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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