Polarimeter 11 01 2010

Preliminary evaluation of the options for polarimetry with nuclear pair production

References

• A photon beam polarimeter based on nuclear e⁺e^- pair production in an amorphous target
• Experimental study of photon beam polarimeter based on nuclear e⁺e^- pair production in an amorphous target

Considered Options

1. Put a an e⁺e^- detector in the beamline before the PS magnet allowing for ~2.5 m lever arm.

Pros

• No magnetic field distortions.

Cons

• Short lever arm.
• Detector is exposed to the photon beam.

1. Put a an e⁺e^- detector in the beamline after the PS magnet allowing for ~4.5 m lever arm, no B-field.

Pros

• Relatively longer lever arm.

Cons

• Detector is exposed to the photon beam.
• Small distortion effects due to incomplete degaussing of the PS magnet.

1. Put separate e⁺ and e^- detectors off the beamline after the PS magnet allowing for ~4.5 m lever arm, small $\int Bdl\sim 0.12$ T m.

Pros

• Relatively longer lever arm.
• Detector not exposed to the beam

Cons

• Magnetic field mixes different momenta and angles at the same position of the detector.
• Distortion due to the fringe fields.

Simulations

• Use GEANT4 based program to simulate the basic features. The geometry may have conflicts with the current Hall D beamline configuration.
• Symmetric Pairs are simulated with $P_{{z}}=4$ GeV, and $\theta =6\times 10^{{-5}}$ rad.
• All pairs pass through a converter which is 6μm thick, positioned at z=0.
• 0.3 T constant dipole magnetic field in vertical direction starting at z=2.3 m with the magnetic length of 40 cm
• Vacuum window 100μm at z=4.45 m (5cm before the detector)
• Some sort of detector made of silicon, 300μm thick. Consists of two pieces, left and right, for positrons and electrons.
• The positions of the hit e⁺ or e^- hits are calculated as the energy-weighed average of individually hits within certain window.