Cell response

The response of the Drift chamber to the crossing of charged particle is evaluated in the routine DchStep.F called when a particle cross the sesitive volumes "Super Layer". The possibilities which can happen when the particles cross the super layers are summarized in the following picture.

When the particles A or B enters in one Super Layer bbsim gives the control to DchStep.F. The management of the tracking will follow the sequence:

1. Get current particle tracking parameters;
2. Find the present cell number;
3. Find the wires of the present cell;
4. Evaluate the wire equations;
5. Find closest distance from all the cell wires;
6. Find intersection with the edge of the cell considering the two wires with the closest distance.
7. Return to Geant with the step corresponding to the change of the cell;
8. Geant call DchStep after the selection of the step and the occurred mechanism; In the first release V00-00-00 the step size is fixed to 1 mm in the Super Layers.
9. If the particle reachs the edge of the cell like particle A in the layer 1:
   1. Store informations in the HIT structure
   2. Save HIT
   3. Reset local parameters
   4. Find the wires of the following cell
   5. Go to step c.
10. If the particle reachs the edge of the cell like particle A in the layer 2:
    1. Estimate multiple scattering due to the crossed wire
    2. Introduce deviations in the Geant data structure
    3. Go to step 1.
11. If the particle decays in the cell like particle B in the layer 3:
    1. Store decay point as last particle point in the cell;
    2. Save HIT;
    3. Reset local parameters;
    4. Give control to Geant.
12. If the particle reachs the edge of the Super Layer like particle A in the layer 3:
    1. Store informations in the HIT structure;
    2. Save HIT;
    3. Reset local parameters;
    4. Give control to Geant.

    In the preliminary version we don't implement the point F), instead we use a fixed small step.

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