Re: Different events with identical kinematics

From: Alfredo Ferrari <>
Date: Mon Jun 16 2008 - 08:40:36 CEST

Hi Joseph

are those identical events originating from neutron interactions below 20
MeV? If so it could well be you get identical (or almost) final states
because of the discrete nature of the group treatment and associated P5
expansion of the angular distribution. That is, given a neutron of energy
E0, it has a finite number of outgoing energies possible, and each one has
three possible polar angles (azimuthal being random).


  On Sat, 14 Jun 2008, Joseph Comfort wrote:

> 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

|  Alfredo Ferrari                ||  Tel.: +            |
|  CERN-AB                        ||  Fax.: +            |
|  1211 Geneva 23                 ||  e-mail:  |
|  Switzerland                    ||                                   |
Received on Tue Jun 17 09:24:39 2008

This archive was generated by hypermail 2.1.8 : Tue Jun 17 2008 - 09:24:40 CEST