ADC spectra
As a follow up to our discussion at the previous meeting, ADC
distributions are shown for different LED thresholds and trigger
sconfigurations. They are for the Left channel of the long TOF paddle which
has the weakest-gain PMT. In the first 2 plots below, this channel was part of
the trigger at 60mV(left) and 120mV(right) LED threshold. Note that
TDC threshold is 30mV lower than trigger threshold in both cases
due to the way JLAB-LED works. Trigger paddles are under 90 degrees
to the long TOF paddle.
In the next 2 plots, this channel was not in the trigger. The right plot
is for the case of 90-degree angle between the TOF and trigger paddles.
About 85% of events do not have a TDC hit (i.e., they are useless for
time-walk corrections) and contribute only to a huge pedestal peak.
In the left plot, all 3 paddles were parallel to each other. This
plot nicely explains the origin of background underneath Landau
peak in the first 3 plots. With 90-degree rotation, there is a
significant number of cosmic rays that went through the top and
bottom trigger paddles but only scratched the edge of the long
TOF paddle. This cannot happen in the parallel configuration and,
therefore, the background is gone in this case.
Time-walk corrections
Different variations of Elton's suggested time-walk corrections
were tried with cosmic data from the 90-degree rotated setup. Three
different functional forms have been tried. In some fits, ADC
pedestals were treated as parameters to fit. Most important variation
was the quantity to be minimized. Two Elton's approaches have been tried:
1) fitting ADC vs t' for each channel individually; 2) minimizing
Sum[(tH-tV)2]. The results mostly looked
similar. However, minimizing
Sum[(tV-tVavg)2 +
(tH-tHavg)2] gave a
slightly better timing resolution than other approaches. Scatter plots
below show the ADC vs t' dependence before and after time walk corrections
for Left TOF channel.
There is a noticable difference with Elton's Monte Carlo, however.
After correction, his resolution for (t'L+t'R)/2
became 80ps - the intrinsic resolution of TOF he put into MC.
In my cosmic data, the resolution impoves only slightly as seen
on the plots below from 10.54 to 10.03 TDC counts, or from 265ps to 250ps.
Perhaps, variation in the angle of cosmic rays accounts for that: there may
be a few centimeter variations in path lengths, accounting for a few
hundred ps variation in time. Note that when time-walk parameters are
used with parallel-setup data, Gilles method gives about 130ps resolution.
Resolution was 110ps under similar conditions with CFD which means that
there should be some room to improve "LED+time-walk" resolution.