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| 8 | <!-- Changed by: Katsuya Amako, 4-Aug-1998 -->
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| 9 | <!-- Changed by: Katsuya Amako, 9-Jul-1998 -->
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| 10 | <!-- Proof read by: Joe Chuma, 2-Jul-1999 -->
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| 11 | <!-- few corrections in 3.18.7: mma, 11-Jan-2001 -->
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| 12 |
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| 13 | <BR>
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| 14 |
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| 15 | <TABLE WIDTH="100%" >
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| 16 | <TR>
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| 17 | <TD>
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| 18 |
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| 19 | </A>
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| 20 | <A HREF="index.html">
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| 21 | <IMG SRC="../../../../resources/html/IconsGIF/Contents.gif" ALT="Contents" HEIGHT=16 WIDTH=59></A>
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| 22 | <A HREF="particle.html">
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| 26 | </TD>
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| 27 | <P>
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| 28 |
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| 29 | <TD ALIGN=RIGHT><FONT COLOR="#238E23"><FONT SIZE=-1>
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| 30 | <B>Geant4 User's Guide</B></FONT></FONT>
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| 31 | <BR><FONT COLOR="#238E23"><FONT SIZE=-1>
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| 32 | <B>For Application Developers</B></FONT></FONT>
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| 33 | <BR><FONT COLOR="#238E23"><FONT SIZE=-1>
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| 34 | <B>Tracking and Physics</B></FONT></FONT></TD>
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| 35 | </TR>
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| 36 | </TABLE>
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| 37 | <P><BR>
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| 38 |
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| 39 | <CENTER><FONT COLOR="#238E23"><FONT SIZE=+3>
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| 40 | <B>5.4 Production Threshold versus Tracking Cut</B></FONT></FONT>
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| 41 | </CENTER>
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| 42 | <P><BR>
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| 43 |
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| 44 | <HR ALIGN="Center" SIZE="7%">
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| 45 | <P>
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| 46 |
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| 47 | <a name="5.4.1">
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| 48 | <H2>5.4.1 General considerations</H2></a>
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| 49 |
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| 50 | We have to fulfill two contradictory requirements. It is the responsibility
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| 51 | of each individual <b>process</b> to produce secondary particles according to its
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| 52 | own capabilities. On the other hand, it is only the Geant4 kernel (i.e., tracking)
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| 53 | which can ensure an overall coherence of the simulation.
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| 54 | <P>
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| 55 | The general principles in Geant4 are the following:
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| 56 | <p>
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| 57 | <OL>
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| 58 | <LI>Each <b>process</b> has its intrinsic limit(s) to produce secondary
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| 59 | particles.</LI>
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| 60 | <LI>All particles produced (and accepted) will be tracked up to <b>zero
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| 61 | range</b>.</LI>
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| 62 | <LI>Each <b>particle</b> has a suggested cut in range (which is converted
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| 63 | to energy for all materials), and defined via a <tt>SetCut()</tt>
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| 64 | method (see
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| 65 | <a href="../GettingStarted/particleDef.html#2.4.2">Section 2.4.2</a>).</LI>
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| 66 | </OL>
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| 67 | <p>
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| 68 | Points 1 and 2 imply that the cut associated with the <b>particle</b> is a
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| 69 | (recommended) <b>production</b> threshold of secondary particles.
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| 70 | <P>
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| 71 |
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| 72 | <HR>
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| 73 | <a name="5.4.2">
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| 74 | <H2>5.4.2 Set production threshold (<tt>SetCut</tt> methods)</H2></a>
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| 75 |
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| 76 | As already mentioned, each kind of particle has a suggested production
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| 77 | threshold. Some of the processes will not use this threshold (e.g., decay),
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| 78 | while other processes will use it as a default value for their intrinsic
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| 79 | limits (e.g., ionisation and bremsstrahlung).
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| 80 | <P>
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| 81 | See <a href="../GettingStarted/particleDef.html#2.4.2">Section 2.4.2</a> to
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| 82 | see how to set the production threshold.
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| 83 | <p>
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| 84 |
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| 85 | <HR>
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| 86 | <a name="5.4.3">
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| 87 | <H2>5.4.3 Apply cut</H2></a>
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| 88 |
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| 89 | The <tt>DoIt</tt> methods of each process can produce secondary particles. Two cases
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| 90 | can happen:
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| 91 | <p>
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| 92 | <UL>
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| 93 | <LI>a process sets its intrinsic limit greater than or equal to the recommended
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| 94 | production threshold. OK. Nothing has to be done (nothing can be done !).</LI>
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| 95 | <LI>a process sets its intrinsic limit smaller than the production threshold
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| 96 | (for instance 0).</LI>
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| 97 | <P>
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| 98 | The list of secondaries is sent to the <i>SteppingManager</i> via a <i>ParticleChange</i>
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| 99 | object.
