class AliTPCROCVoltError3D: public AliTPCCorrection

AliTPCROCVoltError3D class

The class calculates the space point distortions due to z offsets of the ROCs via the residual voltage technique (Poisson relaxation) in 3D. Since the GG (part of the ROCs) represents the closure of the FC in z direction, every misalignment in z produces not only dz distortions but also electrical field inhomogeneities throughout the volume, which produces additional dr and rd$\phi$ distortions.

Each ROC can be misaligned (in z direction) in three ways. A general z0 offset, an inclination along the x and an inclination along the y axis. The z-misalignment's can be set via the function SetROCData(TMatrixD *mat) for each single chamber (ROC). The array size has to be (72,3) representing the 72 chambers according to the offline numbering scheme (IROC: roc$<$36; OROC: roc$\geq$36) and the three misalignment's (see the source code for further details).

Internally, these z offsets (unit is cm) are recalculated into residual voltage equivalents in order to make use of the relaxation technique.

One has two possibilities when calculating the $dz$ distortions. The resulting distortions are purely due to the change of the drift velocity (along with the change of the drift field) when the SetROCDisplacement is FALSE.
For this class, this is a rather unphysical setting and should be avoided.
When the flag is set to TRUE, the corresponding offset in z is added to the dz calculation of the outer ROCs.
For the Alice TPC gas, both effects are of similar magnitude. This means, if the drift length is sufficiently large, a z-offset of a chamber appears to have (approx.) twice the magnitude when one looks at the actual dz distortions.

In addition, this class allows a correction regarding the different arrival times of the electrons due to the geometrical difference of the inner and outer chambers. The impact was simulated via Garfield. If the flag is set via the function SetElectronArrivalCorrection, the electron-arrival correction is added to the dz calculation.

   

   Date: 08/08/2010    
                                                 
   Authors: Jim Thomas, Stefan Rossegger