Environmental Engineering Reference
In-Depth Information
The G-M tubes of some G-M counters are covered with a thin mica window which
allows less energetic b emitters such as
14
C to be detected.
Gas-flow proportional counter: The principle of the gas flow counter is similar
to that of the G-M counter. The gas flow counter has a constant supply of gas and can
have a thin window or windowless (internal proportional counter). Thin window or
windowless gas flow counters will detect a particles and even the weakest b emitters.
The gas-flow proportional counter can be used to measure gross a- and b-particles,
228
Ra,
89
Sr,
134
Cs, and
131
I.
Liquid scintillation counters: Liquid scintillation counting is the method of
choice for measuring b emitters, particularly, weak b emitters such as
14
C,
3
H, and
35
S. The sample is dissolved or suspended in a solution called cocktail. The
commercially available cocktails contain an organic chemical which are able to
absorb radioactive ionization and reemit (fluoresce) it as UV light. The radiation is
thus converted into pulses of light that are detected by a photomultiplier. The amount
of light produced is related to the energy of the b emitter.
Gamma scintillation spectrometry: X-rays pass freely through the solutions
used for liquid scintillation counting. They are, therefore, detected by using solid
fluors containing atoms of high atomic numbers. The most commonly used solid
scintillator is a sodium iodide (NaI) crystal containing thallium ions as an
intentionally added impurity. Impinging rays excite electrons that produce light
when they return to their ground state. The light is detected by a photomultiplier.
The U.S. EPA has prescribed detailed procedures for the detection and
measurement of radioactive chemicals in environmental samples such as radio-
nuclides in drinking waters and radon gas in atmosphere. Interested readers should
refer to these procedures for details, such as EPA 600 and 900 series methods for
water and 9000 series methods for hazardous materials.
12.3.2 Surface and Interface Analysis
The characterization of surface and interfaces is of great concern to many areas of
environmental studies, such as heterogeneous reactions involving solids (adsorption,
diffusion, corrosion), nano-materials, membranes, and aerosol particles, to just name
a few. Surface layers can be in the thickness of less than one atomic layer up to
several micrometers. Both physical (topology and morphology) and chemical
(elemental composition, chemical bonding, and geometric and electronic structure)
techniques have been available for the qualitative and quantitative analysis of
surface and interfaces. At present, there are more than 30 methods that are actually
in use for surface and interface analyses. We only briefly introduce three common
microscopic techniques used to characterize the physical nature (image) of the
surface and interface, namely, scanning electron microscopy (SEM), scanning
tunneling microscopy (STM), and atomic force microscopy (AFM).
The conventional light microscopes have a resolution limit of approximately
250 nm (0.25
), which is approximately the size of a bacterium. This resolution in
nanometer is also approximately the wavelength of the incoming visible light
m
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