7.2. Position Sensitive Detection 445
resolution can be obtained by using several chambers in succession such that the
wires of each of them are tilted at a different angle.
7.2.C MultiwireDriftChamber
A multiwire drift chamber is a tracking device that was built to track particles
in high energy physics collisions. It uses the timing information to determine the
position of an ionizing particle. In principle it is similar to MWPC we just discussed
but uses another detector to make the relative time measurements. A drift chamber
has two other features as well that distinguish it from a conventional multiwire
proportional chamber. One is the anode wire spacing, which in this case is generally
a few centimeters as opposed to a few millimeters in case of a MWPC. The other
is the shape of the cathode. In a drift chamber the cathode is made up of either
closely spaced wires or strips that are kept at distributed potentials. The advantage
of this design is that the electrons produced by the incident radiation are directed
towards the nearest anode wire with a constant drift velocity.
The design and principle of operation of a multiwire drift chamber are shown
in Fig.7.2.5. The incident radiation produces electron ion pairs inside the active
volume that drift towards the nearest anode and cathode. After passing through
the chamber, the radiation is detected by a fast detector, such as a scintillation
counter, which produces a pulse. This pulse initiate a logic-enable pulse that starts
a timing counter as shown in Fig.7.2.5(b). The counter keeps on counting until it
gets a stop signal that is generated when the electrons eventually reach the anode,
produce avalanche, and produce an anode pulse. As shown in the figure, a logic
enable pulse is initiated that stops the timer. The number of ticks of the counter
determines the time it has taken the electrons to reach the anode wire. Now, if
the drift velocity of the electrons in the chamber gas is known, one can determine
the distance traveled by the electron and hence position at which the electron was
produced by the radiation.
7.2.D MicrostripGasChambers
Microstrip gas chambers are essentially based on the concept of MWPC. Here instead
of anode wires stretched between cathode planes, alternating anode and cathode
strips are realized on a suitable substrate. On top of this whole structure a drift
plane is provided and the chamber is filled with gas. In most modern MSGCs the
anode and cathode strips are formed through the process of photo-lithography. The
electrodes are generally made of gold of chromium, though in principle one can use
any high conductivity material. Although with this technique one can form strips
as narrow as a few microns, but the typical widths of anode and cathode strips are
10 μmand 100μmrespectively. The electrode to electrode distance depends on the
position resolution desired and can range from a few tens of micormeters to more
than a hundred micrometer. The thickness of the electrodes is generally less than 2
μm. A typical MSGC is shown in Fig.7.2.6.
MSGCs are known to show high radiation hardness, which makes them suitable
for use in hostile radiation environments. Together with this advantage they also
offer very good position resolution. Detectors with 20μmto 30μmresolution have
been developed and operated at high energy physics research facilities. Although