# Re: simple problem with input attached

From: Francesco Cerutti (Francesco.Cerutti@cern.ch)
Date: Tue Aug 14 2007 - 18:04:56 CEST

• Next message: Yuanjie Bi: "the dose rate at a negative decay time"

Dear Jay,

> Right now I'm trying to create simulation that calculates the energy
> spectrum (i.e Neutron Energy vs Neutron Fluence) for a beam of neutrons
> 20 cm after interacting with a steel target. I'm not entirely sure which
> detector to use in this case, because I am getting slightly confused by
> the detector definitions in the manual. Could you help me decide which
> detector to use and possibly explain to me how to set it up in a simple
> way? I have used a usrbdx, with two adjacent cubes defining the upstream
> and downstream of the detector. Since that didn't work, I've tried to
> create a usrcoll detector defined by a simple vacuum region.

if you want the (double) differential fluence of neutrons entering a
given surface perpendicular to the beam direction at z=40 cm, you can
define that surface this way:

* body
RPP B4 -2.5 2.5 -2.5 2.5 40. 41.
[...]
* region
DETEC 5 +B4

and then use USRBDX with your VA2 region as the first region and DETEC as
the second one. The considered boundary is made by the 6 RPP faces, but in
reality the neutrons will enter only through the front face. Note that if
you change the x/y limits of the surface, in principle the spectrum will
change as well, since with a larger surface you will intercept neutrons
impinging with larger angles (and less energy).
On the other hand, if you want the differential fluence averaged over a
given volume (e.g., where a real detector will be put), you have to
use USRTRACK with the region defining the detector. Since USRCOLL involves
weighting with the mean free path, giving a quantity proportional to the
number of collisions in the volume, you will find always zero if the
material is vacuum.

By the way, in the input you sent, you disabled through FLAIR the B5 body
definition and then you use it. The same for the region named detector.
Moreover, you ask scoring over a neutron kinetic energy interval going
from 1 MeV to 110 MeV, but your 100 MeV/c (MOMENTUM) neutron beam has a
kinetic energy of about 5 MeV - falling into the multigroup treatment
region - .

Finally, a remark that may be of general interest. If you re-define a
pre-defined FLUKA material and assign to it a number GREATER than the
pre-defined FLUKA material number (it is not your case: 4<6), whenever
you refer to it through its name (i.e. ALWAYS if using FLAIR!) the
original one will be taken and not that you defined.

Cheers

Francesco

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

• Next message: Yuanjie Bi: "the dose rate at a negative decay time"

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