Compton (d2n)
Contents
Background
The Hall A Compton polarimeter uses Compton scattering between the electron beam and polarized photons confined within a Fabry-Perot cavity to monitor the polarization of the electron beam. The Compton asymmetry between events where the electron and photon polarizations are parallel and antiparallel is proportional to the beam polarization <math>P_e</math>:
<math>A_{exp} = \frac{S^+ - S^-}{S^+ + S^-} = \langle A_l \rangle P_{\gamma} P_e</math>
The Compton cross-section is low enough that this polarization measurement can be taken simultaneously with a running experiment (e.g. d2n) downstream. Ideally, we can detect both scattered electrons and scattered photons in coincidence.
During d2n running, active Compton polarimetry was limited to the scattered-photon side (no electron polarimetry data are available). The laser feeding the Fabry-Perot cavity (and hence providing photons for Compton scattering) has a wavelength of 1064 nm and the power in the cavity was in the 400 W range throughout the experiment, requiring periodic re-tuning over time. The cavity runs in a cycle:
- Laser on, cavity locked, photons right-circularly polarized (~ 90 seconds)
- Laser off for background measurements and polarization switch (~ 30 seconds)
- Laser on, cavity locked, photons left-circularly polarized (~ 90 seconds)
- Laser off for background measurements and polarization switch (~30 seconds)
During d2n running, we had two separate DAQs running:
- Original (Saclay) DAQ, computing asymmetries based on counting rates
- New (CMU) FADC DAQ, computing asymmetries in energy-weighted, integrated signal
The photon detector was a GSO crystal, 6 cm in diameter by 15 cm in length, which was installed in the hall in December.
Calibration
Detector Response Function
The relation between the Compton asymmetry <math>A_{exp}</math> and the electron beam polarization <math>P_e</math> depends in part on the analyzing power <math>A_l</math>. Understanding how the detector responds to light is necessary to understanding <math>A_l</math>, and hence to arriving at a polarization number from a Compton asymmetry.
To understand the detector response function, we rely on GEANT simulations and results from tests undertaken at HIGS.
