Re: [fluka-discuss]: Flux as a function energy and time

From: Anna Ferrari <>
Date: Sun, 04 Oct 2015 14:59:08 +0200

Dear Ali,

I would like only to add something to the observations of Vittorio.

Considering the MCNPX language, I think that you want to score the fluence
rate (following ICRU definition fluence rate can be referred as particle flux
density, I come back later on this point). As Vittorio wrote, it is possible
to score it as a function of energy and/or time by using the USRYIELD card.
USRYIELD performs a double differential scoring with respect to 2 quantities,
ie and ia. Among the different possibilities you can chose kinetic energy
 (|ie| or |ia| = 1) and/or time (|ie| or |ia| = 33), but take care: for one of
the two only one interval can be specified.
This means that if you want N1 interval in energy and N2 in time, you have to
use for example N1 cards with the correct time binning, one for each energy
 interval you want to have (or, as alternative, N2 cards with the complete
energy binning).
USRYIELD can score a double differential fluence yield (i.e. weighted by
1/cos(theta) ) by using ixa=6 in the what(6) entry (see the manual).
This should correspond to the MCNPX scoring you indicated. Take care also
about the normalization: for example, with respect to energy, fluence results
are normalized "per GeV" (and not "per bin" as -as it seems to me to
remember...- MCNPX tally does... you find all this in the manual).

As last point, I would like to spend some words on the importance of a correct
language about radiometric quantities. In two post of 2006 Alberto Fasso' and
Alfredo Ferrari explained the common misunderstanding between
flux/current/fluence definitions. Maybe it is useful to remind it again here:

The recent (2011) Report 85 of the International Commission on Radiation Units
in chapter III, sections 3.1.2, 3.1.3 and 3.1.4, again defined flux, fluence
and fluence rate:
  - "The flux N is the quotient of dN by dt, where dN is the increment of the
particle number in the time interval dt, thus: N' = dN/dt [s-1] "
  - "The fluence, Phi is the quotient of dN by da, where dN is the number of
particles incident on a sphere of cross-sectional area da, thus:Phi = dN/da
[m-2] "
To have a deeper physical insight: "in dosimetric calculations, fluence is
frequently expressed in terms of the lengths of the particle trajectories. It
can be shown that the fluence Phi is given by Phi = dl/dV, where dl is the sum
of the lengths of particle trajectories in the volume dV (Papiez and Battista,
“Radiance and particle fluence,” Phys. Med. Biol. 39, 1053 –1062 (1994) )
  - "The fluence rate, Phi', is the quotient of dPhi by dt, where dPhi is the
increment of the fluence in the time interval dt, thus: Phi' = dPhi/dt [m-2
Flux and fluence rate are therefore different quantities and should be not
confused. To take into account, however, a long-term use of the term flux in
radiation transport literature, ICRU says in the section 3.1.4 that the
quantity fluence rate has been also termed *particle flux density*. This use
is however deprecated: "Because the word density has several connotations,
 the term fluence rate is preferred".

..hope it helps,


Am Sun, 4 Oct 2015 08:49:06 +0200 schrieb Vittorio Boccone
> Dear Ali,
> sorry for the late reply.
> A time integrated flux density is indeed a fluence, therefore to my
> understanding a fluence as a function of time it should be enough for you.
> If you want to get the flux as a function of time you will just need to
> evaluate the variation of the flux by a simple differentiation of your
> output results.
> So option 1 is USRYIELD (as you weren't very specific on the kind of
> problem you have to face). The point is that you will have a fluence (and
> not a current) therefore your estimator will always have the cos\theta
> factor coming from angle of the trajectory crossing the surface. If you
> have a problem where you want to count specific particles generated from a
> target you could easily use the *yield *(d_2 N/ dx_1 x_2) instead and
> normalize it to the surface of you boundary.
> Option 2 would be a fluscw routine customizing a USRYIELD (as a function of
> time and energy) or USRTRACKs (one per each time bin) scoring.
> V.
> On Sat, Sep 26, 2015 at 4:55 AM, Ali Koosha <> wrote:
>> Thank you Vittorio
>> I simply need “flux” as a function of energy and time. Simply I want to
>> evaluate f(E,t).
>> In MCNPX it is a matter of requesting the code to report the flux for both
>> of variables, e.g.,
>> f4:e 1
>> e4 0.0 98i 10.0
>> t4 0.0 98i 10.0
>> However, I can not find such an option in FLUKA. Indeed, even there is no
>> direct method to calculate flux as a function of time, e.g.
>> f4:e 1
>> t4 0.0 98i 10.0
>> I think, the last possibility is to write a user routine (possibly FLUSCW
>> or COMSCW), but you know that the devil is in the detail.
>> Cheers,
>> AK

Anna Ferrari
Institute of Radiation Physics
Helmholtz-Zentrum Dresden-Rossendorf e.V.
Tel. +49 351 260 2872

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Received on Sun Oct 04 2015 - 16:29:03 CEST

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