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User name Parno/Franklin
Log entry time 16:34:12 on March10,2009
Entry number 265559
keyword=FADC Compton AsymmetriesGregg and I have been working on extracting Compton asymmetries from the FADC data taken over the past month. (The FADC has been running continuously since early in d2n, in parallel with the original Compton DAQ, so we have lots and lots of data.) We are also running with the GSO crystal in place as the photon detector.
The figure shows the Compton asymmetry, as measured by the FADC, from 00:30 to 13:24 on February 18, 2009. We chose this time period because there were three changes of insertable half-waveplate status in 13 hours, providing a nice check of the asymmetry sign.
Hall A was receiving five-pass beam during this time (an energy of 5.9 GeV), at a nominal current of 15 uA and a Wien angle of 22 degrees. We placed cuts on the data so that these asymmetries are only calculated for events where the current was between 14.99 uA and 15.16 uA. A Moller measurement taken the next day gave a beam polarization of about 74%.
The asymmetries plotted below are computed from the background (laser off)-subtracted signal in Accumulators 0 (the "All" accumulator) and 2 (the "Window" accumulator). The thresholds of the window accumulator were set to approximately 10 channels from pedestal on the near side and about 500 channels from saturation on the far side, so as to exclude pedestal noise and very small (as well as very large) pulses. We plot separate asymmetries for laser left and right states; these should be roughly symmetric about 0, and should flip sign with a change of the IHWP (as, indeed, they do.) The error bars shown are statistical only, and are calculated with the assumption that errors in the accumulator signals for different laser and helicity states are not correlated.
The timing of the IHWP changes is approximate and taken from the HALOG. We verified from the timestamps of the FADC runs that no single run spanned two IHWP states. The shortest run (60563) lasted 58 minutes and the longest (60562) lasted two and a half hours.
FIGURE 1
