source: trunk/documents/UserDoc/DocBookUsersGuides/ForApplicationDeveloper/xml/GettingStarted/physicsDef.xml@ 905

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<!--  [History]                                               -->
<!--    Changed by: Katsuya Amako, 15-Jul-1998                -->
<!--    Proof read by: Joe Chuma,  15-Jun-1999                -->
<!--    Changed by: Hisaya Kurashige, 28-Oct-2001             -->
<!--    Changed by: Dennis Wright, 29-Nov-2001                -->
<!--    Converted to DocBook: Katsuya Amako, Aug-2006         -->
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<!-- ******************* Section (Level#1) ****************** -->
<sect1 id="sect.HowToSpecPhysProc">
<title>
How to Specify Physics Processes
</title>

<!-- ******************* Section (Level#2) ****************** -->
<sect2 id="sect.HowToSpecPhysProc.PhysProc">
<title>
Physics Processes
</title>

<para>
Physics processes describe how particles interact with materials.
Geant4 provides seven major categories of processes:

<itemizedlist spacing="compact">
  <listitem><para>
  electromagnetic,
  </para></listitem>
  <listitem><para>
  hadronic,
  </para></listitem>
  <listitem><para>
  transportation,
  </para></listitem>
  <listitem><para>
  decay,
  </para></listitem>
  <listitem><para>
  optical,
  </para></listitem>
  <listitem><para>
  photolepton_hadron, and
  </para></listitem>
  <listitem><para>parameterisation.
  </para></listitem>
</itemizedlist>
</para>

<para>
All physics processes are derived from the <emphasis>G4VProcess</emphasis>
base class. Its virtual methods

<itemizedlist spacing="compact">
  <listitem><para>
  <literal>AtRestDoIt</literal>,
  </para></listitem>
  <listitem><para>
  <literal>AlongStepDoIt</literal>, and
  </para></listitem>
  <listitem><para>
  <literal>PostStepDoIt</literal>
  </para></listitem>
</itemizedlist>

and the corresponding methods

<itemizedlist spacing="compact">
  <listitem><para>
  <literal>AtRestGetPhysicalInteractionLength</literal>,
  </para></listitem>
  <listitem><para>
  <literal>AlongStepGetPhysicalInteractionLength</literal>, and
  </para></listitem>
  <listitem><para>
  <literal>PostStepGetPhysicalInteractionLength</literal>
  </para></listitem>
</itemizedlist>

describe the behavior of a physics process when they are
implemented in a derived class. The details of these methods are
described in 
<xref linkend="sect.PhysProc" />.
</para>

<para>
The following are specialized base classes to be used for simple
processes:

<variablelist>
  <varlistentry>
    <term><emphasis>G4VAtRestProcess</emphasis></term>
    <listitem>
    Processes with only <literal>AtRestDoIt</literal>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis>G4VContinuousProcess</emphasis></term>
    <listitem>
    Processes with only <literal>AlongStepDoIt</literal>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term><emphasis>G4VDiscreteProcess</emphasis></term>
    <listitem>
    processes with only <literal>PostStepDoIt</literal>
    </listitem>
  </varlistentry>
</variablelist>
</para>

<para>
Another 4 virtual classes, such as
<emphasis>G4VContinuousDiscreteProcess</emphasis>, are provided for complex
processes.
</para>

</sect2>

<!-- ******************* Section (Level#2) ****************** -->
<sect2 id="sect.HowToSpecPhysProc.ManagingProc">
<title>
Managing Processes
</title>

<para>
The <emphasis>G4ProcessManager</emphasis> class contains a list of processes that
a particle can undertake. It has information on the order of
invocation of the processes, as well as which kind of <literal>DoIt</literal>
method is valid for each process in the list. A
<emphasis>G4ProcessManager</emphasis> object corresponds to each particle and is
attached to the <emphasis>G4ParticleDefiniton</emphasis> class.
</para>

