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Model description

If you are interested to know exactly these procedures have been implemented, please take a look at the technical_description

ICRU sphere water phantom

Our data specifies for each of the relevant particles (see below) the energy dependent efficiency of energy deposition within a water ICRU sphere. It is a simple step function with the mapping: $$(\rm{PDG},E)\quad\Longrightarrow\quad f,$$ where $f$ is the efficiency factor.

For stable particles, $f$ is 1 at lower energies, where the particle always gets completely absorbed within the phantom. At higher energies, the tunneling probability increases, and therefore $f<1$ (for gammas, protons, electrons, heavy ions). Note that this effect is indirect. An alpha particle has a diminishing probability of tunneling through the phantom, however it is stopped through production of energetic secondary particles which themselves have higher tunneling probabilities.

Unstable particles can release additional energy through decay processes. While normal antiparticles (anti-neutrons, anti-protons, positrons) can release energy through annihilation, if they encounter their alternative. In case of anti-muons, this does not happen since our atmospheric model contains no anti-muons.

Regarding the choice of Physics List

Important resources and facts:

  • The Geant4 Physics Reference manual1)
  • The Geant4 Physics List guide2)
  • The current default physicslist is “FTFP_BERT”.

The geant4 simulation toolkit provides for many physical processes several models. At previous times the numerical experimenter had to carefully pick which models are relevant for the problem under his investigation and to glue these models together in to a physics list. Nowadays Geant4 comes with quite a few reference physics lists, which have been tailored by to tackle different classes of problems. To put it simply, the physics list prototype includes:

  • electromagnetic physics;
  • extra physics processes for gamma and leptons;
  • decay;
  • hadron elastic;
  • hadron inelastic;
  • stopping particles capture processes;
  • ion nuclear interactions;
  • step limiters;
  • others.

The reference physics lists mainly switch between:

  • hadronic interaction component (FTFP, QGSP, …)
  • Intranuclear cascade component (BERT, BIC, …
  • neutron precission component (_HP)
  • Electromagnetic component (_EMV,_EMX,_EMY,_EMZ,_LIV,_PEN,_GS, _SS)

In AtRIS, the user must specify the physics list as the first program argument:

./AtRIS FTFP_BERT_HP simname mode

Recently, the default physics list has been switched from QGSP_BERT to FTFP_BERT. Since neutrons play a significant role in atmospheric ionization and dose rate determination, we always recommend using the high precision neutron module (by adding the suffix _HP to the physics list name). Finally, the user can modify the electromagnetic component by adding one of the above suffixes after “_HP”. We do not consider this to be of benefit for large scale atmospheric simulations. Any additional fine tuning of physics has to be done either via the macro messenger or by editing the source code.

To summarize, for AtRIS we recommend the use of “FTFP_BERT_HP”.

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