Introduction to the section PHYS

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| Geant 3.15  |               GEANT User's Guide              | PHYS001  ##
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Author(s) : M. Maire                                   Submitted: 24.09.84
Origin    : GEANT3                                       Revised: 06.01.92

Summary of the Physics Processes

The computer simulation of particles traversing an experimental setup, has to take into account the interactions of these particles with the matter of the setup. GEANT is able to simulate the dominant processes which can occur in the energy range from 10 KeV up to 10 TeV.

Simulating a given process means:

  1. Evaluate the probability of occurrence of the process, by sampling the total cross-section of the process.
  2. Generate the final state after interaction, by sampling the differential cross-section of the process.
  3. In case of (quasi-)continuous processes, e.g. energy losses or multiple scattering, compute the mean values of some characteristic quantities.

    In the Table below we summarize all the processes currently implemented in GEANT3, with a reference to the corresponding routines. 

    
+---------------------------------------+-------------------------------------------
|                                       |                       |                  |
|                                       | Computation of        | Generation of    |
|                                       | total cross-section   | the final state  |
+---------------------------------------+-and/or-energy-losses--+-particles--------+
|                                       |                       |                  |
| Processes involving the Photon        |                       |                  |
|   +  -                                |                       |                  |
| (e ,e  )pair conversion               | PHYS 210              | PHYS 211         |
| Compton collision                     | PHYS 220              | PHYS 221         |
| Photo electric effect                 | PHYS 230              | PHYS 231         |
| Photo fission of heavy elements       | PHYS 240              | PHYS 240         |
|                                       |                       |                  |
| Rayleigh effect                       | PHYS 250              | PHYS 251         |
|                                       |                       |                  |
|                      -  +             |                       |                  |
| Processes involving e /e              |                       |                  |
| Multiple scattering                   |                       | PHYS 320 or325   |
| Ionisation and delta rays production  | PHYS 330              | PHYS 331 or 332  |
| Bremsstrahlung                        | PHYS 340              | PHYS 341         |
| Annihilation of positron              | PHYS 350              | PHYS 351         |
|                                       |                       |                  |
|                      -   +            |                       |                  |
| Processes involvingmu /mu             |                       |                  |
| Decay in flight                       | CONS 310              | PHYS 400         |
| Multiple scattering                   |                       | PHYS 320 or 325  |
|                                       |                       |                  |
| Ionisation and delta rays production  | PHYS 430              | PHYS 331 or 332  |
| Bremsstrahlung                        | PHYS 440              | PHYS 441         |
|          +  -                         |                       |                  |
| Direct (e ,e )pair production         | PHYS 450              | PHYS 451         |
| Nuclear interaction                   | PHYS 460              | PHYS 460         |
|                                       |                       |                  |
| Processes involving Hadrons           |                       |                  |
| Decay in flight                       | CONS 310              | PHYS 400         |
| Multiple scattering                   |                       | PHYS 320 or325   |
|                                       |                       |                  |
| Ionisation and delta rays production  | PHYS 430              | PHYS 331 or 332  |
+-Hadronic-interactions-----------------+-PHYS-500-or-510-------+-PHYS-500-or-510--+
|                                       |

    Simulated Processes

    Hadronic Interactions

    To simulate the interactions of hadrons with the nuclei of the matter traversed, two alternatives are provided:

    1. The generator of the FLUKA hadron shower montecarlo and the interface routines to GEANT3. See [PHYS520] for more information.
    2. The generator of the GHEISHA hadron shower montecarlo and the interface routines to GEANT3. See [PHYS510] for more information.

      The code both of the GHEISHA and of the FLUKA generators is contained in the GEANT3 library. Users should be aware that the routines of these packages do not follow the GEANT naming conventions and therefore they can clash with the names of user procedures.

      Electromagnetic Processes

      By mean of systematic fits to the existing data, the cross-sections of the electromagnetic processes are well reproduced (within few percents) from 10KeV up to 100GeV, both for light (low Z) and for heavy materials.

      This feature, together with the use of the interface with one of the hadronic shower generators available, should make GEANT useful for careful shower simulations even in a gas.

      The Muon interactions

      These are taken into account up to 10TeV, making GEANT useful for cosmic rays studies.

      Ionisation by Charged Particles

      Two alternatives are provided to simulate this process:

      1. Sampling of a Landau distribution around the mean value for the energy loss ([PHYS332]).
      2. Explicit generation of delta-rays (see [PHYS330/331/430]) and restricted Landau fluctuation below the energy threshold for the production of delta-rays.

        Full Landau fluctuations and generation of delta-rays cannot be used together to avoid double counting of the fluctuations. An automatic protection has been introduced in GEANT3 to this effect. See [PHYS333/332] and [BASE040] for further information.

        Multiple Scattering
        Two methods are provided:

        1. Gaussian approximation ([PHYS320]).
        2. Moli`ere distribution ([PHYS325]).

          The JMATE data structure

          In order to save time, the inverse of the total cross-section and the dE/dX of all processes involving photons, electrons and muons and the dE/dx values for protons, are tabulated at initialization for each material used in the tracking as a function of the kinetic energy of the incident particle. The actual value of the interaction length for a given process (i.e. the inverse of the macroscopic cross section) is then obtained via a linear interpolation in the tables. See [PHYS100] and [CONS199] for a more information on these tables.

