Subsurface Gamma Radiation
Detection
Description
Field instruments commonly used for detecting radiological
contaminants rely on the detection of gamma-ray emissions from the radionuclides of
interest. Above-ground gamma-ray detectors typically employ one of two types of
solid crystals that interact with gamma rays to produce a detectable signal:
sodium iodide (NaI) scintillators and high-purity germanium (HPGe)
semiconductor-type detectors.
A scintillator is a material that gives off light when a
charged particle passes through it. Typically it is a form of plastic, produced
with traces of certain elements that are readily excited by the passing charged
particle and then rapidly decay, thereby producing light.
NaI crystals are generally easier and less expensive to grow
to large sizes than HPGe crystals. This large size is an advantage, since
larger crystals convert more gamma rays to detector counts. NaI scintillators
have good efficiency for the conversion of gamma rays; however, their low
resolution makes spectrometric measurements of mixtures difficult.
The NaI detector is generally used in a scanning mode to
cover large areas quickly. An HPGe detector is generally used to make
high-quality stationary measurements. Thus NaI and HPGe detectors are complementary
in characterizing radiologically contaminated soils.
Yet
at many U.S. Department of Energy (DOE) sites, restoration planning requires
characterization of radiation fields below the surface for both for
contaminated soil and groundwater. The crucial component of any
gamma-measuring device is the detector as described above. The following are descriptions of the
most recent innovations that DOE has demonstrated and evaluated to detect gamma
radiation in the subsurface.
á
Spectral
Gamma Probe. The
Spectral Gamma Probe is designed for the in-situ detection of subsurface radionuclides. The gamma radiation
detection system is driven into the subsurface using a Site Characterization
and Analysis Penetrometer System (SCAPS) or other cone penetrometer (CPT) truck. The sensor uses a NaI
scintillation crystal to detect gamma radiation in the subsurface at the probe
tip. This technology can be used anywhere to characterize underground gamma
radioactivity assuming that the subsurface is conducive to cone penetrometer
exploration and characterization.
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The
Vadose Zone Characterization System (VZCS).
The ability to obtain additional
characterization data on contaminated plumes at new locations beneath tank farms
is limited by poor access. The VZCS uses a CPT with gamma sensors to
characterize gamma contamination in the vadose zone under tank farms. A
truck-mounted VZCS can be driven into tank farms and located in any area the
size of a parking space. Gamma contamination can be characterized to a depth of
150 ft. The VZCS has several sensors within a CPT. A magnetometer in the CPT
tip reduces the probability of contacting subsurface utility or instrument
lines during use. A screening module uses X-ray fluorescence
(XRF) and gamma
spectroscopy (GS) sensors to detect radioactive contaminants. A standard CPT tip module
identifies soil stratigraphy.
A soil moisture module enables the measurement of soil moisture content and
soil resistivity, both important in determining soil stratigraphy and
contaminant migration.
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Slim-Hole
Gamma Ray Log
.
Slim-hole gamma ray tools using sodium iodide (NaI) scintillation crystal
detectors are best suited for high radioactivity environments. At Lawrence
Livermore National Laboratory (LLNL), Livermore, California, a slim-hole gamma
ray drilling log detector was improved upon by doubling the scintillation
crystal volume. This approach is of value in low radioactivity environments.
Limitations
and Concerns
á
Spectral
gamma probe. The
use of the spectral gamma probe is currently limited to sites where a cone
penetrometer can penetrate the subsurface to the desired depth. Its use is
normally restricted to about 150 feet. The spectral gamma probe generally does
not perform well in geologic environments other than clays and sandy sediments.
Sites that have radioactivity levels that span wide ranges could present
problems for quantitative analyses. The NaI detector used in the present
spectral gamma probe has a relatively high detection efficiency but has a
relatively poor energy resolution, and its light output varies with
temperature. Measurements made where high radiation levels are present need
significant post-measurement corrections.
á
The
Vadose Zone Characterization System (VZCS).
The probes cannot be used where the soil is
laden with rocks and boulders because of the potential for probe or pipe
breakage. Field maintenance is needed after CPT deployment. This includes
assembly and disassembly, probe calibration, and repair, as needed. Field
verification is needed.
Applicability
The technologies described detect
gamma radiation in the subsurface environment.
Technology
Development Status
All of the subsurface
technologies described are in the demonstration phase of development.
Web
Links
http://www.itrcweb.org/Documents/RAD_4Web.pdf
http://www.frtr.gov/pdf/itsr2364.pdf
Other
Resources and Demonstrations
See the
descriptions of Surface Gamma
Radiation Detectors,
BetaScintTM, Cone Penetrometer, SCAPS, and XRF.
See http://www.xrfcorp.com/technology/radiation_detection.html for a description of common
radiation detection methods.
Radioactivity, or ionizing radiation, is
the spontaneous disintegration of unstable atomic nuclei. Ionizing radiation
can take the form of alpha, beta, or gamma
particles.
See http://www.osti.gov/bridge/servlets/purl/14627-2k1e0c/native/
See http://www.frtr.gov/pdf/itsr12.pdf for a demonstration of several
radiation detection sensors at Miamisburg.
See http://www.clu-in.org/download/char/402-r-06-007.pdf
for a report on radiological detection methods.
See http://www.frtr.gov/site/8_2_2.html
for ex-situ gamma analysis.