Re: Why is FLUKA slower than MCNPX?

From: Alberto Fasso' <>
Date: Fri, 29 Jul 2011 15:37:45 +0200

If I am not mistaken, MCNPX is based on Class I condensed histories,
while FLUKA uses Class II. That means that MCNPX makes no distinction
between "soft" electron collisions and "hard" ones (i.e. production of
delta rays).
Because FLUKA produces and transports delta rays, of course it takes
more time. And if you score the electron current on the outer surface,
you don't get only the primary electrons, but also the delta rays.

You can check this by setting the threshold for delta ray production
higher than the source energy. (Use command EMFCUT with SDUM=PROD-CUT
and WHAT(1) larger than the maximum source energy). That will convert
FLUKA into a Class I code that can be compared with MCNPX.


On Thu, 28 Jul 2011, wrote:

> Dear FLUKA Experts,
> I am now making some intercomparisions of FLUKA and MCNPX. The simulation
> uses a Y-90 electron source with a 1 mm plexiglass + 1 mm lead shielding.
> The EM-CASCA option was used in the simulation. The cut values and
> production-cut are 1 keV for electrons and gammas.
> It turned out that FLUKA runs much slower than MCNPX. Is this normal or I
> should use some other options?
> I have recorded the electron current on the outer surface of the
> shielding. For the tally of the first channel (0 - 11.5 keV), FLUKA's
> result is much higher than MCNPX's. The FLUKA spectrum show a obvious peak
> at the first channel. Does it make sense?
> Thanks a lot!
> FLUKA Newbie
Received on Fri Jul 29 2011 - 16:13:36 CEST

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