| 1 | \chapter{Transportation}
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| 2 |
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| 3 | The transportation process is responsible for determining the
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| 4 | geometrical limits of a step. It calculates the length of step with which a
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| 5 | track will cross into
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| 6 | another volume. When the track actually arrives at a boundary, the
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| 7 | transportation process locates the next volume that it enters.
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| 8 |
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| 9 | If the particle is charged and there is an electromagnetic (or
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| 10 | potentially other) field, it is responsible for propagating the particle in
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| 11 | this field. It does this according to an equation of motion. This equation
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| 12 | can be provided by Geant4, for the case a magnetic or EM field,
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| 13 | or can be provided by the user for other fields.
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| 14 |
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| 15 | The transportation updates the time of flight of a particle, utilising its
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| 16 | initial velocity.
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| 17 |
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| 18 | \textit{Some additional details on motion in fields:}
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| 19 |
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| 20 | In order to intersect the model Geant4 geometry of a detector or setup, the
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| 21 | curved trajectory followed by a charged particle is split into 'chords segments'. A
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| 22 | chord is a straight line segment between two trajectory points. Chords are
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| 23 | created utilizing a criterion for the maximum estimated distance between a
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| 24 | curve point and the chord. This distance is also known as the sagitta.
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| 25 |
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| 26 | The equations of motions are solved utilising Runge Kutta methods.
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| 27 | Runge Kutta methods of different can be utilised for fields depending on the
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| 28 | numerical method utilised for approximating the field. Specialised methods
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| 29 | for near-constant magnetic fields are under development.
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| 30 |
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