[Halld-cal] Input to the next iteration on electronics

[semenov at jlab.org]

>> 2. I am not sure I understand the muon/cosmics requirement for
>> powering in layers.  If we want the whole inner layer on, we power
>> both lines 1 and 2.  We can then leave 3 and 4 off.  Bias 1 powers
>> cells 1 and 2, and the second row in the double sum (9-12).  Bias 2
>> powers cells 3 and 4, and the first row in the triple sum (13-16). So
>> there is no conflict in operating individual rows in sums. For muons I
>> would expect nominally all units to be on, with the exception of runs
>> were we want to sample the individual ones.  Layer biasing looks nicer
>> but I don't see what we lose as it is.  Obviously, the two
>> arrangements lead to different patterns (e.g the current system cannot
>> power the inner layer and the 1st row in the double sum at the same
>> time), but the current one respects load on line Fernando can probably
>> provide numbers on load. Am I missing something that cannot be done?

Zisis:

The cosmics tests are pretty good opportunity to balance the gains of
SiPMs (because of well-predicted and almost uniform energy depositions
from muons); but for these runs we need to provide the following
conditions:
1. It should be 1 active SiPM per readout cell,
2. we want to be sure that the cell to the left and to the right of the
cell of interest have no energy depositions => we want whole raw active,
3. We want to have many of these rows preferably distributed over whole
thickness of the BCAL to made a track.

All this is achievable with 4 runs with level-wise distribution of biases
(viz., U1-D1, U2-D2, U3-D3, U4-D4).

If we go for the present bias distribution scheme (with mirrored
distribution on the opposite side), we will need more runs:
U1-D1, U2-D2, U3-D3, U4-D4 to figure out the gains in the outer part of
the calorimeter, and U1-U2-D1-D2 and U3-U4-D3-D4 for inner layers (so, 50%
more time on cosmics that might be non-negligible).

But this is not the biggest problem. With U1-D1, U2-D2, U3-D3, U4-D4 runs
to calibrate 3rd and 4th readout layers, we will have no good "tracking"
of muon in the first 2 readout layers (only half of each inner layer will
be active) => we can not guarantee that we will work with muons but not
with protons producing the showers...

Elton: Discussing gain in the timing channels, I can not understand these
arguments in favor of working with saturated pulses. First of all, the
saturation is bad because your amplifiers are overloaded => extra heat
problems, failure problems etc. Secondly, the saturated pulses have 
distorted shape and (that is more important) this shape is unstable (for
example, the subject of the local load - viz., frequency and type and
energy and trajectory of the particles that came before and after the
particle of interest). Also I'm not sure how stable will be delay between
the input pulse and output pulse for overloaded amplifier. All this means
that we probably should forget about reliable and precise timing for the
biggest-energy-deposited particles in BCAL if we go for saturated pulses.
(Even time-walk correction will be a nightmare.)
And finally, I would like to see the faces of people who will read the
chapters in our papers where we will declare that we intentionally set our
detectors to work with saturated pulses when we had perfect opportunity to
work with normal pulses from non-overloaded amplifiers. I don't think that
any "flexibility" arguments will help us then...

Andrei