MCNPX Mesh Tally Problem: Offset of proton flux at surfaces

[Andreas Weber]

Dear all,
we are using MCNPX for the simulation of proton beam interactions at our proton therapy facility. But now we found a very strange behavior within a RMESH1:h flux tally:
In an even very simple geometry we see a constriction or rather offset of the Proton flux at surfaces (surface 1118, Proton beam is coming from left, see below) which are crossed by the beam. The file and a Screen shot are attached below. Does anybody know this Problem or even know how to solve it?
Thank you very much in advance,
yours
Andreas Weber

c simple case
c ++++++++++++++++++++++++++++++++++++++++++++++++++++++
c -- cell card --
1 0 1008:-1006:1082 $ outer world
2 3 -0.00126 1006 -1082 -1008 #3 $ air filled universe
3 3 -1 1118 -1119 -1008 $ water filled degrader

c -- surface cards --
1001 so 70
1006 pz 10 $ begin universe
1007 pz 10.02 $ beam starting surface
1008 cz 5 $ universe
1082 pz 65 $ end universe
1118 pz 48.5 $ begin water tank
1119 pz 50 $ end water tank
1120 pz 1.04
1121 pz 3.04

c
c ++++++++++++++++++++++++++++++++++++++++++++++++++++++
c DATA
c -- material cards --
C Material #1: Water
M1 1001 0.6666
8016 0.3334
C Material #3 air 2% water
M3 1001 0.0013
6012 0.0130
7014 0.7200
8016 0.2657
C -- Datenkarten ---------------------
mode h n e p
imp:h,n,e,p 0 1 1
c phys:n 75 0 0 -1 75 0 0
phys:h 75 0 0 J 0 J 0
c lca 2 1 1 0023 1 1 0 1 0
c lea 1 4 1 0 1 0 0 1
c leb
prdmp 0 5000000 0 1
C prdmp j -30 j 1
C
C -- Source Definition --
c
sdef erg=68 sur=1007 pos=0 0 10.02 vec=0 0 1 dir=1 par=h
c
tmesh
c
rmesh1:h flux
c
cora1 -5 149i 5
corb1 -0.2 0.2
corc1 0 324i 65
c
endmd

 

[Vailanor]

Flux Tally Number 1+---------------------+---------------------+-------------------+| | Mesh 1 | Total |+---------------------+---------------------+-------------------+| Energy Group Bins | Flux (1/cm2-s-MeV) | Flux (1/cm2-s) |+---------------------+---------------------+-------------------+| 1.000000E-08 - | 8.265705E+02 | 8.265705E+04 || 4.330000E-01 - | 6.068767E+01 | 1.566156E+03 || 1.000000E+00 - | 4.789483E+02 | 4.789483E+04 || 2.250000E+00 - | 6.845824E+02 | 1.539056E+05 || 5.120000E+00 - | 4.919091E+02 | 2.489546E+05 || 1.000000E+01 - | 3.229898E+02 | 3.229898E+05 || 2.000000E+01 - | 2.376786E+02 | 4.753571E+05 || 4.000000E+01 - | 1.381050E+02 | 5.524199E+05 || 7.622500E+01 - | 2.356605E+01 | 1.792852E+05 || 1.500000E+02 - | 5.344437E+00 | 8.016656E+04 || 3.000000E+02 - | 4.081477E-01 | 1.224443E+04 || 5.000000E+02 - | 1.788456E-02 | 8.942280E+02 || 7.000000E