Re: FLUKA: About GEOTRK


To "FLUKA LIST @CERN" <fluka-discuss@listbox.cern.ch>
From Alberto Fasso' <Alberto.Fasso@cern.ch>
Date Wed, 7 Nov 2001 16:56:58 +0100 (CET)
In-Reply-To <3BE949E3.A864E7BB@in2p3.fr >
Reply-To "Alberto Fasso'" <Alberto.Fasso@cern.ch>
Sender owner-fluka-discuss@listbox.cern.ch

On Wed, 7 Nov 2001, Laurent Aphecetche wrote:

> Hi,
> 
> Following the nice documentation about "22} How to write, compile and
> link a user routine" A. Fasso sent on this list (I guess it was in
> January), I would like to use the GEOTRK entry (in geoden.f) to get, for
> instance, neutron fluences in a R-Z binning. I know USRBIN is able to do
> it, but the idea behind this choice is to be able to :
> 
> 1) have my own output of fluxes (directly within ROOT files) instead of
> usual fluka outputs.
> 2) compute a mean and a sigma for each value scored (to get an idea of
> statistical errors)
> 
> (BTW I did the same thing for the per-region scoring in GEODEN, and it
> worked fine).
> 
> Now my problem : I thought I just have to sum over the RULL variable
> (=track-length*weight = cm, right ?) in any R-Z cell I'd want to have,
> in case IJ=8=neutron, and then to divide by my cell volume (=cm^-3), to
> get my fluxes. But by doing that and looking at the output of a regular
> USRBIN, e.g. :
> 
> *        |_________|_________|_________|_________|_________|_________|
> USRBIN          11.0       8.0      71.0    111.47       0.0     187.0
> MODE
> USRBIN         11.47       0.0     -63.0       1.0                 2.0 &
> 
> I got different results. So my question is : what am I doing wrong here
> ? Is the USRSTB routine doing more complicated things than just a sum of
> RULL ?

In principle no. Or better yes, it does things like multiplying by
FLUSCW, and finding the address of the bin which has been dynamically
allocated: but this should not affect the result in your case.
The problem is, it is nice for a user to do his own scoring (or biasing,
or whatever), but it makes very difficult for us to debug it. There
are many points where one can introduce an error, and they are all outside
our control. This is exactly the reason why we encourage everybody to 
use as much as possible the standard scoring facilities, which have been
ultra-debugged by years of our own mistakes :-)

All help I can give you is to suggest some obvious checks:
1) Is the ratio between the standard results and yours constant for all
   bins? If yes, the ratio could point to a normalization error (wrong
   volume, wrong total weight of primaries). Fortunately you have only one
   bin in R, so all the bins have the same volume.
2) Check the same after having shifted by 1 bin your results in both
   directions (possibility that you have miscalculated the bin number). 
3) Some histogramming packages (I don't know about ROOT) don't work
   in the same way as the binnings of FLUKA in the case where the content
   of some bins is zero: they try to "fill" the empty intervals by 
   making bins of non-uniform width.
4) Are you scoring only total fluence or are you histogramming also versus
   energy or other variables? In this case, how do you compare with
   standard scoring? Be careful about the neutron group energy structure,
   ROOT will probably make energy histograms of uniform width, which are
   incorrect in this case: and when summing, you may get a wrong result.
5) FLUKA variables are all in double precision. With programs such as
   HBOOK and PAW it is necessary to convert the scored values to single
   precision before scoring. How about ROOT?

This is everything I can think about for the time being.

Alberto 



-- 
Alberto Fassò
CERN-EP/AIP, CH-1211 Geneve 23 (Switzerland)
Phone: (41 22) 767 2398    Fax: (41 22) 767 9480   Alberto.Fasso@cern.ch


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