Download - Status of Moeller Polarimeter Peter Otte April 7, 2008 11th Collaboration Meeting, Dubrovnik
Status ofMoeller Polarimeter
Peter OtteApril 7, 200811th Collaboration Meeting, Dubrovnik
Why built a Moeller Polarimeter?
Aim: measuring electron-beam polarization
Using Moeller scattering
What is Moeller scattering? (1/2)
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Spin component from beam and radiator
involved: 2 electrons(Composed by 3 amplitudes in 1st order QED)
cross section:
e-
e-
tensor gives analysis powerMoeller c.s. for unpol.
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 distributionof scattering angle:
BeamEEE 21
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Bremsstrahlung
Moeller scattering°
Tagging system
NN
NNA
BeamEEE 21 setup:
finding several pairs of ladder channels with a sum of EBeam
→ coincidental detection of both Moeller electrons
measurement:
(arrows indicate spin alignment of beam and target)
Background processes
Processes that produce the same footprint on the ladder:
Moeller scattering always:
E1+E2 = EBeam
time correlated
Bremsstrahlung accidental coincidences not time correlated
Pair production(3 body process.)
sometimes: E1 + E2 = EBeam
time correlated
How can we reduce triggers generated by bremsstrahlung and pair production events during our measurement to receive better statistics?
Better results by…
forcing the right energy sum→ this becomes the trigger condition
forcing correlation in time→ done in offline analysis usingtime-spectrum
attaching additional detectors
Erik Heid 2006
Rom
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Beam profile measurements
…
16 fibre detectors
View towards tagger ladder:
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
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0
0
200
400
600
800 measurement fit of exp. data geant simulation
Experiment 16/2/2008
cou
nts
horizontal position in cm towards Beam Dumptowards high energy photons
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5
0
200
400
600
800
2 measurements measuremt, reference @ 0cm measuremt, reference @ -1,6cm fit of exp. data Geant simulation
cou
nts
vertical position in cmtowards basement towards ceiling
Experiment 16/2/2008
horizontal vertical
8mm 9mm
vertical profile measurement behind the taggerwith radiator
(Planned: Simulation with radiator)
-2 -1 0 1 2 3 4 5 6 7 80
100
200
300
400
500 measurement between: -2 and 4 cm 0 and 6 cm 2 and 8 cm Fit of data around peak
a
rb. r
ate
/Hz
vertical position in cmtowards basement towards ceiling
FWHM:2.5cm
Why a new Moeller Polarimeter?
why demanding a new one? new gap width higher rate: aiming for 108 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
Status so far
Logic board ready programmed First tests in beam next week
additional detectors (out of plane) necessary?
appendix
Comparison with old Moeller Polarimeterwhy demanding a new one? new gap width higher rate: aiming for 108 tagged photons per sec ability to adapt the trigger logic to tagger calibration
more easily
old (~1995) new (2008)
Trigger in real-time yes yes
Reconfigure the logic
by wire by software
Coincidences 3 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
Size big: standard electronic
small: one sheet of paper and power consumption of about 4 Watts