Source Specifications in MCNP Presented by: F. Rahmani Shahid Beheshti University Department of Radiation Application

Input to Output Flow Diagram:

Source Commands : SDEF KCODE SSW / SSR User subroutine Fixed Reactor There are four possible source types: SDEF KCODE SSW / SSR User subroutine Fixed Reactor MCNP generated surface source write your own

Source Commands : There are four possible source types: Fixed (or general ) source (SDEF card) MCNP generated surface source (SSW to write and SSR to read). Criticality Source (KCODE Card); needs also an initial source (KSRC, SDEF card or SRCTP file from a previous calculation written by the SSW card). User supplied source (if SDEF, SSR and KCODE are all missing)

General Cards : These general cards can be used with any of the above sources: SI (source information), SP (source probability), SB (source bias), DS (dependent source) MODE card implied the type of source particle.

MODE card : MCNP4c can be run in several different modes: N — neutron transport only (default) N P — neutron and neutron-induced photon transport P — photon transport only E — electron transport only P E — photon and electron transport N P E — neutron, neutron-induced photon and electron transport And can be run in similar way with other particles according to it’s version.

Source Specifications : A particle source has: Intensity, Energy, Direction, Shape, Temporal characteristics, A position somewhere within the phase space of the problem We are concerned here with fixed sources, or primary sources; not secondary sources such as those generated by fission, neutron capture, or electron recoil. The source strength in represented in MCNP by the starting weight, WGT, which is usually assigned to unity to represent a normalized source. The frequency of occurrence of a particle of a particular attribute is determined by the source’s probability density function (pdf).

General Source Card (SDEF) : The SDEF command with its many variables is one of the more complex MCNP commands and is capable of producing an incredible variety of sources The SDEF command has many variables or parameters that are used to define all the characteristics of all sources in the problem The source and type of radiation particles for an MCNP problem is defined by the SDEF command Only one SDEF card is allowed in an input file

SDEF variables:

SDEF variables:

SDEF variables defaults: POS = x y z default is 0 0 0; CEL = starting cell number ERG = starting energy default is 14 MeV; TME = time default is 0; PAR = source particle type 1 for N, N P, N P E; 2 for P, P E; 3 for E and other particles

SDEF variables: Values of variables can be specified at three levels: (1) explicitly (e.g., ERG=1.25), (2) with a distribution number (e.g., ERG=d5), (3) as a function of another variable (e.g., ERG=Fpos). Specify variables at levels 2 and 3 requires the use of four other source cards: SI (source information), SP (source probability), SB (source bias), DS (dependent source)

Sdef par=2 erg=1.25 dir=1 vec=1 0 0 pos=0 0 0 Sdef pos=10 -2 0

Important Point When developing a new source definition, always check and recheck that source particles are truly being generated where you think they should be. HINT: Always use the VOID card and the PRINT 110 statement somewhere in data block of the input file. The PRINT 110 causes the starting locations. directions, and energies of the first 50 particles to be printed to the output file. Examine this output table to convince yourself that particles are being generated as you expect.

Spatial Distribution (Shape): In general, use SUR for a surface source and CELL for a volume source. Volume Sources : SUR=0 (default value). Cartesian (These ace specified with X, Y, and Z.) Point: X, Y and Z are all constant. Line: one variable, the others are constant, Rectangular Plane: Fix one variable and vary the other two. Rectangular Polyhedron: vary the three variables.

Spatial Distribution (Shape): Spherical (These are specified by POS (center of sphere) and RAD (radius), do not specify XYZ and AXS.) Point: RAD=0, or not specified at all. Spherical shell: specify two radii for RAD on an SIn card. Cylindrical (These are specified with POS (point on axis), AXS (direction cosines of axis), RAD (radius of cylinder), and EXT (distance form POS). Disk :set EXT=0, which provides a source with circular symmetry on a plane.

Spatial Distribution (Shape): Surface Sources: SUR ≠0, Sampled values of X Y and Z determine position (make sure that the point is on surface). IF X Y Z are not specified, the position is sampled from SUR Plane (SUR defines a name of a plane, POS must be a point on plane. The position of sampled uniformly on the circle of radius RAD centered around). Cylindrical (This must be specified with as a volume source, specify two equal radii for RAD on an SIn card).

Spatial Distribution (Shape): Cylindrical (This must be specified with as a volume source, specify two equal radii for RAD on an SIn card). Spherical (SUR is the name of a spherical surface. If AXS is not specified, position is sampled uniformly on surface. If AXS is specified, EXT in used for the cosine of the angle between the direction AXS and the vector from the center of the sphere to the position point (EXT can have a distribution). Spheroidal SUR is the name of ellipse revolved around one of its axis. A spheroid must have its axis parallel to one of the coordinate axis.

