A basis signal is a composite waveform of the charge signals recorded on the 36 segments and the central contact of the detector for a unit charge placed at a given location in the detector. Such a basis signal can be calculated from the electric field, weighting potentials and impurity concentration in the crystal. By calculating a set of basis signals on a three-dimensional grid of points which span the volume of the crystal, one can perform least-square fitting of linear combinations of basis signals against the observed signal, a process we call signal decomposition. This gives the positions gamma-ray interaction point in the crystal and their relative charge deposition.
This set of basis signals calculated over a grid is simply referred to as a "basis" and it is a simple file. A description of the basis file format is given here . For GRETINA, the grid used is non-uniform and reflects the position sensitivity of the detector. The average spacing of grid points is roughly 1 mm resulting in approximately 250000 grid points for each detector crystal. The picture below shows the positions of a fraction of the grid points used in the non-uniform GRETINA basis grid.
A basis derived solely from the electric fields and weighting potentials is called a "raw basis". In order for it to be used in, the electronic response characteristics of the system must be applied. As the operating voltage, impurity concentration, and geometry (A,B) of the crystals differ, a raw basis must be calculated for each crystal. From the raw basis for each crystal a response-corrected basis can be constructed, which is utilized in experiments.