Physics and Engineering of Radiation Detection

442 Chapter 7. Position Sensitive Detection and Imaging

Reflection

Reflected Pulse Time Difference

Direct Transmission Reflected Direct

HV−In and

Signal−Out

Incident Particle

Figure 7.2.1: Position sensitive detection from a timing device. The signal gen-

erated in the central anode wire is transmitted in both directions. The direct and

reflected pulses arrive at the processing electronics (such as an oscilloscope) at dif-

ferent times. The time difference is then used to determine the position of the pulse

generation provided the system has been calibrated.

The spacing between the anode wires, also called pitch, determines its position
resolution. A pitch of 2mmto 3mmis typical of most wire chambers. It should,
however, be remembered that the position resolution is generally better than this
spacing. The cathode planes are generally separated by a distance of approximately
six times the pitch. A point worth noting here is that the cathode plane can also
consist of closely spaced wires or strips to enhance the position resolution. However
it complicates the design and is typically not done.
Typical anode wire diameter is about 20μm, which produces a high potential
gradient with respect to the cathode plane. The potential at any point in the active
volume of the chamber can be calculated from

V(x, y)=−

q

4 π 0

ln

[

4sin^2

(πx

s

)

+4sinh^2

(πy

s

)]

, (7.2.1)

which represents the distribution of potential around an array of parallel line charges
q measured inC/malongz-axis and located at (x, y)=(i×s, i×s)withi =
0 ,± 1 ,± 2 .... Naturally the potential near the anode wires should be high enough
to cause avalanche multiplication required by proportional counters. The electric
field intensity in a typical multiwire chamber is shown in Fig.7.2.3. It is evident
that the field intensity is very high near the anode wire and that is where the
avalanche begins. The details of the multiplication process that leads to an output
pulse proportional to deposited energy has already been discussed in the chapter on
gas filled detectors and therefore will not be repeated here. The interested reader
is encouraged to go through the section on proportional counters in that chapter.
Here we will concentrate on the aspects particular to the multiwire operation of the
chamber.
As with single wire proportional counters, the radiation passing through the
chamber volume produces electron ion pairs. The ions move towards the cathode
while the electrons rush toward the anode wires. The high electric potential at
the anode causes the electrons to gain enough energy between collisions to produce
secondary ionization and ultimately avalanche. A major concern in multiwire pro-
portional counters is the spreading of this avalanche since it can potentially cause
deterioration of the position resolution. The reader might recall, that in single wire