11 Detector Simulation
501
model [82] is implemented in DPMJET [21] and used in FLUKA. The Quark Gluon
String (QGS) Model [83] in different variants is used in MARS/MCNPX, GEANT4
and in the QGSJET event generator [84]. A third model, the Fritiof model, is used
and extended in the FTF model in GEANT4.
Hadrons are produced in the initial collisions and the decay of QCD strings,
tubes composed of compressed gluonic fields [10] generated by the separation of
colored quarks. Models implement a Lund-like string. The proto-hadrons generated
by string decay, once past their formation time, together with outgoing nucleons, and
the remaining cluster of nucleons are handed to a cascade for additional scattering
in the nucleus (e.g. in FLUKA and in GEANT4 QGS_BIC and FTF_BIC physics
lists using the Binary cascade), or passed directly to a pre-compound module (in the
GEANT4 QGSP_BERT and FTFP_BERT physics list).
Developments over the last 10–15 years include a high-energy extension of
the PEANUT cascade in FLUKA to undertake reactions handled previously by
DPMJET; the connection of the Binary cascade in GEANT4 with the QGS model
(as QGS_BIC) to re-scatter the slow products of high-energy model. More recent
developments include the extensive improvement of the Fritiof-based FTF model in
GEANT4 to model light anti-nucleus interactions at low energies and at rest [85],
and to include an internal Reggeon cascade, and changes in the production ratio of
different types of diquarks [86].
11.3.10 Treatment of Low-Energy Neutron Interactions
Amongst particles created by hadronic interactions, neutrons survive a longer
time and are among those which travel the furthest. Also, they are amongst the
most numerous. This makes their treatment important for many applications and
correspondingly expensive computationally. Most neutrons are emitted in the deexcitation phase of a reaction and have energies of order MeV.
Neutrons produced in high-energy interactions (above 20 MeV) also lose energy
in elastic and inelastic interactions (which release protons, alpha particles or light
ions—or create gamma rays) before being captured by nuclei. Only part of this
process occurs on fast timescales and others are much slower (μs–ms). By tracking
time, it is possible to emulate the time dependence of the signal. This also allows the
simulation tool to use a time threshold, and abandon neutron tracks after this time,
in order to save computation time.
Neutrons’ contribution to the visible energy measured in a detector comes via
the transfer of energy to charged particles and from capture and other reactions with
nuclei that generate gammas. Elastic scattering is particularly important in organic
scintillators, where interactions with hydrogen transfer significant parts of energy
and momentum to the recoil proton.
In some cases, are treatment of neutron interactions at a greater level of detail
is required, potentially down to thermal energies. In HEP it is needed in special
cases, such as the study of activation of materials for radiation safety purposes.
501
model [82] is implemented in DPMJET [21] and used in FLUKA. The Quark Gluon
String (QGS) Model [83] in different variants is used in MARS/MCNPX, GEANT4
and in the QGSJET event generator [84]. A third model, the Fritiof model, is used
and extended in the FTF model in GEANT4.
Hadrons are produced in the initial collisions and the decay of QCD strings,
tubes composed of compressed gluonic fields [10] generated by the separation of
colored quarks. Models implement a Lund-like string. The proto-hadrons generated
by string decay, once past their formation time, together with outgoing nucleons, and
the remaining cluster of nucleons are handed to a cascade for additional scattering
in the nucleus (e.g. in FLUKA and in GEANT4 QGS_BIC and FTF_BIC physics
lists using the Binary cascade), or passed directly to a pre-compound module (in the
GEANT4 QGSP_BERT and FTFP_BERT physics list).
Developments over the last 10–15 years include a high-energy extension of
the PEANUT cascade in FLUKA to undertake reactions handled previously by
DPMJET; the connection of the Binary cascade in GEANT4 with the QGS model
(as QGS_BIC) to re-scatter the slow products of high-energy model. More recent
developments include the extensive improvement of the Fritiof-based FTF model in
GEANT4 to model light anti-nucleus interactions at low energies and at rest [85],
and to include an internal Reggeon cascade, and changes in the production ratio of
different types of diquarks [86].
11.3.10 Treatment of Low-Energy Neutron Interactions
Amongst particles created by hadronic interactions, neutrons survive a longer
time and are among those which travel the furthest. Also, they are amongst the
most numerous. This makes their treatment important for many applications and
correspondingly expensive computationally. Most neutrons are emitted in the deexcitation phase of a reaction and have energies of order MeV.
Neutrons produced in high-energy interactions (above 20 MeV) also lose energy
in elastic and inelastic interactions (which release protons, alpha particles or light
ions—or create gamma rays) before being captured by nuclei. Only part of this
process occurs on fast timescales and others are much slower (μs–ms). By tracking
time, it is possible to emulate the time dependence of the signal. This also allows the
simulation tool to use a time threshold, and abandon neutron tracks after this time,
in order to save computation time.
Neutrons’ contribution to the visible energy measured in a detector comes via
the transfer of energy to charged particles and from capture and other reactions with
nuclei that generate gammas. Elastic scattering is particularly important in organic
scintillators, where interactions with hydrogen transfer significant parts of energy
and momentum to the recoil proton.
In some cases, are treatment of neutron interactions at a greater level of detail
is required, potentially down to thermal energies. In HEP it is needed in special
cases, such as the study of activation of materials for radiation safety purposes.