[Halld-pid] [New Logentry] TOF PMT TDC response

[Alexander Ostrovidov]

On Friday, March 06, 2015, zihlmann at jlab.org wrote:
> 
> Logentry Text:
> --
> in the following 3 examples/PMTs of 3 different paddles are shown 
> The first plot shows in plane 1 (do not know at this point vertical or horizontal) where one can clearly see the main TOF TDC peak at about 400ns. 
> There is a small peak later at about 530ns which is about 130ns later. This would correspond to  about 40 feet of cable (only very rough guess)

Indeed, we measured the cable delay to be at 70 plus/minus a few nanoseconds 
per 50ft of cable leading to about 140ns delay of reflections.

> and could be reflections from somewhere and back to the TDC.
> However the second plot shows the right(-bottom?) PMT TDC response from paddle 16 in plane 1. Here we see a double peak structure (650ns and 800ns) after the
> main TOF TDC peak (about 400ns). Not this double peak structure is not see in the PMT on the opposite side of the paddle!!!!!!!

Interesting that this channel (South-15) is the only one showing such 
delayed double-reflections. But it is not just TDC. This is the same channel 
which developed an fADC problem toward the end of the run - see log 
entry https://logbooks.jlab.org/entry/3314060. Its pedestal has jumped 
from 200 to 260 fADC counts for no apparent reason. Something is not
right with this channel. May be, some DC offset from its PMT base or something. 
I'll be at JLAB on March 21 and test it. But it would be nice if someone can look
on the signal from S15 with a scope now.

> The next neighbouring paddle, paddle 15, with PMT right (-bottom?) does hardly show any structure after the main TOF TDC peak. only in a log plot one notices the little
> bump at 560ns more easily.
> I think what I want to say with all this one has to look at all individual 176 PMT TOF response histograms and identify possible issues one by one.
> 

On a different subject. Is anyone doing anything with TOF discriminator thresholds now?
I don't see any entry in the logbooks but I noticed a sharp drop in the scaler rates due to
increased discriminator thresholds. We at FSU are in dark here if this was spontaneous
change or someone is doing some kind of a study.

Sasha