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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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| 90 |
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