Re: Re: Why is FLUKA slower than MCNPX?

From: Alberto Fasso' <>
Date: Fri, 29 Jul 2011 17:54:34 +0200

> Thanks a lot for your answer!
> I think, if I am remember correctly ,the EM-CASCA option already turned off
> the delta-ray production.

No, you don't remember correctly. "Delta rays", in the case of electrons
are called Moller scattered electrons. Their production is regulated by
EMFCUT with SDUM=PROD-CUT, as I explained in my previous mail.

> In my case, if the production cut of electrons is
> large then the maximum source energy, the electron tally is always zero.

Wrong again. If the production cut of electrons is large than the maximum
source energy, the incident electron deposits its energy according to the
full stopping power, instead of the restricted stopping power plus delta ray
production. The energy loss is the same, but the energy deposition is
different: concentrated along the electron path instead of partly
deposited far from the path by the delta rays.
The electron tally (whatever you mean by that) will still include the
electron, although it does not include the delta rays.

> In addition, the very high number in the first channel still bothers me.

Sorry, I don't understand what you mean by "the first channel". Please

> When I use the following MULSOPT card, the first channel looks OK, but it
> needs even more time. How can I eliminate the "wrong" tally?
MULSOPT 0.0 0.0 0.0 1.0 1.0 99999999.GLOBAL

Again, I don't understand your reference to "the first channel".
The MULSOPT command you are showing switches on single scattering, and of
course the run will take much more time.


> Thanks again for your help!
>> -----原始邮件-----
>> 发件人: "Alberto Fasso'" <>
>> 发送时间: 2011年7月29日 星期五
>> 收件人:
>> 抄送:
>> 主题: Re: Why is FLUKA slower than MCNPX?
>> 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.
>> Alberto
>> 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 - 19:46:55 CEST

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