Live time Calculations


This document discusses the two types of live time (trigger and energy) returned by Handel for XIA's x-ray products (excluding the microDXP). The intended audience for this document are users interested in understanding more about this particular data acquisition statistic. This document expects some familiarity with the decimation, thresholds and run statistics for the x-ray product line. (For more information, consult the user manual for any of XIA's x-ray products.)

If you want to skip the detailed discussion and have come here to learn which live time to use in your analysis, the answer -- in general -- is trigger live time. In reality, most users do not need to know the live time at all as the pile-up correction factor is based on the Input Count Rate (ICR) and the Output Count Rate (OCR), which are both returned directly by Handel:

Trigger Livetime

Trigger live time is measured by counting clock ticks when the primary trigger filter is below threshold and the detector signal is in range. For reset firmware, the fast filter is the primary trigger filter for the shortest peaking time range (DECIMATION 0); for the longer peaking times (DECIMATION 2 and greater), the baseline filter is the primary trigger filter. The RC-feedback firmware, which does not support a baseline filter, uses the fast filter as the primary trigger filter for all DECIMATIONs.

For DECIMATIONs greater then 0 it is possible, though not recommended, to disable the baseline threshold in which case the firmware falls back to using the fast filter as the primary trigger filter. For most running environments it is not necessary or desirable to disable the baseline threshold.

The primary trigger filter's main function, however, is to measure threshold crossings or triggers. Since ICR and OCR are defined as:

XIA is able to provide ICR as a quantity measured purely from the primary trigger filter.

Energy Livetime

Energy livetime, on the other hand, is a completely calculated value, as derived below. By definition,

From (2),

(3) is calculated while the run is active using the measured values of events, triggers and trigger livetime.

Which one to use?

As stated in the Introduction, you almost always want to use the trigger live time -- when you actually need a live time. If you are only interested in applying a deadtime correction factor to your data, use equation (1). It is worth noting that the deadtime correction factor using XIA's ICR -- measured to a high degree of precision with the primary trigger filter -- allows for excellent pile-up correction and accuracy.

So why do we even calculate the energy live time? Historically, analog processors have used the term "live time" to mean energy live time. If you want to compare the results of a system of analog electronics with an XIA system, you can take a preset run on both systems using a fixed energy live time length and the resulting data should be similarly scaled.