Physics and Engineering of Radiation Detection

450 Chapter 7. Position Sensitive Detection and Imaging

simple double sided silicon detector. Here, the strips on both sides are orthogonal
to each other giving a two-dimensional position resolution. Depending on the appli-
cation it may as well be desired to keep the two sides at another angle with respect
to each other. The purpose of the guard rings shown in Fig.7.2.8 is to reduce the
possibility of electrical breakdown. Generally several guard rings are implanted on
a detector module such that the high voltage gradually decreases instead of a large
abrupt change.

Al Strip SiO 2 Pitch

n−Type Silicon p−Stop

Guard Rings

HV Bias Resistor
Al Strip

n+ Implant

p+ Implant

Output

Output

Figure 7.2.8: Design of a

simple double sided silicon

microstrip detector.

7.3 ImagingDevices

At the most basic level, creating an image of an object requires exposing it to a
radiation field, such as x-rays or light photons, and then detecting the reflected, re-
fracted, and transmitted particles on a two dimensional position sensitive detector.
Common examples of imaging devices are the familiar medical x-ray system and the
digital camera. Broadly speaking, imaging devices can be divided into two cate-
gories: conventional and electronic. The film based x-ray system is a conventional
device while digital camera is an electronic imaging system. In the following sections
we will look at some of the important devices belonging to both of these categories.

7.3.A ConventionalImaging

As stated earlier, by conventional imaging we mean the devices that do not acquire,
process, and store the signals electronically.

A.1 X-rayPhotographicFilms...................

X-rays are the most widely used means of producing images of objects hidden behind
other objects that are opaque to visible light. For example the images of internal
organs of human body are routinely taken for medical diagnostic purposes. To make
such an image, x-rays are made to fog a photographic film after passing through
the sample. If the sample has many different materials (such as tissues and bones
in human body), then the x-rays will be variably attenuated depending on the
attenuation constant of the materials. Since the photographic film is sensitive to the
intensity of the x-rays therefore a two dimensional image of the object is formed. In
this way, the photographic film acts as a position sensitive x-ray detector.
The base material of photographic films is generally polyester or celluloid. The
film is coated with an emulsion made of some light sensitive material, such as silver