# MCNP: How to specify a small source in a large lattice

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1. Dec 6, 2014

### enakker

I am working on an input file in MCNPX/6 that uses a CT scan lattice geometry. I want to specify a small source in a large universe (lung). Right now I have a source uniformly distributed through the universe. The existing documentation is vague on this topic. Is it possible to contain the source in just a few of the voxels? Thanks for any suggestions or help!

Source definition:
SDEF PAR=D1 ERG=D2 CEL=D3 X=D4 Y=D5 Z=D6 $Particle dist Energy dist C SI1 L 34 3 3 34 SP1 .42 .25 .16 .17 C SI2 L 6.00 .7117 2.1318 7.69 SP2 .42 .25 .16 .17$ Particle energy short-lived
C
SI3 L (4<6<8) \$ 4 is the lung universe, 6 is the lattice unit cell, and 8 contains the lattice
SP3 1
C
SI4 H -25.048828 -24.658203
SP4 D 0 1
C
SI5 H -50.048828 -49.658203
SP5 D 0 1
C
SI6 H 21.625 21.875
SP6 D 0 1
C

2. Dec 11, 2014