting, Dubrovnik

2. Why built a Moeller Polarimeter? Aim: measuring electron-beam polarization Using Moeller scattering

3. What is Moeller scattering? (1/2) Spin component from beam and radiator involved: 2 electrons (Composed by 3 amplitudes in 1 st order QED) cross section: e-e- e-e- tensor gives analysis power Moeller c.s. for unpol.

4. What is Moeller Scattering? (2/2) Key features of this two-electron process: huge energy transfer, T~1/m energy sum of both electrons sums up to beam energy: Angular distribution of scattering angle: taken from „Diplomarbeit Leukel“ 1995 Bremsstrahlung Moeller scattering °

5. Tagging system setup: finding several pairs of ladder channels with a sum of E Beam → coincidental detection of both Moeller electrons measurement: (arrows indicate spin alignment of beam and target)

6. Background processes Processes that produce the same footprint on the ladder: Moeller scatteringalways: E 1 +E 2 = E Beam time correlated Bremsstrahlungaccidental coincidencesnot time correlated Pair production (3 body process.) sometimes: E 1 + E 2 = E Beam time correlated How can we reduce triggers generated by bremsstrahlung and pair production events during our measurement to receive better statistics?

7. Better results by… forcing the right energy sum → this becomes the trigger condition forcing correlation in time → done in offline analysis using time-spectrum attaching additional detectors Erik Heid 2006 Roman Leukel 1995

8. Beam profile measurements … 16 fibre detectors View towards tagger ladder:

9. Beam profile measurements behind the tagger with primary beam detector collected electrons with approx. 400 MeV Result: FWHM of 8 / 9 mm in x- and y-direction horizontalvertical 8mm 9mm

10. vertical profile measurement behind the tagger with radiator (Planned: Simulation with radiator)

11. Why a new Moeller Polarimeter? why demanding a new one? new gap width higher rate: aiming for 10 8 tagged photons per sec ability to adapt the trigger logic to tagger calibration more easily The new trigger board: (Uppsala board) faster smaller more flexible

12. Status so far Logic board ready programmed First tests in beam next week additional detectors (out of plane) necessary?

13. appendix

14. Comparison with old Moeller Polarimeter why demanding a new one? new gap width higher rate: aiming for 10 8 tagged photons per sec ability to adapt the trigger logic to tagger calibration more easily old (~1995)new (2008) Trigger in real-timeyes Reconfigure the logic by wireby software Coincidences3 pairs à 16 latter channels between arbitrary latter channels, not only pairs up to 256 channels can be handled Decision time?faster: 20ns, less cables and electronic Sizebig: standard electronic small: one sheet of paper and power consumption of about 4 Watts