POLARIMETRY of MeV Photons and Positrons Overview Beam Characterization – undulator photons –...

POLARIMETRYof MeV Photons and Positrons

Overview• Beam Characterization

– undulator photons

– positrons

• Basics of the Transmission Method– for photon polarimetry

– for positron polarimetry

• Description of the Layouts and Hardware– for the photon polarimeter

– for the positron polarimeter

• Expected Polarimeter Performance

SLAC EPAC12 June 2003

E166 Proposal PresentationK.P. Schüler

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Undulator Photon Beam



undulator basics (1st harmonic shown only)

E166 undulator parameters

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Undulator Photon Beam


E166 undulator: photon spectrum, angular distr. and polarization E166 Proposal Presentation

K.P. Schüler

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Positron Beam Simulation

distributions behind the converter target (0.5 r.l. Ti)based on polarized EGS shower simulations by K. Flöttmann



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Low-Energy PolarimetryCandidate Processes• Photons: Compton Scattering on polarized electrons

– forward scattering (e.g. Schopper et al.)– backward scattering– transmission method (e.g. Goldhaber et al.)

• Positrons: all on ferromagnetic = polarized e- targets– Annihilation polarimetry (e+e- ) (e.g. Corriveau et al.) – Bhabha scattering (e+e- e+e-) (e.g. Ullmann et al.)– brems/annihilation (e+ ) plus -transmission (Compton) polarimetry

Principle difficulties of e+ polarimetry:– huge multiple-scattering at low energies even in thin targets– cannot employ double-arm coincidence techniques or single-event counting due to poor machine duty cycle – low energies below 10 MeV, very vulnerable to backgrounds

All of the candidate processes have been explored by us: the transmission method is the most suitable



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Transmission Polarimetry of (monochromatic) Photons

Pecomp

paircompphot

PP

0

M. Goldhaber et al.Phys. Rev. 106 (1957) 826.

all unpolarized contributions cancel in the

transmission asymmetry (monochromatic case)



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Transmission Polarimetry of Photons

monochromatic case

But, undulator photons are not at all monochromatic: Must instead use integrated numbers or energies

Analyzing Power:



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Transmission Polarimetry of Positrons2-step process:

• re-convert e+ via brems/annihilation process– polarization transfer from e+ to proceeds

in well-known manner

• measure polarization of re-converted photons

with the photon transmission method discussed earlier– infer the polarization of the parent positrons

from the measured photon polarization

experimental challenges:

• huge angular distribution of the positrons

at the production target: – e+ spectrometer collection & transport efficiency

– background rejection issues

• huge angular distribution of the re-converted photons– detected signal includes large fraction of Compton scattered photons

– requires extensive simulations to determine the effective Analyzing Power

formal procedure:

Fronsdahl & Überall; Olson & Maximon;

Page; McMaster



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Polarimeter Layout Overview



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Analyzer Magnets

g‘ = 1.919 0.002 for pure iron Scott (1962)

Error in e- polarization is dominated by knowledge in effective magnetization M

along the photon trajectory: 05.0/

07.0

ee

e

PP

P

active volumePhoton Analyzer Magnet: 50 mm dia. x 150 mm longPositron Analyzer Magnet: 50 mm dia. x 75 mm long



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Photon Polarimeter Detectors

Si-W Calorimeter Aerogel threshold Cerenkov



12

Positron Polarimeter Layout



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Positron Transport System

e+ transmission (%)through spectrometer

photon background

fraction reaching CsI-detector



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CsI Calorimeter Detector

Crystals: from BaBar ExperimentNumber of crystals: 4 x 4 = 16Typical front face of one crystal: 4.7 cm x 4.7 cmTypical backface of one crystal: 6 cm x 6 cmTypical length: 30 cmDensity: 4.53 g/cm³Rad. Length 8.39 g/cm² = 1.85 cmMean free path (5 MeV): 27.6 g/cm² = 6.1 cmNo. of interaction lengths (5 MeV): 4.92Long. Leakage (5 MeV): 0.73 %

Photodiode Readout (2 per crystal): Hamamatsu S2744-08with preamps



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Expected Photon Polarimeter Performance

62.0

07.0

0266.0

E

e

A

P

Si-W Calorimeter energy-weighted mean:

)(/)( EEEE

EA

Expected measured energy asymmetry

and energy-weighted analyzing power

determined through analytic integration and. with good agreement, through special polarized GEANT simulation

Aerogel Cerenkov See Table 12

all measurements very fast only syst. Error of should matter eP



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Expected Positron Polarimeter Performance

Simulation based on modified GEANT code

which correctly describes the spin-dependence of the Compton process

Photon Spectrum & Angular Distr.

number & energy-weightedAnalyzing Power vs. Energy

10 Million simulated e+ per point & polarity on the re-conversion target



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Table 13



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Analyzing Powervs. Target Thickness

Analyzing Powervs. Energy Spread



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Spin-Dependent Compton Scattering



POLARIMETRY of MeV Photons and Positrons Overview Beam Characterization – undulator photons –...

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