NB: these are unedited notes

Sparse notes from morning session

  • Ghosts
    • look into CAL trigger info, probably only for high energy ghosts: it may be possible to look for a missing CAL-HI primitive
    • slide 14: yellow layers are inferred ghosts, i.e. layers that did trigger, while the other blue layers did not trigger (but had a request either in the TEM diagonistics or from a 3-in-a-row)
    • slide 16, comment from Bill: might be a residual activation from exiting the SAA, check with Neil Johnsono about average energy expected from such events; Eric/Luca: we have indications of such activations from several monitoring plots, found specific quantitites that display such behaviour
    • on ghosts from CAL (Steve): can we get ghosts in CAL by looking at the inconsistency between the readout energy and the selected readout range (e.g. an out of time ghost whose signal decays in time and latched a higher energy readout range)
  • CR Tracking
    • essentially available in GR; Leon: need to think about restructuring, we now have different type of tracks and need to decide which to present to subsequent steps; Bill: need to work on the ED too; Eric: need to find a way not to step on each other, like a key system in the TDS; Tracy: easy to do, need to change the track list from a vector to a map; Eric: need to change ACDValTools accordingly, it does not currently know about this. Leon: implementation issue, need to talk over this; Philippe: do we have a hint of how oftern this finds a new track? Bill: not really; Leon: it should find the same ghosts tracks that we find; Leon: there is a sense that adding all this will slow down recon, but it may not be so, since, e.g. tracks that are found by the CR tracking may not have to be reconstructed by the gamma track finder
  • Tree-based Track fitting
    • processing time for combo pat rec (current) is artificial and determined by choices in recon that limit the combinatorics
    • slide 25: currently no mechanism to merge two trees, so in the case of a photon converting high up the Tree-based track fitting misses the direction; Luca B: any bias from the method with the angle? Bill: namenly which method has a larger fish-eye effect?
      Bill: I b=think ultimately this approach will impact much the bkg rejection, since it gives a full picture of the shower develpoment, so I don't expect too much improvement on the PSF, but we can think to many metrics of the tree we can move to CTs to train them; incidentally, our bkg rejection sufffer at high energy, which is where this tool works best
  • PSF/Eric: I would say this is most you can do with Merit; Luca B: I scanned some 100s events, high energy photons > 5-10 GeV, and compared residuals of the TKR direciton vs the CAL direction, and found a better picture wrt my expectations, need to check; you do not need to scan many events where 1/2 xtal in the cal screw direction and centroid; Eric: this is not an effect of isolated xtals that pull the centroid, we are talking about longitudinal position being off and impacting the centroid measurement
    Eric: this plot tells me it is a matter of longitudinal position, things are fine in the middle of a tower, when you move closer to twr boundaries data and MC disagree more
    Toby: we should start always specifying what happens for Front and Back PSF separately
  • CTBCORE: this is mostly related to Pass7, but I want to discuss it here since we will have to face similar validations for Pass8.

Notes from afternoon Tagup

  • discussed more specific developments this morning, will need to think about knitting all these together
  • Tracy managed to compile Geant4 v9.3 in GR!
  • BT paper: Luca and Philippe discussed plans for the BT paper; it would be worth working on the results on energy scale from Melissa and on issues related to longitudinal CAL position measurement (aka CTBCORE issue) using BT data and try to add them to the paper; we think this would make the BT paper more interesting as opposed to just compiling a summary of quite old results and state conclusions unrelated to the LAT calibration
  • Leon: started scanning a skim of periodic trigger events with some people (Philippe, Luca B, Johan?) and found them to belong to two piles: 1 has an energy release consistent with MIPs, which is surprising, the other has completely uncorrelated tracks and energy deposits in the CAL - investigations are ongoing
  • several people installed and started workin on TMine (Steve, Luca L ?..) , and initiated discussions with developers (Eric, Alex)
  • discussion about CAL longitudinal position calibration: Sasha reports it is currently not applied as it requires a special handling a layer7 (question), since it has low stat; Eric and Sasha report that there we are currently using the same identical calibration for MC and recon, which implies that systematics from the real CAL are not in the MC and its response is bound to be better

