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
Beijing, Feb 3rd. 2007 LEPOL 3
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
Beijing, Feb 3rd. 2007 LEPOL 4
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
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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
Beijing, Feb 3rd. 2007 LEPOL 6
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)
Beijing, Feb 3rd. 2007 LEPOL 7
Considered Processes
• 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
Beijing, Feb 3rd. 2007 LEPOL 8
Considered Processes
• 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
Beijing, Feb 3rd. 2007 LEPOL 9
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
Beijing, Feb 3rd. 2007 LEPOL 10
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
Beijing, Feb 3rd. 2007 LEPOL 11
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
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Regions of maximum asymmetry
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Bhabha Polarimeter
50MeV < E < 150 MeV 20MeV < E < 150 MeV 0.04 < < 0.120 0.04 < < 0.120
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Bhabha Polarimeter
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Bhabha PolarimeterSignificance
-
Energy range: 50 – 150 MeV 20 – 150 MeV cosθ: 0.04 – 0.12 rad 0.04 – 0.21 rad
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Photon distributions
cosθ: 0.08 – 0.4 rad no energy cut
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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
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~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
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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
Beijing, Feb 3rd. 2007 LEPOL 20
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%
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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