Re: [fluka-discuss]: Proton shielding

From: Francesco Cerutti <Francesco.Cerutti_at_cern.ch>
Date: Sun, 25 Mar 2018 20:32:01 +0200

Dear Valerio,

your input displays some basic misunderstanding in scoring and geometry.

For instance, in the region definition

SPHERE3 5 +vacsph -outsph -insph -target

the last two terms are clearly useless, since those bodies are contained
inside the second, which is subtracted (meaning that the region is the
intersection between the space inside vacsph and the space outside outsph)

Yours is not a geometry error, still it's an ill geometry definition.

Then, it does not seem to be true that you score before and after the
concrete. You always score on the outsph sphere, i.e. the boundary between
the region SPHERE2 and the region SPHERE3, as you ask for in your USRBDX
cards. And its area is about 10^9 cm^3, dramatically larger than the
varying (?) numbers you typed in the USRBDX area field. See below for the
angle actual meaning.

Moreover, you asked for a pointlessly excessive resolution in angle and
energy (what's the point for 500 solid angle bins over an already limited
solid angle interval?). By the way, this is also the reason for the
ALL-PART error when post-processing the results. In fact, in its Output
frame, Flair reports the error message from the post-processing utility
usxsuw (applying to USRBDX results):

Subscript out of range on file line 237, procedure usxsuw.f/MAIN.
Attempt to access the 300001-th element of variable gmstor.

500 angular bins times 500 energy bins yield 250,000 elements, which is
still less than the 300,000 limit mentioned above, but in case of ALL-PART
one has to add to those also the ones of low energy neutrons, which are
bound to a fixed energy grid below 20 MeV (conversely, in case of NEUTRON,
these predefined energy bins are used between 1 MeV - your scoring lower
limit, why? - and 20 MeV, while above 20 MeV your (210 - 1)/500 = 0.418
MeV binning applies, and this eventually does not break the 300,000
limit).

I have the impression that you missed the meaning of the USRBDX angle.
This is the angle between the particle direction at the crossing point
with the scoring surface and the normal to the scoring surface at the same
point. In your case the normal is the radial direction, obviously varying
over the sphere, implying that a particle crossing perpendicularly the
sphere, no matter where, has angle zero.

I suggest you implement a suitable USRBIN fluence (as well as directly
ambient dose DOSE-EQ if you like) scoring, to visually appreciate the
fluence attenuation, as well as a USRYIELD scoring (where the angle rather
refers to the beam direction), the latter applied both to the surface
before the concrete (boundary between SPHERE1 and SPHERE2) and the one
after the concrete (boundary between SPHERE2 and SPHERE3).

Kind regards

Francesco

**************************************************
Francesco Cerutti
CERN-EN/STI
CH-1211 Geneva 23
Switzerland
tel. +41 22 7678962
fax +41 22 7668854

On Fri, 23 Mar 2018, Valerio Santoro wrote:

>
> Dear Fluka experts,
>
> I am trying to simulate a concrete shielding for a proton beam of 200 MeV impinging on an iron
> target. I am using the point-source line of-sight model, building a concrete sphere around my
> target. This simulation aims to score the fluence before the shielding and outside the
> shielding, in order to compare the results with the analytical Moyer's model. Outside my
> shielding I create a sphere filled with vacuum, in order to create a scoring region outside the
> shielding. What I am doing is to estimate the neutron fluence, function of the angle in sr in
> the USRBDX scoring option, in order to get the fluence in each bin from the tab_lis output file
> to convert in ambient dose if it is possible. I am suppose to do a "simple" simulation, just to
> find a kind of agreement with the literature, but from my results I get the following:
>
> 1. When I try to score in USRBDX the fluence of All-part, I get an error, I can only see
> neutron fluence plots;
> 2.  The values provided in the tab_lis file are the same before the shielding, and after the
> shielding.
>
> Probably different things in the set-up are wrong, in the definition of the geometry, or I have
> to activate other cards for biasing or for neutron transport, but could anyone help me to fix
> the errors, in order to solve the problem in a simple way? I attach the input and flair files,
> thank you very much in advance,
>
>
> Valerio Santoro
>
>
>

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Received on Sun Mar 25 2018 - 23:00:39 CEST

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