Flash ADC Timing

[Matthew Shepherd <mashephe@indiana.edu>]

Hi,

Richard, Mihajlo, Beni, and I had a short talk today to discuss what  
drives the FCAL FADC timing resolution.  I will try briefly summarize  
this below so that there is a record of this discussion.

The FCAL timing resolution serves two functions:

1.  to suppress clusters or blocks that are due to beam related  
interactions that are out of time with the pertinent physics event
2.  in certain classes of events, like all neutral forward particles,  
the FCAL timing resolution is needed to unambiguously determine the  
beam bunch that produced the physics event so that the relevant  
tagger hit can be identified

Resolution needed for 1 depends on the background rate in the FCAL  
and the feeling is that something on the scale of several ns is  
sufficient -- this is probably easily achievable and essentially  
"comes for free" with the FADC, but we have to be careful about  
getting too sloppy because:

Resolution need for 2 depends really on the tagger rate and the  
feeling here is that, due to the high rates, we really need quite  
pure tagging of the beam bunch.  This likely requires an FCAL event  
time resolution on the scale of a couple hundred ps; however, this  
does *not* mean a single channel resolution of a couple hundred ps  
(see below).

A rough sketch of the noise suppression/event building would be:

- trigger notices spike in energy sum due to all neutral event
- FADCs and tagger paddles are read out over some time window around  
the trigger
- processing of event offline removes noise of type (1) above by  
looking for hits out of time with energy spike
- times from all blocks that are in time with the physics are averaged
- average time is used to determine event time and select tagger signal

There is an advantage that these high-energy all-neutral events will  
light many blocks, each of which generates a (nearly) independent  
event time determination.  (Actually, the slightly varying cable  
lengths and transit times probably dither the signals across the  
synchronized FADCs.)  In any case, these can all be averaged to help  
determine the event time.  So the resolution must be good enough on a  
single channel to remove noise of type 1 and also, when averaged  
across many channels, the average needs to be precise enough for  
application 2.  We should be able to check this with a simulation  
which includes noise and then independently verify the resolution  
using a PMT and real FADC.  Our instinct though is that something on  
the scale of 800 ps - 1 ns (some sort of simple FADC algorithm) is  
probably sufficient.

I should add that this feature of the flash and FCAL was always  
central to discussions about a beam test (even before I got fully  
engaged at IU).  This was likely due to the tough experience with  
FCAL noise in RadPhi (which is a comparable environment to GlueX).   
Seeing a tight FCAL - tagger time resolution in a beam test using  
flash electronics would be great proof that this works for all  
neutral channels as advertised.

-Matt