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0001 -------------------------------------------------------------------
0002
0003 =========================================================
0004 Geant4 - an Object-Oriented Toolkit for Simulation in HEP
0005 =========================================================
0006
0007 field01
0008 -------
0009
0010 Example that enables investigation of the accuracy and performance of the
0011 tracking in a magnetic field.
0012
0013 The key Geant4 capabilities demonstrated in this example are:
0014 - creating a uniform magnetic field interactively using the field
0015 messenger,
0016 - choosing the type of Runge Kutta stepper used for integration of the
0017 motion of charged particles in the magnetic field,
0018 - controlling the thresholds that determine which looping particles are
0019 killed by G4Transporation.
0020
0021 Some of these capabilities are available via interactive commands,
0022 implemented in F01FieldMessenger.
0023
0024 A. The magnetic field is defined in the ConstructSDandField() method
0025 in the F01DetectorConstruction class using the G4FieldBuilder class.
0026
0027 The interactive commands, under the /field directory, are created with
0028 the instantiation of G4FieldBuilder in the F03DetectorConstruction constructor,
0029
0030 B. Choosing the type of stepper -
0031
0032 The basic capabilities of choosing the stepper type are demonstrated in the
0033 field.in macro file:
0034
0035 /field/stepperType TsitourasRK45 ## Choose a stepper type ( Tsitouras )
0036
0037 /field/stepperType RK547FEq1 ## Choose an FSAL stepper ( FEqRK1 )
0038
0039 /field/setMinimumStep 0.1 mm ## Smaller steps always succeed
0040
0041 /field/update ## Initialise using parameters above
0042 ## if they were changed after initialization
0043
0044 In addition it is possible to choose to use a new type of stepper, known
0045 as 'First Same as Last' or FSAL, which in each step obtains the field value
0046 at the step endpoint and evaluates the 'right hand size' of the equation
0047 for the next integration step. This reduces the number of calls to the field
0048 evaluation, which can be one the most computationally expensive methods,
0049 while providing similar accuracy.
0050
0051 There are several potential choices of the stepper type. Here are some
0052 suggestions:
0053 ===========================================================================
0054 Number Name of Stepper Comments
0055 ===========================================================================
0056 Recommended - default since Geant4 10.4:
0057
0058 DormandPrince745 : Uses a pair 4th & 5th order formulae (like other 4/5
0059 well-known and very efficient embedded method
0060 methods); their difference is the error estimate.
0061 Highly recommended in literature, including
0062 Hairer & Wanner, & Numerical Recipes
0063 Used in several established RK code (e.g. DOPRI5)
0064 ===========================================================================
0065 Good choices for reasonably smooth fields:
0066
0067 BogackiShampine45 : more efficient embedded 4/5 pair
0068 Used in many applications, including
0069 RKSUITE suite.
0070
0071 TsitourasRK45 : potentially the most efficient embedded 4/5
0072 pair - found in expanded search of parameter
0073 space.
0074
0075 DormandPrinceRK56 : higher order embedded method from authors of DoPri5.
0076 Uses a pair 5th & 6th order formulae.
0077
0078 DormandPrinceRK78 : higher order embedded method from authors of DoPri5.
0079 Uses a pair 7th & 8th order formulae.
0080
0081 NystromRK4 : a specialised Nystrom method for magnetic fields.
0082 Reuses the field value at the mid-point of the step,
0083 and also provides an analytical estimation of the
0084 integration error based on numerical evaluation of
0085 fourth order variation in the equation for
0086 magnetic field.
0087 ===========================================================================
0088 The new 'First Same as Last' (FSAL) steppers can be chosen in addition:
0089 RKFEq1 : FSAL stepper with improved equilibrium properties.
0090 When kinks or other anomalies are encountered,
0091 and at the start of integration when the best
0092 step size is not known, this type of stepper
0093 converges faster and more smoothly to good
0094 step sizes.
0095 ===========================================================================
0096 The old default and old first alternative -
0097
0098 ClassicalRK4 : original Runge-Kutta method, very robust but slower )
0099 ( obtains error estimate by doing 2 half steps )
0100 Good baseline for comparison - long experience of use.
0101 May be good alternative for less smooth fields.
0102
0103 CashKarpRKF45 : The oldest 'embedded' RK method in Geant4 -
0104 also fairly robust.
0105 Faster than ClassicalRK4 for smoother fields,
0106 as it does not need two half steps to estimate error.
0107 Available since Geant4 1.0
0108
0109 ===========================================================================
0110 Other potential choices for non-smooth fields (with kinks, abrupt changes):
0111
0112 SimpleHeum : low order, with error obtained from half-steps
0113 BogackiShampine23 : lower order embedded method (new in 10.3-beta)
0114 ===========================================================================
0115
0116
0117
0118 C. Controlling the killing of looping particles
0119
0120
0121 Occasionally tracks 'looping' in a strong magnetic field, making little
0122 progress even over hundreds of integration steps. This is due to a
0123 combination of a strong magnetic field and a thin material (gas or vacuum)
0124 in which the size of a physics step is substantially larger than the radius
0125 of curvature of the track.
0126
0127 Since the amount of CPU time which can be consumed by one or few such tracks
0128 is very large, it is important to limit the number of integration steps
0129 spent on these tracks. The module for propagation in field in Geant4
0130 flags tracks which take more than a certain number (default 1,000) integration
0131 steps without reaching the requested end of the step size, which was
0132 determined by the physics and geometry.
0133
0134 The Geant4 G4Transportation and G4CoupledTransportation processes are tasked
0135 to select which of the looping tracks are killed and which survive. To
0136 balance the potential significant cost of integrating looping particles,
0137 three thresholds exist
0138
0139 The 'Warning' Energy: a track with energy below this value that is found to
0140 loop is killed silently (no warning.)
