Environmental Monitoring Using
Gamma-ray Spectrometers
The energies of gamma-rays produced by radioactive decay
are characteristic of the decaying nuclide. For example potassium-40 (40K)
decays to argon-40 (40Ar) with the emission of gamma-rays at 1460
keV. Gamma-ray spectrometers are designed to measure the intensity and
energies of gamma-rays and hence measure the distribution of particular
radioactive nuclides.
Airborne gamma-ray spectrometry commenced in the late
1960´s, primarily for uranium exploration. Since the mid-1970´s, the method
has been applied extensively in support of geological mapping and mineral
exploration. Airborne gamma-ray spectrometry can also be used for
environmental monitoring. Radioactive debris from the Russian nuclear
satellite Cosmos 954 was successfully located by airborne gamma-ray
spectrometry. It has also been used for mapping radioactive contamination
from the Chernobyl nuclear accident and for mapping radioactive plumes from
nuclear reactors.
Sander Geophysics has over forty years experience
conducting high resolution gamma-ray spectrometer surveys in Canada and
internationally. The company offers airborne environmental mapping of
natural and anthropogenic radiation using gamma-ray spectrometers flown in
fixed-wing aircraft and helicopters, as well as ground surveys using
suitable vehicles.
Aerospatiale Astar 350D equipped for environmental monitoring
Survey Platform
Due to their slow speed, helicopter-borne surveys are
capable of defining small anomalies and result in data of high resolution
and sensitivity. However, fixed-wing aircraft have lower operating costs and
some specialized fixed-wing aircraft, like Sander´s BN2B Islander, can
safely survey at speeds as low as 90 knots (175 km/h), resulting in
economical high resolution data. Ground-based vehicle surveys offer
increased resolution at a low cost, and are ideal for relatively small areas
with good ground access.
Spectrometer System
All of Sander Geophysics´ gamma-ray spectrometer systems
use modern NaI crystals and offer full 256 channel recording. This ensures
that the exact nature and energy level of the recorded radiation can be
determined accurately, and quantitative maps of individual nucleides such as
K, U, Th, Cs and others can be made.
Sander Geophysics´ fixed-wing aircraft can accommodate 50
litres (3000 cu in) of downward facing crystals and 8.4 litres (500 cu in)
of upward facing crystals. Helicopter systems accommodate up to 33 litres
facing down and 8.4 litres facing up. Our ground-based systems use up to
16.8 litres depending on the desired resolution and vehicle speed. In
addition, all our aircraft and ground vehicles are equipped with an
integrated navigation system, SGNav, utilising a NovAtel 24-channel GPS
(Global Positioning System) receiver, for precise navigation and accurate flight path recovery.
Specialized Processing
Sander Geophysics has implemented a spectral component
analysis technique, based on the method of Hovgaard and Grasty (of
Exploranium and Gamma-Bob respectively). This noise reduction technique,
called Noise Adjusted Singular Value Decomposition (NASVD), uses the full
spectrum data to enhance the resolution of radiometric data. Sander also
uses a combination of spectrum fitting and NASVD to produce maps of
cesium and other man-made nuclides from the 256 channel radiometric data.
A Case Study
Recently, airborne gamma-ray spectrometry has been used
to identify and quantify the distribution of natural and man-made gamma
emitting isotopes in the vicinity of nuclear power plants in order to assess
the plant´s dose impact on members of the public. In addition, the data
provide an environmental baseline of the nuclear operating site and the
surrounding community. In the event of an accidental release of radiation
from the facility, a subsequent survey could then be used to determine any
increase in dose to the public.
Ideally, a gamma-ray spectrometer survey of a nuclear
facility will demonstrate that the gamma radiation produced at the facility
is localized to the containment structures and the waste storage sites, and
that no radiation from the reactor or waste storage operations is detected
outside the plant boundary. Confidence in the facility is further enhanced
if the natural radiation levels in the areas outside the plant boundaries
are shown to be typical of the levels found within the rest of the country.
In the fall of 1999, Sander Geophysics flew an airborne
gamma-ray survey over the primary zone of the Pickering Nuclear Generating
Station in Ontario, Canada. The survey was designed to address public
concern of possible soil contamination resulting from the operation of the
facility, and was flown under contract to Ontario Power Generation (OPG),
formerly Ontario Hydro.
The survey showed that the radiation levels in the
surrounding community are comparable to the values reported by the
Geological Survey of Canada (GSC) as typical for Ontario. Within the plant
boundary, several radioactive isotopes resulting from normal plant
operations were detected within acceptable limits. These isotopes included
cobalt-60 (60Co), cesium-137 (137Cs) and nitrogen-16 (16N).
Shown above is a map of the 16N distribution within the survey
area, with an aerial photograph as a background. Clearly visible are the
four operating Pickering "B" reactor units, which show up as a localized
area of high intensity (yellow-red) on the map. To the left of this high are
the Pickering "A" reactor units which are shut down. Also apparent, is the
generally low intensity (green-blue) of the area immediately adjacent to the
plant and out into the surrounding community.
The results of the survey have familiarized the public
with existing levels of both natural and man-made gamma radiation, and
verified that there are no gamma emitting radioactive materials in the
surrounding residential community due to the operation of the nuclear
generating station.
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