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Gain intercalibration procedure
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The procedure for CAL gain intercalibration at PS and SPS uses the [CalTuple|http://www.slac.stanford.edu/exp/glast/ground/software/status/documentation/EngineeringModel/latest/CalXtalResponse/latest/] of all 4-range readout runs taken at different scanning positions. *It is based on the signal (in MeV) stored in the CalXtalFaceSignalAllRange\[tower\]\[layer\]\[column\]\[side\]\[range\] array*. At SPS we checked the constants obtained at PS (towers 2 and 3), especially for HEX8 for which the lever arm in energy deposit was not so large. We also calibrated tower 1 for which LEX1/HEX8 was not well calibrated at PS. |
To intercalibrate gains of each given log end, we summed almost (see below) all runs available (to maximize the statistics) and fitted the profile histogram of R_i (signal for gain #i) vs R_i+1 (signal for gain #i+1) to get the slope. This profile is limited to the events where 1/ R_i does not saturate 2/ R_i+1 < saturation value of R_i and 3/ the ratio R_i/R_i+1 is close to 1 within 30%.
In addition, for the logs at the center of a module, we first ignored the runs corresponding to scan positions near their end to avoid direct illumination of the diodes due to beam and shower spread (in that case the scatter plots are a mess...). Despite these criteria, some weird features were observed, e.g. for the log end T2-L6-X5-S0 (i.e. tower 2, layer 6, column 5, side 0): in this file the 4 plots at the top are the R_i spectra (summed over all runs) in each energy range (R_0=LEX8, R_1=LEX1, R_2=HEX8, R_3=HEX1) while the 6 plots at the bottom are R_0 i vs R_i+1 (scatter plot on the left, superimposed on the linear fit of the profile histogram in red) and R_0/R_1 (distribution on the right), R_1 vs R_2 and R_1/R_2, R_2 vs R_3 and R_2/R_3. Two populations are clearly visible in the R_1/R_2 plot (with a difference of ~7% in ratio). The "bad" population was found to arise from one specific scanning position. Since other similar cases were found, new runs were taken at the same scanning positions, but the effect remains and is still not understood. However, we removed it using the following run selection:
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