Status of Møller Polarimeter Peter-Bernd Otte September 23, th Collaboration Meeting, Mainz

Why built a Møller Polarimeter? Aim: measuring electron-beam polarization Using Møller scattering

What is Møller Scattering? (1/2) Spin component from beam and radiator Electron-Electron-scattering (Composed of 2 amplitudes in 1 st order QED) cross section: e-e- e-e- tensor gives analysis power Møller c.s. for unpol.

What is Møller Scattering? (2/2) Key features of this two-electron process: energy sum of both electrons sums up to beam energy: Angular distribution of scattering angle: Both electrons are emitted coincidently Bremsstrahlung Møller scattering

Using the tagger: setup: several pairs of tagger channels with a sum of E Beam → coincidental detection of both Møller electrons measurement: (arrows indicate spin alignment of beam and target) Trigger logic

Background processes Processes that produce the same footprint on the ladder: Møller 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 Reduce bremsstrahlung and pair production events

Better results by… forcing: …correct energy sum → this becomes the trigger condition (reduces pair production events) …correlation in time → done in offline analysis using time-spectrum (reduces Bremsstrahlungs events) Possible: attaching additional detectors near the focal plane (→ see Dubrovnik talk)

Why a new Møller Polarimeter? why demanding a new one? new Tagger gap width (2.5cm) adapt the trigger logic more easily The new trigger board: Uppsala board, FPGA based faster smaller more flexible

Status and Measurements (1/2) Møller Trigger works fine, test measurement with following logic:  32 low energy channels  each with corresponding 17 high energy channels, centred around (E’ 1 +E’ 2 ) = beam energy low energy channel 17 corresponding high energy channels Trigger signal Tagger channels more…

Status and Measurements (2/2) First test with unpol. beam:  duration: 14 min.  490nA, 1508 MeV  1,5kHz Interrupts  19% Livetime Uppsala board Level adapters

Results 1/6 Trigger logic works fine, Multiplicity in tagger: Within time window of +/- 50ns around trigger signal All recorded events multiplicity number of recorded events *10 4 Only events with multiplicity=2 used for analysis

Results 2/6 Task: Count the number of events for each channel pair (using time difference) For example: 15 time difference histograms for channel 33 and no hits Møller peak coincident background time difference in ticks counts

Results 3/6 Doing this for each low energy channel, e.g : no hits Møller peak coincident background counts energy sum E’ 1 +E’ 2 in MeV measured energy shift ~17MeV beam energy

Results 4/6 Shift of Møller peak (E’ 1 +E’ 2 ) - beam energy in MeV tagger channel of low energy electron preliminary simulation with simplified field: only 2 MeV shift, but same direction. Due to bigger scattering angle than bremsstrahl electrons → remember Talk from A. Polonskij Dubrovnik

Results 5/6 All channels combined, rebinned to 2.1MeV beam energy = 1508MeV measured energy shift of Møller peak 17,2(4) MeV counts energy sum E’ 1 +E’ 2 in MeV

Results 6/6 Percentage of background under peak? Within 2  around Møller peak: 96(4)% Møller events counts energy sum E’ 1 +E’ 2 in MeV Møller events covered under normal distribution background events

Outlook Measuring of Beam Polarisation on Wednesday (tomorrow). New simulations necessary: for correction due to coincident background Still current: mounting additional detectors (suppress background)

appendix

Simulated Tagger Optic 1/3 Setup with effective magnetic field edges focal plane hom. magnet field with effective edges electron trajectory reference entry point

Simulated Tagger Optic 2/3 Construction of focal- and detector plane (20 cm space to focal plane) Using crossing point of two electrons trajectories (with +/-  small 

Simulated Tagger Optic 3/3 Calculated shift in energy sum (E’ 1 +E’ 2 ) - beam energy in MeV Møller electron energy in units of beam energy mean value