Current modelling of the background in the PCA is based on the rate of
events simultaneously triggering 6 discriminators in a PCU (There are 8
possible discriminators in each PCU including the 6 good xenon anodes, the
xenon veto and the propane layer veto.) A plot of the good xenon rate with
respect to the 6 discriminator coincidence rate shows that the distribution
is not single valued.
The points in this plot from one arm of this distribution are colored red to identify them in the following plots.
It was initially considered that electrons might be a source of significant
background but a plot of the good xenon rate with respect to the rate of
coincidences between the propane layer veto and either of the layer 1 good
xenon anodes shows that electrons are not a significant source of
background and that further they are not the source of the red colored
Again, the points in this plot with high electron rates are colored green to identify them in the other plots.
Finally, a plot of these data in geographic coordinates provides a clue to
In this image, the satellite moves to the right. The empty region it the SAA exclusion region where voltage is turned off to the PCA due to the high density of trapped electrons. The red points lie along the trajectory of the satellite as it leaves the SAA. In the region with Longitude less than 100 and Latitude greater than 0 it can be seen that when the satellite is on a northward trajectory (having just left the SAA) the points are red while on the southward trajectory, the points are not colored. The green points are concentrated in regions of high magnetic latitude.