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There several ideas how to decrease the systematic uncertainties in absolute energy scale calibration of the CAL, noticed in data/MC comparison for 2006 beam test.

  1. We can use proton and GCR tracks crossing a crystal at small angles alpha to horizontal  (i.e. with big angles theta to vertical axis) to intercalibrate the peaks of energy depositions from nuclei with different charge:
    1. On the step 1, using the energy deposition of vertical protons as a reference, we measure the nonlinearity for energies 10-40 MeV and get the quenching factor for He
      • protons at alpha=90 degrees (theta=0) have most probable energy deposition 10.6 MeV in a crystal 
      • protons at alpha=0.25  rad (14 degrees) to horizontal plane have the most probable energy deposition ~40 MeV (very precisely measured for crystals in layer 0 because angle alpha is measured by tracker and multiple scattering in CsI is small yet)
      • by looking at the evolution of pathlength-corrected energy deposition versus full (uncorrected) energy deposition we'll get the measurement of CAL nonlinearity in the energy range 10-40 MeV, completely independent from charge injection calibration 
      • by finding the alpha angle at which protons have the same most probable energy deposition as vertical He we'll measure the quenching factor of He for on-orbit energy spectrum independently of charge injection calibration.
    2. on the step 2, using the same procedure and  comparing the signals of He for alpha angle  varying from 90 degrees to 14 degrees, we'll measure nonlinearity in the energy range 40-160 MeV and get the quenching factor for Li ions (with most probable energy deposition ~90 MeV)
    3. on the step 3 Applying the same procedure to Li ions, we'll get nonlinearity in the region 90-360 and quenching factor for Be (MPV~160 MeV) and B (MPV ~250 MeV)
    4. on the step 4 we reach 1000 MeV and will get quenching factors for C, N and O
    5. on the step 5 we reach 4000 GeV and and get quenching factors for all nuclei up to Si (MPV ~2.5 GeV)
    6. on the step  6 we measure nonlinearity up to 10 GeV and get quenching factor for Fe (MPV ~8 GeV)
    7. on step 7 we measure nonlinearity upto 40 GeV, or even upto 70 GeV (the end of dynamic range) if using minimum alpha angle ~0.1 rad (~5 degrees)
  2. Systematic uncertainty on each step is to be defined, but it is definitely  much less than 1%,  as we always compare energy depositions having exactly the same spectrum shape (just scaled). If this uncertainty will be on each step ~0.3%, the resulting uncertainty at 70 GeV per crystal  will be ~2% in the worst case.
  3.  Recently Eric Grove found a paper confirming and explaining "antiquenching" effect in CsI crystals:
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