7.2. Position Sensitive Detection 441
7.2 PositionSensitiveDetection.......................
7.2.A TypesofPositionSensitiveDetectors..............
Broadly speaking, position sensitive detectors can be grouped into three categories:
array devices, scanning devices, and timing devices.
A.1 ArrayDevices..........................
The position sensitive detectors that are made up of a large number of individual
detectors arranged in the form of an array fall into this category. The individual
detectors of the array can be of any shape, such as linear, circular, or square. The
example of linear arrays are multiwire proportional counters and silicon microstrip
detectors, both of which were originally developed for high energy experiments. We
will discuss these and other types of array devices later in the chapter.
A.2 ScanningDevices........................
It is in principle possible to perform position measurement by scanning a single
detector over the object. However such devices are generally used for imaging and
therefore their discussion is deferred to the section on imaging devices.
A.3 TimingDevices.........................
The array devices we discussed earlier can not always be made very large. To un-
derstand this point let us suppose that the detector is to be used to find the position
of a particle within a precision of a few centimeters along a linear dimension that
stretches for several meters. Furthermore suppose that the incident particle can
come from any direction. Building an array detector for this purpose would require
enclosing the whole volume with small detectors, which may not be practically pos-
sible. The solution to this problem is to build the so called timing device in which
the position is measured by determining the time of arrival of the pulse generated
by the incident particle. This concept is graphically depicted in Fig.7.2.1.
Let us now discuss some of the most commonly used position sensitive detectors.
7.2.B Multiwire Proportional Chambers (MWPCs)
A multiwire proportional counter is simply a gas filled wire chamber consisting of
a large number of anode wires stretched in a plane for position measurement. The
original motivation of developing such detectors came from the need to track particles
in high energy physics experiments. The wire chambers thus developed worked
amazingly well and helped scientists make great discoveries at particle accelerators.
Although these chambers are still used in some laboratories but since the advent of
high resolution silicon detectors their utility has somewhat diminished.
Fig.7.2.2 shows the sketch of a typical multiwire proportional chamber. The
closely spaced thin anode wires are stretched in a plane that is midway between
two cathode planes. The cathodes are kept at negative potential with respect to
anode wires that are kept at ground potential. Each of the anode wires acts as
an independent proportional counter and must therefore be read out independently.