Re: Question about the neutron fluence scored by USRBDX

From: Alberto Fasso' <fasso_at_SLAC.Stanford.EDU>
Date: Wed, 21 Sep 2011 06:44:01 -0700 (PDT)

Dear Lee,

the neutron fluence in a given volume is not the number of neutrons in that
volume, but the sum of all neutron paths in that volume. It is quite common
that a moderator such as polyethylene increases the "traffic" of neutrons.
Take into account that the same neutron can cross the volume more than once,
every time contributing to fluence.
If you have ever seen a neutron spectrum inside a shielding or exiting from it,
there is a thermal neutron peak much higher than the neutron fluences at other
energies.

Alberto

On Wed, 21 Sep 2011, lzfneu wrote:

>
> Dear fluka users,
>
> I want to calculate the neutron fluence after the thermal neutron
> (2.535e-11 GeV) penetrating through the polyethy. I used the USRBDX card
> to score the neutron fluence before and after the neutron penetrating. Two
> files were generated after the calculation, they were "jyx_usrbdx_21_sum"
> and "jyx_usrbdx_22_sum" respectively.
>
> The results are as follows:
>
> before thermal neutron penetrate through the shielding material, the total
> respond (in the file "jyx_usrbdx_21_sum") is ( --> (Part/pr)
> 1.002745 +/- 1.5818005E-02 % )
>
> after the thermal neutron penetrate through the polyethy, the total
> respond (in the file "jyx_usrbdx_22_sum") is ( --> (Part/pr)
> 1.802868 +/- 3.6832288E-02 % )
>
> I don't understand why did the neutron fluence increase after the thermal
> neutron penetrate through the polyethy or what's wrong with my calculation?
>
> Could you please help me and any help will be appreciated! Thanks in advance!
>
> Lee
>
> My input file is as follows:
>
> TITLE JYX
> GLOBAL 1. 1.0
> DEFAULTS CALORIME
> BEAM -2.53E-11 0.0 0.0 0.0 -1.0NEUTRON
> BEAMPOS 0.0 0.0 -0.1
> GEOBEGIN 0.0 COMBNAME
> 0 0
> * Black body
> SPH blkbody 0.0 0.0 0.0 10000000.0
> XYP v1 500.0
> XYP v4 -500.0
> YZP v2 4.0
> YZP v5 -4.0
> XZP v3 13.0
> XZP v6 -13.0
> XYP r4 0.0
> YZP r2 4.0
> YZP r5 -4.0
> XZP r3 13.0
> XZP r6 -13.0
> * sample thickness
> XYP r1 0.5
> END
> * Black hole
> BLKBODY 5 +blkbody -(v1-v4 +v2 -v5 +v3 -v6 )
> * air around
> air 5 (v1-v4 +v2 -v5 +v3 -v6)-(r1-r4 +r2 -r5 +r3 -r6)
> * sample
> Sample 5 r1-r4 +r2 -r5 +r3 -r6
> END
> GEOEND
> * 104 Air dry (near sea level)
> MATERIAL .00120484 AIR
> COMPOUND -1.248E-4 CARBON -.755267 NITROGEN -.231781 OXYGENAIR
> COMPOUND -0.012827 ARGON AIR
> * Polyethylene (C2_H4)n
> MATERIAL .94 POLYETHY
> COMPOUND -.143711 HYDROGEN -.856289 CARBON POLYETHY
> * ..+....1....+....2....+....3....+....4....+....5....+....6....+....7..
> ASSIGNMA BLCKHOLE BLKBODY
> ASSIGNMA AIR air
> ASSIGNMA POLYETHY Sample
> USRBDX 101. NEUTRON -21. air Sample 1.I0
> USRBDX 1E-14 1000. &
> USRBDX 101. NEUTRON -22. Sample air 1.I
> USRBDX 1E-14 1000. &
> RANDOMIZ 1.0
> START 1E6
> STOP
>
>
>

-- 
Alberto Fasso`
SLAC-RP, MS 48, 2575 Sand Hill Road, Menlo Park CA 94025
Phone: (1 650) 926 4762   Fax: (1 650) 926 3569
fasso_at_slac.stanford.edu
Received on Wed Sep 21 2011 - 16:58:51 CEST

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