glasgow university seminar october 2006 a novel photon polarimeter joseph santoro cua glasgow...

Glasgow University Seminar October 2006Glasgow University Seminar October 2006

A Novel Photon Polarimeter

Joseph SantoroCUA

Glasgow University SeminarOctober, 2006


WARNING

IN THIS TALK, POLARIZATION WILL BE SPELLED WITH A “Z”!!


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


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


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


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


Drift Chambers

TOF

Cherenkov

Electromagnetic Calorimeter

A touch of CLAS


Polarized Photon Beamline

Tagger

Photon Polarimeter

Goniometer

CLASPairSpectrometer

ElectronBeam

e-

e+

Actually bendsin z


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


Polarimetry


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


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


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

±


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


Silicon Microstrip Detectors• 2x256 and 2x384 strips

• 50m pitch • On-board preamplifier chips


MSD Housing


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


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


Triggering on e+e- pairs

e+

e-

delayed

Active Target

Pair

Spectrometer

MSD

~200 ns

~1ms readout / deadtime


Trigger Flow Chart


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


Distribution of pairs

Circular distributionreflects the photon beam profile


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


-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!!


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


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 a novel photon polarimeter joseph santoro cua glasgow...

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