Moeller Polarimeter - uni
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Transcript Moeller Polarimeter - uni
Status of
Moeller Polarimeter
Peter Otte
April 7, 2008
11th Collaboration Meeting, Dubrovnik
Why built a Moeller Polarimeter?
Aim: measuring electron-beam polarization
Using Moeller scattering
What is Moeller scattering? (1/2)
e
involved: 2 electrons
(Composed by 3 amplitudes
in 1st order QED)
e
cross section:
d 0
d
B R
1 a jk Pj Pk
d CM d CM
j ,k
Moeller c.s. for unpol.
tensor gives analysis power
j, k x, y, z
Spin component from
beam and radiator
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:
°
Moeller scattering
Bremsstrahlung
taken from „Diplomarbeit Leukel“ 1995
E1 E2 EBeam
Tagging system
setup:
finding several pairs of ladder
channels with a sum of EBeam
→ coincidental detection of both
Moeller electrons
measurement:
N N
A
N N
(arrows indicate spin alignment of beam and target)
E1 E2 EBeam
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
sometimes:
E1 + E2 = EBeam
time
correlated
(3 body process.)
How can we reduce triggers generated by bremsstrahlung and
pair production events during our measurement to receive
better statistics?
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
Better results by…
Beam profile measurements
View towards tagger ladder:
…
16 fibre detectors
Beam profile measurements behind
the tagger with primary beam
horizontal
vertical
800
Experiment 16/2/2008
Experiment 16/2/2008
2 measurements
measuremt, reference @ 0cm
measuremt, reference @ -1,6cm
fit of exp. data
Geant simulation
measurement
fit of exp. data
geant simulation
800
600
400
counts
counts
600
8mm
400
9mm
200
200
0
0
0,0
0,5
1,0
towards high energy photons
1,5
2,0
2,5
3,0
horizontal position in cm
3,5
4,0
towards Beam Dump
0,0
0,5
towards basement
1,0
1,5
2,0
2,5
3,0
vertical position in cm
detector collected electrons with approx. 400 MeV
Result: FWHM of 8 / 9 mm in x- and y-direction
3,5
4,0
4,5
towards ceiling
vertical profile measurement
behind the tagger
with radiator
500
measurement between:
-2 and 4 cm
0 and 6 cm
2 and 8 cm
Fit of data around peak
arb. rate /Hz
400
FWHM:
2.5cm
300
200
100
0
-2
-1
towards basement
0
1
2
3
4
vertical position in cm
(Planned: Simulation with radiator)
5
6
7
8
towards ceiling
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
smaller
The new trigger board:
(Uppsala board)
more flexible
faster
Status so far
Logic board ready programmed
First tests in beam next week
additional detectors (out of plane) necessary?
appendix
Comparison with old 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
old (~1995)
new (2008)
Trigger in real-time
yes
yes
Reconfigure the
logic
by wire
by software
Coincidences
3 pairs à 16
between arbitrary latter channels, not only pairs
latter channels 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