MCNP Guide
Physics Models in MCNP
Essential physics settings for accurate simulations
Physics Control Overview
MCNP physics cards control how particles interact with materials. Proper settings ensure accurate results while maintaining computational efficiency.
Key Physics Cards
MODE
Particle types to transport
PHYS
Physics options and energy limits
CUT
Time and energy cutoffs
MT
Thermal scattering data
Neutron Physics
Neutron transport is the foundation of reactor physics and criticality calculations. Choose settings based on your energy range and accuracy requirements.
Basic Reactor Physics
c Standard reactor physics setup
MODE n $ Neutrons only
PHYS:n 20 0 0 -1 $ 20 MeV max; dnb=-1 analog delayed n
CUT:n J 0 0.25 $ Default time, no E cutoff, WC1=0.25
c Thermal scattering (essential for moderated systems)
MT1 lwtr.20t $ Light water S(α,β)
MT2 grph.20t $ Graphite S(α,β)
MT3 poly.20t $ Polyethylene S(α,β)This setup covers most reactor applications. With the 2nd entry (emcnf) at 0, implicit capture is used at all energies — the default, which improves efficiency by avoiding explicit absorption tracking. The 4th entry (dnb = −1) samples delayed neutrons analogically, also the default.
High-Energy Applications
c Accelerator or fusion applications
MODE n p $ Neutrons and photons
PHYS:n 150 0 0 -1 0 $ 150 MeV max (model physics above tables)
PHYS:p 150 $ Photon physics to 150 MeV
c Track specific reactions
c LCA and LCB control intranuclear cascade (INC) model
c parameters for high-energy physics (Bertini/CEM options).
c Photon production from neutrons is automatic in MODE n p.
c Energy splitting for efficiency (pairs: factor, energy)
ESPLT:n 2 1.0 4 0.1 $ Split 2-for-1 below 1 MeV, 4-for-1 below 0.1 MeVExtended energy ranges and coupled transport handle high-energy physics. Energy splitting improves statistics for important energy ranges.
Photon Physics
Photon transport is crucial for shielding, dosimetry, and detector response calculations. Enable features based on your specific application needs.
Shielding Calculations
c Coupled neutron-photon for shielding
MODE n p $ Both particle types
PHYS:n 20 $ Neutron physics to 20 MeV (photon
c production is automatic in MODE n p)
PHYS:p 20 $ Photons with coherent scattering ON (default)
c Energy boundaries for dose calculations
E0 0.01 0.1 1.0 10.0 $ Photon energy bins (MeV)Coupled transport captures gamma rays from neutron interactions. Essential for accurate dose calculations through shields.
Detailed Dosimetry
Hover over each card to see what every numbered parameter controls.
c High-fidelity photon physicsPHYS:p 20 0 0 -1 0 $ All physics effects enabledc Explicit electron transport (MODE p e)PHYS:e 1.0 0 0 0 0 1 $ Electron physics if needed
Annotated Physics Cards
Hover over a line to see what each numbered parameter controls.
Full physics treatment for precise dosimetry work. Includes all interaction mechanisms and secondary particle production.
Thermal Neutron Treatment
Thermal neutron scattering in bound materials requires special treatment. Use S(α,β) data for accurate results in moderated systems.
c Common thermal scattering materials
MT1 lwtr.20t $ Light water (H2O)
MT2 hwtr.20t $ Heavy water (D2O)
MT3 grph.20t $ Graphite
MT4 beo.20t $ Beryllium oxide
MT5 poly.20t $ Polyethylene
MT6 zrh.20t $ Zirconium hydride
c Temperature-dependent data (pick ONE per material):
c MT1 lwtr.21t $ H2O at 350K
c MT1 lwtr.26t $ H2O at 600K
c Free gas thermal treatment is the default when
c no S(alpha,beta) data is specified via MT cards.Match S(α,β) data to your actual materials and temperatures. Free gas thermal treatment is the automatic default when no S(α,β) data is provided.
Performance Optimization
Physics settings significantly impact computational efficiency. Balance accuracy with speed based on your problem requirements.
Efficiency Settings
Implicit capture
On by default for neutrons (emcnf=0 on PHYS:n): particles survive absorption with reduced weight instead of being killed.
Energy cutoffs
Set reasonable lower energy limits. Don't track particles that won't contribute.
Physics selection
Enable only needed effects. Extra physics features cost computational time.
Particle modes
Transport only necessary particle types. More particles = longer runtime.
Common Applications
Reactor Physics
MODE: n
Energy: 0-20 MeV
Special: Thermal scattering
Focus: k-effective, flux
Shielding Analysis
MODE: n p
Energy: 0-20 MeV
Special: Coupled transport
Focus: Dose rates
Accelerator Physics
MODE: n p e
Energy: 0-1000 MeV
Special: High-energy reactions
Focus: Activation, heating
Detector Response
MODE: n p
Energy: 0-10 MeV
Special: Detailed photon physics
Focus: Energy deposition
Physics Verification
Always verify physics settings with benchmark problems or analytical solutions. Small changes in physics options can significantly affect results.
Start with default settings and add complexity only when needed. Document your physics choices and the reasoning behind them.