Beijing, Feb 3 rd, 2007 LEPOL 1 Low Energy Positron Polarimetry for the ILC Sabine Riemann (DESY) On behalf of the LEPOL Collaboration

Beijing, Feb 3rd, 2007 LEPOL 1

Low Energy Positron Polarimetry for the ILC

Sabine Riemann (DESY)

On behalf of the LEPOL Collaboration

Beijing, Feb 3rd. 2007 LEPOL 2

Outline

Low Energy Polarimeter (LEPOL)

Task within EUROTeV WP4 (Polarized Positron Source)

- Why & where do we need it ?

- Available processes Polarimeter options- Our suggestion: Bhabha polarimeter- Backup: Compton Transmission Polarimeter- Summary


Measurement of positron polarization at the source

Control of polarization transport

Optimization of positron beam polarization

Commissioning

Desired: non-destructive method with accuracy at few percent level


e+ Polarisation Measurement near the source

Polarization measurement measure asymmetries !

Find a process with• sensitivity to longitudinal polarization of positrons (electrons)• good signal/background ratio• significant asymmetryIn the energy range 30 MeV … 5000 MeV

Desired: non-destructive easy to handle fast (short measuring time)

e+ beam parameters e+ / bunch Ne+ 2·1010

bunches / pulse 2820Rep. Rate R 5 HzEnergy E 30 – 5000

MeVEnergy spread ΔE/E 10 %Normalized emittance ε*~ 3.6 cm

radBeam size σx,y ~ 1 cm


Considered Processes

• Compton Scattering (ex.: SLC, HERA)– Laser backscattering on beam– Preferred polarimeter option at IP – Not an option for the LEPOL (very small rates due to large beam size)– after damping rings beam size smaller this

option is under study by Tel Aviv U (G. Alexander) – But: very far from source

• Mott– Transverse polarized positrons– Mott~E-4 Moller~E-2 Bhabha~E-2

high background at relevant energies


Considered Processes• Compton Transmission (ex.: E166, ATF)

– Reconversion of e+ to in target– Polarization dependent transmission of through

magnetized Fe – Small, simple setup– Can deal with poor beam quality– Destructive Beam absorbed in relatively thick

target– Less efficient with increasing beam energy (E < 100 MeV)



• Synchrotron radiation (ex.: VEPP-4 storage ring) (S.A. Belomestnykh et al., NIM A 227 (1984) 173

– Transverse polarization needed– Angular asymmetries of synchrotron radiation in

damping ring– Relative simple setup– Non-destructive, non intrusive– Very small signal: Asymmetry < 10-3

– position far from source



• Laser Compton Scattering • Compton Transmission Experiment• Mott Scattering• Synchrotron radiation • Bhabha/Møller

magnetized iron target;

e- polarization in Fe: ~7%, angular distribution of

scattered particles

corresponds to e+

polarisation


Bhabha Polarimetry• As Møller polarimeter already widely used (SLAC, VEPP)• Cross section:

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

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

42422

42

20 coscos67coscos69

sin16

cos1

eePPr

d

d


Bhabha PolarimetryWorking point:

– After pre-acceleration 125 MeV – 400 MeV– First design studies done for 200 MeV

Used for simulations: Polarized GEANT4, release 8.2contact: A. Schälicke and collaborateurs,http://www-zeuthen.desy.de/~dreas/geant4


Studies for a Bhabha Polarimeter

Ebeam = 200 MeV

Ebeam = ±10%2·108 positronsang. spread =±0.5o

80 m Fe target

PFe = 100%

Pe+ = 100%

electrons(+) electrons(-)positronsphotonsafter Bhabha scattering


Regions of maximum asymmetry


Bhabha Polarimeter

50MeV < E < 150 MeV 20MeV < E < 150 MeV 0.04 < < 0.120 0.04 < < 0.120


Bhabha Polarimeter


Bhabha PolarimeterSignificance

-

Energy range: 50 – 150 MeV 20 – 150 MeV cosθ: 0.04 – 0.12 rad 0.04 – 0.21 rad


Photon distributions

cosθ: 0.08 – 0.4 rad no energy cut


Bhabha Polarimetry• Best significance using asymmetries of scattered Bhabha

electrons • Working point:

– After pre-acceleration and separation of e+ beam: 120 MeV – 400 MeV

• Asymmetries:

– Detection of scattered Bhabha electrons is sufficient

– Detection of scattered Bhabha positrons for checks

– Use of photons (annihilation in flight)

Conclusion for layout

• Separation of energy range spectrometer

• Separation of e+ and e- magnetic field (spectrometer

• Separation of angular range masks

• Target


~5m

e+

~2.5m

e+

Angular range large enough no bend needed to kick out the scattered e-,e+,

Detector size: O(40*60 cm2) signal rate: O(109) per second for 80 m Fe foil

detectormask, shieldingexit window (?)

spectrometer

higher energy, lower

lower energy, higher

electronspositronsphotons+background

Side view

top view


Magnetized Iron TargetHeating of the target -> Magnetization decreases

– Simulation for 30 µm– Cooling by radiation– TC (Fe) = 1039 K; melting point 1808 K

Ongoing considerations on target layout– ΔT ΔM ΔP ΔAsy– Magnetization (monitoring, tilted target?)– Cooling in real

Multiple scattering additional angular spread of ≤4%

Target temperature vs. time

Magnetisation vs. Temperature


Compton Transmission Method ?• Destructive !• Working point: Ee

+ < 100 MeVideal after capture section O(~30 MeV)

Dimensions O(1m) Experiences from E166, ATF

Thick Target (1 to 3 X0), with high energy deposition O(~kW) Small asymmetries O(<1%) at higher energies

Example: Ebeam 30 MeV, Pe-=7.92%, Target: 2X0 W, Absorber 15cm Fe

A(Pe+=30%) ~ 0.4% A(Pe+=60%) ~ 0.8%


SummaryRecommended for polarimetry near the source – but not yet tested: Bhabha

polarimeter at ~400 MeV

• In principle, the Bhabha polarimeter could work during ILC operation

• Backup possibility: Compton Transmission

• After DR: Compton polarimeter

To be discussed: - where will we need to check the e+ pol?

- when? Commissioning/ operation

type of polarimeter

Our plan:

Finalize the design study, think about target tests, suggest a polarimeter design

LEPOL Collaboration:

DESY: K. Laihem, S. Riemann, A. Schälicke, P. Schüler

HU Berlin: R. Dollan, T. Lohse

NC PHEP Minsk: P. Starovoitov

Tel Aviv U: G. Alexander

Documents

Beijing, Feb 3 rd, 2007 LEPOL 1 Low Energy Positron Polarimetry for the ILC Sabine Riemann (DESY) On behalf of the LEPOL Collaboration