We'd like to especially highlight that point-wise neutron interactions below 20 MeV are automatically activated for the DAMAGE, NEW-DEFA, and PRECISIOn DEFAULTS (as well as in the absence of the DEFAULTS card, corresponding to the NEW-DEFA implicit choice), requiring the download and installation of neutron data libraries as detailed here.

This major release includes powerful technical improvements, featuring support for unstructured-mesh geometries (externally generated from CAD/CAE geometries) and a beta version of an automatic importance biasing algorithm, as well as a series of relevant physics improvements, among which enhanced gamma cascades from thermal-neutron capture. See the list below for a detailed account of all physics and technical improvements. Bug fixes are listed in the dedicated FLUKA news, as well as in the release notes.

PHYSICS IMPROVEMENTS:

• Gamma emission from thermal-neutron capture events is now initiated from ENSDF data blocks for 168 isotopes, duly taking into account the competition between the emission to discrete levels and to the continuum.
• Furthermore, in the gamma de-excitation module, a crude fallback estimate of the energies of the first and second nuclear levels has been deprecated in favor of actual nuclear level data when available, thereby removing spurious gamma lines.
• Point-wise interactions of low-energy neutrons for which the evaluated nuclear data prescribe a residual nucleus break-up are now explicitly treated by dedicated call to the FLUKA Fermi break-up module. In our present point-wise neutronics databases, this concerns non-elastic channels for 6Li, 7Li, 10B, 12C, and 14N. NB: for residual 8Be (in the ground state or in any excited level) or 12C (in excited levels only), a call to Fermi break-up is issued regardless of break-up flags. This improvement leads to an observable suppression of spurious gammas in favor of particle emission; residual nuclei production from the aforementioned channels is thereby also improved.

TECHNICAL IMPROVEMENTS:

• A new capability has been added to support unstructured meshes (UM) extracted from CAD/CAE files in TetGen, Abaqus, or ANSYS cdb format via the UMESH card. This memory-efficient, high-performance CPU implementation is based on Flair’s GeoViewer module, enabling users to include multiple UMs in a single input while allowing transformations and CSG operations on UMs. UM regions can be treated on equal footing to standard FLUKA CSG regions. For details on usage, refer to the dedicated manual page, as well as the Unstructured Mesh guide in our documentation section. To fully benefit from this feature, please ensure you have the latest version of Flair (3.4.*) installed.
• An automatic importance biasing algorithm has been introduced as an experimental feature, greatly facilitating the creation and use of importance biasing schemes at the minimal expense of issuing the newly included card AUTOIMBS. Please refer to the manual page of the latter for its usage, as well as for relevant notes and a disclaimer elucidating its current experimental status.
• A new command-line utility (fluresnucle_formatter.py) has been added in the bin folder for the conversion of tab.lis results files from RESNUCLE scorings into the following formats: txt, tex, json, csv, xlsx (Excel).
• In past FLUKA versions, the absence of a RANDOMIZe card would imply that every cycle in a run would start with the same seed. This behavior has now been forbidden: absence of a RANDOMIZe card is now equivalent to having a RANDOMIZe card with default WHATs, thereby ensuring that subsequent cycles continue on the random-number sequence prescribed in the first cycle and avoiding the pointless repetition of identical cycles observed on several occasions in this forum.
• The magnetic horn has been included among the analytical magnetic fields of cards MGNCREATe and MGNFIELD. See the manual page of the MGNCREATe card for the mplemented analytical expression of this field.
• In the CRYSTAL card, a reference momentum is now offered as an option for crystal property initialization at a momentum other than that of the BEAM card (the default value).
• Mac OSX compilation and linking:
  - The build issue reported in this forum thread has been tentatively fixed by the explicit addition of the `-ld_classic` linker flag, an option which will be dropped in the near future.
  - Optimize-sibling calls have been disabled at compilation for the function performing point-wise neutron inelastic interactions in order to avoid a miscompilation.
• The peak RAM usage since the start of the run is now logged in the .out file.
• The codebase of FLUKA's Fermi break-up and gamma de-excitation modules has been migrated to a modern C++ framework, in view of the ongoing evolution towards the next FLUKA generation.