MCNPX Mesh Tally Problem: Offset of proton flux at surfaces

In summary: E+03 | 8.000000E+03 || 6.000000E+02 - | 1.4000000E+02 | 9.000000E+02 In summary, 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:We see a constriction or rather offset of the Proton flux at surfaces (surface 1118, Proton beam is coming from left, see
  • #1
Andreas Weber
1
0
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
upload_2017-11-13_12-5-19.png

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
 

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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
 

1. What is the MCNPX Mesh Tally Problem?

The MCNPX Mesh Tally Problem refers to an issue that can arise when using the Monte Carlo N-Particle (MCNP) code to simulate the transport of particles, such as protons, through a material or system. Specifically, it occurs when trying to tally (or measure) the flux of particles at a surface, and the results do not match the expected values.

2. What causes the offset of proton flux at surfaces?

There are several factors that can contribute to the offset of proton flux at surfaces in MCNPX. One potential cause is improper meshing, where the mesh size or placement is not optimal for accurately capturing the particle flux. Another possibility is the use of coarse energy bins, which can result in an underestimation of the flux. Additionally, the physical characteristics of the system being simulated, such as scattering or absorption, can also impact the calculated flux at surfaces.

3. How can the MCNPX Mesh Tally Problem be resolved?

There are several techniques that can be employed to address the MCNPX Mesh Tally Problem. One approach is to refine the mesh, either by decreasing the mesh size or adjusting its placement in the system. Another strategy is to increase the number of energy bins used in the simulation, which can improve the accuracy of the flux tally. Additionally, adjusting the scattering and absorption properties of the materials in the system can also help to mitigate the offset of proton flux at surfaces.

4. Are there any limitations to using MCNPX for accurately tallying proton flux at surfaces?

While MCNPX is a powerful and widely used tool for simulating particle transport, there are some limitations to its accuracy when it comes to tallying proton flux at surfaces. These limitations can arise due to the simplifications and assumptions made in the underlying physics models, as well as errors or uncertainties in the input data used for the simulation. It is important for users to carefully evaluate the results and consider potential sources of error when using MCNPX for this purpose.

5. How can the results of MCNPX simulations be validated?

Validating the results of MCNPX simulations, including the tally of proton flux at surfaces, is a critical step in ensuring the accuracy and reliability of the data. This can be accomplished through a variety of methods, such as comparing the results to experimental data or using other simulation tools to cross-check the results. It is also important to thoroughly review and verify the input data and parameters used in the simulation to ensure they are appropriate for the desired outcome.

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