7.3. Imaging Devices 457
7.3.F MicrostripandMultiwireDetectors
The microstrip and multiwire detectors we discussed earlier in the chapter can be
used as imaging detectors. However since these devices provide only a one dimen-
sional spatial information therefore in order to use them as imaging devices one
should do either of the following.
The design can be modified such that there are two planes of readout detectors
that are kept orthogonal to each other. Both MSGCs or SMSDs can be modified
in this way. The reason is that they are produced by implanting strips on a
substrate, which can be done on the other side of the substrate as well. We
have seen earlier that double sided silicon detectors, where readout strips on
one side are normally implanted orthogonal to the other side, are a reality and
constitute one of the most commonly used position sensitive detectors. On the
other hand, modifying the multiwire proportional or drift chambers in this way
carries a number of engineering and operational difficulties.The other method is to use two or more one-dimensional detectors in succession
such that the readout strips or wires are orthogonal or at some other angle with
others. Multiwire proportional chambers are operated in this configuration,
though to obtain good position resolution and not as imaging devices.7.3.G Scintillating Fiber Detectors
In the chapter on scintillation detectors we saw that certain materials produce light
when exposed to radiation. One of the problems with these systems is that, in order
to avoid absorption of the light produced by them, one must use other phosphors
to change the light wavelength. This process has its own efficiency, which decreases
the overall efficiency of the detector. Scintillating fibers, on the other hand, are
specially designed solids that can not only produce scintillation light but also guide
the photons with minimal self absorption. Imaging detectors based on such fibers
are a reality now as their variations have been shown to perform amazingly well in
particle physics research as well as medical imaging.
Fig.7.3.5 shows a typical scintillation fiber having two components: a scintillation
core and an optical cladding. Radiation passing through the scintillation core pro-
duces light photons. The optical cladding ensures that most of these photons travel
down the fiber through the process of total internal reflection. Most commercially
available scintillation fibers produce blue or green light and are therefore suitable for
detection by commonly available photomultiplier tubes. The diameter of a typical
fiber is around 1mm^2 but fibers of 100μmor less in diameter are also available.
The fibers can be made in any cross sectional shape but round and square shaped
fibers are the most popular.
Most imaging devices use photomultiplier tubes to detect scintillation photons
produced in the fibers. Since a typical imaging device contains several tens of thou-
sands of scintillation fibers therefore a single channel PMT is not suitable for the
purpose. Other PMT structures such as microchannel PMTs are therefore often
used. The scintillation light is transported through an optical guide, such as clear
fibers, to the photocathode of the PMT. Fig.7.3.6 shows the conceptual design of
an imaging system based on scintillating fibers. Here two sets of fiber bundles are