Question about the neutron fluence scored by USRBDX

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
                                               
Received on Wed Sep 21 2011 - 14:35:25 CEST