History of ToF Corrections for EVP Data

The ToF corrections are an important part of the COMPTEL data processing largely because a ToF selection is one of the fundamental data selections that is imposed to optimize the signal-to-noise of the data. The ToF correction scheme has evolved over the years. Here is a summary of the evolution of the ToF corrections using the nomenclature first introduced by van Dijk.

ToF-0

Raw ToF data with no corrections applied.

ToF-I

This level of ToF correction incorporated differences between minitelescopes and the dependence on D1 energy. Differences in the average separation of the D1 and D2 modules for each minitelescope, along with differences in cable lengths for each minitelescope, resulted in shifts of up to 20 channels. In addition, there is an observed dependence on both D1 and D2 energy. This first-order correction resulted in a forward peak at channel 120 and a backward peak at channel 80.

ToF-II

Even after applying the ToF-I corrections, there were observed to be significant variations in the forward peak position as a function of both D1 and D2 energy deposits. The ToF-II corrections incorporated these dependencies. The correction scheme to generate ToF-II data (developed by Boer and van Dijk) was incorporated into a routine known as TOFCOR.

ToF-III

Completely evised the ToF corrections. These corrections are based on the PMT pulseheights, rather than the derived D1 and D2 energy deposits. Hence, these corrections require the use of REM data. In addition, corrections were included to account for temperature variations of the detectors and the FCC, the pathlength differences between minitelescopes (and the module-specific electronics), the D2 hardware threshold, pathlength corrections based on interaction locations in D1 and D2, and the variations of the rise time of the signal as function of interaction location in D1. This particular version of ToF corrections is designed to align the forward ToF peaks of each minitelescope. (This scheme developed by van Dijk.)

ToF-IV

Similar as ToF-III, but the corrections were designed to align the backward ToF peak. This correction scheme was never applied within COMPASS. (This scheme developed by van Dijk.)

ToF-V

Similar as ToF-III, but the corrections were designed to align simultaneously both the backward and forward ToF peaks. This correction scheme was never applied within COMPASS. (This scheme developed by van Dijk.)

ToF-VI

This scheme starts with the ToF-III data and imposes an additional correction based on location in D2. Empirical correction factors were derived using data in several localization bins within each D2 module. The correction factors are contained in the COMPASS dataset type TCC. These corrections are not applied to events that are located in excluded regions of D2 modules with dead PMTs. At the present time (Jan, 1998), these corrections cannot be applied to about 5-10 percent of the data. The latest EVPRNN code assigns events which have these correction a CPL value (REFLAG) of 11. Events for which no valid ToF-VI correction could be applied have a CPL value (REFLAG) of 10. (The correction algorithm developed by Weidenspointner, COM-RP-MPE-DRG-174.)

Last Update: February 17, 1998