Direct Energy, 2018a

238 10.3 Radiation Detectors
high energy, short wavelength electromagnetic radiation. When these three
types of radiation interact with air or another gas, they can excite or ionize
the atoms of the gas. Flowing ions are a current, so this process converts
the radiation to electricity. Types of radiation detectors include ionization
chambers, Geiger counters, scintillation counters, and photographic lm
based detectors [37].
Ionization chambers and Geiger counters work on the same principle.
In both cases, a gas is enclosed in a chamber or tube, and a voltage is
applied across the gas [37]. Incoming alpha particles, beta particles, or
gamma rays, ionize the gas. Due to the applied voltage, positive ions
ow to one of the electrodes, and negative ions ow to the other electrode
thereby forming a current. Geiger counters operate at higher voltages than
ionization chambers. The voltage between the electrodes in an ionization
chamber may be from a few volts to hundreds of volts while the voltage is
a Geiger counter is typically from 500 V to 2000 V [37]. Many smoke detectors
are ionization chambers [152]. When no smoke is present, radiation
from a weak radiation source ionizes air between the electrodes, and a current
is detected on the electrodes. When smoke is present, it scatters the
radiation, so no current is detected [152]. In an ionization chamber, each
incoming radioactive particle causes a single atom to ionize. In a Geiger
counter, an incoming radioactive particle causes an atom to ionize. Then,
the ions formed ionize additional atoms of the gas, and these ions ionize
additional atoms forming a cascading reaction powered and maintained by
the voltage gradient which accelerates and separates the ion pairs. Geiger
counters are often more sensitive due to this amplication of the current
produced.
Scintillation counters and photographic lm based detectors involve an
additional step in converting radiation to electricity. A scintillation counter
is often made from a crystalline material such as sodium iodide [37]. Sometimes
a phosphor is also used [5, p. 166]. Incoming radioactive particles
excite, but do not ionize, the atoms of the material. These atoms then
decay and emit a photon. Semiconductor or other types of photodetectors
convert the photons to electricity [37]. In photographic lm based detectors,
incoming radioactive particles expose the lm thereby changing its color
[37]. Materials used in the lm include Al 2 O 3 and lithium uoride [153].
Again, photodetectors are used to convert the information recorded on the
lm to a measurable signal. Scintillation counters can be higher sensitivity
than other types of radiation detectors, and they can be used to determine
the energy of incoming radiation by spectroscopy [154]. The lm based
detectors can be worn as a ring or badge. These type of detectors are used,
for example, by radiology technicians and by nuclear power plant employ-