Different events with identical kinematics

I am doing a beamline simulation, running particles (e.g., neutrons)
through a set of collimators. At various points along the way, I produce
a file of particle kinematics (X,Y,Z, momentum components, age, weight,
and Fluka case number). I am mainly interested in what gets through to
the end. (The particle source is actually a set of similar Fluka output
files from earlier stages of the simulation, with some selection criteria
imposed.)

During some post-processing, I noticed that many groups of events had
identical output kinematic data, differening only in the Fluka case
number. More specifically, I did 5 reclycles of the input data. For one
set of final position/momentum data, I had 2 identical outputs with
different case numbers in one of the cycles; the entire output including
the case number was duplicated into a separate cycle; some other cycles
had the same output data, with still different case numbers.

In looking at the history of such events, I could see that they had gone
through the collimator with identical kinematic values at each of the
inspection points. But the events prior to that were quite different.

I would not expect such similarities if the random number sequence is
anywhere near random. I start the 5 cycles with the default random
number, and the values between cycles are properly written. It seems very
peculiar to me. One would normally notice such duplication. I found it
because my post-processing imposed some very tight constraints on the
events (i.e., did the particles scatter from something upstream).

The issue is important because I count the number of particles that reach
the end of the line to determine production rates. I'm not sure how to
interpret the duplications.

Thank you
Joe Comfort
Received on Sun Jun 15 12:09:56 2008