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

[Zisis Papandreou]

Hi Andrei:

Yes, indeed, the cosmics are extremely important in understanding the 
behaviour of the SiPMs, to balance the gains, and in to compare their 
signals versus LED system.

The mirroring of the U and D boards was discussed by Fernando.  My take 
from that is that it is doable with some logistical impact but would not 
require any major modifications.  Changing to layer-wise biasing could 
have (significant) labour and time impact and would pose a major 
challenge in space (to accommodate extra cables if load balancing cannot 
be achieved).  Hearing Eugene give us the overview of lab activities and 
budgets, there is little to no appetite in getting extra money, and 
certainly the justification would have to be quite compelling, requiring 
more labour (just like the extra ring effort).

In order to clarify things, we should distinguish between cosmics bench 
tests and cosmics tests when the BCAL is in the solenoid.
For the former, although the current system may lead to more cosmics 
runs, there is nothing pressing on the schedule for these, assuming that 
we come to a consensus that there are no outstanding questions that 
require input from cosmics tests.  The only pressure is for the LED 
system, to get first article approval.  For bench tests, we can also set 
up suitable trigger counters if need be.

The barrel tests will be different in the sense that we will have 
definition of tracks from opposite sides of the calorimeter, in the 
initial stages, before the CDC is inserted.  And after the CDC is 
inserted, we will continue taking cosmics with both detectors working 
together; the CDC can give us precise tracks and we can verify particle 
patterns.  These with the solenoid turned off.  Once the solenoid is on, 
we can take cosmics to see curvatures, but we can also take them 
periodically when the solenoid is off for maintenance.  Always, we would 
compare with MC.

As for proton showers, their numbers are lower and we can study their 
signature. Since the inner layers will be on top (bench test) muons will 
punch through them and cells below (including any segmented counters we 
have under the mini BCAL, eg 4 counters, one under each column of 
SiPMs), whereas any shower near the upper/inner layer should give a 
recognizable pattern in the layers below it.  If the shower starts on 
the aluminum plate above, this should also be recognizable.  These are 
hand-waving arguments, that can be tied down via MC.

I agree many bias combinations are needed and we should plan these as 
efficiently as possible.  Runs should be priorized based on what input 
we believe is required quickly to answer or verify any outstanding 
questions.

I look forward to more discussions at coffee on Friday.

Cheers and thanks for all comments and ideas, Zisis...

