source: trunk/documents/UserDoc/DocBookUsersGuides/ForApplicationDeveloper/xml/Fundamentals/unitSystem.xml @ 1013

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<!--  [History]                                               -->
<!--    Converted to DocBook: Katsuya Amako, Aug-2006         -->
<!--    Changed by: Katsuya Amako, 14-Jul-1998                -->
<!--    Changed by: Katsuya Amako, 30-Nov-1998                -->
<!--    Changed by: Dennis Wright, 20-Nov-2001                -->
<!--    Proof read by: Joe Chuma,  28-Jun-1999                -->
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<!-- ******************* Section (Level#1) ****************** -->
<sect1 id="sect.SysUni">
<title>
System of units
</title>

<!-- ******************* Section (Level#2) ****************** -->
<sect2 id="sect.SysUni.BasicUni">
<title>
Basic units
</title>

<para>
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. Internally it uses a consistent set on units
based on the <literal>HepSystemOfUnits</literal>:

<informalexample>
<programlisting>
                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)
</programlisting>
</informalexample>
</para>

<para>
All other units are defined from the basic ones.
</para>

<para>
For instance:

<informalexample>
<programlisting>
                 millimeter = mm = 1;
                 meter = m = 1000*mm;
                 ...
                 m3 = m*m*m;
                 ...
</programlisting>
</informalexample>
</para>

<para>
In the file
<literal>source/global/management/include/SystemOfUnits.h</literal> you will
find all of these definitions. That file is part of CLHEP.
</para>

<para>
Moreover, the user is free to change the system of units to be
used by the kernel.
</para>

</sect2>

<!-- ******************* Section (Level#2) ****************** -->
<sect2 id="sect.SysUni.InputData">
<title>
Input your data
</title>

<!-- ******************* Section (Level#3) ****************** -->
<sect3 id="sect.SysUni.InputData.AvoidHardCoded">
<title>
Avoid 'hard coded' data
</title>

<para>
You <emphasis role="bold">must</emphasis> give the units for the 
data you are going to
introduce:

<informalexample>
<programlisting>
 G4double Size = 15*km, KineticEnergy = 90.3*GeV, density = 11*mg/cm3;
</programlisting>
</informalexample>
</para>

<para>
Indeed, the full Geant4 code is written respecting these
specifications, and this makes it independent of the units chosen
by the user.
</para>

<para>
If the units are not specified, it is understood that the data
is implicitly in the internal G4 system, but this is strongly
discouraged.
</para>

<para>
If the data set comes from an array or from an external file, it
is strongly recommended to set the units as soon as the data are
read, before any treatment. For instance:

<informalexample>
<programlisting>
  for (int j=0, j&lt;jmax, j++) CrossSection[j] *= millibarn;
  ...
  my calculations
  ...
</programlisting>
</informalexample>
</para>

</sect3>

<!-- ******************* Section (Level#3) ****************** -->
<sect3 id="sect.SysUni.InputData.Interactive">
<title>
Interactive commands
</title>

<para>
Some built-in commands from the User Interface (UI) also require
the units to be specified.
</para>

<para>
For instance:

<informalexample>
<programlisting>
    /gun/energy 15.2 keV
    /gun/position 3 2 -7 meter
</programlisting>
</informalexample>
</para>

<para>
If the units are not specified, or are not valid, the command is
refused.
</para>

</sect3>
</sect2>


<!-- ******************* Section (Level#2) ****************** -->
<sect2 id="sect.SysUni.OutputData">
<title>
Output your data
</title>

<para>
You can output your data with the units you wish. To do so, it is
sufficient to <emphasis role="bold">divide</emphasis> the data by the 
corresponding unit:

<informalexample>
<programlisting>     
      G4cout &lt;&lt; KineticEnergy/keV &lt;&lt; " keV";
      G4cout &lt;&lt; density/(g/cm3)   &lt;&lt; " g/cm3";
</programlisting>
</informalexample>     
</para>

<para>
Of course, <literal>G4cout &lt;&lt; KineticEnergy</literal> will print the
energy in the internal units system.
</para>

<para>
There is another way to output your data. Let Geant4 choose the
most appropriate units for the actual numerical value of your data.
It is sufficient to specify to which category your data belong
(Length, Time, Energy, etc.). For example

<informalexample>
<programlisting>     
      G4cout &lt;&lt; G4BestUnit(StepSize, "Length");
</programlisting>
</informalexample>     
</para>

<para>
<literal>StepSize</literal> will be printed in km, m, mm, fermi, etc.
depending of its actual value.
</para>

</sect2>


<!-- ******************* Section (Level#2) ****************** -->
<sect2 id="sect.SysUni.IntroNewUnits">
<title>
Introduce new units
</title>

<para>
If you wish to introduce new units, there are two methods:

<itemizedlist spacing="compact">
  <listitem><para>
    You can complete the file <literal>SystemOfUnits.h</literal>

    <informalexample>
    <programlisting>       
      #include "SystemOfUnits.h"
      
      static const G4double inch = 2.54*cm;
    </programlisting>
    </informalexample>      

    Using this method, it is not easy to define composed units. It
    is better to do the following:
  </para></listitem>
  <listitem><para>
    You can instantiate an object of the class
    <emphasis>G4UnitDefinition</emphasis>

    <informalexample>
    <programlisting>       
      G4UnitDefinition ( name, symbol, category, value )
    </programlisting>
    </informalexample>       

    For example: define a few units for speed

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

    The category "Speed" does not exist by default in
    <emphasis>G4UnitsTable</emphasis>, but it will be created automatically.
    The class <emphasis>G4UnitDefinition</emphasis> is located in
    <literal>source/global/management</literal>.
  </para></listitem>
</itemizedlist>
</para>

</sect2>

<!-- ******************* Section (Level#2) ****************** -->
<sect2 id="sect.SysUni.PrintUnits">
<title>
Print the list of units
</title>

<para>
You can print the list of units with the static function:
<literal>G4UnitDefinition::PrintUnitsTable();</literal>
</para>

<para>
or with the interactive command: <literal>/units/list</literal>
</para>


</sect2>
</sect1>