7.3. Imaging Devices 451
nitrate or silver halide salts. After the film has been exposed to x-rays it must go
through a series of chemical processes to make the image visible.
The main advantage of photographic films is that they can cover large areas. On
the other hand, they have several disadvantages, some of which are listed below.
Low Sensitivity: The sensitivity of photographic films to x-rays is so poor
that a typical x-ray film absorbs only about 1% of the radiation.Low Flux Detection Limit:Typical photographic films can not detect x-ray
flux of less than 0.1 photons perμm^2.Logarithmic Response: Photographic film emulsions respond logarithmi-
cally to the incident x-ray flux. Consequently the image contrast needs very
careful interpretation.Film Development:The photographic films have to be chemically developed
to view the image. Although with modern technology the process is not any
more time consuming or labor intensive but still it can not match the real time
data acquisition and processing offered by electronic imaging systems.Electronic Archiving: Saving films electronically on a computer memory
device for archiving and electronic transfers is difficult, if not impossible.A.2 ThermoluminescentDetectorArrays
We have already discussed the detectors based on thermoluminescence phenomenon
in the chapter on solid state detectors. As a reminder, thermoluminescent materials
have the ability to store energy delivered by radiation and then release it after being
exposed to heat. It is, in principle, possible to develop an imaging system based on
an array of these devices. This is routinely done at high energy physics experiments.
An number of closely spaced TLDs are installed at the desired location, for example
near a beam pipe. The TLDs keep on accumulating the dose until they are retrieved
and then read out. An image reconstruction software can then be used to construct
the spatial radiation profile of that area. Use of TLDs in such a way at particle
detector experiments is not uncommon. The main advantage of such a system is
that it does not require any electronic circuitry and is therefore fairly easy to install.
The downside is that it can only provide a spatially coarse radiation profile.
7.3.B ElectronicsImaging
Electronic imaging is one of the fastest growing fields mainly due to its applicability
in consumer electronics. Digital cameras have now literally replaced the conventional
film cameras due to their cost effective availability and high resolution. These cam-
eras use the so called charged coupled devices (CCDs) to register photons coming
from the object. The resulting signals are then used by a processor to reconstruct
the object’s image. The CCDs also have applications in medical diagnostics. In
other areas, such as astronomy and particle physics, the electronic imaging devices
have seen enormous progress. The fast paced development of such devices has also
added a fair bit of complexity in not only their hardware but also the software.