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 GSO detector response function, we rely on GEANT simulations and results from tests undertaken at HIGS.
Detector Linearity
Apart from the response of the GSO detector, the PMT/base/GSO combination may have additional nonlinearities. CMU grad student Megan Friend has worked on mapping these out.
Identification of Cavity States
Correct identification of laser cavity states is crucial for computing Compton asymmetries, since Compton events only occur during cavity-on states and we use the cavity-off states to perform background corrections. A careful separation between events with left-circularly polarized photons and those with right-circularly polarized photons is also essential. Misidentifications will introduce systematic errors and dilute the asymmetries.
Cavity-state identification relies on signals from EPICS variables and real-time bits.
We recently discovered that the real-time bit meant to indicate whether the laser cavity is on or off is unreliable. This may be true only of the copy sent to the CMU DAQ, or it may also affect the original DAQ -- we don't know yet.
