FLUKA version: 4-4.0, released on: 2024-02-14

We'd like to especially highlight that point-wise neutron interactions below 20 MeV are henceforth 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.

PHYSICS IMPROVEMENTS:

• A new model for the nuclear elastic scattering of protons has been implemented, relying on partial-wave analyses and experimental angular distributions, significantly improving FLUKA's description of proton elastic scattering near Coulomb barrier and up to 250 MeV for target nuclei with A>1. Proton-proton nuclear elastic scattering remains unaltered with respect to previous FLUKA versions.
   
• The reaction cross section for protons on 11B at low energies has been further refined, and now features a broad peak of 900 mb at ~650 keV as well as a narrow peak of 100 mb at ~150 keV.
   
• A correction has been made to FLUKA's deuteron reaction model below 150 MeV/n: in earlier versions, when a neutron was stripped to the continuum, the remaining proton was subject to an excessive Coulomb barrier when attempting to penetrate the target nucleus. This spurious effect (now corrected) led to a significant underestimation of channels such as (d,2n), especially on heavy target nuclei at deuteron energies up to 10s of MeV, as reported in this and this forum thread.
   
• Point-wise thermal neutron scattering laws S(a,b,T) can now be imported directly from ACE-formatted files (see the LOW-PWXS card documentation in the manual). FLUKA neutron library packages do not provide ACE files. It is up to the user to retrieve them if needed, e.g. from the following repositories
  - https://www.oecd-nea.org/dbdata/jeff/jeff33/tsl.html
  - https://github.com/highness-eu/NJOY-NCrystal-Library
  to name but a few. Use of external ACE files should be duly quoted and referenced when employed through FLUKA.
   
• Correlated gamma emission spectra following the capture of thermal neutrons have been included from ENSDF for the following isotopes: 1H, 12C, 14N, 16O, 17O, 21Ne, 24Mg, 28Si, 29Si, 39K, 41Ca, 42Ca, 43Ca, 46Ti, 48Ti, 49Ti, 52Cr, 53Cr, 57Fe, 60Ni, 61Ni, 67Zn, 83Kr, 90Zr, 91Zr, 94Mo and 95Mo, now better reproducing experimental gamma line energies and intensities, thus overcoming the issue raised in this forum thread.
   
• Point-wise (n,inl) interactions reaching a final state with excitation energy above the neutron separation energy are now resampled to avoid excessive excitation energy going into the gamma de-excitation stage.

TECHNICAL IMPROVEMENTS:

• The (already discouraged) use of WHAT(2) in the MATERIAL card (material atomic weight) has been deprecated due to frequent inadvertent misuse as mass number. Should your inputs have this field filled, please clear it, or refer to the manual to intentionally overwrite default values.
   
• The AUXSCORE card has been provided with the additional option RESNUCLEi, enabling the scoring of residual nuclei filtered by the projectile particle species.
   
• A profiler is now included, allowing the user to resolve the CPU time spent per region and per particle species on every cycle. See the PROFILE card documentation in the manual.
   
• In the source_newgen.f user routine:
  - A new sampling function has been added to sample from external files containing discrete energy lines and their relative intensities.
  - From now on it is possible to use the same sampling function with up to 100  files simultaneously.
   
• A new command-line utility, fluprocess.py, is available in the bin/ folder as an additional tool for a simplified post-processing of FLUKA scoring of the output files.

Several bugs were fixed (see release notes for a complete list).