Subsurface Gamma Radiation Detection
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.
á 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.
á 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.
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.
Other Resources and Demonstrations
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.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.