A Novel Photon
Polarimeter
Joseph Santoro
CUA
Glasgow University Seminar
October, 2006
Glasgow University Seminar October 2006
WARNING
IN THIS TALK, POLARIZATION
WILL BE SPELLED WITH A “Z”!!
Glasgow University Seminar October 2006
Overview
1.
Intro: Polarized photons and pair production
2. Polarimetry: Polarization estimation vs.
polarization measurements
3. The Hall B polarimeter
4. Results from g8b polarized photon run
Glasgow University Seminar October 2006
Polarized Photons
Linear Polarization
Circular 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
y
X
P
y
Glasgow University Seminar October 2006
X
Pair production
e+e- production plane is
correlated with the polarization
of the incoming photon
Yang, Berlin, and Madansky 1950
 3
 1  F (q)2 4
 2

 X unp  P1  X pol
4
E     8
k
q


Olsen, Maximon 1961
k : photon energy ; E / E : electron/positron energy ;   k / 2 E E min. momentum transfer
    : elements of solid angles for the electron/positron; P1 : photon polarizati on
  r02 Z 2 : fine structure constant,classical electron radius, atomic number of the converter;


1/ 3 2 

q : momentum of the recoil nucleus ; F (q)  1 / 1  111 qZ
: Atomic Form Factor


Glasgow University Seminar October 2006
Why do we care about Polarized
Photons?
A polarized photon beam is important for Baryon
spectroscopy experiments (such as those @ CLAS)
 Theoretical calculations can include photon polarization
 Asymmetry S (polarization-dependent) can be more
sensitive than 
 S measurement is experimentally easier than 
(No acceptance calculation !!)
 One part of obtaining double polarization observables
(Beam-Target and Beam-Recoil or Beam-Meson)
 Extraction of 6 more SDME’s than unpolarized data
 Much higher sensitivity to N* interference effects
Glasgow University Seminar October 2006
A touch of CLAS
Drift Chambers
TOF
Cherenkov
Electromagnetic Calorimeter
Glasgow University Seminar October 2006
Polarized Photon Beamline
Tagger
Pair
Spectrometer
e+
Goniometer
Electron
Beam
CLAS
Actually bends
in z
ePhoton
Polarimeter
Glasgow University Seminar October 2006
Polarized Photons at CLAS
Coherent bremsstrahlung γ-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.
θCOH
θINCOH
Glasgow University Seminar October 2006
Polarimetry
Glasgow University Seminar October 2006
The polarization can be estimated
by a fit to the coherent energy
spectrum
Enhancement
Estimating Photon Polarization
Data for PERP
Calculation
Coherent Spectrum
Primary coherent edge
Polarization
Polarization from calculation
Photon Energy (MeV)
K. Livingston
Photon Energy (MeV)
Glasgow University Seminar October 2006
Measuring Photon Polarization
Measure g polarization Pg from pair production
 p   0 (1  Pg  A  cos(2))
 ||   
S
 ||   
or more specifically…..
Pair polarimeters based on magnetic separation (circa 1960)
of the e+e- 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
Need a minimum of
3 MSDs to resolve
ambiguity in position
of e+e- pair
e
g
e+
Glasgow University Seminar October 2006
Hall B Photon Polarimeter
Top View Schematic
The crossing points of the e+e-(P,N)
and the angle between the polarization
plane and the e+e- 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.
±
Glasgow University Seminar October 2006
Active Target and Veto Counter
Veto counter
Active Target
Entrance window
•VC 1x1cm, 100 mm-thick scintillator
•AT 1x1cm, 50 mm-thick scintillator
•Photo-coupling to the PMTs through
a reflective light box
Glasgow University Seminar October 2006
Silicon Microstrip Detectors
• 2x256 and 2x384 strips
• 50mm pitch
• On-board preamplifier chips
Glasgow University Seminar October 2006
MSD Housing
Glasgow University Seminar October 2006
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
re+e- ~ 800mm
g
Glasgow University Seminar October 2006
Electronics and Readout
1. 2(3) PCB-mounted preamplifier chips
2. Custom-made signal control box (Ribbon to LEMO)
3. 2 CAEN V550 flash ADC cards and V551 B
sequencer
Each strip has one FADC channel and is read out
sequentially
Glasgow University Seminar October 2006
Triggering on e+e- pairs
g
Pair
e+
Active
Target
e-
Spectrometer
delayed
MSD
~200 ns
~1ms readout / deadtime
Glasgow University Seminar October 2006
Trigger Flow Chart
Glasgow University Seminar October 2006
Position Distributions
One-track events used
for independent
detector alignment
Need at least 3 working
detectors to
simultaneously resolve
the position of e+e- pair
DEAD
Optimal
Optimal
Glasgow University Seminar October 2006
Noisy
strips
Distribution of pairs
Circular distribution
reflects the photon
beam profile
Glasgow University Seminar October 2006
From Data to Asymmetry
We need to know the polarization a priori at least
once before we use the polarimeter!!!
Polarimeter was “calibrated” with 100% polarized
Compton back scattered photons at Spring-8 facility
to obtain an “Analysing Power” A=0.11
Analysing power is a
combo of MSD and
converter thickness
(multiple scattering)
K. De Jager, et. al. “A pair polarimeter
for linearly polarized, high-energy photons”
Eur Phys J A (2004) 19, 275-278
S
We actually measure
product of analysing
power and asymmetry

Glasgow University Seminar October 2006
-Asymmetry
Independent measurement of
PARA and PERP polarization
was desirable but unattainable
Seff
Proof of principle experiment
 p   0 (1  Pg  A  cos(2))
S eff
PARA  PERP

PARA  PERP
A = 0.119 (from SPRING-8 test)
Pg 
S eff
A
 94 %

CHECK OUT CLAS NOTE 2006-01!!
Glasgow University Seminar October 2006
Polarimeter++
Polarimetry is highly desirable for the upcoming FROST
experiment but first some things need to be fixed:
AT/VC need to be redesigned to provide better light collection
Motor control assembly and detector housing needs to be redesigned,
with two different motors and optical encoder position readback
Replace 4 panel single-sided MSD design with 2 panel design
incorportating a double-side and single sided panel
Glasgow University Seminar October 2006
Summary and Conclusions
•A scheme for photon polarimetry, based on field-free detection of an
e+e- 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 future polarimeter design (FROST) will incorporate a new housing
design, motor rotation mechanism and 2 MSD panels instead of 4.
Glasgow University Seminar October 2006