History: Livetime

+ The electronic or "front-end" deadtime deals with the effective masking of signals/triggers within the front-end electronics (PMTs, discriminators, etc.) before reaching the ADC/TDC/Trigger Supervisor. If a PMT fires such that the discriminator puts out a pulse 50ns long, and another signal comes within that 50ns it will be "hidden".

+ Let us take the "true" input trigger rate as -+~~red:m~~+-, and number of triggers in a period -+~~red:T~~+- is -+~~red:k~~+-, and the time the electronics are dead for each event is -+~~red:d~~+-. Then the relation between the real trigger count and the observed trigger count is (for the non-updating case):
+ ::~pp~m T = k + m (k d)~/pp~::
+ where -+~~red:m (k d)~~+- accounts for the number of triggers missed due to the electronics being "dead". Then the fractional "deadtime" -+~~red:D~~+- is:
+ ::~pp~D = ( m (k d) ) / (m T) = (k/T) d .~/pp~::
+ Since -+(k/T)+- is the observed rate, we see that, for relatively small deadtime-fractions and in the non-updating case, the fractional deadtime is linear in the observed rate.
+
+ For GEn, the electronic dead-time was measured by using a pulser to send an analog signal to the BigHand and BigBite trigger electronics to generate an artificial T3 trigger; these were the T7 events. A separate copy of the T7 signal was sent to the trigger supervisor and the 1877 TDC which recorded the relative timing of all input trigger signals. By looking at the time difference between the T7 and T3 signals in the TDC, one can look for the corresponding artificial T3 that should be present with each T7 and very tightly correlated in time, and therefore measure the fraction of the time the T3 is missing. This fraction is the "front-end" deadtime, ~~red:D~~.

+ ::~pp~LT_DAQ_T3 = prescale_T3 * N_T3_datafile / N_T3_scaler~/pp~::

+ How this is accomplished electronically is that an exact copy of the inputs sent to the trigger supervisor is also sent to a Lecroy 1877 TDC with a COMMON-STOP from the BB trigger. For the standard GEn analysis, these TDC channels are read into and stored as the D.bit1 through D.bit9 variables in the ROOT-file.