Environmental Engineering Reference
In-Depth Information
make evaluating radioactivity in water assets in the field somewhat difficult.
Alpha particles, for example, can only travel short distances, and they cannot
penetrate through most physical objects; therefore, instruments designed to
evaluate alpha emissions must be specially designed to capture emissions at
a short distance from the source, and they must not block alpha emissions
from entering the detector. Gamma radiation does not have the same types
of physical properties, so it can be measured using different detectors.
Measuring different types of radiation is further complicated by the rela-
tionship between the intrinsic properties of the radiation and the medium
in which the radiation is being measured. Gas-flow proportional counters
are typically used to evaluate gross alpha and beta radiation from smooth,
solid surfaces, but because water is not a smooth surface and because alpha
and beta emissions are relatively short range and can be attenuated within
the water, these types of counters are not appropriate for measuring alpha
and beta activity in water. An appropriate method for measuring alpha and
beta radiation in water is using a liquid scintillation counter; however, this
requires mixing an aliquot of water with a liquid scintillation cocktail. The
liquid scintillation counter is a large, sensitive piece of equipment and is not
appropriate for field use; therefore, measurements for alpha and beta radia-
tion from water assets are not typically made in the field.
Unlike the problems associated with measuring alpha and beta activity
in water in the field, the properties of gamma radiation allow it to be mea-
sured relatively well in water samples in the field. The standard instrumen-
tation used to measure gamma radiation from water samples in the field is a
sodium iodide (NaI) scintillator.
Although the devices outlined above are the most commonly used for
evaluating total alpha, beta, and gamma radiation, other methods and other
devices can be used. In addition, local conditions (e.g., temperature, humid-
ity) or the properties of the specific radionuclides emitting the radiation may
make other types of devices or other methods more optimal to achieve the
goals of the survey than the devices noted above. In such cases, experts or
individual vendors should be consulted to determine the appropriate mea-
surement device for any specific application.
An additional factor to consider when developing a program to monitor
for radioactive contamination in water assets is whether to take regular grab
samples or to sample continuously. Portable sensors can be used to analyze
grab samples at any point in the system, but they have the disadvantage that
they provide measurements only at one point in time. On the other hand,
fixed-location sensors are usually used as part of a continuous, online moni-
toring system. These systems continuously monitor a water asset and could
be outfitted with some type of alarm system that would alert operators if
radiation increased above a certain threshold; however, the sampling points
are fixed, and only certain points in the system can be monitored. In addi-
tion, the number of monitoring locations required to capture the physical
and radioactive complexity of a system can be prohibitive.