          Probability of Interaction

          The total cross-section of each process is used at tracking time to evaluate the probability of occurrence of the process expressed as the the distance to the point where the interaction will occur. See [PHYS010] for an explanation of the method used.

          Note: The section PHYS is closely related to the section CONS. The user which would like to have a complete overview of the physics processes included in GEANT3 should read both sections.

          Control of the physical processes

          For most of the individual processes the default option (indicated below) can be changed via data cards [BASE030/040]. The processes are controlled via a control variable which is in the common /GCKING/. Below are reported the mnemonic names of the mechanisms and the corresponding control variables. If not otherwise noted, the meaning of the control variable is ithe following:

          = 0
          The process is completely ignored.
          = 1
          The process is considered and possible secondary particles generating from the interaction are put into the /GCKING/ common. If the interacting particle disappears in the interaction, then it is stopped with ISTOP=1 (common /GCTRAK/)
          = 2
          The process is considered. If secondary particles result from the interaction, they are not generated and their energy is simply added in the variable DESTEP (common /GCTRAK/. If the interacting particle disappears in the interaction, the variable ISTOP is set to 2.

          Below are listed the data card keywords, the flag names and values, and the resulting action.

          Keyword
          Related process
          DCAY
          Decay in flight. The decaying particles stops. The variable IDCAY controls this process.
          IDCAY =0
          No decay in flight.
          =1
          (D) Decay in flight with generation of secondaries.
          =2
          Decay in flight without generation of secondaries.
          MULS
          Multiple scattering. The variable IMULS controls this process.
          IMULS =0
          No multiple scattering.
          =1
          (D) Multiple scattering according to Moli`ere theory.
          =2
          Same as 1. Kept for backward compatibility.
          =3
          Pure Gaussian scattering according to the Rossi formula.
          PFIS
          Nuclear fission induced by a photon. The photon stops. The variable IPFIS controls this process.
          IPFIS =0
          (D) No photo-fission.
          =1
          Photo-fission with generation of secondaries.
          =2
          Photo-fission without generation of secondaries.
          MUNU
          Muon-nucleus interactions. The muon is not stopped. The variable IMUNU controls this process.
          IMUNU =0
          No muon-nuclues interactions.
          =1
          (D) Muon-nucleus interactions with generation of secondaries.
          =2
          Muon-nucleus interactions without generation of secondaries.
          LOSS
          Continuous energy loss. The variable ILOSS controls this process.
          ILOSS =0
          No continuous energy loss.
          =1
          Continuous energy loss with generation of delta-rays above DCUTE (common /GCUTS/) and restricted Landau fluctuations below DCUTE.
          =2
          (D) Continuous energy loss without generation of delta-rays and full Landau-Vavilov-Gauss fluctuations. In this case the variable IDRAY is forced to 0 to avoid double counting of fluctuations.
          =3
          Same as 1, kept for backward compatibility.
          =4
          Energy loss without fluctuation. The value obtained from the tables is used directly.
          PHOT
          Photo-electric effect. The interacting gamma is stopped. The variable IPHOT controls this process.
          IMUNU =0
          No photo-electric effect.
          =1
          (D) Photo-electric effect with generation of the electron.
          =2
          Photo-electric effect without generation of the electron.
          COMP
          Compton scattering. The variable ICOMP controls this process.
          ICOMP =0
          No Compton scattering.
          =1
          (D) Compton scattering with generation of - e .
          =2
          Compton scattering without generation of - e .
          PAIR
          Pair production. The interacting gamma is stopped. The variable IPAIR controls this process.
          IPAIR =0
          No pair production.
          =1
          (D) Pair production with generation of - + e /e .
          =2
          Pair production without generation of - + e /e .
          -
          BREM
          Bremstrahlung. The interacting particle (e , + + - e , mu , mu ) is not stopped. The variable IBREM controls this process.
          IBREM =0
          No bremstrahlung.
          =1
          (D) Bremstrahlung with generation of gamma.
          =2
          Bremstrahlung without generation of gamma.
          RAYL
          Rayleigh effect. The interacting gamma is not stopped. The variable IRAYL controls this process.
          IRAYL =0
          (D) No Rayleigh effect.
          =1
          Rayleigh effect.
          DRAY
          delta-ray production. The variable IDRAY controls this process.
          IDRAY =0
          No delta-rays production.
          =1
          (D) delta-rays production with generation - of e .
          =2
          delta-rays production without generation of - e .
          +
          ANNI
          Positron annichilation. The e is stopped. The variable IANNI controls this process.
          IANNI =0
          No positron annichilation.
          =1
          (D) Positron annichilation with generation of gamma's.
          =2
          Positron annichilation without generation of gamma's.
          HADR
          Hadronic interactions. The particle is stopped in case of inelastic interaction, while it is not stopped in case of elastic interaction. The variable IHADR controls this process.
          IHADR =0
          No hadronic interactions.
          =1
          (D) Hadronic interactions with generation of secondaries.
          =2
          Hadronic interactions without generation of secondaries.