PMT timing offsets

The goal here is to set time offsets for all PMT's such that t=0 corresponds as closely as possible to the moment photons imping on the target.

First, I used the clean photon peak from a CF252 run to synchronize all the detectors. Here is what the timing spectra look like after these offsets are applied:

Next, I take the difference between per pulse normalized TDC spectra of empty target runs and Al runs. Next, a single offset is applied to all detectors so that the photon peak lies at 4 ns - or the time it takes light to travel the distance from target to detector.