Physics - NonMC

In many cases, the commands used to set up non-MC calculations are identical to traditional (MC) MULASSIS commands, e.g., to define a geometry and its materials, source particle distribution and analysis output controls, albeit subject to the constraints discussed below. Several additional commands have been added that allow:

  • User definition of a NIEL function to apply to the primary ion species (to be used for the non-MC calculation only).

  • Specification of a global error to apply to the results.

  • Control of the numerical integration process, if the default values are considered inappropriate.

  • To initiate the a non-MC calculation.

The non-MC simulation mode in MULASSIS v2.0 allows shielding and effects analysis results to be generated rapidly compared to (statistically meaningful) MC simulation outputs. However, this mode involves a number of approximations and assumptions which impact the accuracy of the calculations. [R21] and [R22] provide the details of the continuous slowing-down approximation (CSDA), straight-ahead algorithm used for the non-MC calculations by MULASSIS. In summary, the principal differences in the types of calculations which may be performed in non-MC mode compared to standard MULASSIS MC simulations are:

  • The non-MC mode treats all ion species but no other particle species, such as electrons, neutrons and photons.

  • Only the primary ion is treated; secondary particle production and energy deposition are ignored.

  • The attenuation of the incident ions by nuclear interactions in the shield is treated. This loss is approximated using Geant4’s total inelastic cross-section models. Non-MC mode also assumes straight-line propagation, i.e., it ignores elastic scattering of the ions.

  • For planar shield geometries, the algorithm can treat unidirectional incident particles with a user-defined angle of incidence or cosine law angular distributions with user-defined maximum and minimum angles to the surface normal.

  • For spherical shield geometries, the results are relevant to the dose and fluence at the centre of the shield only, and no results are provided for any intervening layers of the geometry.

  • Non-MC simulations provide calculations of fluence, total ionising dose (TID) and total non-ionising dose (TNID, sometimes referred to as “NIEL” analysis). Pulse-height energy deposition spectrum analysis (PHS), Equivalent dose and MC-SCREAM analyses can not be treated in non-MC mode.

  • If particle fluence output is required, the fluence analysis type must be omnidirectional, i.e., the fluence is proportional to number of particles crossing each boundary divided by the cosine of the angle to the surface normal. This is the default fluence analysis mode for MULASSIS for MC calculations. The alternative fluence analysis type is “planar”, which currently is not permitted for non-MC cases.

  • If TNID (NIEL) analysis is selected for a non-MC calculation, only the TNID of the primary particle will be provided in the CSV file, and the value will be non-zero provided that either:

    • The primary species are protons, since MULASSIS includes NIEL coefficient datasets for protons in different materials. The appropriate dataset is selectable by the user during the run. However, MULASSIS does not currently include coefficients for heavier ions.

    • The user provides suitable NIEL coefficients for the incident ion species at run-time.

    In comparison, MULASSIS in MC simulations will output TNID results for protons, neutrons, electrons, charged pions as well as provide the total TNID from these particle species. Other particle types are not currently treated by the MC calculation.

  • Non-MC simulation of geometries with any layer less than the equivalent thickness of 10 microns Si or Al is not recommended. Indeed, even at these shield thicknesses, some caution should be applied in the selection of the Romberg numerical integration tolerance (QRtol), which should not be greater than 10-7 (see /nonMC/integration/rombergTolerance command in NonMC Integration Commands). For shields with layers >=1 mm Al equivalent, tolerances of 10-4 have been used successfully for the majority of the test cases. The MULASSIS default Romberg integration tolerance is 10-7 and the minimum value of 10-10 can be used for maximum fidelity.

Mulassis/ml-v02-00/r342