Physics and Engineering of Radiation Detection

302 Chapter 5. Solid State Detectors

precision machinery.

Pixel Detectors

Pixel detectors are position sensitive devices having a two dimensional array of
small rectangular detector elements or pixels. Each of these pixels has a linear
dimension of less than a millimeter^3. The technology is somewhat similar to the
microstrip detectors we studied earlier. The pixel array is developed by selectively
doping a semiconductor wafer.

Other Position Sensitive Detectors

A number of variants of the simple microstrip and pixel detectors we just dis-
cussed exist, some of which are listed below.

Monolithic Pixel Detectors

Hybrid Pixel Detectors

Charged Couple Devices (CCDs)

MOS CCDs

Deep Depletion CCDs

Drift Detectors

5.1.L Radiation Damage in Semiconductors

A serious problem with semiconductors is their vulnerability to structural damage
caused by radiation. Both instantaneous and integrated doses contribute to the
overall damage. Although some of these effects can be compensated by increasing
the reverse bias but eventually the bulk of the damage renders the detector material
useless. Hence deployment of semiconductor detectors specially in hostile radiation
environments, such as particle accelerators, needs careful consideration and regular
monitoring.

L.1 DamageMechanismandNIELScaling............

The most damaging defects caused by radiation are the result of atomic displace-
ments that destroy the lattice structure. These defects could be dispersed or clus-
tered around the incident radiation track. The dispersed atomic displacements away
from each other cause the so calledpoint defects. A cluster of such displacements
in close vicinity to each other is calledcluster defect. The distinguishing feature of
these clustered defects is that they render a large region of the bulk of the mate-
rial unusable. The main damage mechanism is the displacement of an atom from
its lattice site by an incident particle. This is calledNon Ionizing Energy Lossor
NIEL. As opposed to the ionizing energy loss, NIEL is not proportional to the total

(^3) The choice of pixel dimension depends on the resolution requirements and engineering details. Practical
systems with as small as 10μmand as large as 200μmpixels have been constructed.