[Halld-cal] BCAL SiPMs, adding channels, and noise

[Elton Smith]

HI Matt,

Thanks for bringing up these concerns. It clearly points to the importance
of Dan's work to develop the algorithms for our proposed readout scheme.
A realistic program will help us to develope the algorithms (or perhaps
guidelines) for sparcification of Bcal hits in the FADC. A second
important question which he should look into is which SiPMs are summed
into the various readout channels. We have briefly talked about this
during our weekly phone conferences. For example, for the "outer" region,
should we continue the tower structure of the "inner" region (i.e. 4x1
arrangement) or modify to a 2x2 scheme?

Note that the expected number of detected photons/GeV is somewhat higher
than originally projected based on single fiber measurements. Tests are
underway to verify these numbers with cosmics and a full module at Regina.
In the interim, Andrei and Irina have generated files with various shower
profiles (and number of photoelectrons in each cell) for 60 MeV and 2.5
GeV energy depositions at the ends of the Bcal (corresponding to limits
that determine the dynamic range). If you think these would be useful, it
might be good to generate showers at the center of the module for, say, 1
GeV showers.

Cheers, Elton.


On Wed, 14 Jul 2010, Matthew Shepherd wrote:

>
> Hi all,
>
> I wanted to follow up on a detail regarding dark rate in the BCAL and
> proposed readout scheme.  Dan Bennett has been working to code up the
> simulation correctly for the new scheme of doing an analog sum of SiPMs
> into a single FADC channel.  This has got us thinking again about these
> issues again.
>
> Recall that the readout threshold within a cell (formerly a single SiPM)
> was set such that for beam off events on average one would have 5% of
> all BCAL cells above threshold.  The 5% is a my guesstimate of what the
> DAQ can handle shipping off the crate and onto disk.
>
> When one goes to clusterizing level one achieves a nice background
> suppression since the clusterizer requires hits on both ends of a cell.
> The probability of this is 5%*5% -- small.
>
> If now we gang together three or four SiPMs several things happen:
>
> If we apply the former single SiPM threshold to the sum, as expected,
> many cells (collections of 3 or 4 SiPMs) are now above threshold.  This
> two implications:  (1) this is a problem for the DAQ since tons of noise
> data is transmitted from the crate across the backplane to disk and (2)
> this also a problem for the reconstruction.  As a simple test, with
> single 700 MeV photons one now reconstructs on average 18.4 photons per
> event in the BCAL.  With the old single-cell readout one gets an average
> of 1.2 photons per event.
>
> The naive solution is to simply increase the cell threshold by a factor
> of three or four (depending on how many SiPMs are being added).  We did
> this and, as expected, the average number of reconstructed photons
> returns to around 1.  However, I'm certain this will have a very
> negative impact on the physics.  The previous threshold was equivalent
> to about 7 MeV deposited in a cell -- a factor of 3-4 means a cell
> energy threshold of 20-30 MeV.  If we start throwing out all cells with
> this much energy then we can anticipate terrible resolution for low
> energy photons (which we know are important).
>
> If it is truly the number of cells and not the fraction of the cells
> that sets the DAQ limit (which is likely) then with a total of a factor
> of 3 fewer cells we should be able to tolerate a higher percentage of
> cells on.  For example about 15% of the current planned number of FADC
> channels is equal to about 5% of the former planned number of FADC
> channels.  This means we can lower the threshold and tolerate the higher
> fraction of channels above threshold, but the reconstruction also needs
> to be able to weed out these noise cells.  Previously we had a fake hit
> cell rate of 5% of 5% -- that would now go to 15% of 15%.
>
> The ideal solution is that the SiPM noise rate drops by a factor
> equivalent to the number of SiPMs that have been added together.  This
> is why I asked this morning if the specification for SiPM noise had been
> changed in light of the newly proposed readout scheme.
>
> I think the bottom line is that this needs careful study.  We can start
> by examining the assumptions that went into the estimation of the
> threshold -- this is documented in GlueX-doc 1231.  We need to verify
> that all those assumptions are still correct -- if they still suggest a
> 20-30 MeV cell threshold then I think we have a problem to solve.  One
> can think of an infinite number of solutions:  time multiplexing if
> SiPMs, forming an AND between cell ends in the FADC, etc.., but we
> should verify it is a problem first.
>
> Cheers,
>
> Matt
>
>
>
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