the design of the gas detector and the applied voltage. In this region, the
total current measured is equal to the number of ionizations caused by the
primary radiation multiplied by the gas amplification factor. In this region,
the current increases with the applied voltage in proportion to the initial
number of ion pairs produced by the incident radiation. Therefore, as in the
case of the region of saturation, the current amplification is relatively pro-
portional to the types of radiations, e.g.,a-,b-, and g-radiations. This region
is referred to as the proportional region(see Fig. 7.2). Proportional coun-
ters are usually filled with 90% argon and 10% methane at atmospheric
pressure. These counters can be used to count individual counts and to
discriminate radiations of different energies. These counters, however, are
not commonly used for g- and x-ray counting because of poor counting
efficiency (<1%).
As the applied voltage is increased further, the current produced by
different types of radiation tends to become identical. The voltage range
over which the current tends to converge is referred to as the region of
limited proportionality. This region is not practically used for detecting
any radiation in nuclear medicine.
With additional increase in voltage beyond the region of limited pro-
portionality, the current becomes identical, regardless of how many ion
pairs are produced by the incident radiations. This region is referred to as
the Geiger region(see Fig. 7.2). In the Geiger voltage region, the current is
produced by an avalanche of interactions. When highly accelerated elec-
trons strike the anode with a great force, ultraviolet (UV) light is emitted,
which causes further emission of photoelectrons by gas ionization and from
the chamber walls. The photoelectrons will again strike the anode to
produce more UV, and hence an avalanche spreads along the entire length
of the anode. The amplification factor can be as high as 10^10. During the
avalanche, however, the lightweight electrons are quickly attracted to the
anode, whereas a sheath of slow-moving heavy positive ions builds up
around the anode. As a result, the voltage gradient falls below the value
necessary for ion multiplication, and therefore the avalanche is terminated.
All this occurs in less than 0.5 microsecond, and the counter is left insensi-
tive and must recover before another event can be counted.
Recovery begins with the migration of the positive ions toward the
cathode (i.e., chamber wall) and takes about 200 microseconds at a gas pres-
sure of 0.1 atmosphere, which is equal to the dead time of the counter that
varies with gas pressure. As the positive ions approach the cathode, sec-
ondary electrons may be emitted from the surface of the cathode, which
then set another discharge just about 200 microseconds after the previous
one. Such repetitive discharges that are due to secondary electrons are inde-
pendent of the types and energy of radiation that the counter is intended
to measure. The emission of secondary electrons is suppressed by a tech-
nique known as quenching to eliminate repetitive counter discharges (see
later).
Principles of Gas-Filled Detectors 73