440 Chapter 7. Position Sensitive Detection and Imaging
may lead to false results specially for low radiation environments. Uniformity is a
term used to describe the fact that all of the pixels show uniform response when
illuminated by an spatially uniform beam of radiation. To ensure this, a mask of
the response of each pixel under uniform and no illumination are obtained and then
subsequently used to correct the actual images.
7.1.F TemporalLinearity........................
Temporal linearity is of course one of the most desirable properties of any imaging
system. However due to aging, radiation damage, and other effects, the detector
response changes with time. This requires periodic calibration of the system, some-
thing that is painstaking but absolutely necessary to ensure proper functioning.
7.1.G Noise and Signal-to-Noise Ratio (S/N).............
Modern position sensitive detectors and imaging systems consist of not only high
resolution detection material but also of highly sensitive analog and digital electron-
ics. All the subsystems of such a device contain their own sources of noise. For
systems that are supposed to work at very high resolutions, characterization of all
of these noise sources is extremely important. If the spread in the signal due to
individual components is known then the total noise can be obtained by taking the
square root of the sum of their squared values, that is
σtotal=[N
∑
i=1σi] 1 / 2
, (7.1.27)
whereσicorresponds to the noise of theithcomponent of the total ofNcomponents.
Although one would ideally want to be able to analytically model all the noise
sources but in practice such a task is extremely difficult, if not impossible. The
general practice, therefore, is to determine the noise experimentally. One point worth
noting here is that there are a few noise sources that are always there irrespective
of the complexity of the system. This makes life a bit easier since one can then
determine at least the physical limit of the overall system resolution. Following is
the list of such known noise sources.
Statistical fluctuations of the incident radiation flux.Statistical fluctuations of the radiation absorption and charge pair production.Thermal or Johnson noise.We will look at these and other noise sources later in the chapter.
The term signal-to-noise ratio, as the name suggests, is the ratio of the signal
to the total noise. It plays a very important role in determining the usefulness of
a particular detection system for a certain application. It should however be noted
that it is not always necessary to maximize this ratio. For example in systems where
resolution is not of much concern, other factors, such as sampling frequency, may
be more relevant thanS/N.