MCNP6: Cell Volume Not Calculated - Assymetrical

[NeutronWrangler]

TL;DR Summary

Trying to run simple attenuation problem but one cell will not calculate the volume.

Hi,

I'm trying to simulate how much lead i need to put in front of my source to reduce low energy scatter from reaching my detector. I have 4 cells currently.
Cell 1: The source cell, im using a Cs137 source (0.662 MeV).
Cell 2: The collimator. I'm simulating a cylinder to be placed in the lead slab in the x-direction.
Cell 3: The lead slab.
Cell 4: The Attenuated Cell. There is where i'm measuring the flux after attenuation.

I keep getting errors/zero volume issues in Cell 3 with the collimator. If I take out the cylindrical hole, it runs fine so i figure i'm having issues with how i set up the geometry but i can't figure for the life of me whats wrong. I'm pretty sure its something so simple so maybe another set of eyes will help.

Here is the code:

Lead Attenuation Requirements
c ---------------------------------
c          CELL CARD
c ---------------------------------
1  2  -0.001225   (10 -11 14 -15 16 -17)       imp:p=1  $ Source Cell
2  2  -0.001225   (-18 11 -12)                       imp:p=1  $ Collimator
3  10 -11.34      (18 11 -12 14 -15 16 -17)    imp:p=1  $ Lead Cell
4  2  -0.001225   (12 -13 14 -15 16 -17)       imp:p=1  $ Attentuated Cell
9999 0            (-10:13:-14:15:-16:17)            imp:p=0  $ External Void
c Blank line to terminate cell card  

c ---------------------------------
c         SURFACE CARD
c ---------------------------------
c 
10 px -50.0  $ X-Boundary
11 px -0.55  $ Attentuator Width
12 px  0.55  $ Attentuator Width
13 px  50.0  $ X-Boundary
14 py -50.0  $ Y-Boundary
15 py  50.0  $ Y-Boundary
16 pz -50.0  $ Z-Boundary
17 pz  50.0  $ Z-Boundary
18 cx  0.1    $ Collimator Radius
c Blank line terminate surface card

c ---------------------------------
c          DATA CARD
c ---------------------------------  
mode p
phys:p
sdef pos= -30 0 0 erg=0.662 vec = 1 0 0
F22:p 11     $ Surface Flex after attentuation  
F32:p 12     $ Surface Flex entering attentuation
sd22 1.05    $ Multiply flux by volume to avoid getting per cm3      
sd32 1.05    $ Multiply flux by volume to avoid getting per cm3
F18:p 4      $ Tally Height in Attentuated Cell
F28:p 1      $ Tally Height in Source Cell
DE18 0.01 0.05 0.1 0.2 0.5 0.662 0.9 1.5 5.0 10.0 
DF18 1 1 1 1 1 1 1 1 1 1 
DE28 0.01 0.05 0.1 0.2 0.5 0.662 0.9 1.5 5.0 10.0 
DF28 1 1 1 1 1 1 1 1 1 1 
c PTRAC file=hdf5 type=p flushnps=1e6 filter=cell 4 event = col,sur
c -----------------------
c        MATERIAL CARD
c -----------------------
c LEAD
m10    82000.14p  1.0
c AIR
m2     6000.14p   0.000124   $CARBON
       7000.14p   0.755318   $NITROGEN
       8000.14p   0.231731   $OXYGEN
       18000.14p  0.012827   $ARGON
nps 1e5       $ particle histories cutoff
print60
c Blank line terminate material card

 

[Alex A]

Hi @NeutronWrangler,

Welcome to Physics Forums. The collimator is a flat bit of lead, 50x50, 1.1 cm thick, and with a tiny hole through the middle of 1mm radius. It's volume can be calculated manually as 50x50x1.1 - Pi*0.1*0.1*1.1 and then "vol=" added to the definition line for cell 3 with the answer. It's nice when it happens automatically but pretty common to need to do this for more complex shapes. Don't bother trying to 'fix' the geometry so that it works.

 

[NeutronWrangler]

@Alex A

Thank you so much. I'm new to MCNP and I thought about setting the volume manually but was worried that it would skew the results. This is helpful to know that i can do this.