> semenov at jlab.org <mailto:semenov at jlab.org>
> 22 May, 2012 6:39 PM
>
> 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
>
>
> Elton Smith <mailto:elton at jlab.org>
> 22 May, 2012 3:43 PM
> Hi Andrei,
>
> In order to keep of record of these discussions as well as make sure 
> no one is excluded from the conversation, I am copying this message to 
> the halld-cal listserver.
>
> I believe Zisis has answered the issue of bias distribution. It seems 
> you are proposing to do what we intended all along for the production.
>
> As far as TDCs, there is no problem in using saturated pulses into the 
> discriminator. (Of course the pulse to the fADC should not saturate). 
> These pulses effectively give you smaller rise times. The timing 
> resolution from leading edge discriminators is proportional to the 
> (pulse height/threshold), so the higher gain allows setting the 
> threshold to a higher value for the same time resolution. That gives 
> more flexibility.
>
> Thanks, Elton.
> Elton Smith
> Jefferson Lab MS 12H5
> 12000 Jefferson Ave
> Suite #16
> Newport News, VA 23606
> (757) 269-7625
> (757) 269-6331 fax
>
> semenov at jlab.org <mailto:semenov at jlab.org>
> 22 May, 2012 1:12 PM
> Elton and Fernando:
>
> I do not agree with Elton's plan, and I did try to explain my ideas in the
> last slide of my yesterday's talk (but it looks like nobody was willing to
> spend 5 more minutes to discuss very serious issues :)
>
> OK, one more time.
>
> The last paragraph in my talk was: "Gains in the timing channels should be
> big enough to provide enough efficiency on the low edge of the dynamic
> range with minimal stable thresholds of discriminators, and should be
> small enough to not saturate the signal shape and not to burn
> discriminators on the high edge of the dynamic range. The test of the
> chosen final timing settings on cosmics is required."
>
> To be specific, I propose to set the threshold of 50 mV as a conservative
> estimation of "minimal stable thresholds of discriminators"; with this
> threshold, the gain in timing channel should be the same as in FADC
> channel to cover the low edge of our dynamic range. Because we want not
> just triggering of discriminator (that is OK if the threshold is just
> below of the peak amplitude) but good timing (that requires the threshold
> to be at least in the middle of the pulse front), I recommend the gain in
> the timing channel 2 times bigger than the one in the FADC channel. Taking
> into account 2-Volt dynamic range of FADC, we will have 2x2V = 4V pulses
> for the events in the high edge of the dynamic range, that is not very
> small but (probably) manageable amplitude for the discriminator.
>
> If we go with Elton's plan, for the events in the high end of dynamic
> range, we will have pulses of 5x2V = 10V amplitude in the input of
> discriminator (sometimes even more), and I don't think this is a good idea
> unless we intend to burn our discriminators.
>
> I do understand the origin of the desire to put bigger signals to the
> timing channels: indeed for PMT operation, the bigger portion of charge
> from PMT anoge to the timing channel means more stable pulse front (just
> because of statistics) => better timing. Unfortunately, these
> considerations don't work for our case: we discussing the amplification of
> already existing pulse, and bigger gain just means bigger amplified
> unstable pulse (viz., the same shape, just bigger amplitude). No win here,
> just disadvantages (if we remember about pulse distortion because of
> saturation effects).
>
> Now about the bias distribution: Fernando, I'm terribly sorry but your
> statement on the meeting about the bias distribution was incorrect. If you
> have a look on pages 1 and 2 of your document "BCAL Readout Tests -
> Milestone" (GlueX-doc-1951), you will see that the bias distribution for
> upstream and downstream boards in Hall B test were exactly the same =>
> with U1,D1 bias (for example), the most inner biased cells look on
> different cells of BCAL (because of the mirror flip). Moreover (just to be
> sure that this is not some error in the paper but the real thing we had in
> the tests), see the event display (in the attachment) for the run 587
> (U1,D1). In the first (inner) layer, the biased upstream and downstream
> readout cells look on different areas of BCAL. If we will use U1,D2 biases
> (that we did not in the test), the inner layers will be OK, but the rest
> of the module will not be seen from both sides again.
>
> Again: I do propose change the biasing distribution to the layer-wise.
> If we will make 2 types of boards (viz., upstream boards with a mirror
> bias distribution in respect to the downstream boards), it will be better
> than our present situation, but there are 2 disadvantages here:
> 1. We will need to develop 2 types of boards;
> 2. In the cosmics tests, it's very useful to have whole layer powered (to
> select muon passage) but not just a half of the layer.
>
> Hope, you will agree with my arguments.
>
> Andrei
>
>
>
> Elton Smith <mailto:elton at jlab.org>
> 21 May, 2012 7:06 PM
> Hi all,
>
> We need to provide Fernando with input to the next (final) iteration 
> on the electronic boards, so these can be prototyped and tested in 
> preparation for production. The following changes are planned for the 
> next set of boards:
> 1. TDC gain (relative to fADC). Presently the boards have been 
> adjusted with a x5 gain (down from x10 for the original boards)
> 2. Operating point for the SiPMs (i.e. nominal overvoltage).
> 3. Electronic amplification of the fADC (depends on the operating 
> voltage).
> 4. Mechanical changes to the assembly to efficiently extract heat from 
> the electronics, including changes to the transition board.
>
> I think we need to provide this information soon, otherwise the next 
> iteration cycle will start impacting the overall schedule. Here is a 
> straw proposal for each of the above, which I offer for discussion and 
> comments:
> 1. I suggest we stay with the x5 amplification. We can take a look at 
> the runs with different thresholds, but we still  have quite some 
> flexibility in what thresholds to use, as we are presently using a 
> nominal threshold of 100 mV.
> 2. I suggest we use Vover=1.2 V. We want to make sure that we can 
> operate the SiPM over a range of bias voltages and this would allow us 
> to run between Vover=0,9 to 1.4 V spanning a fairly broad range. It 
> also gives us higher PDE with a nominal increase in dark rate. If we 
> run at 0.9 V, we loose PDE and I do not think we will want to run much 
> lower than this, so we would be limited at the low end of 
> adjustability. Yi and Serguei can comment on the dark rate/PDE trade-off.
> 3. Pick the electronic amplification for the Vover=1.2 V setting that 
> reasonably matches the dynamic range at the 5 deg setting.
> 4. These changes will simplify assembly and likely make heat removal 
> more efficient. They improve the overall design.
>
> Comments and feedback are welcome.
>
> Thanks, Elton.
>
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