MCNP Guide
Source Definitions in MCNP
Defining where and how particles enter your simulation
Understanding Sources
Sources tell MCNP where particles originate and their initial properties. The SDEF card (source definition) controls particle type, position, energy, and direction. Without proper source specification, even perfect geometry and materials won't produce meaningful results.
Basic Source Structure
c Simple neutron point source
SDEF PAR=n $ Particle type (neutron)
POS=0 0 0 $ Position (x,y,z)
ERG=2.45 $ Energy in MeV
c The four essential parameters:
c PAR = particle type (n, p, e, etc.)
c POS = starting position
c ERG = initial energy
c DIR = initial direction (optional, default=isotropic)Every source needs at least these parameters. MCNP will reject particles that start in void regions or undefined cells, so verify your source positions are inside valid geometry.
Point Sources
Point sources emit particles from a single location. They're perfect for small radioactive sources, neutron generators, or when source size is negligible compared to your problem dimensions.
Common Laboratory Sources
c Cf-252 neutron source (spontaneous fission)
SDEF PAR=n POS=0 0 0 ERG=d1
SI1 H 0.1 0.5 1.0 2.0 4.0 6.0 8.0 10.0
SP1 D 0.05 0.15 0.25 0.30 0.15 0.07 0.02 0.01
c Am-Be neutron source (α,n reaction)
SDEF PAR=n POS=5 0 0 ERG=d2
SI2 H 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0
SP2 D 0.02 0.08 0.15 0.20 0.22 0.18 0.10 0.03 0.01 0.01
c Co-60 gamma source (medical/industrial)
SDEF PAR=p POS=0 0 10 ERG=d3
SI3 L 1.173 1.333 $ Two gamma energies
SP3 D 1.0 1.0 $ Equal intensityThese examples show realistic source spectra using SI/SP cards. The SI card defines energy bins (H=histogram, L=list), while SP cards specify probabilities (D=discrete, C=cumulative). Always verify that probabilities sum to 1.0.
Directional Sources
c Collimated neutron beam
SDEF PAR=n POS=0 0 0 ERG=14.1 VEC=1 0 0 DIR=1
c Cone beam (±15 degrees)
SDEF PAR=p POS=0 0 0 ERG=6.0 VEC=0 0 1 DIR=d1
SI1 -1 0.966 1 $ cos(165°) to cos(0°)
SP1 0 0 1 $ Only forward cone
c Isotropic with forward bias
SDEF PAR=n POS=0 0 0 ERG=2.0 DIR=d2
SI2 -1 -0.5 0 0.5 1 $ Cosine bins
SP2 1 1 1 1 1 $ Isotropic sampling
SB2 1 2 4 8 16 $ Forward bias factorsThe VEC parameter sets the reference direction, while DIR controls the angular distribution. DIR=1 creates a perfect beam, while distributions allow controlled spreading. The SB card biases sampling for efficiency without changing physics.
Volume Sources
Volume sources distribute particles throughout regions of space. They're essential for reactor cores, activated materials, or any situation where the source has significant spatial extent.
Cylindrical Sources
c Fuel pin fission source
SDEF PAR=n POS=0 0 0 AXS=0 0 1 RAD=d1 EXT=d2 ERG=d3
SI1 0 0.4096 $ Fuel radius (cm)
SP1 -21 1 $ r^1 for uniform volume
SI2 0 365.76 $ Active length (cm)
SP2 0 1 $ Uniform height
SI3 S 4 $ Watt fission spectrum
SP3 -3 0.966 2.842 $ U-235 parameters
c Cylindrical waste container
SDEF PAR=p POS=0 0 0 AXS=0 0 1 RAD=d4 EXT=d5 ERG=0.662
SI4 0 30 $ Container radius
SP4 -21 1 $ Uniform volume
SI5 0 100 $ Container height
SP5 0 1 $ Uniform heightThe AXS parameter defines the cylinder axis, while RAD and EXT control radial and axial distributions. The SP1 entry "-21 1" ensures uniform volumetric sampling (proportional to r for cylinders).
