As the applied voltage is increased beyond the Geiger region, a single
ionizing event produces a series of repetitive discharges leading to what is
called spontaneous discharge. This region is called the region of continuous
discharge because the gas may be ionized in the absence of radiation at this
high voltage (see Fig. 7.2). Operation of a detector in this region may cause
damage to the detector.
Ionization Chambers
Ionization chambers are operated at voltages in the saturation region that
spans 50–300 V. The detector is a cylindrical chamber filled with air or a gas,
sometimes at high pressure. A central wire and the chamber act as the elec-
trodes and the current is measured by an electrometer. The detection effi-
ciency of the ionization chambers for x-rays and g-rays is very low (<1%)
and depends on the energy of these radiations. Ionization chambers are
primarily used for measuring high-intensity radiation such as x-ray beams
and high activity of radiopharmaceuticals. Cutie Pie meters, dose calibra-
tors, and pocket dosimeters are the common ionization chambers used in
nuclear medicine.
Cutie Pie Survey Meter
The Cutie Pie survey meter is made of an outer metallic cylindrical elec-
trode and a central wire. It uses air for ionization and is operated with a
battery. It is used to measure the radiation exposure in the range of mR/hr
to R/hr. It is primarily used to monitor the exposure at high radiation levels
such as those from x-ray beams and^99 Mo–99mTc generators. It does not have
a dead time.
Dose Calibrator
The dose calibrator is an ionization chamber and one of the most essential
instruments in nuclear medicine for measuring the activity of radionuclides
and radiopharmaceuticals. Since it measures the current produced by activ-
ity, it does not have deadtime effects. It is a cylindrically shaped, sealed
chamber with a central well and is filled with argon and traces of halogen
at high pressure (~5–12 atmospheres). Its operating voltage is about 150 V.
A typical dose calibrator is shown in Figure 7.3.
Because radiations of different types and energies produce different
amounts of ionization (hence current), equal activities of different radionu-
clides generate different quantities of current. For example, the amount of
the current produced by 1 mCi (37 MBq) of 99mTc differs from that produced
by 1 mCi (37 MBq) of^131 I. Isotope selectors provided on the dose calibra-
tor are the feedback resistors to compensate for the differences in ioniza-
74 7. Gas-Filled Detectors