minimc_engine

MiniMC engine cfg-strings

Engine options control aspects of the simulation which are not directly tied to source or geometry. Most importantly, this includes control of which quantities to tally and with what binning to do so. Most users are not really expected to change anything else than this, but other parameters affects aspects like how many threads to use for the simulation, or how to account for multiple scatterings and absorption. For more details refer to the specific parameters below.

Parameter reference

absorption : Whether to include absorption effects.

Default value: "1"
Description: Whether to include absorption effects. If set to 1 (the default) they will be included implicitly through attenuation of neutrons weights. If set to 0, the absorption cross section will be artificially set to 0barn.
Example values: "0", "1"

ignoremiss : Whether to exclude neutrons missing the geometry from tallies.

Default value: "0"
Description: Whether to exclude neutrons missing the geometry from tallies. Set to 1 to exclude them and 0 to include them.
Example values: "0", "1"

nscatlimit : Maximum number of scatterings to allow for each neutron.

Default value: "none"
Description: Maximum number of scatterings to allow for each neutron. If set to none (the default), no limit will by explicitly applied. If set to any other non-negative integral value, the scattering cross section of a neutron will essentially become 0 just after a neutron has undergone that many scatterings. A typical usage might be to set this value to 1, in order to exclude multiple scattering effects.
Example values: "none", "0", "1", "2", "10", "999"

nthreads : Number of threads to use for simulation.

Default value: "auto"
Description: Number of threads to use for simulation. Either accepts an integer value >=0 or the special keyword "auto".
Example values: "auto", "1", "2", "20"

roulette : Roulette parameters for the neutron termination strategy.

Default value: "0.1,0.01,2"
Description: Roulette parameters for the neutron termination strategy. At each simulation step inside the geometry, neutrons are split into a transmitted (and tallied) part as well as a part undergoing a forced collision. Weights are handled appropriately, decreasing in magnitude after each step. To maximise utility of simulation time, a roulette choice is made for neutrons having undergone at least N scatterings and whose weights are below a threshold value W. The choice allow the neutron to survive with a probability of P, and if it survives its weight will be increased by a factor of 1/P. This allows the simulation to avoid using most time on insignificant neutrons, while still providing the correct tally distributions in the limit of large statistics. The roulette parameter thus needs three comma-separated values, encoded as "P,W,N".
Example values: "0.5,1e-4,5"

seed : Seed value for the random number stream used for the simulation.

Default value: "0"
Description: Seed value for the random number stream used for the simulation. Accepts any 64 non-negative integral value that fits into a 64bit unsigned integer. Note that even with the same seed, results are not expected to be reproducible unless also using nthreads=1.
Example values: "1000", "123456789"

tally : Quantities to tally (available: de,e,l,mu,nscat,nscat_uw,q,theta,w).

Default value: "theta"
Description: Comma separated list of quantities to tally. The quantities available for tally are: de (energy loss), e (energy), l (wavelength), mu (cosine scattering angle), nscat (number of scatterings), nscat_uw (unweighted number of scatterings), q (scattering vector), theta (scattering angle), w (weight).
Example values: "q", "mu,q,e,l", "q"

tallybins : Modify binning of tally histograms.

Default value: ""
Description: Modify binning of tally histograms. This is supplied as a comma separated list of ",,,". If a tally is enabled via the tally keyword, it will use the binning specified in this list, if any. If a binning is not specified, an automatic choice is made for the binning. Special entries "+" or "-" can be used to generally increase or decrease binning in such cases.
Example values: "", "q:1000:0.0:20.0", "q:1000:0.0:20.0,l:200:3.0:5.0", "+", "-", "q:1000:0.0,+"

tallybreakdown : Whether to also tally per-component.

Default value: "1"
Description: If set to 1 (the default), tallies will also include per-component breakdowns, allowing to distinguish contributions depending on number of scatterings (0, 1, 2+) and whether the scatterings were elastic or not.
Example values: "0", "1"

tallyref : Definition of incoming quantities in tallies.

Default value: "truth"
Description: Definition of incoming quantities in tallies. This affects the calculation of tally quantities like "q", "theta", or "mu" which needs parameters from the incoming neutron state. The default value of "truth" implies that the actual original quantity of each individual neutron as it came from the source is used. A value of "src" implies that the reference value will be taken as the mean value provided by the source. In the special case of energy specified as a wavelength (via the "wl" source parameter), the mean wavelength rather than energy will be used where that makes a difference.
Example values: "truth", "src"

Tally overview

Currently 9 different quantities are available for tallying in histograms as the neutron leaves the geometry during a MiniMC simulation. The "tally" parameter is used to select which such histograms to produce. Refer also to the parameters "tallybins", "tallyref", and "tallybreakdown" which all affects the details of how quantities are defined and histogrammed.

The available tally quantities are:

• de : energy loss (eV)
• e : energy (eV)
• l : wavelength (Aa)
• mu : cosine scattering angle
• nscat : number of scatterings
• nscat_uw : unweighted number of scatterings
• q : scattering vector (1/Aa)
• theta : scattering angle (degree)
• w : weight