Polarimetry Report Sabine Riemann on behalf of the DESY/HUB group January 24, 2008 EUROTeV Annual Meeting, Frascati.

Polarimetry Report

Sabine Riemann on behalf of the DESY/HUB groupJanuary 24, 2008

EUROTeV Annual Meeting, Frascati

24 Jan 2008, EUROTeV Annual Meetiing, Frascati Polarimetry Report 2

Outline

ILC Positron Polarization: Pe+ ≈ 30% (baseline design)

Pe+ ≈ 60% (update)

E166 Proof of principle polarimetry at low energies

Status G4 with polarization Polarimetry at low ILC energies (LEPOL)

Update of Bhabha polarimeter studies Compton polarimetry after DR?

Polarized e+ source modeling Summary


E166 @ SLAC

Helical undulator:

energy and polarization of photons

Simulation of• Conversion into e+ e- pairs in thin W target, • polarization transfer to high energetic leptons

46.6 GeV e- beam


e+ polarimetry at low energies

• Mott polarimeter rotate longitudinal spin to transverse

• Bhabha (Moller) polarimetry (E(e+) > 100 MeV)

• Compton polarimeter need small beam size

• E166:Compton transmission polarimeter ( and e+)– Reconversion of e+ to polarized photons (bremsstrahlung,

annihilation)

– Transmission of photons through magnetized iron ( and ) Measurement of transmission with CsI calorimeter

– Polarization dependent Compton cross section asymmetry

– Method suitable for E(e+) ≤ few tens MeV

reconversion magnetized transmitted detector

target iron photons

e+


E166: Compton transmission polarimetry

• Measurement of transmission in CsI calorimeter (3x3 crystals)

good signal-background separation

» Asymmetry for parallel / antiparallel polarized iron

• e+ polarization• (PFe = 7%)

• Analyzing power Ae+ from simulation Geant4 with polarized prozesses

ii

iii

EE

EE

eFe

eAP

P


E166 results (preliminary)

E166 publication coming soon...

preliminary


Geant4 with polarized processes• Polarization is taken into account for

– Pair-production– Bremsstrahlung– Compton scattering– Moller/Bhabha scattering– Positron annihilation into 2 photons

• Included since Geant4 release 8.2 (Dec 06)– Main focus: longitudinal (or circular) polarization (extension to

transversal is foreseen)– Energy range of interest: MeV … ~5 GeV

• Applications:– Polarization transfer (target, E166)– Polarimetry (Bhabha/Moller, Compton transmission – e.g.

E166)– ILC polarized positron source

• Cross checks with EGS, WHIZARD/O’mega

Authors: A. Schälicke, K. Laihem, P. Starovoitov



Polarimetry at/near the e+ source(LEPOL)

• Requirements:– Suitable for low energy range– Suitable for large positron beam size– Fast– Non-destructive– Accuracy O(few %)

• Possibilities:– Bhabha polarimeter (125-500 MeV)– Compton – after DR (beamsize!)– Compton transmission (~E166): after capture section but only

for few bunches

• People:– DESY/HUB: R. Dollan, T. Lohse, S. Riemann, A. Schälicke, P. Schuler, A. Ushakov

Minsk: P. Starovoitov, Tel Aviv U: G. Alexander


Bhabha Polarimetry• As Møller polarimeter already widely used

• Cross section:

• e+ and e- must be polarized• maximal asymmetry at 90°(CMS) ~ 7/9 ≈ 78 %

Example: Pe+= 80%, Pe-= 7% Amax ~ 4.4 %

42422

42

20 coscos67coscos69

sin16

cos1

ee

PPrd

d

e+

MagnetizedIron

Detector


Bhabha Polarimetry Centre-of-mass system Lab system

Position for Bhabha polarimeter:– After pre-acceleration and beam separation: 400 MeV– Studies done for Ee+ = 400 MeV – Target foil heating (<30 m) is in acceptable limit (simulation)


Example: e- distribution and e- asymmetry

angular range of interest: 0.03 – 0.1 rad Asymmetry in the angular range: Ae- ~50 % (Ae+ ~5%, Aγ ~-15%)

Pz(Target) = -100%

Pz(Target) = +100%

• 30 μm magnetized Fe foil• Ebeam: 400 MeV (10 % spread)• Angular spread: 0.5º• 100% e+ and e- polarisation assumed

e- asymmetry (analyzing power)

e- distribution


scattered e- in detector plane

(beam: 1*109 e+, E: 400 MeV (10 % spread), ang spread: 0.5º, int. B-gield 0.1 Tm

asymmetry (analyzing power)

e- distribution

e- distribution

- run

+ run

Distance analyzing magnet – detector ~ 5m

Optimization is ongoing


Compton Polarimetry after DRLarge e+ beam size no Compton polarimetry before the DRAfter DR: small beam size, 5GeV, transverse polarization (DR…Turnaround) ~longitudinal polarization (spin rotation after turnaround) taking into account earth curvature


Compton Polarimetry after DRPossible option:

– Compton polarimetry after turnaround (5GeV)

• First considerations for (5 GeV): – see G. Alexander, P. Starovoitov, LC-M-2007-014

• Use laser of laser wireLaser wire:

• /2 crossing angle between e and • e beam size larger than laser beam waist

– Compton:• ~ crossing angle between e and • small e beam size for high rate

– Discussions with ‘instrumentation’ WP leader of former EDR started….


Polarized positron source modeling

• Activation and radiation damage studies(A. Ushakov et al., EPAC06: EUROTEV-REPORT-2006-052,

ILC07: arXiv:0712.2343)

• Production and tracking of polarized e+ at the source:– Use G4 (spin tracking) and FLUKA, ASTRA (acc.)– PAC07: PAC07-THPMN0– Optimization of capture efficiency and polarization

(see next slide)


Polarized positron source modelling


Polarized positron source modeling

• Recent study of beam displacement with photon collimator polarization is ‘robust’

• up to now `ideal’ undulator spectrum as input used• Fast helicity reversal

– RDR design not sufficient, – need helicity flip with frequency as for electrons


Summary & Conclusion• Lessons from E166 for low energy polarimetry

– Detailed simulations are essential – but time consuming

• Tools for design/performance studies:– G4 with polarization extension:

• Ongoing: validation, spin tracking, transverse pol.• e+ polarimeter:

– Compton transmission (few bunches, E < 50 MeV)– Bhabha (~400 MeV)

• Ongoing simulation studies for polarimeter layout – Compton (5GeV, longitudinal polarization)

• Further studies only in cooperation with machine and Lw people – Report on LEPOL will be delivered end of 2008– No prototype

• Source modeling – status report will be delivered end of 2008• Fast helicity flip to match the experimental precision for physics

– Design exists (seeSLAC-TN-05-045), work on alternatives is planned

Polarimetry Report Sabine Riemann on behalf of the DESY/HUB group January 24, 2008 EUROTeV Annual Meeting, Frascati.

Documents

e reconversion of e

e e pairs

e sourcelepolrequirements

e distribution

e asymmetryangular range

mevcompton polarimeter

polarization transfer

low energies mott polarimeter