Re: Biasing neutrons

 Hi Hantao,

   you use the biasing card to increase the statistics of the
   neutrons, that are back-scattered from your target: this is a
   problem, that should be studied with attention. In order
   to help you, it would be useful to have your input card, with
   the description of your target and of the detector you used for the
   scoring (I'm not sure that the input, that you have sent in attachment
   posting your question about the warning messages in the .err file,
   is exactly the same...).

   In general, there are some things that you can do, before the
   application of the biasing card, in order to properly set the
   biasing. First: you can study, using for example a USRBDX card at the
   boundary crossing between the rear part of your target and the material
   around (air, vacuum), how the neutron radiation in the backward
   direction comes out from your target (I would put a USRBDX card also
   at the front part of your target...).
   At this point you know more about your problem and you can optimize the
   dimensions of your detector in backward direction and, finally, optimize
   the biasing (for example acting also on the radiation produced in the
   forward direction...).
   So, please send your input card....

    cheers,

     Anna

> Hello, everyone,
> Recently, I calculate the back-scattering neutron spectra from targets
> by using fluka. I find that I just score few neutrons
> at the place of 2500 centimeters away from the target at backward
> direction. These results are useless due to too few events. I know
> that
> it is because of too few primaries, but at this time I have used
> 1000000 primaries(protons in my simulation). So I consider using the
> biasing card to obtain more neutrons, for example,
> * bias: set 1 everywhere
> BIASING 1.0 1.0 1.0 1.0 35.0
> PRINT
> * now bias in steps of 2
> BIASING 1.0 1.0 3.0 DE4 DE4
> BIASING 1.0 1.0 9.0 DE5 DE5
> BIASING 1.0 1.0 27.0 DE6 DE6
> BIASING 1.0 1.0 81.0 DE7 DE7
> BIASING 1.0 1.0 243.0 DE8 DE8
> BIASING 1.0 1.0 729.0 DE9 DE9
>
> The biasing card is working, but I still score the few neutrons. In
> fact I want to place a moderator for back-scattering fast neutrons
> at 2500 centimeters away from the the target at backward
> direction. Maybe there are three reasons for my problem, the first
> is too few primaries, the second is too small area for the
> detector, the last one is too far distant from the target to the
> detector. If I increase the primaries, it will need more cpu
> time. Now the 1000000 primaries simulation will spend half of a
> day. Someone can given me some experiences for dealing with this
> embarrassing situation. Thank you very much!
>
> Jing Hantao
> Jinght_at_mail.ihep.ac.cn
> 2008-11-10
>
Received on Mon Nov 24 2008 - 10:28:41 CET