A Novel Photon Polarimeter

1
A Novel Photon Polarimeter

• Joseph P. Santoro
• Catholic University of America/JLab
• December 27, 2006

2
Overview

• Intro Polarized photons and pair production
• Polarimetry Polarization estimation vs.
  polarization measurements
• The Hall B polarimeter
• Results from g8b polarized photon run

3
Polarized Photons
Circular Polarization
Linear Polarization
t
t
Peaks and troughs are 0? out of phase
Peaks and troughs are 90? out of phase
Produced via coherent brem. (goniometer) or via
Compton backscattering
E
E
Produced as long as the incident electron beam
is polarized
P
P
X
y
X
y
4
Pair production
ee- production plane is correlated with the
polarization of the incoming photon Yang, Berlin,
and Madansky 1950
Olsen, Maximon 1961
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Why do we care about Polarized Photons?

• A polarized photon beam is important for Baryon
• spectroscopy experiments (such as those _at_ CLAS)
• Theoretical calculations can include photon
  polarization
• Asymmetry S (polarization-dependent) can be more
  sensitive than s
• S measurement is experimentally easier than s
• (No acceptance calculation !!)
• One part of obtaining double polarization
  observables
• (Beam-Target and Beam-Recoil or Beam-Meson)
• Extraction of 6 more SDMEs than unpolarized data
• Much higher sensitivity to N interference
  effects

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JLab accelerator CEBAF
Contiuous Electron Beam Accelerator
Facility Newport News, VA

• 7/8 mile racetrack
• Energy 0.8 - 5.7 GeV
• 200 mA, polarization 75
• 1499 MHz operation
• Simultaneous delivery 3 halls

Hall B CLAS
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A touch of CLAS
Drift Chambers
TOF
Cherenkov
Electromagnetic Calorimeter
8
Polarized Photon Beamline
Pair Spectrometer
CLAS
Tagger
e
Goniometer
e-
Electron Beam
Actually bends in z
Photon Polarimeter
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Polarized Photons at CLAS
Coherent bremsstrahlung ?-rays produced by an
electron beam on 50µm diamond crystal mounted
on Goniometer. The Goniometer is aligned to
the incident electron beam through a series of
photon intensity scans.
The polarized photons are produced in one of two
linearly polarized orientations, which are
parallel or perpendicular to the horizontal
plane in the lab. The fine adjustment required
to produce the coherent photon spectrum (on the
order of µradians) is achievable with the
Goniometer.
?COH
Electron beam
?INCOH
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Polarimetry
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Estimating Photon Polarization
The polarization can be estimated by a fit to the
coherent energy spectrum
Data for PERP Calculation
Enhancement
Coherent Spectrum
Polarization from calculation
Primary coherent edge
Polarization
Photon Energy (MeV)
Photon Energy (MeV)
K. Livingston
12
Measuring Photon Polarization
Measure g polarization Pg from pair production
or more specifically..
Pair polarimeters based on magnetic separation
(circa 1960) of the ee- pair sacrifice analying
power Microstrip detectors get rid of the
magnetic field!!!!

B. Wojtsekhowski, et. al. A pair polarimeter For
linearly polarized photons NIM 515 (2003) 605-613
e-
Need a minimum of 3 MSDs to resolve ambiguity
in position of ee- pair
e
g
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Hall B Photon Polarimeter
Top View Schematic
The crossing points of the ee-(P,N) and the
angle between the polarization plane and the ee-
production plane
The photon with polarization e, parallel to the
X-axis, is incident on the thin converter along
the z-axis and produces an e e- pair.
w
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Active Target and Veto Counter
Veto counter
Active Target
Beam
Entrance window

• VC 1x1cm, 100 mm-thick scintillator
• AT 1x1cm, 50 mm-thick scintillator
• Photo-coupling to the PMTs through
• a reflective light box

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Silicon Microstrip Detectors

• 2x256 and 2x384 strips
• 50mm pitch
• On-board preamplifier chips

e-/
Top View (cross section)
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MSD Housing
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All put together
The idea is take measurements with the detector
at 0? and rotated by 90? at each Eg
q- mec2/Eg Eg 2.5 GeV ree- 800mm
g
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Electronics and Readout

• 2(3) PCB-mounted preamplifier chips
• Custom-made signal control box (Ribbon to LEMO)
• 2 CAEN V550 flash ADC cards and V551 B sequencer
• Each strip has one FADC channel and is read out
• sequentially

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Triggering on ee- pairs
Pair Spectrometer
MSD
e
g
Active Target
e-
delayed
Trigger Control
200 ns
1ms readout / deadtime
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Trigger Flow Chart
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Position Distributions
One-track events used for independent detector
alignment Need at least 3 working detectors to
simultaneously resolve the position of ee- pair
DEAD
Optimal
Noisy strips
Optimal
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Distribution of pairs
Circular distribution reflects the photon beam
profile
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From Data to Asymmetry
We need to know the polarization a priori at
least once before we use the polarimeter!!! Polar
imeter was calibrated with 100 polarized
Compton back scattered photons at Spring-8
facility to obtain an Analysing Power A0.11
Analysing power is a combo of MSD and converter
thickness (multiple scattering) We actually
measure product of analysing power and asymmetry
K. De Jager, et. al. A pair polarimeter for
linearly polarized, high-energy photons Eur
Phys J A (2004) 19, 275-278
S
w
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w-Asymmetry
Proof of principle experiment Independent
measurement of PARA and PERP polarization was
desirable but unattainable
Seff
A 0.119 (from SPRING-8 test)
w
CHECK OUT CLAS NOTE 2006-01!!
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Summary and Conclusions

• A scheme for photon polarimetry, based on
  field-free detection of an
• ee- pair was presented
• Tested for the first time during the g8b
  experiment and due to
• technical difficulties, the test turned into a
  proof-of-principle
• experiment
• Comparisons were made between a calculation/fit
  to the coherent
• spectra and an effective asymmetry
• A possible future polarimeter design will
  incorporate a new housing
• design, motor rotation mechanism and 2 MSD
  panels instead of 4.