Spatial Distribution (Shape): Cylindrical (This must be specified with as a volume source, specify two equal radii for RAD on an SIn card). Spherical (SUR is the name of a spherical surface. If AXS is not specified, position is sampled uniformly on surface. If AXS is specified, EXT in used for the cosine of the angle between the direction AXS and the vector from the center of the sphere to the position point (EXT can have a distribution).

Energy Spectrum: ERG= starting energy (default 14 MeV): Use an SP card to define a distribution from Built-In Functions for Source Probability and Bias Specification

Directional distribution: The default is isotropic distribution. VEC defines a reference vector (which can be itself a distribution); the default for a surface source is the normal to the source in a direction defined by NRM. DIR defines the cosine of the polar angle; cosine distribution for a surface source is the default. The azimuthal angle is sampled uniformly. DIR=l gives a mono-directional source (beam) in the ,direction of VEC. DIR can be biased to a preferred direction,

Temporal distribution: TME = time (in units of shakes, default=O). SP with f=-1 defines a Gaussian distribution with time. Alternatively, DS can be used to define a discrete distribution.

variance reduction allowed with F8 tallies having energy bins. Biasing: It allows the production of more source particle, with suitably reduced weights, in the more important regimes of each variables. Source biasing is the only variance reduction allowed with F8 tallies having energy bins.

See manual for : Others: Repeated source structure (use CEL with a path from level 0 to level n). Surface source write (SSW card). Surface source read (SSR card) Criticality source (KCODE and KSRC cards). User supplied source (Subroutines SOURCE and SRCDX).

Examples: Point Isotropic Sources: Isotropic Volumetric Sources: Two Point Isotropic Sources at Different Positions SDEF ERG=1.00 PAR=2 POS=d5 SI5 L -10 0 0 10 0 0 $ SP5 .75 .25 Point Isotropic Source with Discrete Energy Photons SDEF POS 0 0 0 ERG=d1 PAR=2 SI1 L 0.3 0.5 1 2.5 $ the 4 discrete energies (MeV) SP1 0.2 0.1 0.3 0.4 $ frequency of each energy Isotropic Volumetric Sources: Surface source read (SSR card) Criticality source (KCODE and KSRC cards). User supplied source (Subroutines SOURCE and SRCDX).

Examples: Point Isotropic Sources: Two Point Isotropic Sources at Different Positions SDEF ERG=1.00 PAR=2 POS=d5 SI5 L -10 0 0 10 0 0 $ SP5 .75 .25 Point Isotropic Source with Discrete Energy Photons SDEF POS 0 0 0 ERG=d1 PAR=2 SI1 L 0.3 0.5 1 2.5 $ the 4 discrete energies (MeV) SP1 0.2 0.1 0.3 0.4 $ frequency of each energy

Work Problems: Write the SDEF card, and associated cards, explaining each instruction, for: An inward directed source cm spherical surface. A monodirectional source emitted from a surface in the direction of the direction positive to the surface. A point source in which the low-energy particles are emitted with a cosine distribution,Criticality source (KCODE and KSRC cards).

SDEF SUR=12 RAD=D2 nrm=-1 ERG=d1 POS=0 15.275 0 SI1 0.1 29i 3 SP1 0 1 30r SI2 0 .5 SP2 0 1

sdef pos=0 0 0 vec=0 1 0 DIR=D1 ERG=FDIR D2 par=2 SI1 -1 -.9 -.8 -.7 -.6 -.5 -.4 -.3 -.2 -.1 0 & .1 .2 .3 .4 .5 .6 .7 .8 .9 1 Sp1 0 2.66E-02 2.66E-02 2.66E-02 2.60E-02 2.56E-02 2.45E-02 2.33E-02 2.12E-02 1.77E-02 1.03E-02 5.22E-03 6.03E-03 7.90E-03 1.14E-02 1.52E-02 1.91E-02 2.41E-02 2.96E-02 3.82E-02 5.89E-02 DS2 s 4 5 6 7 c Sc4 cosine=-1 to -0.9 SI4 0 1 2i 4 Sp4 0 2.38E-02 2.37E-03 4.04E-04 1.80E-05 sb4 0 1 3r Sc5 cosine=-0.9 to -0.8 SI5 0 1 2i 4 SP5 0 2.38E-02 2.36E-03 4.39E-04 2.70E-05 Sb5 0 1 3r Sc6 cosine=-0.8 to -0.7 SI6 0 1 2i 4 SP6 0 2.35E-02 2.66E-03 4.75E-04 3.30E-05 Sb6 0 1 3r Sc7 cosine=-0.7 to -0.6 SI7 0 1 2i 4 SP7 0 2.26E-022.89E-03 5.12E-04 4.60E-05 Sb7 0 1 3r