History: D.ctimeL1A

To link together the "times" measured with the Neutron Detector Electronics (relative to the L1A) and those measured with the BigBite electronics (relative to the BBtrig), it helps to understand the way the two trigger systems worked. In all cases, the relative time difference between the two reference times (common signals) are recorded by the Lecroy 1875A TDC corresponding to the D.ctimeL1A[0] variable in the TTree.



Times recorded in the neutron detector are relative to L1A, which means that:

ND_recorded_time = ND_real_time - L1A.

So to relate a time in the ND to something from BigBite, the "L1A" has to be replaced by "BBtrig", so one does:

  ND_time_relative_BB = ND_real_time - BBtrig
                      = ND_real_time - L1A + (L1A - BBtrig)
                      = ND_recorded_time + (L1A - BBtrig)

  ND_time_relative_BB = ND_recorded_time + 0.05*D.ctimeL1A[0]

Likewise, to get a BB time relative to the ND trigger to compare against some ND variable, just subtract 0.05*D.ctimeL1A[0].

The features of D.ctimeL1A[0]
The NeutronDetector generated triggers using an energy sum over multiple modules; this trigger is designated as T1. The BigBite spectrometer generated its own trigger signal, called T2, from a threshold placed upon the energy deposited in its calorimeter. The T1 and T2 signals were then passed to a coincidence unit, such that a logical AND of the signals formed the coincidence trigger T3. The leading edge of T3 was determined by when the overlap of T1 and T2 occurred. The trigger supervisor would then form the L1A with a time relative to the accepted trigger. The ASCII figure below tries to demonstrate this for a normal "good" T3 event.

 T1  ___________         _________________
               |        |
               |________|
                
 T2  ______                      _________
           |                     |
           |_____________________|

 T3  __________             ______________
               |           |
               |___________|

 L1A __________
               |                      
               |__________________________

Prior to the experiment, the relative timing of the T1 and T2 trigger signals into the discriminator were set such that T1 should come after T2, and so determine the time of the T3 and the L1A. The width of the T1 and T2 signals going into the coincidence unit 40-45ns for T1, and 95 to 100ns for T2 for kinematics 3 and 4 at least, with the T1 coming ~35ns after the T2.

Events were collected with a mix of other relative-timings between T1, T2, T3, and the L1A. First, if T1 came "too early" (perhaps due to an accidental coincidence), then the timing diagram looks like this:

 T1  ________         __________________
            |        |
            |________|
                
 T2  __________                       __
               |                     |
               |_____________________|

 T3  __________             ____________
               |           |
               |___________|

 L1A __________
               |                      
               |________________________

where T3 and the L1A are timed exactly relative to T2.

To generate the L1A's timing, a copy of the T2 and the T3 signal were sent to a retiming module. If the T3 came "too late" after the T2, the L1A would come at T2 + fixed_offset. So in the extreme case, if the T3 signal was missing or was very late, the diagram could look like this:

 T1  ______________________          _____
                           |        |
                           |________|
                
 T2  __                       ____________
       |                     |
       |_____________________|

 T3  ______________________             __
                           |           |
                           |___________|

 L1A __________________
                       |                      
                       |__________________


       |<-           ->|
          fixed delay

This results in D.ctimeL1A and D.bit3 distributions that look like in the following plot.