Physics and Engineering of Radiation Detection

476 Chapter 8. Signal Processing

8.2.A TypeofCable

A typical signal cable consists of a central metallic wire with a metal shield around
it. No matter how good the wire of the cable is, it still has some resistance given by

R=ρ

Lc

Ac

, (8.2.1)

whereρis the resistivity of the metal,Lcis its total length, andAc is its cross
sectional area. The voltage drop across the cable due to this resistance can simply
be calculated by Ohm’s law, which gives

V=IR=ρI

Lc

Ac

. (8.2.2)

HereIis the current flowing through the wire. This voltage drop must be kept as
low as possible as compared to the minimum signal voltage. It is apparent from this
equation that this can be done in essentially three ways:

Low Resistivity Material:Resistivityρis material and temperature depen-

dent. Silver is a very good conductor with a resistivity of 1. 62 × 10 −^8 Ωcmbut

is quite expensive as compared to other materials, such as copper or aluminum.

Copper has a resistivity of 1. 69 × 10 −^8 Ωcm, which is slightly higher than that

of silver but with two obvious advantages: it is much cheaper and can be easily

shaped into conductors of different thicknesses. Another good material is alu-

minum with a resistivity of 2. 75 × 10 −^8 Ωcm. This can be cheaper than copper

but has a slightly higher resistivity.


Small Cable Length: Keeping the length small is the most simplistic way

to decrease signal attenuation due to cable. However there are normally engi-

neering constraints to the minimum possible length. For example in a detector

working in high radiation field, the data processing units are generally located

a few hundred feet away from the detector and proper routing of cables puts a

minimum limit on the cable length.


Large Cross Sectional Area: The technical term describing the cross sec-

tional area of a wire is itsgage. The lower the gage, the thicker the wire.

Resistivity and hence the voltage drop in low gage wires is definitely small,

but there may be practical considerations, such as cost and cable routing (the

lower the gage the stiffer the cable), that may put a limit on the wire thickness

for a particular system.

It is a general practice to use standard cables in radiation detection systems,
even though a custom made cable is expected to make signal transportation a bit
better. There are two reasons for that. Firstly the cost of custom made cables is
significantly higher than the standard cables, and secondly a custom made cable has
to be available in large quantities to replace the damaged cables. We will discuss
here some of the important types of standard cables available.

Example:

A50mlong single wire copper cable is used to transport signal current. The

wire has a cross sectional area of 0.5mm^2. Assume that the resistivity of