1. Polarized Positrons at Jefferson Lab
Jonathan Dumas (JLab/LPSC), Joe Grames (JLab), Eric Voutier (LPSC)
Outlook
The Experiment
We investigate the idea of generating polarized positrons using the CEBAF polarized electron photoinjector.
- low energy (e- beam energy ≤ 8.5 MeV)
- bremsstrahlung photons
- low radiation level
- thin LOW/HIGH power target
A polarized positron source is intended for the ILC. E166 has demonstrated feasibility of a polarized positron source by pair creation:
- high energy e- beam (≈50 GeV)
- synchrotron photons
- thin low power target
Electron Beam Energy
Electron Driver
The goal is to measure electron momentum with precision better than 0.5%. Magnetic field map and operational range increased to 8.5 MeV to support this goal.
The design goal for the electron driver is to produce a high average current (1 mA), high polarization (85%) with energy up to ~ 10 MeV electron beam. The CEBAF photoinjector provides the necessary characteristics:
Electron Gun
GaAs/GaAsP superlattice photocathode
High QE ~1% at 780 nm (~6 mA/mW)
Test surface charge limit
High Polarization ~85% at 780 nm
=> Test polarization at high current
1.5 GHz fiber laser ( nm)
Test new 1.5GHz rf control module
100kV DC Load Lock Electron Gun
demonstrated lifetime >250 C at 1 mA
(J. Grames et al., in Proc. of the 2007 Particle Accelerator Conference, THPMS064, p )
=> Field emission is limiting lifetime
Photoinjector
“G0” high bunch charge demonstrated
MHz  ~2 mA at 1.5 GHz
Restore G0 setup
Test pulsed mode DF~0.1%
Consider 200 kV gun option?
Normal & SRF cavities => ~8.5 MeV
Target spot size set by quadrupoles
2008: Develop simulation tools and design experiment
2009: Build and install experiment beam line, demonstrate e+ production
2010: Characterize e+ (polarization, yield, energy and angular distributions)
Goal = Deliver simulation tools, experimental data, baseline design for e+ source
Electron Polarimetry
High precision (~1%) Mott measures electron polarization (at higher current) and calibrates positron polarimeter.
Transfer polarization from e- to e+
Positron Polarimetry
- Transmission Compton
- Well suited for low momentum
- Rapid relative monitor
Successfully used, e.g., E166
(P.Schuler et al., in Proc. of the 17th international spin physics symposium, p )
In a material, e- radiate gs (Bremsstrahlung). gs (>1.022MeV) can create a e+/e- pair.
Bremsstrahlung
e- longitudinal g circular
Pair creation
g circular e+ longitudinal
Polarization transfer depends on:
-e- energy Ee-
-g energy Eg
-Target atomic number: Z
-g scattering angle
Polarization transfer depends on:
- g energy Eg
-e+ energy Ee+
-Target atomic number: Z
-e+ scattering angle
Electromagnetic shower :
-Bremsstrahlung
- Pair creation
The target: a Tungsten foil
Particle Distributions
Particle distributions (scattering angle vs energy) after the foil for:
Tungsten foil:
- Thickness: 0.25 mm
Electron beam:
- 1 bunch (0.67 pC-1 mA at 1.5 GHz)
- energy of 5 MeV
- transverse size of 1 mm
Polarization transfer calculation for both interactions.
Olsen & Maximon, Phys. Rev. 114 (1957)
Use tungsten because:
High Z => greater EM shower
Low power target => Low cost
Thin target => better beam quality
Good thermal properties => extends deposited power limit
Considerations:
Positron yield (foil thickness optimization)
Power deposition (melting foil)
Geant4 simulation: polarization distribution at the creation vertex
Ee-= 5 MeV
e- longitudinal = 100%
Z=74 (Tungsten)
g scattering angle=all
Eg= 5 MeV
g circular= 100%
Z=74 (Tungsten)
e+ scattering angle=all
at the exitof the foil
Electron distribution
e- momentum ~ 4.7 MeV/c
Scattering angle < 30o
Photon distribution
Large amount of photons
Low energy
Positron distribution
Low momentum (0.5->2 MeV/c)
~58% at 2 MeV
Eg= 5 MeV
g circular= 100%
Z=74 (Tungsten)
e+ scattering angle=all
Target thickness=0.25mm
Ee-= 5 MeV
e- longitudinal = 100%
Z=74 (Tungsten)
scattering angle=all
Target thickness=0.25mm
Ee-= 5 MeV
e- longitudinal = 85%
Z=74 (Tungsten)
e+ scattering angle=all
Target thickness=0.25mm
Fig.7: Positron yield after the foil and power deposit in the target.