Zero boundary crossing result for a monoenergetic source in vacuum

From: Alberto Fasso' (Alberto.Fasso@cern.ch)
Date: Mon Apr 14 2003 - 15:30:09 CEST


The reason for your zero result is that you have asked for a scoring upper
energy limit which is IDENTICAL to the energy of your monoenergetic
source. Although in theory all photons should be inside the requested scoring
energy range, due to numerical rounding the compiler "sees" an incident energy
which is slightly larger (even if by a veryy small amount), and therefore
outside the requested energy range.
You can check this by replacing the line
USRBDX 4.5E-3 0.0 1.0 &
with
USRBDX 4.50001E-3 0.0 1.0 &
This minimal change in the upper scoring range is sufficient to obtain a
meaningful result.
This happens because you have simulated a very unrealistic problem, where
there is only vacuum an no matter at all. In practice, such a problem would
never arise, because most photons would lose at least a tiny amount of energy
before being scored. In any case, as I have shown, it is sufficient to add
some tiny margin when defining the scoring energy limits.

A similar problem was found by another user some time ago, where the rounding
affected the limit of an angular scoring range. In that case, however, the
problem was more serious, since pi (or 2pi, I don't remember) was an actual
natural geometrical limit, rather than one set by the user. For this reason,
the code has been corrected and the correction will be avilable in the next
release. In your case, a correction does not seem justified.

   Alberto FassÚ

On Tue, 11 Mar 2003, wuzhen wrote:

> Dear fluka users,
> In My example there is a point source which direction cosine with respect to
> z-axis is 1,which position is (0,0,0),and which energy is 4.5Mev. I want to
> get the fluence of a sphere which radius is 100cm. the number of energy
> intervals from 0.0Gev to 4.5E-3Gev is 1, and the number of angular bins from 0
> to 2Pi is 1. I think the result should be 1/(4.5E-3*2Pi). But the result got
> from fluka is 0. Please explain it to me. Thanks a lot. The following is my
> input file.
> TITLE
> Text
> DEFAULTS NEW-DEFA
> *---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----*
> BEAM -4.5E-3 0.0 0.0 0.0 0.0 1.0PHOTON
> BEAMPOS 0.0 0.0 0.0 0.0 0.0
> *---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----*
> GEOBEGIN 0.0 COMBINAT
> Text
> SPH 1 0.0 0.0 0.0 200.0
> SPH 2 0.0 0.0 0.0 100.0
> END
> *-reg-----or-----or-----or-----or-----or-----or-----or-----or-----or-----
> BL1 5 +1 -2
> AI2 5 +2
> END
> GEOEND
> *---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----*
> MATERIAL 82.0 207.2 11.35 17.0 0.0 0.0LEAD
> MATERIAL 26.0 55.847 7.877 11.0 0.0 0.0IRON
> *---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----*
> ASSIGNMA 1.0 1.0
> ASSIGNMA 2.0 2.0
> *---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----*
> OUTLEVEL 1.0 7.0
> *---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----*
> USRBDX 101.0 7.0 35.0 2.0 1.0 1.0PHOTON
> USRBDX 4.5E-3 0.0 1.0 &
> *---+----1----+----2----+----3----+----4----+----5----+----6----+----7----+----*
> RANDOMIZ 1.0
> START 1.0E+4 1.0E+8
> STOP
>
> best regards,
>
> Zhen wu
>

-- 
Alberto FassÚ
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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