The position of a hit in a detector cell ( and in figure ) is determined by the timing difference between the phototube signals from each end. Because the cell should be uniformly illuminated by cosmic rays the hit position spectra for each cell should be, roughly, a square box whose width corresponds to the physical length of the cell. Figure shows position spectra for all the cells in one plane.
Figure: Sample position spectra for all the cells in plan NC1 overlayed. The center of the cell is defined as x = 0.
The position offsets used in the software are adjusted so that the sharp edges of that ``box'' (determined by differentiating the spectrum) are equidistant from the center of the cell. The position offset calibration is extremely important if the detector is to be used as a polarimeter. In order to extract neutron polarization the scattering angle must be found which, in turn, requires that the position of the hits in both the front and back planes be determined accurately.