Status ofMøller Polarimeter

Peter-Bernd OtteSeptember 23, 200812th Collaboration Meeting, Mainz

Why built a Møller Polarimeter?

Aim: measuring electron-beam polarization

Using Møller scattering

What is Møller Scattering? (1/2)

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Rk

Bjjk

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d

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0

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Spin component from beam and radiator

Electron-Electron-scattering(Composed of 2 amplitudes in 1st order QED)

cross section:

e-

e-

tensor gives analysis powerMøller 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 distributionof scattering angle:

Both electrons are emittedcoincidently

BeamEEE 21

Bremsstrahlung

Møller scattering

BeamEEE 21

Using the tagger: setup:

several pairs of tagger channels with a sum of EBeam

→ coincidental detection of both Møller electrons

measurement:

(arrows indicate spin alignment of beam and target)

Trigger logic

NN

NNA

Background processes

Processes that produce the same footprint on the ladder:

Møller scattering always:

E1+E2 = EBeam

time correlated

Bremsstrahlung accidental coincidences not time correlated

Pair production(3 body process)

sometimes: E1 + E2 = EBeam

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 usingtime-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

multiplicitynum

ber

of r

ecor

ded

even

ts *

104

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 forchannel 33 and 315..329

no hits

Møller peak

coincident background

time difference in ticks time difference in ticks time difference in ticks

coun

tsco

unts

coun

tsco

unts

coun

ts

Results 3/6

Doing this for each low energy channel, e.g. 33..35:

no hitsMøller peak

coincidentbackground

coun

ts

energy sum E’1+E’2 in MeV

measuredenergy shift

~17MeVbe

am e

nerg

y

Results 4/6

Shift of Møller peak

(E’ 1+

E’ 2)

- b

eam

ene

rgy

in M

eV

tagger channel of low energy electron

preliminary

simulation withsimplified 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

bea

m e

nerg

y =

150

8MeV

measuredenergy shift of Møller peak17,2(4) MeV

coun

ts

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

coun

ts

energy sum E’1+E’2 in MeV

Møller eventscovered 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 edgesfocal plane

hom. magnet fieldwith effective edgeselectron trajectory

reference entrypoint

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)

- b

eam

ene

rgy

in M

eV

Møller electron energy in units of beam energy

mean value