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| 100 | </UL>
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| 101 | <p>
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| 102 | BEFORE being recopied to the temporary stack for later tracking, the particles
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| 103 | below the production threshold will be kept or deleted according to the
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| 104 | safe mechanism explained hereafter.
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| 105 | <p>
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| 106 | <UL>
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| 107 | <LI>The <i>ParticleDefinition</i> (or <i>ParticleWithCuts</i>) has a boolean data member:
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| 108 | <tt>ApplyCut</tt>.</LI>
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| 109 | <LI><tt>ApplyCut</tt> is OFF: do nothing. All the secondaries are stacked (and then
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| 110 | tracked later on), regardless of their initial energy. The Geant4 kernel respects
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| 111 | the best that the physics can do, but neglects the overall coherence and
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| 112 | the efficiency. Energy conservation is respected as far as the processes
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| 113 | know how to handle correctly the particles they produced!</LI>
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| 114 | <LI><tt>ApplyCut</tt> in ON: the <i>TrackingManager</i> checks the range of each secondary
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| 115 | against the production threshold and against the safety. The particle is
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| 116 | stacked if <TT>range > min(cut,safety).</TT></LI>
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| 117 | <UL>
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| 118 | <LI>If not, check if the process has nevertheless set the flag ``good for tracking''
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| 119 | and then stack it (see Section 5.4.4 below for the explanation of the
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| 120 | <tt>GoodForTracking</tt> flag).</LI>
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| 121 | <LI>If not, recuperate its kinetic energy in the <TT>localEnergyDeposit</TT>,
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| 122 | and set <TT>tkin=0</TT>.</LI>
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| 123 | <LI>Then check in the <i>ProcessManager</i> if the vector of <i>ProcessAtRest</i> is not
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| 124 | empty. If yes, stack the particle for performing the ``Action At Rest'' later.
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| 125 | If not, and only in this case, abandon this secondary.</LI>
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| 126 | </UL>
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| 127 | With this sophisticated mechanism we have the global cut that we wanted,
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| 128 | but with energy conservation, and we respect boundary constraint (safety)
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| 129 | and the wishes of the processes (via ``good for tracking'').
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| 130 | </UL>
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| 131 | <P>
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| 132 |
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| 133 | <HR>
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| 134 | <a name="5.4.4">
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| 135 | <H2>5.4.4 Why produce secondaries below threshold?</H2></a>
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| 136 |
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| 137 | A process may have good reasons to produce particles below the recommended
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| 138 | threshold:
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| 139 | <p>
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| 140 | <UL>
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| 141 | <LI>checking the range of the secondary versus geometrical quantities like
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| 142 | safety may allow one to realize the possibility that the produced particle,
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| 143 | even below threshold, will reach a sensitive part of the detector;</LI>
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| 144 | <LI>another example is the gamma conversion: the positron is always produced,
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| 145 | even at zero energy, for further annihilation.</LI>
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| 146 | </UL>
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| 147 | <p>
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| 148 | These secondary particles are sent to the ``Stepping Manager'' with a flag
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| 149 | <tt>GoodForTracking</tt> to pass the filter explained in the previous section
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| 150 | (even when <tt>ApplyCut</tt> is ON).
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| 151 | <P>
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| 152 |
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| 153 | <HR>
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| 154 | <a name="5.4.5">
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| 155 | <H2>5.4.5 Cuts in stopping range or in energy?</H2></a>
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| 156 |
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| 157 | The cuts in stopping range allow one to say that the energy has been released
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| 158 | at the correct space position, limiting the approximation within a given
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| 159 | distance. On the contrary, cuts in energy imply accuracies of the energy
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| 160 | depositions which depend on the material.
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| 161 | <P>
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| 162 |
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| 163 | <HR>
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| 164 | <a name="5.4.6">
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| 165 | <H2>5.4.6 Summary</H2></a>
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| 166 |
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| 167 | In summary, we do not have tracking cuts; we only have production thresholds
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| 168 | in range. All particles produced and accepted are tracked up to zero range.
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| 169 | <P>
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| 170 | It must be clear that the overall coherency that we provide cannot go
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| 171 | beyond the capability of processes to produce particles down to the recommended
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| 172 | threshold.
