Minutes of the May 25 Meeting
Present: Eduardo do Couto e Silva, Bob Hartman, Tune Kamae, Hartmut Sadrozinki
1) There were few changes proposed to the test beam photon matrix. From 17 to 24 MeV (depending on whether it is not a burden due the radiation safety since we need tritium), from 50 and 100 MeV to 70 (geometric mean between 20 and 200 MeV) and 200 MeV. One should bear in mind that changes are strongly dependent on beam availability. We also agreed that in the plan we will include phi angle runs especially for off axis runs. We start with 30 degrees and if schedule permits we will add more angles.
2) Tune presented results from Ted Fieguth based on an old experiment at SLAC that used channeling to produce the low energy photons. It requires a positron beam from 4 to 10 GeV to produce one photon per pulse with energies from 26 MeV to 120 MeV. Tune will gather more information for next week.
3) Hartmut presented a calculation of the FOV based on the GLAST proposal numbers. These indicate that we should take data at 30, 50 and 70 degrees. The motivation for 30 deg is because for this polar angle we expect most of the data, ~50 deg is needed because it corresponds to half of the effective area on axis, 70 deg (instead of 80 deg) so that we do not lose to much statistics. Energy dependence needs to be checked.
4) We had a note from Eric Grove on the calorimeter that emphasized on a few points. We need good tagging for low energy photons since we have not yet had a good energy resolution measurement at low energies (<100 MeV) . The low energy positron beam is most likely not going to be used for gain and linearities (since this is better measured in a hadron beam). Nevertheless the positron beam can be used for studying showers between towers and Benoit is preparing a test matrix and a presentation for next week's meeting. Tune mentioned that for Geant 4 they already learned how to correct for offsets in the center of the shower. This shall be incorporated in the future to address the discrepancies seen between Geant 3 and data.
5) Bob collected from Alex Moiseev a presentation on Backsplash from ACD (very much like J. Ormes' talk at IDT). Essentially what is needed is more knowledge on the backsplash as a function of distance to the tiles. It is clear that one should as soon as possible check Geant4 simulations against data from test beams. Few questions remain, how accurate is the empirical parametrization as compared to the MC simulations ? What exactly is needed for the beam test plan. We still need an ACD plan.
6) Eduardo presented a discussion about placing the entire LAT in a beam. While not practical for obvious reasons (cost, schedule and risk) we thought we should at least discuss the implications for not doing it. To trigger discussion Eduardo wrote a note that discusses the parameter space to be addressed: data throughput and asynchronous rate, energy depositions, hit multiplicity and distributions. These may not be easily simulated in a cosmic ray set up. However if we believe that the cosmic ray set up can address all these issues then we should document it and pass it through the DAQ and electronics group for revision.
7) We meet again next THURSDAY at 9 am PDT at VRVS SUN