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53 | <H1><A NAME="SECTION02300000000000000000"> |
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54 | Transportation</A> |
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55 | </H1> |
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56 | |
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57 | <P> |
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58 | The transportation process is responsible for determining the |
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59 | geometrical limits of a step. It calculates the length of step with which a |
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60 | track will cross into |
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61 | another volume. When the track actually arrives at a boundary, the |
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62 | transportation process locates the next volume that it enters. |
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63 | |
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64 | <P> |
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65 | If the particle is charged and there is an electromagnetic (or |
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66 | potentially other) field, it is responsible for propagating the particle in |
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67 | this field. It does this according to an equation of motion. This equation |
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68 | can be provided by Geant4, for the case a magnetic or EM field, |
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69 | or can be provided by the user for other fields. |
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70 | |
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71 | <P> |
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72 | The transportation updates the time of flight of a particle, utilising its |
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73 | initial velocity. |
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74 | |
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75 | <P> |
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76 | <I>Some additional details on motion in fields:</I> |
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77 | |
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78 | <P> |
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79 | In order to intersect the model Geant4 geometry of a detector or setup, the |
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80 | curved trajectory followed by a charged particle is split into 'chords segments'. A |
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81 | chord is a straight line segment between two trajectory points. Chords are |
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82 | created utilizing a criterion for the maximum estimated distance between a |
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83 | curve point and the chord. This distance is also known as the sagitta. |
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84 | |
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85 | <P> |
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86 | The equations of motions are solved utilising Runge Kutta methods. |
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87 | Runge Kutta methods of different can be utilised for fields depending on the |
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88 | numerical method utilised for approximating the field. Specialised methods |
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89 | for near-constant magnetic fields are under development. |
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