424 Chapter 7. Position Sensitive Detection and Imaging
spacing of these points and how well the detector is able to separately record radia-
tion intensities at the points determine the spatial resolution of the system. Position
resolution is a critical criterion used to characterize the usefulness of the detection
system for a particular application and can therefore be said to be application de-
pendent. For position sensitive detectors, the requirement for spatial resolution are
generally much more stringent than for imaging systems. For example in detectors
used in high energy experiments, a position resolution of a few tens of micrometers is
not uncommon. On the other hand, in a CT scanner, which is an imaging system, a
spatial resolution of a few millimeters is considered practical for diagnostic purposes.
Most modern position sensitive detectors consist of a large number of very thin
but long and closely spaced detectors. For example a multiwire proportional counter
comprises of a large number of very closely spaced thin anode wires. Similarly a
multistrip silicon detector consists of very large number of a few micrometer wide
and a few centimeters long silicon detectors doped on a single crystal sheet.The
spatial resolution of such detectors is dependent on the individual detector elements
and the free space between them.
Let us now have a look at the imaging systems. In principle, a position sensitive
detector can be used for imaging applications and vice versa. However, due to dif-
ferent requirements of spatial resolution and frequency response the general practice
is to design the systems for either position resolution or imaging. A drawback of
the imaging systems is that no matter how precise they are, there is always some
discrepancy between an object and its image. The reason for this discrepancy lies
in the way imaging detectors are constructed. Most of the modern imaging systems
consist of a number of discrete elements, generally known aspixels, each having some
shape and all lumped close together to form an array of detectors. Each of these
pixels is able to record the intensity of the incident radiation. Ideally there should
not be any image deterioration if the pixels are small enough and are kept very close
to each other. However there are two distinct effects that can cause degradation of
the spatial resolution. One is the crosstalk and the other is aliasing. The crosstalk
can affect the position sensitive detectors as well while aliasing is specific to imaging
systems. These two effects are discussed below.
A.1 Crosstalk
The term crosstalk has actually been borrowed from electronics where it refers to
the pickup of electrical signal by nearby signal lines and devices. Since position
sensitive detectors and imaging devices are made up of large number of individual
detector elements therefore it is possible that a part of the signal received by one
of the channels (that is, one of the detectors) gets transfered to the neighboring
channels. The detectors based on a single bulk of the material are most affected by
crosstalk. The reason is that such detectors provide alternative paths to the signal
charge carriers after they get scattered. The charges that go astray are thus picked
up by the neighboring channels.
A.2 AliasingandAntialiasing
Aliasing is an effect that causes blurring of the image. The image appears blurred
when it acquires extra information that the original object did not contain. This
unwanted information is artificially produced either by wrong sampling or by wrong