Re: [fluka-discuss]: Biasing Air Activation

From: Alberto Fasso' <>
Date: Tue, 21 Apr 2015 02:07:17 -0700 (PDT)

Dear Felix,

a factor of 10^9 is certainly overbiased! A normal biasing factor for
photonuclear interaction is about 100: you could force it a little, but
certainly not more than 1000.
That you could not get any O-15 or N-13 with 5E9 electrons is not believable.
You must have done some mistake in your input (I think perhaps that you could
have forgotten the command PHOTONUC to activate photonuclear interactions).
A lot could depend also on the geometry of the problem: you say "a large
air volume (some m^3)", but what matters is not the volume but the thickness
of air in the direction of the beam. Could you please send your input file?
You could have made an error also in the command RESNUCLE. You say that you get
a good statistics of activation by photoneutrons: but where do those neutron
come from, if not from O-16(gamma,n)O-15 and N-14(gamma,n)N-13 reactions?

Apart from that, there is a much more efficient way to calculate air
activation. Score a differential photon energy spectrum (USRTRACK in the
volume of interest), and fold off-line that spectrum with the energy-dependent
(gamma,n) cross sections of oxygen and nitrogen.


On Mon, 20 Apr 2015, Felix Ernst Horst wrote:

> Dear FLUKA experts,
> I'm trying to determine air activation from photonuclear interactions in a
> bremsstrahlung field generated by 18 MeV electrons.
> I'm scoring via RESNUCLE over a large air volume (some m^3). For CPU time
> reduction i have set a high EMFCUT (8 MeV).
> I tried to bias the production of O-15 (production threshold 15.7 MeV) and
> N-13 (production threshold 10.5 MeV) with LAM-Bias, but even with a reduction
> of the interaction length for photons in air by a factor of 10^9 i still
> can't see any O-15 in the output and N-13 has still a large statistical
> uncertainty after 5E9 primary electrons.
> The activation products generated in the air volume by capture of
> photoneutrons have good statistics although i did not apply any biasing here.
> That is my LAM-BIAS card:
> Is a factor of 10^9 overbiased or still too low here?
> Are there any hints for biasing such problems?
> Thank you in advance and regards
> Felix

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Received on Tue Apr 21 2015 - 13:03:16 CEST

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