5 Solid State Detectors
149
inclination and the Lorenz force in a magnetic field, the charge of one track may be
distributed over two or more strips. This can be exploited to improve the accuracy
of the position measurement well below the value given by the strip pitch. It is then
limited by fluctuations of the ionization process and in particular the generation of
delta electrons and the electronics noise. A measurement precision down to about
1 μm has been achieved.
5.6.1 Strip Detector Readout
In the conceptually simplest version each strip is connected to its own electronic
readout channel and the position is determined by the number of the strip providing
a signal.
Binary (yes/no) readout may be used if no energy information is required and if
the position accuracy given by the strip pitch is sufficient. One also does not lose
position resolution compared with analogue readout if the strip pitch is large with
respect to the width of the diffusion cloud.
Analogue (signal amplitude) readout of every channel may lead to a substantial
improvement of the position measurement precision if the strip spacing matches
the charge spread due to diffusion during collection. (Charge spread can also be
due to track inclination or the Lorenz angle in a magnetic field.) In addition, the
simultaneous measurement of energy loss becomes possible.
Charge division readout reduces the number of readout channels as only a
fraction of the strips is connected to a readout amplifier (Fig. 5.7). Charge collected
at the other (interpolation) strips is divided between the two neighbouring readout
channels according to the relative position. Charge division is due to the capacitors
between neighbouring strips. For charge division to work, it is necessary to
hold the intermediate strips at the same potential as the readout strips. This can
be accomplished by adding high ohmic resistors or with other methods. If the
intermediate strips were left floating, they would adjust themselves to a potential
Fig. 5.7 Charge division readout. The interstrip capacitors between the readout strips act as
capacitive charge divider. The high-ohmic resistors are required to keep all strips at the same
potential
149
inclination and the Lorenz force in a magnetic field, the charge of one track may be
distributed over two or more strips. This can be exploited to improve the accuracy
of the position measurement well below the value given by the strip pitch. It is then
limited by fluctuations of the ionization process and in particular the generation of
delta electrons and the electronics noise. A measurement precision down to about
1 μm has been achieved.
5.6.1 Strip Detector Readout
In the conceptually simplest version each strip is connected to its own electronic
readout channel and the position is determined by the number of the strip providing
a signal.
Binary (yes/no) readout may be used if no energy information is required and if
the position accuracy given by the strip pitch is sufficient. One also does not lose
position resolution compared with analogue readout if the strip pitch is large with
respect to the width of the diffusion cloud.
Analogue (signal amplitude) readout of every channel may lead to a substantial
improvement of the position measurement precision if the strip spacing matches
the charge spread due to diffusion during collection. (Charge spread can also be
due to track inclination or the Lorenz angle in a magnetic field.) In addition, the
simultaneous measurement of energy loss becomes possible.
Charge division readout reduces the number of readout channels as only a
fraction of the strips is connected to a readout amplifier (Fig. 5.7). Charge collected
at the other (interpolation) strips is divided between the two neighbouring readout
channels according to the relative position. Charge division is due to the capacitors
between neighbouring strips. For charge division to work, it is necessary to
hold the intermediate strips at the same potential as the readout strips. This can
be accomplished by adding high ohmic resistors or with other methods. If the
intermediate strips were left floating, they would adjust themselves to a potential
Fig. 5.7 Charge division readout. The interstrip capacitors between the readout strips act as
capacitive charge divider. The high-ohmic resistors are required to keep all strips at the same
potential