source: trunk/documents/UserDoc/DocBookUsersGuides/ForApplicationDeveloper/xml/TrackingAndPhysics/physicsTable.xml @ 904

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<!--    Created by: Hisaya Kurashige,      2-Dec-2004         -->
<!--    Converted to DocBook: Katsuya Amako, Aug-2006         -->
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<!-- ******************* Section (Level#1) ****************** -->
<sect1 id="sect.PhysTab">
<title>
Physics Table
</title>

<!-- ******************* Section (Level#2) ****************** -->
<sect2 id="sect.PhysTab.Gene">
<title>
General Concepts
</title>

<para>
In Geant4, physics processes use many tables of cross sections,
energy losses and other physics values. Before the execution of an
event loop, the <literal>BuildPhysicsTable()</literal> method of
<emphasis>G4VProcess</emphasis> is invoked for all processes and 
cross section tables are prepared. "Standard" electromagnetic 
processes calculate cross section and/or energy loss values for 
each material and for each production cut value assigned to each 
material. A change in production cut values therefore require these 
cross sections to be re-calculated. Cross sections for hadronic 
processes do not depend on the production cut.
</para>

<para>
The <emphasis>G4PhysicsTable</emphasis> class is used to handle cross section
tables. <emphasis>G4PhysicsTable</emphasis> is a collection of instances of
<emphasis>G4PhysicsVector</emphasis> (and derived classes), each of which has
cross section values for a particle within a given energy range
traveling in a material.
</para>

</sect2>


<!-- ******************* Section (Level#2) ****************** -->
<sect2 id="sect.PhysTab.MateCoup">
<title>
Material-Cuts Couple
</title>

<para>
Users can assign different production cuts to different regions
(see <xref linkend="sect.CutReg" />). 
This means that if the same material is used in regions with different 
cut values, the processes need to prepare several different cross
sections for that material.
</para>

<para>
The <emphasis>G4ProductionCutsTable</emphasis> has 
<emphasis>G4MaterialCutsCouple</emphasis>
objects, each of which consists of a material paired with a cut
value. These <emphasis>G4MaterialCutsCouple</emphasis>s are numbered with an
index which is the same as the index of a <emphasis>G4PhysicsVector</emphasis>
for the corresponding <emphasis>G4MaterialCutsCouple</emphasis>in the
<emphasis>G4PhysicsTable</emphasis>. The list of 
<emphasis>MaterialCutsCouple</emphasis>s used
in the current geometry setup is updated before starting the event
loop in each run.
</para>

</sect2>


<!-- ******************* Section (Level#2) ****************** -->
<sect2 id="sect.PhysTab.Fio">
<title>
File I/O for the Physics Table
</title>

<para>
Calculated physics tables for "Standard" electromagnetic processes
can be stored in files. The user may thus eliminate the time
required for the calculation of physics tables by retrieving them
from the files.
</para>

<para>
Using the built-in user command "<emphasis
role="bold">storePhysicsTable</emphasis>"  
(see <xref linkend="sect.BuiltinCom" />), 
stores physics tables in files. Information on materials and
cuts defined in the current geometry setup are stored together with
physics tables because calculated values in the physics tables
depend on <emphasis>MaterialCutsCouple</emphasis>. Note that physics tables are
calculated before the event loop, not in the initialization phase.
So, at least one event must be executed before using the
"<emphasis role="bold">storePhysicsTable</emphasis>" command.
</para>

<para>
Calculated physics tables can be retrieved from files by using
the "<emphasis role="bold">retrievePhysicsTable</emphasis>" command. Materials 
and cuts from files are compared with those defined in the current geometry
setup, and only physics vectors corresponding to the
<emphasis>MaterialCutsCouple</emphasis>s used in the current setup are restored.
Note that nothing happens just after the
"<emphasis role="bold">retrievePhysicsTable</emphasis>" command is issued. 
Restoration of physics tables will be executed in parallel with the calculation 
of physics tables.
</para>

</sect2>


<!-- ******************* Section (Level#2) ****************** -->
<sect2 id="sect.PhysTab.Build">
<title>
Building the Physics Table
</title>

<para>
In the <literal>G4RunManagerKernel::RunInitialization()</literal> method,
after the list of <emphasis>MaterialCutsCouple</emphasis>s is updated, the
<literal>G4VUserPhysicsList::BuildPhysicsTable()</literal> method is invoked
to build physics tables for all processes.
</para>

<para>
Initially, the <literal>G4VProcess::PreparePhysicsTable()</literal> method
is invoked. Each process creates <emphasis>G4PhysicsTable</emphasis> objects as
necessary. It then checks whether the <emphasis>MaterialCutsCouple</emphasis>s
have been modified after a run to determine if the corresponding
physics vectors can be used in the next run or need to be
re-calculated.
</para>

<para>
Next, the <literal>G4VProcess::RetrievePhysicsTable()</literal> method is
invoked if the <literal>G4VUserPhysicsList::fRetrievePhysicsTable</literal>
flag is asserted. After checking materials and cuts in files,
physics vectors corresponding to the <emphasis>MaterialCutsCouple</emphasis>s
used in the current setup are restored.
</para>

<para>
Finally, the <literal>G4VProcess::BuildPhysicsTable()</literal> method is
invoked and only physics vectors which need to be re-calculated are
built.
</para>


</sect2>
</sect1>