Complex Geometries
c Reactor core with multiple assemblies
SDEF POS=d1 ERG=d10
SI1 L 0 0 0 $ Assembly 1 center
21.5 0 0 $ Assembly 2 center
0 21.5 0 $ Assembly 3 center
SP1 D 0.4 0.3 0.3 $ Power distribution
c Spherical core with radial zones
SDEF PAR=n POS=0 0 0 RAD=d2 ERG=FPOS=d3
SI2 0 50 100 150 $ Radial zones (cm)
SP2 -21 2 $ r^2 for sphere volume
DS3 S 4 5 6 $ Zone-dependent spectra
SI4 H 0.1 1.0 10.0 $ Inner zone (hard)
SP4 D 0.1 0.6 0.3
SI5 H 0.1 1.0 10.0 $ Middle zone
SP5 D 0.2 0.5 0.3
SI6 H 0.1 1.0 10.0 $ Outer zone (soft)
SP6 D 0.4 0.4 0.2Multiple positions create distributed sources, while FPOS links energy to position. The DS card selects different distributions based on location, enabling realistic spatial variations in source properties.
Surface Sources
Surface sources emit particles from geometric surfaces. They're useful for boundary conditions, beam problems, or when using MCNP's surface source write/read capability for multi-stage calculations.
c Detector calibration source (disk)
SDEF PAR=p SUR=10 NRM=1 ERG=0.662 DIR=d1
SI1 0 1 $ Forward hemisphere only
SP1 -31 1 $ Cosine distribution
c Beam port source
SDEF PAR=n SUR=20 NRM=-1 RAD=d2 ERG=d3
SI2 0 5 $ Port radius (cm)
SP2 -21 1 $ Uniform area
SI3 H 0.1 1.0 10.0 $ Beam energy spectrum
SP3 D 0.2 0.6 0.2
c Surface source write/read
SSW 100 $ Write crossing particles
c In follow-up calculation:
SSR old=100 new=200 $ Read and map to new surfaceThe SUR parameter specifies the surface, while NRM controls direction (1=outward, -1=inward). Surface source write (SSW) saves particle data for reuse in subsequent calculations, enabling efficient multi-stage problems.
Criticality Sources
Criticality calculations use special source cards (KCODE and KSRC) instead of SDEF. These provide initial neutron distributions that evolve toward the fundamental mode during the calculation.
c Criticality calculation setup
KCODE 10000 1.0 50 250 $ particles/cycle, k-guess, skip, total
KSRC 0 0 0 $ Initial source point
10 0 0 $ Additional points
0 10 0
c Multiple starting points for large systems
KSRC -50 -50 0 -50 50 0 50 -50 0 50 50 0
-25 -25 0 -25 25 0 25 -25 0 25 25 0
0 0 0 0 0 100 0 0 200
c Initial source distribution from file
SRCTP -50 -50 -100 50 50 100 $ Bounding box
KSRC 0 0 0 $ At least one point requiredKCODE specifies the criticality parameters, while KSRC provides starting locations. Use multiple KSRC points for large or complex geometries to ensure good coverage. The SRCTP card can initialize from previous calculations.
Verification and Testing
Source Validation
Always verify your source definitions before running full calculations. Use short runs with VOID cards to check particle starting positions and energies. MCNP's Print Table 110 shows source sampling statistics.
c Test source with void transport
VOID $ No transport, just source sampling
SDEF PAR=n POS=d1 ERG=d2
SI1 L 0 0 0 10 0 0 0 10 0 $ Test positions
SP1 D 0.5 0.3 0.2 $ Probability weights
SI2 H 1.0 2.0 4.0 $ Energy bins
SP2 D 0.3 0.5 0.2 $ Energy probabilities
c Check with mesh tally
FMESH4:n GEOM=xyz ORIGIN=-20 -20 -20
IMESH=20 IINTS=20 JMESH=20 JINTS=20
KMESH=20 KINTS=20The VOID card stops transport after source sampling, letting you verify distributions. Mesh tallies can visualize source spatial distributions. Always check that rejection rates are reasonable and distributions match expectations.
Common Source Mistakes
Verify that source positions are inside valid cells - MCNP rejects particles starting in void. Check that probability distributions sum to 1.0 and use appropriate sampling schemes (uniform area vs. uniform volume).
For efficiency, avoid sources with high rejection rates. Use cell-based rejection (CEL parameter) or carefully design distributions to match your geometry. Document all distribution parameters for reproducibility.