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This effect looks smaller than effect of the error in ground calibration procedure, but it affects all crystals. The change of asymmetry slope demonstrate substantial variations from crystal to crystal, but strongly correlated with the time drift of energy scale
for trhe same crystal, as shown on the following plot:
This correlation suggests that the drop of light yield is caused by increase of light absorption in the crystal due to radiation damage. In the crystal most sensitive to the radiation damage the systematic bias in longitudinal position is ~3 mm per segment.The evolution of asymmetry calibration in this crystal during first 1 year is shown on the following plot:
Each curve corresponds to period of 6 Ms (~2 month)
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- we can't use one calibration file for while 2 -year period, we have to update it every 2-3 month.
- 6 files were generated for the period august 2008 - october 2009
- we really need to generate asymmetry calibration for the period starting from October 16 2009 (since this date GCR files become empty due to software bug)
- no problems for earth limb data reprocessing - we can use August 2008 calibration file.
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