11 Detector Simulation
495
and beam particles do not interact. The methods for modelling transport of photons
and electrons used in Monte Carlo codes are based on sampling of differential cross
sections obtained from approximate theoretical calculations. A recent review offered
a comprehensive description of the principles and approximations [47] for models
of electron and photons up to 1 GeV, documenting widely used models and those of
the precise modern electron–photon code Penelope [48].
11.3.4 Interactions of Photons
Photon interactions are ‘discrete’ interactions, that occur at a point and can be
modeled this way. This makes them much simpler than modeling the interactions
of charged particles. Interactions considered including photoelectric, Compton
incoherent scattering, electron–positron pair production and potentially Rayleigh
coherent scattering.
The cross section for each interaction is sampled from measured or theoretical
distributions. In some cases a simplified form is used, to reduce the cost of
computation with a simplified description of the energy and Z dependence. Else,
the values for each material at particular values can be pre-computed and stored in
tables for interpolation.
Once the type of interaction is chosen, its products are sampled from the
appropriate distributions. Pseudorandom numbers are used to sample the energy,
angles and momenta from the differential distributions [37]. The original particle’s
state, if it survives, is altered to preserve energy and momentum.
The interaction of energetic photons with nuclei is discussed in the hadronic
section below. Often specialized tools are used to simulate optical photons and their
collection. It is possible, though, to generate optical photons and model reflection,
refraction and absorption on different types of surfaces. GEANT4 is able to do this.
11.3.5 Interactions of Charged Particles
The simulation of the electromagnetic (EM) interactions of charged particles is
complicated by the large cross section for elastic interactions and of ionization,
which produces low-energy electrons (delta electrons).
In a few cases it is useful to simulate every single interaction of a charged particle
in a medium, including its elastic collisions with nuclei, the ionization of atoms
and creation of delta electrons, and the ‘hard’ interactions, which create photons or
electron–positron pairs.
All production simulation tools estimate the cumulative effect of the elastic
scattering off nuclei. It is modeled in several different ways. Many utilize the
multiple-scattering approach pioneered by Goudsmit and Sanderson [49] as their
basis. One simple way to sample angular deviation over a short step is Moliere’s
495
and beam particles do not interact. The methods for modelling transport of photons
and electrons used in Monte Carlo codes are based on sampling of differential cross
sections obtained from approximate theoretical calculations. A recent review offered
a comprehensive description of the principles and approximations [47] for models
of electron and photons up to 1 GeV, documenting widely used models and those of
the precise modern electron–photon code Penelope [48].
11.3.4 Interactions of Photons
Photon interactions are ‘discrete’ interactions, that occur at a point and can be
modeled this way. This makes them much simpler than modeling the interactions
of charged particles. Interactions considered including photoelectric, Compton
incoherent scattering, electron–positron pair production and potentially Rayleigh
coherent scattering.
The cross section for each interaction is sampled from measured or theoretical
distributions. In some cases a simplified form is used, to reduce the cost of
computation with a simplified description of the energy and Z dependence. Else,
the values for each material at particular values can be pre-computed and stored in
tables for interpolation.
Once the type of interaction is chosen, its products are sampled from the
appropriate distributions. Pseudorandom numbers are used to sample the energy,
angles and momenta from the differential distributions [37]. The original particle’s
state, if it survives, is altered to preserve energy and momentum.
The interaction of energetic photons with nuclei is discussed in the hadronic
section below. Often specialized tools are used to simulate optical photons and their
collection. It is possible, though, to generate optical photons and model reflection,
refraction and absorption on different types of surfaces. GEANT4 is able to do this.
11.3.5 Interactions of Charged Particles
The simulation of the electromagnetic (EM) interactions of charged particles is
complicated by the large cross section for elastic interactions and of ionization,
which produces low-energy electrons (delta electrons).
In a few cases it is useful to simulate every single interaction of a charged particle
in a medium, including its elastic collisions with nuclei, the ionization of atoms
and creation of delta electrons, and the ‘hard’ interactions, which create photons or
electron–positron pairs.
All production simulation tools estimate the cumulative effect of the elastic
scattering off nuclei. It is modeled in several different ways. Many utilize the
multiple-scattering approach pioneered by Goudsmit and Sanderson [49] as their
basis. One simple way to sample angular deviation over a short step is Moliere’s