10 Signal Processing for Particle Detectors
445
Fig. 10.5 Weighting field (potential) for pixel electrode configuration
10.2.3 Weighting Potential and Induced Charge in Co-Planar
Electrodes
Coplanar grid readout was introduced for unipolar charge sensing by Luke [15]
and it is commonly used with Cadmium Zinc Telluride (CZT) detectors. In such
materials only electrons are collected (from the ionization produced by gamma rays
or x-rays), the holes suffering from very low mobility and trapping. With parallel
plane electrodes the induced charge for single carrier collection is dependent on the
position (depth) where the ionization took place. In the coplanar grid concept one set
of alternate strips is biased slightly more positively with respect to the other set of
strips. This results in a drift field such that the signal electrons are collected on one
set of strips only, Fig. 10.6. The weighting potential (field) for both sets of strips
is identical, Fig. 10.7. The induced charges and currents are quite different, Figs.
10.8 and 10.9. Their respective differences are independent of the position, as can
be concluded by following the weighting potential plot from any point on the planar
sloped part of the plot to unity weighting potential for the collecting electrode, and
(across the saddle) to zero potential for the non-collecting electrode, Fig. 10.9.
10.3 Noise: Origin and Properties
10.3.1 Noise Process and Noise Variance
The basis of a noise process can be represented as a sequence of randomly generated
elementary impulses that has a Poisson distribution in time and mean rate of
occurrence n. Upon acting on a physical system with impulse response much
445
Fig. 10.5 Weighting field (potential) for pixel electrode configuration
10.2.3 Weighting Potential and Induced Charge in Co-Planar
Electrodes
Coplanar grid readout was introduced for unipolar charge sensing by Luke [15]
and it is commonly used with Cadmium Zinc Telluride (CZT) detectors. In such
materials only electrons are collected (from the ionization produced by gamma rays
or x-rays), the holes suffering from very low mobility and trapping. With parallel
plane electrodes the induced charge for single carrier collection is dependent on the
position (depth) where the ionization took place. In the coplanar grid concept one set
of alternate strips is biased slightly more positively with respect to the other set of
strips. This results in a drift field such that the signal electrons are collected on one
set of strips only, Fig. 10.6. The weighting potential (field) for both sets of strips
is identical, Fig. 10.7. The induced charges and currents are quite different, Figs.
10.8 and 10.9. Their respective differences are independent of the position, as can
be concluded by following the weighting potential plot from any point on the planar
sloped part of the plot to unity weighting potential for the collecting electrode, and
(across the saddle) to zero potential for the non-collecting electrode, Fig. 10.9.
10.3 Noise: Origin and Properties
10.3.1 Noise Process and Noise Variance
The basis of a noise process can be represented as a sequence of randomly generated
elementary impulses that has a Poisson distribution in time and mean rate of
occurrence n. Upon acting on a physical system with impulse response much