Physics list

remage uses a custom modular physics list built on top of Geant4’s standard physics constructors. It combines:

• Configurable electromagnetic physics (standard, low-energy, Livermore, Penelope, polarized)

• Optional optical physics (scintillation, Cherenkov, absorption, Rayleigh, WLS)

• Configurable hadronic physics lists, including the Geant4 high-precision neutron (NeutronHP) models

• Radioactive decay with extended control over decay time thresholds

• Multiple custom processes:

  • Inner Bremsstrahlung for beta decay

  • Custom neutron capture with gamma cascades loaded from external files

  • Custom wavelength-shifting (WLS) optical process

• Production cuts are managed explicitly per-region, including a dedicated SensitiveRegion.

This physics list is designed and adjusted for:

• Low-energy nuclear and particle physics

• Underground or low-background experiments

• Precision gamma and beta spectroscopy

• Optical detector simulations

• Long-lived radioactive decay chains

Electromagnetic physics

The electromagnetic physics is configurable via the /RMG/Processes/LowEnergyEMPhysics command.

Available options:

• None → G4EmStandardPhysics

• Option{1,2,3,4} → G4EmStandardPhysics_option{1,2,3,4}

• Penelope → G4EmPenelopePhysics

• Livermore → G4EmLivermorePhysics (default)

• LivermorePolarized → G4EmLivermorePolarizedPhysics

A description of the different modes can be found in the upstream documentation.

Some additional EM features from G4EmExtraPhysics are always enabled:

• Synchrotron radiation

• Gamma-nuclear interactions

• Muon-nuclear interactions

• e± nuclear interactions

Optical physics

Optical physics is optional and can be enabled by /RMG/Processes/OpticalPhysics.

This option enables the following physics processes: Refraction, reflection, scintillation, Cherenkov radiation, optical bulk absorption, Rayleigh scattering and wavelength shifting. The wavelength shifting (WLS) is either the default implementation or a custom WLS process.

Global optical settings:

• Track scintillation secondaries first

• Enable scintillation by particle type

• Boundary process invokes sensitive detectors

Custom optical wave length shifting (WLS) process

An optional custom wavelength-shifting process RMGOpWLSProcess is available and enabled by default. It can be disabled with /RMG/Processes/OpticalPhysicsMaxOneWLSPhoton, if necessary.

It wraps the standard G4OpWLS process, but ensures at most one secondary photon is produced per WLS interaction. Most materials do not emit photons with an efficiency of 100%, but a lower WLSE. In the default optical physics of Geant4, setting such an efficiency enables the sampling of the number of emitted photons from a distribution Poisson(WLSE). This will lead to the emission of multiple photons in some cases.

We perceive this to be not the case in reality, but that the number of emitted photons should rather be distributed along Bernoulli(WLSE). The custom WLS process ensures this assumption holds true.

Hadronic physics

Hadronic physics can be completely disabled or configured via predefined physics lists.

If enabled, the following processes are included:

• Hadronic elastic scattering (G4HadronElasticPhysicsHP)

• Optional thermal neutron scattering (G4ThermalNeutrons)

• Hadronic inelastic physics (selectable)

• Stopping physics (G4StoppingPhysics)

• Ion physics (G4IonPhysics)

Hadronic Physics Options

A hadronic physics option can be selected with /RMG/Processes/HadronicPhysics. Available options are

Option	Description
None	No hadronic physics (default)
QGSP_BIC_HP	Quark-Gluon String + Binary Cascade + HP neutrons [1]
QGSP_BERT_HP	QGSP with Bertini cascade + HP neutrons [2]
FTFP_BERT_HP	Fritiof string model + Bertini + HP neutrons [3]
Shielding	Optimized shielding list with HP neutrons [4]

The Geant4 physics reference contains descriptions of the Fritiof and Bertini cascade models.

High-precision neutron cross sections (NeutronHP [5]) is enabled when it is included in the individual options.

Tip

For cosmogenic simulations, we recommend the Shielding hadronic option.

An addition option, /RMG/Processes/EnableNeutronThermalScattering, enables thermal scattering for low-energy neutrons.

Custom Grabmayr gamma cascades

When enabled with /RMG/Processes/UseGrabmayrsGammaCascades, remage replaces the standard neutron capture process (nCapture) with a custom one that will generate gamma cascades for specific isotopes from files as provided by P. Grabmayr et al. [6] and calculated with MAURINA. For all other isotopes without a registered override file, this process exactly behaves the same as the builtin process.

This option is primarily intended for simulation scenarios that require precision gamma spectroscopy following neutron capture.