<para>
In order to validate processes, they should be registered with
the particle's <emphasis>G4ProcessManager</emphasis>. Process ordering
information is included by using the <literal>AddProcess()</literal> and
<literal>SetProcessOrdering()</literal> methods. For registration of simple
processes, the <literal>AddAtRestProcess()</literal>,
<literal>AddContinuousProcess()</literal> and <literal>AddDiscreteProcess()</literal>
methods may be used.
</para>

<para>
<emphasis>G4ProcessManager</emphasis> is able to turn some processes on or off
during a run by using the <literal>ActivateProcess()</literal> and
<literal>InActivateProcess()</literal> methods. These methods are valid only
after process registration is complete, so they must not be used in
the <emphasis>PreInit</emphasis> phase.
</para>

<para>
The <emphasis>G4VUserPhysicsList</emphasis> class creates and attaches
<emphasis>G4ProcessManager</emphasis> objects to all particle classes defined in
the <literal>ConstructParticle()</literal> method.
</para>

</sect2>


<!-- ******************* Section (Level#2) ****************** -->
<sect2 id="sect.HowToSpecPhysProc.SpecPhysProc">
<title>
Specifying Physics Processes
</title>

<para>
<emphasis>G4VUserPhysicsList</emphasis> is the base class for a "mandatory user
class" (see <xref linkend="sect.HowToDefMain" />), in which all physics 
processes and all particles required in a simulation must be registered. 
The user must create a class derived from
<emphasis>G4VUserPhysicsList</emphasis> and implement the pure virtual method
<literal>ConstructProcess()</literal>.
</para>

<para>
For example, if just the <emphasis>G4Geantino</emphasis> particle class is
required, only the transportation process need be registered. The
<literal>ConstructProcess()</literal> method would then be implemented as
follows:

<example id="programlist_HowToSpecPhysProc_1">
<title>
Register processes for a geantino.
</title>
<programlisting>
  void ExN01PhysicsList::ConstructProcess()
  {
    // Define transportation process
    AddTransportation();
  }
</programlisting>
</example>

Here, the <literal>AddTransportation()</literal> method is provided in the
<emphasis>G4VUserPhysicsList</emphasis> class to register the
<emphasis>G4Transportation</emphasis> class with all particle classes. The
<emphasis>G4Transportation</emphasis> class (and/or related classes) describes
the particle motion in space and time. It is the mandatory process
for tracking particles.
</para>

<para>
In the <literal>ConstructProcess()</literal> method, physics processes
should be created and registered with each particle's instance of
<emphasis>G4ProcessManager</emphasis>.
</para>

<para>
An example of process registration is given in the
<emphasis>G4VUserPhysicsList</emphasis>::<literal>AddTransportation()</literal> 
method.
</para>

<para>
Registration in <emphasis>G4ProcessManager</emphasis> is a complex procedure
for other processes and particles because the relations between
processes are crucial for some processes. Please see 
<xref linkend="sect.PhysProc" /> and the example codes.
</para>

<para>
An example of electromagnetic process registration for photons
is shown below:

<example id="programlist_HowToSpecPhysProc_2">
<title>
Register processes for a gamma.
</title>
<programlisting>
void MyPhysicsList::ConstructProcess()
{
  // Define transportation process
  AddTransportation();
  // electromagnetic processes
  ConstructEM();
}
void MyPhysicsList::ConstructEM()
{
   //  Get the process manager for gamma
  G4ParticleDefinition* particle = G4Gamma::GammaDefinition(); 
  G4ProcessManager* pmanager = particle-&gt;GetProcessManager();

  // Construct processes for gamma
  G4PhotoElectricEffect * thePhotoElectricEffect = new G4PhotoElectricEffect();      
  G4ComptonScattering * theComptonScattering = new G4ComptonScattering();
  G4GammaConversion* theGammaConversion = new G4GammaConversion();

  // Register processes to gamma's process manager
  pmanager-&gt;AddDiscreteProcess(thePhotoElectricEffect);
  pmanager-&gt;AddDiscreteProcess(theComptonScattering);
  pmanager-&gt;AddDiscreteProcess(theGammaConversion);
}
</programlisting>
</example>
</para>


</sect2>
</sect1>