DATA
The CAL routine (cal_mpc_calib_channel) takes the non-walk corrected
time (KPMT_PT1) and applies a correction by linearly interpolating between
charge points in the MPCA titles file. The MPCA time is stored in
KPMT_PTM. As of SNOMAN 5_0082 the first photon time spread is given by a function fit to the MPCA bank 1 data. This time spread is stuffed into KPMT_PTMS. The time
and estimate of the tube RMS are only valid if the tube was really
hit by multiple photons! If the multiple photon PCA is applied to
single photon data it will overcorrect the tube times, so be sure that
the data you are studying is really multi-pe.
MC
KPMT_PTM and KPMT_PTMS are stuffed in mc_fec. If the MC first photon time spread is smaller than the value of the function fit to data then the PTM time is smeared so that it is as large as it is for data. The value in KPMT_PTMS is the same in data and MC for the same input charge.
ENABLING/DISABLING THE CORRECTION
The multiphoton correction is currently disabled by default. It is enabled by setting the MPCA charge threshold to a value different from 0. This can be acomplished by adding the following line to your command file:
$set_mpca_threshold 1.5
With this setting KPMT_PTM will store the multiphoton PCA time for PMTs with charges greater than 1.5pe. If the PMT has a charge less than the threshold KPMT_PTM will be set equal to KPMT_PT.
NOTES:
The MPCA time should only be used on data that is clearly multi-pe. Charge alone is not always an indicator of the number of photons that struck a tube - a single photoelectron hit with a PCA calibrated charge of 5pe will have a time which is on average quite different from that of a five photoelectron hit with a charge of 5pe.
The MPCA correction is based upon gain calibrated charge. CAL will not produce a time for KPMT_PTM unless the PCA gain calibration is turned on (TCAL 1 word 4 to 54).
The MPCA correction uses the combined charge KPMT_QM, so the combined charge must be turned on ($combined_q_mode [mode]).