System of units

HepSystemOfUnits

GEANT4 offers the user the possibility to choose and use the units he prefers for any quantity. In fact the Geant4 kernel takes care of the units. Internaly it uses a consistent set on units based on:

                millimeter              (mm)
                nanosecond              (ns)
                Mega electron Volt      (MeV)
                positron charge         (eplus)
                degree Kelvin           (kelvin)
                the amount of substance (mole)
                luminous intensity      (candela)
                radian                  (radian)
                steradian               (steradian)

All the others units are defined from the basic ones.
In the file: source/global/management/include/SystemOfUnits.h you will find all these definitions. That file is part of CLHEP.

      G4UnitDefinition::PrintUnitsTable();
      Idle> /units/list

Input your data

Avoid 'hard coded' data

You must give the unit of the data you are going to introduce:

G4double Size = 15*km, KineticEnergy = 90.3*GeV, density = 11*mg/cm3;

If the unit is not specified, it is understood that the data is implicitely in the internal G4 system, but this is discouraged.

If the data set comes from an array or from an external file, it is strongly recommanded to set the unit as soon as the data are read, before any treatement. For instance, immediatly after to have read a file of cross sections expressed in millibarn:

  for (int j=0, j<jmax, j++) CrossSection[j] *= millibarn;
  ........................................................
  ......  my calculations .................................

interactive commands

Some built-in commands from the User Interface (UI) also require the unit.

For instance:

    /gun/energy 15.2 keV
    /gun/position 3 2 -7 meter

If the unit is not specified or not valid the command is refused.

Output your data

• You can output the data on the unit you wish. To do so it is sufficient to DIVIDE the data by the corresponding unit:

       
        G4cout << KineticEnergy/keV << " keV" ;
        G4cout << density/(g/cm3)   << " g/cm3" ;
  

  (of course: G4cout << KineticEnergy will print the energy in the internal system of units)

• There is another way to output the data; lets Geant4 chooses the most approriate unit to the actual numerical value of your data. It is sufficient to specify to which category your data belong (Length,Time, Energy ..etc..) for example

  
       G4cout << G4BestUnit(StepSize, "Length");
  

  StepSize will be printed in km, m, mm or ... fermi depending of its actual value.

Introduce new units

you may wish introduce new units.

• you can complete the file SystemOfUnits.h :
  
  #include "SystemOfUnits.h"
  static const G4double inch = 2.54*cm;

  In this way it is not easy to define composed units.

• you can instantiate an object of the class G4UnitDefinition:
  
  G4UnitDefinition ( name, symbol, category, value)

  example 1: define a few units for speed

             
  G4UnitDefinition ("km/hour" , "km/h", "Speed", km/(3600*s) );
  G4UnitDefinition ("meter/ns", "m/ns", "Speed", m/ns );
  

  The category "Speed" does not exist by default in G4UnitsTable, but it will be created automatically.

  example 2: introduce pseudo-length and pseudo-energy

             
  G4UnitDefinition ("g/cm2" , "g/cm2" , "speudoLength",  g/cm2);
  G4UnitDefinition ("mg/cm2", "mg/cm2", "speudoLength", mg/cm2);
           
  G4UnitDefinition ("MeV.g-1.cm2", "MeV.g-1.cm2", "speudoEnergy", MeV*cm2/g );
  

  The class G4UnitDefinition is located in source/global/management

Change the internal system of units

The user is free to change the system of units to be used internally by the G4 kernel.
To do so, one must redefine the basic units in SystemOfUnits.h and recompile the full Geant4 kernel, since the units are heavily used elsewhere (especially in the phycics sector).
Indeed the full Geant4 code is written respecting the above conventions and this makes it independent of the units chosen by the user.
The file source/global/management/include/G4UnitsTest.hh is an example of redefinition of the units: International System of units.
example of a shower profile

more examples can be found in source/global/management/test/G4UnitsTableTest.cc

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