586 INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS
nitrogen temperature, 196 C. At this low temperature ther-
mal electronic noise is reduced and the lithium is prevented
from diffusing into the central zone of intrinsic semiconduc-
tor material affording increased resolution capabilities. The
energy required for an electron-hole pair generation for sev-
eral radiation detectors are as follows: 2.9 eV (eV, electron
volt) for germanium, 30 eV for a gas ionization chamber and
500 eV to produce a photoelectron in a NaI(T1) scintillation
detector. Thus the germanium detector produces 170 times
the number of ion pairs than the scintillation detector per
unit of eV and the resolution is better by 13 times. Different
pulse widths from each type of detector caused by the ran-
domness of the process lead to the differences in energy
resolution. For electrons, x-rays, and photons the energy
resolution in the germanium detector is 3.8, 0.6 and 20 keV,
respectively. The line widths for the germanium and scintil-
lation detectors are 3.3 and 46 keV (keV, 1000 ev), respec-
tively. The germanium detector in conjunction with a pulse
height analyzer can be used to measure x- and gamma rays
in energy dispersive spectrometers. In these systems radia-
tion with a spectrum of energies can be resolved, since the
germanium detector produces a current pulse proportional to
the energy of the radiation. One disadvantage of this system
is that radiation with a wave-length above one Angstrom isbetter resolved in a crystal spectrometer. Energy dispersive
systems have excellent resolution for wavelengths below one
Angstrom but demonstrate poor resolution for wavelengths
above 1 Å.(b) Scintillation counters
Some inorganic crystals and organic crystals or mol-
ecules emit a pulse of light on interaction with ionizing
radiation. The energy of the radiation causes ionization or
activation of the scintillating substance; it relaxes emitting a
fluorescent or phosphorescent light pulse with life times of
about 10^ ^8 and 10^ ^4 seconds, respectively, in the visible or
near uv region. The number of photons emitted in each light
pulse is proportional to the energy of the radiation event.
Some crystals, namely sodium iodide, are doped with an
activator to shift the light pulse to a longer wavelength.(i) Crystal scintillation counters
Scintillation counters utilize a crystal optically coupled
to a photomultiplier tube that converts the light pulse to a
current pulse (see Section III,B,1,b, (4), ( a ),(iii)). Further
coupling to a pulse height analyzer results in an energy dis-
persive spectrometer. Inorganic scintillation crystals of alkali
halides doped with thallium, lead or europium as activators,
are commonly used. Potassium halides containing the natu-
ral radioisotope,^40 K, which radiates electrons, positrons, and
g rays, are not used in scintillation detectors.
A sodium iodide crystal doped with 0.1 to 1% of thal-
lium (I) iodide, NaI(T1), is most widely used. The NaI has
a high density that absorbs gamma radiation and the iodide
ion provides an efficient conversion of radiation to light. The
radiation event first activates the iodide ion that emits a light
pulse in the uv region. The uv pulse excites the thallous ion
that on relaxation emits fluorescent light at about 410 nm. It
is compatible to the photomultiplier tube. Since NaI is hygro-
scopic, the crystal must be sealed well. For most efficient
counting a “well type” detector is frequently used. Isotopes
emitting x-, gamma, beta, and alpha radiation are detected
by NaI(T1), and in particular x- and gamma radiation are
most beneficially measured. Energy dispersive spectrom-
eters using this detector are employed in x- and gamma-ray
spectrometers.(ii) Liquid scientillation counters
Liquid scintillation counting is a convenient and effi-
cient means of detecting low levels of radiation from small
amounts of samples. The liquid scientillation solution con-
sists of a primary, or a primary and secondary scintillator or
phosphor dissolved in an organic solvent. The radioactive
sample is dissolved in the scintillation solution. The radia-
tion excites the organic solvent molecules, such as toluene,
xylene, terphenyl, etc. The excited molecules transfer their
energy, mainly non-radiatively, to the primary scintillator
causing emission of a fluorescent light pulse detected by the
photomultiplier tube.
The newer models of liquid scintillation counters con-
tain photomultiplier tubes that respond to the emissions of a
primary scintillator. PPO (2,5-diphenyloxazole), a primaryTo preamplifierLithium-drifted
Intrinsic regionN-type regionP-type region
(dead layer-
~0.1mm)X-raysGold contact
surface (~200 Å)Gold contact
surface (~2000 Å) –500VElectronsHolesFIGURE 40 Cross-section of a typical lithium-drifted silicon,
Si(Li), detector, x-rays create electron-hole pairs in the intrinsic
region of the semiconductor; these charge carriers then migrate
to the electrodes under the influence of an applied bias voltage.
(Courtesy of the Kevex Instruments, Inc.)C009_005_r03.indd 586C009_005_r03.indd 586 11/23/2005 11:12:29 AM11/23/2005 11:12:29