Re: [fluka-discuss]: 10 MeV proton+Si, biasing neutron

From: Santana, Mario <msantana_at_slac.stanford.edu>
Date: Fri, 27 Sep 2013 18:11:47 -0700

Hi Yuanjie,

1) a factor of 0.005 may be OK, maybe towards the low side. Typically I
use 0.001 to 0.05, but it will depend on the type of reaction and
specifics of your simulation. In the past I saw some simulations crashing
when the factor was too low (e.g. 0.005 for photo nuclear reactions in
thin metals). According to the manual:

'8) Reduction factors excessively large can result in an abnormal increase
of the number of secondaries to be loaded on the stack, especially at high
primary energies. In such cases, FLUKA issues a message that the secondary
could not be loaded because of a lack of space. The weight adjustment is
modified accordingly(therefore the results are not affected) but if the
number of messages exceeds a certain limit, the run is terminated."

So check the output file to make sure the run will not be terminated once
you shoot for a long simulation

2) You already have a geometrical biasing step factor of 4. All you could
do is increase it to 5. If you increase it anyway further FLUKA will
reduce the factor to the maximum (5). Therefore you don't have much
potential gain from that side. I'd suggest that you split your shielding
in more layers. Ideally, you would need to choose small enough layers so
as to compensate for the neutron attenuation in the shielding with the
geometric splitting so that the population of neutrons (unweighted) would
stay constant.
Please note that when using strong biasing you will have large weight
dispersions so you may want to consider stabilizing your weight through
the weight window commands, otherwise the simulation may not be efficient
and the statistics may get ruined by tracks with very high importance.

Mario

On 9/27/13 12:50 AM, "毕远杰" <biyuanjie_at_tsinghua.org.cn> wrote:

>Dear fluka users
>
>I have 10 MeV proton+Si target, and a shielding of 50 cm concrete in the
>front. Since the low neutron yield( About 3.6e-6 particle/sr in the
>forward), I need to biasing the neutron.
>1) I use LAM-BIAS with a factor a 0.005, is that OK?
>
>2) I divide the shield to 5 pieces, and use region biasing for it. But
>still few neutron at the end of the shielding for 1e9 primary. Should I
>increse the basing factor?
>
>BIASING 0.0 4. shied1
>BIASING 0.0 16. shied2
>BIASING 0.0 64. shied3
>BIASING 0.0 256. shied4
>BIASING 0.0 1024. shied5
>LAM-BIAS 0.005 SILICON PROTON
>INEALL
>LAM-BIAS 0.005 SILICON NEUTRON
>INEALL
>
>Best wishes
>Yuanjie
>
Received on Sat Sep 28 2013 - 04:31:02 CEST