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The TDC threshold tuning procedure is described below. In the interest of brevity, here is a table of proposed
TDC values for flight:
The Details:
We want to set the TDC0-3 crossing thresholds such that each PMT triggers on a constant light level. Nominally,
these will be at:
THR0 and THR1:THR0 and THR1 look at the anode pulse. THR0 is typically set to 100 channels, corresponding to about 0.2 MIP, and is required to form the ZLo trigger. THR1 is set by choosing a TDC value that corresponds to the muon peak in ADC0. Since a muon is a minimum ionizing partilce, this sets TDC1 to trigger on a 1 MIP signal. To find the value for THR1 from the data, we plot the ratio of (# THR1 crossings)/(# THR0 crossings) versus THR1. This ratio should roll off from 1 (where THR0=THR1), to smaller values as THR1 increases. (See below for plots from each PMT.) To determine the value of the ratio that corresponds to a 1 MIP signal, a Landau fit to the ADC0 plot is used. We make the assumption that the (#THR1/#THR0) ratio should be the same as the ratio of the area under Landau fit of ADC0 from 0 to the peak to the area from 0 to infinity. More consicely: THR2 and THR3:TDC2 and TDC3 look at the 11th dynode pulse. Traditionally, with the ADC0 muon peak at 1000 channels, TDC3 is set to 100 channels, corresponding to about 4 MIPs. TDC2 is used in conjunction with the CD to form the ZHi trigger, and is usually set to around 20 MIPs. Both of these will have to be fine tuned in flight, but we can take a reasonable crack at them using the THR1 values determined above. We make the assumption that THR2 and THR3 will be larger (or smaller) than the goal THR value (600 for THR2 and 100 for THR3) by the same amount that the THR1 value for that tube is larger (or smaller) than the mean of all THR1 values. In equation form: Results:Results from the calibration runs on the ice using the methods described above are contained in the following spreadsheet (pdf OpenOffice). For those tubes with projected THR3 value less than 100 channels, a value of 100 is used to avoid triggering on noise.Note that the S3 PMTs do not show the same slow roll-off seen in the other PMTS. This indicates that the light level from S3 is very low, but the technique outlined should still locate the 1 MIP peak assuming that it is on-scale. For S3, the THR1 and THR3 thresholds are required for the S3Lo and S3Hi triggers. At the moment, I do not know what number of MIPs we want this to correspond to, but the 1 MIP signal seems to be 140 for both PMTs.
(# THR1 crossings)/(# THR0 crossings) Plots:                S3 PMTs   | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||