Diamond Detectors for Ionizing Radiation - HEPHY

CHAPTER 8. DETECTOR GEOMETRIES 49
Figure 8.1: CVD diamond samples with a simple dot (substrate side) and with a guard ring
(growth side). The scale ticks to the left represent millimeters.
areas are the equilines of potential (each shade corresponds toa15Vinterval), while the
arrows show the negative gradient of the potential, i.e., the electric eld. In total, the
fringe eld is signicantly reduced by the guard structure, being restricted basically to a
small range between the top and guard electrodes. Furthermore, the charges drained by
the guard ring do no longer contribute to the signal, which is measured at the bottom
electrode.
Guard rings are also used for silicon detectors, however, the reason there is primarily
to reduce surface leakage currents.
8.2 Strips
Strip detectors have a large number of narrow strip implants on one side, while the
opposite side is provided with a single, large electrode, called backplane. Normally, each
of these strips is wire-bonded to a separate amplier channel, allowing to detect the
track position in one dimension. In some detector designs, only every second (or even
third) strip is connected to an amplier, while the remaining \intermediate strips" are
terminated with high impedance. As there is a capacitive coupling, signals on these
intermediate strips are partially transfered to the readout strips. With proper geometric
design, the number of readout channels can be dramatically reduced while only little SNR
is sacriced. Strip and pixel detectors are often referred to as \trackers", as their intention
is the track reconstruction.
8.2.1 Spatial Resolution
The principal idea of not simply applying dots on both sides is to gain position information.
At the cost of more amplier channels and more complicated readout, the spatial
resolution gets better with smaller electrode areas, forming strips or pixels. With a strip
detector, the simplest case of data processing is to reduce the position information to
the readout channel with the highest observed signal. Thus, the position information is
digitized in steps of the strip pitch 1 p. Similar to the intrinsic noise of an ADC, one gets
1 distance from one strip center to the neighbor strip center