to Dave's research index.
Relative Thresholds from TOTs
10 May 2001
to Dave's research index.
Please see the latest report on this analysis.
Albrecht and I have developed a new method for determining the relative threshold (or the amplitude of the prompt pulse) from the TOT information. This method, and the information it provides, would be useful for AMANDA-II simulations. Previously, the only way determine the prompt pulse amplitude was by measuring the ratio of the prompt to delayed amplitude (see Steve Barwick's message on ABS under Data Related -> Delay/undelay calibration 2000 ). If this new method is successful, it can be used on previous years' data for which no direct measurements of prompt vs. delayed amplitude is available. The method can also be used to determine a possible time dependance of relative threshold from the data.
We have compared our results to the average amplitude measurements taken during the summer 2000-2001 season. The relative thresholds as deduced from the measurements disagree with the those calculated by our method by 30 to 50 percent, and the reason why is unclear. The discrepancy could have a large impact on AMANDA-II simulated 1 p.e. detection efficiency and thus must be resolved.
Our goal is to determine the most probable relative threshold for each channel from it's TOT distribution. Relative threshold is simply absolute threshold / amplitude. The method is straight-forward. We start by determining the relative threshold of a pulse as a function of TOT. Then, using actual data, we book a histogram of TOTs and record the most probable TOT for each channel. Finally, using our calculated function, we can determine the most probably relative threshold.
To determine the relative threshold as a function of TOT, we assume that all standard AMANDA-II optical fiber pulses are identical in shape. Furthermore, we assume pulses vary only in their amplitudes, and are not stretched on in the time axis. We believe this to be a reasonable assumption because each electron propagates through the dynode stack independently.
I obtained a digitized average pulse from Stephan's AMANDA-II webpage, and wrote a program which calculates the relative threshold vs. TOT numerically. Shown below are plots of the pulse and the calculated threshold vs. TOT
Using 2001 minimum bias data, I plotted the TOT distributions for individual AMANDA-II modules. A few samples are shown here.
For each AMANDA-II module on strings 14, 15, 16, and 19, my kumac picks off the most probable TOT and then computes the relative threshold using the calculated fit. The results are available in this ascii text file.
Below is a histogram of the resulting relative thresholds compared to the measurement taken in summer 2000-2001. (To obtain the relative threshold from the measurement, I used 50 mV/Fast Pulse Amp. ). In this plot and subsequent ones, modules for which there was no measured value were removed, however, no extra care was taken beyond this to remove noisy/anomolous modules.
The new method gives an average relative threshold of around 0.32 whereas the measurements indicate a lower relative threshold of around 0.18.
Below is a plot of measured vs. calculated threshold. It is not entirely clear whether it is correct to compare this calculation to this particular measurement. The measurement relies on average pulse amplitudes whereas the calculation relies on most probably TOT. There does appear to be some weak correlation, but better agreement, or an explanation for their disagreement is certainly desired.
to the research index.