2 Comments

  1. J. Eric Grove

    Ghosts

    • look into CAL trigger info, probably only for high energy ghosts: it may be possible to look for a missing CAL-HI primitive
    • slide 16, comment from Bill: might be a residual activation from exiting the SAA, check with Neil Johnsono about average energy expected from such events; Eric/Luca: we have indications of such activations from several monitoring plots, found specific quantitites that display such behaviour
    • on ghosts from CAL (Steve): can we get ghosts in CAL by looking at the inconsistency between the readout energy and the selected readout range (e.g. an out of time ghost whose signal decays in time and latched a higher energy readout range)

    Regarding Bill's question about activation in SAA, the dominant long-lived radioactive decay from activation in the SAA will be 128I from the CAL, which has a beta endpoint of about 2.1 MeV and a half-life of 25 min.  Those 2 MeV betas overwhelmingly range out in CAL, of course.  I don't know what gets activated in the W of the TKR, but we'll run that simulation here in the coming days.

    Regarding Steve's comment about energy and best range in CAL, see my email from yesterday, quoted here.

    On 25 May 2010 (Day 145), at 4:13 AM EDT, Steven Ritz wrote:

    Hi Neil and Eric:
    We're here at the Pass8 workshop, dealing with ghosts, and I was wondering if we have additional information from the CAL to help identify a subset of the ghost events.  In particular, we might look at consistency of the range selection and the measured energy.  You've probably already thought about this, but the idea is roughly as follows.  A large enough amount of energy is deposited to cause a higher range readout to be selected,  but there is no valid trigger.  The shaped front-end signal then decays away but, before the decay is complete, along comes a valid trigger.  The tail of the shaped signal is then digitized, making a ghost.

    So, when do the range selection discriminators fire and what is the one-shot level duration (if there is a one-shot)?  If they are "live" all the time, then this has a chance of being useful (a higher-energy scale is selected, but the ADC counts are very low and the pattern is inconsistent (the xtal is isolated); however, if they fire only at readout time (after a trigger), then it won't.  

    There is no one-shot.  It's a simple comparator that's latched when the GCFE state machine reaches HOLD after a valid TACK.  If there is residual shaped signal, it should appear equally in the range selection and ADC output.  I don't think that a ghost would cause a mismatch between range selection and ADC value.

    It could also be that if a higher energy scale is selected there should also be a CAL-LO trigger from that crystal.  That leads to another question, suggested by Luca B: maybe we can play a similar game with the CAL as we are doing with the TKR, looking for inconsistencies between the readout and the trigger primitive input bits from the TEM diagnostics.

    The CALLO (which at this level is called FLE) discriminator runs off the fast shaper, so indeed it's possible for FLE to fire when the output of the ADC would suggest that it shouldn't.  The signals are completely separate after the preamp.  I'd suspect that this will likely go the other direction, though: the slow-shaped output will be anomalously high, riding on the tail of a ghost, while the fast-shaped FLE will not have been asserted.  Remember, too, that trigger primitives are ORed together on a layer (so you only know layer-end, not individual GCFE) and that you'll need to compare a GCFE's ADC output to its corresponding layer-end trigger primitive (i.e. be sure not to mix or confuse the AFEE boards).

    We've done this sort of comparison to look for diode hits, where the fast signal in the Si is nicely shaped by the fast-shaped FLE but not by the slow shaper.  This leads to FLEs or FHEs that don't correspond to ADC output (i.e. unexpected assertion of FLE or FHE).

     Any thoughts you have in this direction would be helpful.  It is likely this won't work out, but it's worth thinking through at least once.  Perhaps you've already done that.
    Thanks,
    Steve

    J. Eric Grove

  2. J. Eric Grove

    You can see the 128I decays in the daily standard rate stripcharts from OSSE that I attached to the CAL Data Monitoring confluence page.  You can find an explanation of the OSSE standard plots on that page.  The 25-min halflife decays are visible in the 50-100 keV and 100-300 keV rate channels shown in two of the three bottom stripcharts.