Physical background

The G4NeutronHP and the G4PhotoEvaporation models are not good at reproducing experimental data [5], since they either didn’t consider energy conservation or do not consider the state density, producing unexpected gammas. The MAURINA models for Ge76 and Gd are fine-tuned for these isotopes.

The MAURINA simulations (here as an example: 76Ge(n,g)) are based on estimates of photon strength functions tuned to data from 76Ge and level densities tuned to the range in which the complete level set is known. The gamma cascade was sampled starting from the sampled spin state of the compound nucleus, which is iteratively decayed according to the calculated transition probabilities until either the metastable or the ground state is reached. For excitation levels below the continuum edge at energy 1.56 MeV in 77Ge (critical energy), the known discrete levels, including their spin, are used. Above this energy, the level densities and spin distributions are used.

Important

The gamma cascade files are not shipped with remage by default, but have to be provided and registered by the user. For this, the command /RMG/GrabmayrGammaCascades/SetGammaCascadeFile can be added to a macro file. See [6] for files to use for this.

Radioactive decays

Geant4 changed the default time threshold for radioactive decays in the 11.2 minor update to 1 year. remage automatically reverts this change and sets the threshold to the old default of Geant4 before 11.2, being 1.0e+27 ns. This threshold can be adjusted using the built-in command /process/had/rdm/thresholdForVeryLongDecayTime after /run/initialize.

See also

The primary particle generation for decay chains is handled by generators.

Gamma angular correlations

When using Geant4 11.3 or higher, the simulation of angular correlations between gammas emitted coincident with a radioative decay are enabled. This can be controlled by /RMG/Processes/EnableGammaAngularCorrelation.

Internal Bremsstrahlung

Internal Bremsstrahlung is a radiative correction to beta decay where a photon is emitted along with the beta particle and neutrino. Unlike external bremsstrahlung (which occurs when the beta particle interacts with matter after emission), internal bremsstrahlung is emitted directly from the nucleus during the decay process itself.

The IB photon energy spectrum is continuous, extending from zero up to the Q-value of the decay. For Ar-39 (Q = 565 keV), the IB photons are typically soft (low energy), with most photons below 0.5 MeV. The spectrum is calculated based on [7].

The Inner Bremsstrahlung process is disabled by default in remage. To activate Inner Bremsstrahlung process, please use the command /RMG/Processes/EnableInnerBremsstrahlung before /run/initialize.

Production cuts and step limits

Production cuts

Production cuts are specified in length units (as usual in Geant4), and applied per particle type (gamma, electron, positron, proton).

Two regions are supported with separate production cuts, a default region (world volume) and a sensitive region.

/RMG/Processes/DefaultProductionCut {LENGTH} [mm]
/RMG/Processes/SensitiveProductionCut {LENGTH} [mm]

Note

• The default cuts are tuned for low-energy applications (≈ 100 keV in Ge)

• The energy range for production cuts is explicitly set for low energy physics (200 eV to 100 GeV).

• All registered sensitive detectors will be automatically added to the sensitive region.

Step Limits

The G4StepLimiterPhysics is always added, but will not have an effect by default. It allows volume-level step limits to be enforced if configured elsewhere (e.g. in a GDML file).

Setting production cuts and step limits via GDML

If the default setup of production cuts in remage is not enough, regions with attached productioon cuts (and optional step limits) can be defined in a GDML file. The information is stored in a specific format in an auxiliary structure.

These region definitions can be created easily with our python tooling. For this, refer to the documentation of this feature in the legend-pygeom-tools package, see G4Regions in GDML.

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[1]

see the “Hadronic Component” and “Related Physics Lists” sections in the QGSP_BIC physics list docs.

[2]

see the “Hadronic Component” and “Related Physics Lists” sections in the QGSP_BERT physics list docs.

[3]

see the “Hadronic Component” and “Related Physics Lists” sections in the FTFP_BERT physics list docs.

[4]

see the “Hadronic Component” and “Related Physics Lists” sections in the Shielding physics list docs.

[5] (1,2)

https://geant4-userdoc.web.cern.ch/UsersGuides/ForApplicationDeveloper/html/TrackingAndPhysics/physicsProcess.html#high-precision-neutron-interactions-neutronhp

[6] (1,2)

See calculations of P. Grabmayr with MAURINA. The data files are available for inclusion with the simulation:

• Cross sections and gamma cascades in 77Ge. publication: 10.1140/epja/s10050-024-01336-0 and data file.

• Gamma cascades in gadolinium isotopes. publication: 10.1140/epjc/s10052-023-11602-y and data file.

• Other such files exist in the wild, but not in a central place.

[7]

Hayen et al., in Rev. Mod. Phys. 90, 015008 (2018). doi: 10.1103/RevModPhys.90.015008. The calculation is described in detail in Section V (Radiative Correction).