glasgow university seminar october 2006 a novel photon polarimeter joseph santoro cua glasgow...
Post on 14-Jan-2016
215 Views
Preview:
TRANSCRIPT
Glasgow University Seminar October 2006Glasgow University Seminar October 2006
A Novel Photon Polarimeter
Joseph SantoroCUA
Glasgow University SeminarOctober, 2006
Glasgow University Seminar October 2006Glasgow University Seminar October 2006
WARNING
IN THIS TALK, POLARIZATION WILL BE SPELLED WITH A “Z”!!
Glasgow University Seminar October 2006Glasgow 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 2006Glasgow University Seminar October 2006
Polarized Photons Circular PolarizationLinear Polarization
Xy
EE
y XP
P
t t
Peaks and troughs are 90◦ out of phase
Produced as long as the incident electronbeam is polarized
Produced via coherent brem. (goniometer) or via Compton backscattering
Peaks and troughs are 0◦ out of phase
Glasgow University Seminar October 2006Glasgow University Seminar October 2006
Pair production
polunp XPXkq
qF
E
14
2
2
3 4)(1
8
e+e- production plane iscorrelated with the polarizationof the incoming photonYang, Berlin, and Madansky 1950
Factor Form Atomic:1111/1)( ; nucleus recoil theof momentum :
converter; theofnumber atomic radius,electron classicalconstant, structure fine:
onpolarizatiphoton : ositron;electron/p for the angles solid of elements :
transfermomentum min. 2/ ;energy ositron electron/p :/ ;energy photon :
23/1
220
1
qZqFq
Zr
P
EEkEEk
Olsen, Maximon 1961
Glasgow University Seminar October 2006Glasgow University Seminar October 2006
Why do we care about Polarized Photons?
A polarized photon beam is important for Baryonspectroscopy experiments (such as those @
CLAS)
Theoretical calculations can include photon polarization
Asymmetry (polarization-dependent) can be more sensitive than
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 2006Glasgow University Seminar October 2006
Drift Chambers
TOF
Cherenkov
Electromagnetic Calorimeter
A touch of CLAS
Glasgow University Seminar October 2006Glasgow University Seminar October 2006
Polarized Photon Beamline
Tagger
Photon Polarimeter
Goniometer
CLASPairSpectrometer
ElectronBeam
e-
e+
Actually bendsin z
Glasgow University Seminar October 2006Glasgow University Seminar October 2006
Polarized Photons at CLAS
The polarized photons are produced in one of The polarized photons are produced in one of two two linearly polarizedlinearly polarized orientations, which are orientations, which are parallel parallel or or perpendicularperpendicular to the horizontal to the horizontal planeplanein the lab. in the lab. The fine adjustment required to produce the The fine adjustment required to produce the coherent photon spectrum (on the order of coherent photon spectrum (on the order of μμradiansradians) is achievable with the Goniometer. ) is achievable with the Goniometer.
Electron beam
Coherent bremsstrahlung γ-raysproduced 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.
θθINCOHINCOH
θθCOHCOH
Glasgow University Seminar October 2006Glasgow University Seminar October 2006
Polarimetry
Glasgow University Seminar October 2006Glasgow University Seminar October 2006
Estimating Photon Polarization
The polarization can be The polarization can be estimated by a fit to the estimated by a fit to the coherent energy spectrum coherent energy spectrum
En
han
cem
eEn
han
cem
en
tn
t
Data for Data for PERP PERP CalculationCalculation
Pola
rizati
on
Pola
rizati
on
Photon Energy (MeV)Photon Energy (MeV)
Polarization from Polarization from calculationcalculation
Coherent Coherent Spectrum Spectrum
Photon Energy (MeV)Photon Energy (MeV)
Primary coherent Primary coherent edge edge
K. Livingston
Glasgow University Seminar October 2006Glasgow University Seminar October 2006
Measuring Photon Polarization
Need a minimum of 3 MSDs to resolve ambiguity in position of e+e- pair
e-
e+
Measure polarization P from pair production
||
||
))2cos(1(0 APp
Pair polarimeters based on magnetic separation (circa 1960)of the e+e- pair sacrifice analying power
Microstrip detectors get rid of the magnetic field!!!!
or more specifically…..
B. Wojtsekhowski, et. al. “A pair polarimeterFor linearly polarized photons” NIM 515 (2003)605-613
Glasgow University Seminar October 2006Glasgow University Seminar October 2006
Hall B Photon PolarimeterTop View Schematic
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.
The crossing points of the e+e-(P,N) and the angle between the polarizationplane and the e+e- production plane
±
Glasgow University Seminar October 2006Glasgow University Seminar October 2006
Active Target and Veto Counter
BeamActive Target
Veto counter
Entrance window
•VC 1x1cm, 100 m-thick scintillator•AT 1x1cm, 50 m-thick scintillator
•Photo-coupling to the PMTs through a reflective light box
Glasgow University Seminar October 2006Glasgow University Seminar October 2006
Silicon Microstrip Detectors• 2x256 and 2x384 strips
• 50m pitch • On-board preamplifier chips
Glasgow University Seminar October 2006Glasgow University Seminar October 2006
MSD Housing
Glasgow University Seminar October 2006Glasgow University Seminar October 2006
All put together
The idea is take measurementswith the detector at 0◦ and rotated by 90◦ at each E
+- ≈ mec2/EE= 2.5 GeVre+e- ~ 800m
Glasgow University Seminar October 2006Glasgow University Seminar October 2006
Electronics and Readout1. 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 outsequentially
Glasgow University Seminar October 2006Glasgow University Seminar October 2006
Triggering on e+e- pairs
e+
e-
delayed
Active Target
Pair
Spectrometer
MSD
~200 ns
~1ms readout / deadtime
Glasgow University Seminar October 2006Glasgow University Seminar October 2006
Trigger Flow Chart
Glasgow University Seminar October 2006Glasgow University Seminar October 2006
Position Distributions
DEAD
Noisy stripsOptimal
Optimal
One-track events used for independent detector alignment
Need at least 3 workingdetectors to simultaneously resolve the position of e+e- pair
Glasgow University Seminar October 2006Glasgow University Seminar October 2006
Distribution of pairs
Circular distributionreflects the photon beam profile
Glasgow University Seminar October 2006Glasgow University Seminar October 2006
From Data to AsymmetryWe need to know the polarization a priori at leastonce 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 acombo of MSD andconverter thickness(multiple scattering)
We actually measure product of analysing power and asymmetry
K. De Jager, et. al. “A pair polarimeterfor linearly polarized, high-energy photons” Eur Phys J A (2004) 19, 275-278
Glasgow University Seminar October 2006Glasgow University Seminar October 2006
-Asymmetry
PERPPARA
PERPPARAeff
))2cos(1(0 APp
eff
Proof of principle experimentIndependent measurement ofPARA and PERP polarizationwas desirable but unattainable
%94
A
P eff
A = 0.119 (from SPRING-8 test)
CHECK OUT CLAS NOTE 2006-01!!
Glasgow University Seminar October 2006Glasgow 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 2006Glasgow 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.
top related