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| 173 | <P>
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| 174 | In other words a process can produce the secondaries down to the recommended
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| 175 | threshold, and by interrogating the geometry, or by realizing when mass-to-energy
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| 176 | conversion can occur, recognize when particles below the threshold have
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| 177 | to be produced.
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| 178 | <P>
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| 179 |
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| 180 | <HR>
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| 181 | <a name="5.4.7">
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| 182 | <H2>5.4.7 Special tracking cuts</H2></a>
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| 183 |
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| 184 | One may need to cut given particle types in given volumes for optimisation
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| 185 | reasons. This decision is under user control, and can happen for particles
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| 186 | during tracking as well.
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| 187 | <P>
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| 188 | The user must be able to apply these special cuts only for the desired
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| 189 | particles and in the desired volumes, without introducing an overhead for
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| 190 | all the rest.
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| 191 | <P>
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| 192 | The approach is as follows:
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| 193 | <p>
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| 194 | <UL>
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| 195 | <LI>special user cuts are registered in the <i>UserLimits</i> class (or its descendant),
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| 196 | which is associated with the logical volume class.</LI>
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| 197 | <P>
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| 198 | The current default list is:
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| 199 | <UL>
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| 200 | <LI>max allowed step size</LI>
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| 201 | <LI>max total track length</LI>
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| 202 | <LI>max total time of flight</LI>
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| 203 | <LI>min kinetic energy</LI>
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| 204 | <LI>min remaining range</LI>
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| 205 | </UL>
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| 206 | The user can instantiate a <i>UserLimits</i> object only for the desired logical
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| 207 | volumes and do the association.<p>
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| 208 | The first item (max step size) is automatically taken into account by the G4 kernel
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| 209 | while the others items must be managed by the user, as explained below.
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| 210 | <P>
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| 211 | <b>Example</b>(see novice/N02): in the Tracker region, in order to force the step size not
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| 212 | to exceed 1/10 of the Tracker thickness, it is enough to put the following code in
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| 213 | <TT>DetectorConstruction::Construct()</TT>:
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| 214 | <PRE>
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| 215 | G4double maxStep = 0.1*TrackerLength;
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| 216 | logicTracker->SetUserLimits(new G4UserLimits(maxStep));
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| 217 | </PRE>
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| 218 | The <i>G4UserLimits</i> class is in <TT>source/global/management</TT>.<p>
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| 219 | <LI>Concerning the others cuts, the user must define dedicaced process(es).
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| 220 | He registers this process (or its descendant) only for the desired particles
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| 221 | in their process manager.
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| 222 | He can apply his cuts in the <TT>DoIt</TT> of this process,
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| 223 | since, via <i>G4Track</i>, he can access the logical volume and <i>UserLimits</i>.
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| 224 | <p>
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| 225 | An example of such process (called <i>UserSpecialCuts</i>) is provided in the repository,
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| 226 | but not inserted in any process manager of any particle.</LI>
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| 227 | <P>
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| 228 | <b>Example: neutrons. </b>
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| 229 | One may need to abandon the tracking of neutrons after a given time
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| 230 | of flight (or a charged particle in a magnetic field after a given total
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| 231 | track length ... etc ...).
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| 232 | <P>
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| 233 | Example(see novice/N02): in the Tracker region, in order to force the total time of
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| 234 | flight of the neutrons not to exceed 10 milliseconds, put the following
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| 235 | code in <TT>DetectorConstruction::Construct()</TT>:
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| 236 | <PRE>
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| 237 | G4double maxTime = 10*ms;
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| 238 | logicTracker->SetUserLimits(new G4UserLimits(DBL_MAX,DBL_MAX,maxTime));
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| 239 | </PRE>
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| 240 | and put the following code in <TT>N02PhysicsList</TT>:
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| 241 | <PRE>
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| 242 | G4ProcessManager* pmanager = G4Neutron::Neutron->GetProcessManager();
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| 243 | pmanager->AddProcess(new G4UserSpecialCuts(),-1,-1,1);
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| 244 | </PRE>
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| 245 | (The default <i>G4UserSpecialCuts</i> class is in
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| 246 | <TT>source/processes/transportation</TT>.)
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| 247 | </UL>
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| 248 | <P>
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| 249 |
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| 250 | <BR><BR>
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| 251 | <HR><A HREF="../../../../Authors/html/subjectsToAuthors.html">
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| 252 | <I>About the authors</A></I>
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