Revision as of 10:16, 1 November 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 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 GeV, and θ=6 x 10^-5 rad in the lab.
• 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.
• Simulated 40K symmetric pairs with the fixed θ-angle and momentum in the lab, and uniformly distributed in φ.

A layout of the detector and the beamline for this simulations.

Some results

Positions of the hits on the detector. From left to right from top to bottom: Directional cosines C_y vs C_x of the original electrons and positrons, the locations of the hits on the face of the detector Y vs X, zoom-in for the electron Y vs X, zoom-in for the positron Y vs X.

• One can see the circles corresponding to the constant momentum and angle. The separation is one the order of 0.4 mm after the magnet.

If we do not use and B-field and leave the detector in the beamline this still will be the separations, B-field does not affect these circles.

• The background will become an issue. If we increase the converted thickness to 50μm the circle will collapse into a blob. We can try to very roughly estimate the background contributions assuming:

1. Converter
2. Silicon detector
3. Coherent-range γ-flux 10⁷ Hz, total flux through the target 10⁸ Hz

• Total background rate is:
• Total pair production rate is:
• We have ~50% background if we leave the detector in the beam line.
• We can reduce it to 5% background at running at signal counting rate of 20Hz (such a rate assumes 100% acceptance).

• Turning magnet on will get rid of the problem, but the magnetic field effects need to be dealt with. More work is needed for that but judging from the Reference 2 above this might be doable without a slit. (slit in this case would have to be less than 1mm wide which will interfere with the beam and not allow to sample the whole beam.