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