The final offset which must be determined is the overall, or global, RF offset which determines the time-of-flight of the neutrons to the first plane. Obviously this offset is critical since this time-of-flight will determine the energy of the neutrons detected in the polarimeter. The gamma peak described above is also used for this calibration. Figure shows a raw time-of-flight spectrum for a target at .
Figure: Raw time-of-flight spectrum for the at a scattering angle of . The gamma peak, caused by pion production and decay in the target, is labelled.
It has been determined that, given the flight path of 160 meters, the gamma peak in the front plane should be centered on channel 12136 in the TOF histogram shown in figure . Therefore, as a final step to the calibration the global RF offset is adjusted in software to bring the gamma peak to the correct position. This calibration is done for every run in order to correct for slight shifts in the timing of the beam or the phase of the corrected RF signal.
A more detailed discussion of the calibration techniques used in the data replay for these types of detectors can be found in [Tad85].