0141
0142 Above the 'Warning Energy', if a track is selected for killing a warning is
0143 generated.
0144
0145 The 'Important' Energy: the threshold energy above which a track will survive
0146 for multiple steps if found looping.
0147
0148 number of 'tracking' steps. They will be only be killed only if they still
0149 loop after than
0150 The number of 'trials': the number of steps that 'important' tracks survive.
0151
0152 Note that currently only stable particles are killed. ( Refinements to enable
0153 toggling whether unstable particles can be killed are in development. )
0154
0155 This example demonstrate choosing different values for these parametes
0156 in the main() method of field01.cc using one of two techniques.
0157
0158 The first method is new in Geant4 release 10.5, and uses the G4PhysicsListHelper
0159 which has methods to choose a pre-selected set of parameter values. The choices
0160 are between a set each of low and high thresholds. Either one can be enabled
0161 by calling correspondingly
0162 - G4PhysicsListHelper::GetPhysicsListHelper()->UseLowLooperThresholds();
0163 or
0164 - G4PhysicsListHelper::GetPhysicsListHelper()->UseHighLooperThresholds();
0165 These methods must be called before the physics is constructed - i.e. typically
0166 before RunManager's Initialise() method is called.
0167 This works only if either
0168 - a modular physics lists is used, or if
0169 - the G4ModularPhysicsList and its AddTransporation method are used
0170 to create and register a common transportation process for all particles
0171 (one for each thread).
0172
0173 ii) Fine grained control (available in Geant4 versions since 7.0)
0174
0175 Fine grained control of the Transportation's parameters for looping particles
0176 is also possible.
0177
0178 This is demonstrated in the F01RunAction's ChangeLooperParameters method,
0179 which is called by the BeginOfRunAction. There the appropriate
0180 Transportation object for the electron is obtained, and its parameters
0181 (if valid) are used to overwrite the thresholds in the G4Transportation class.
0182
0183 For example, to ensure that only looping particles with energy 10 keV are
0184 killed silently we change the value of the 'Warning' Energy:
0185
0186 runAction->SetWarningEnergy( 10.0 * CLHEP::keV );
0187
0188 [ This is passed along to the registered G4Transportation or
0189 G4CoupledTransportation object by the F01RunAction's ChangeLooperParameters.]
0190
0191 As a result the killing of any (stable) looping track with energy over 10 keV
0192 will generate a warning.
0193
0194 A second configurable energy threshold enables tracks above it to survive a
0195 chosen number of 'tracking' steps. They will be only be killed only if they
0196 still loop after than number of tracking steps. F01RunAction's methods are
0197 used to configure these parameters:
0198
0199 runAction->SetImportantEnergy( 0.1 * CLHEP::MeV );
0200 runAction->SetNumberOfTrials( 30 );
0201
0202 which the run action passes to the G4Transportation or
0203 G4CoupledTransportation object registered for the electron.
0204
0205 Note that for all pre-configured and modular physics lists share a single
0206 Transportation process for all types of particles. So the parameters for
0207 killing loopers will be shared by all particle types in this case.
0208
0209
0210 Background Information
0211
0212 1- GEOMETRY DEFINITION
0213
0214 The "Absorber" is a solid made of a given material.
0215
0216 Three parameters define the absorber :
0217 - the material of the absorber,
0218 - the thickness of an absorber,
0219 - the transverse size of the absorber (the input face is a square).
0220
0221 The volume "World" contains the "Absorber".
0222 In this test the parameters of the "World" can be changed , too.
0223
0224 In addition a transverse uniform magnetic field can be applied.
0225
0226 The default geometry is constructed in F01DetectorConstruction class,
0227 but all the parameters can be changed via
0228 the commands defined in the F01DetectorMessenger class.
0229
0230 2- AN EVENT : THE PRIMARY GENERATOR
0231
0232 The primary kinematic consists of a single particle (electron, Ekin = 0.5 GeV)
0233 which hits the
0234 absorber perpendicular to the input face. The type of the particle
0235 and its energy are set in the F01PrimaryGeneratorAction class, and can
0236 be changed via the G4 build-in commands of G4ParticleGun class (see
0237 the macros provided with this example).
0238
0239 It is also possible to change the position of the primary particle vertex
0240 or activate its randomization via the commands defined in the
0241 F01PrimaryGeneratorMessenger class.
0242
0243 A RUN is a set of events.
0244
0245 3- DETECTOR RESPONSE
0246
0247 The spatial distribution of charged particles transported in magnetic
0248 field is envistigated.
0249 A HIT is a record, event per event , of all the
0250 informations needed to simulate and analyse the detector response.
0251
0252 In this example a F01CalorHit is defined as a set of 2 informations:
0253 - the total energy deposit in the absorber,
0254 - the total tracklength of all charged particles in the absorber,
0255
0256 Therefore the absorber is declared
0257 'sensitive detector' (SD), see F01CalorimeterSD, which means they can contribute to the hit.
0258
0259 4- PHYSICS LIST
0260
0261 The particle's type and the physic processes which will be available
0262 in this example are set in the FTFP_BERT physics list. This physics list
0263 requires data files for electromagnetic and hadronic processes.
0264 See more on installation of the datasets in Geant4 Installation Guide,
0265
0266 5- HOW TO START ?
0267
0268 - Execute field01 in 'batch' mode from macro file e.g.
0269 % ./field01 field01.in
0270
0271 - Execute field01 in 'interactive' mode with visualization e.g.
0272 % ./field01
0273 ....
0274 Idle> /run/beamOn 1
0275 ....
0276
0277
0278
