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Characterization of uranium contamination in surface soils
J. Environ. Radioactiviry 26 (1995) 147-156 01995 Elsevier Science Limited Printed in Ireland. ELSEVIER
All rights reserved 0265-931X/95/$9.50
0265-931X(94)00007-7
Characterization of Uranium Contamination in Surface Soils
A. J. Schilk, K. H. Abel & R. W. Perkins Pacific Northwest (Received
Laboratory,
5 August
Richland,
Washington
99352, USA
1993; revised version received 5 April 1994; accepted 15 April 1994)
ABSTRACT Traditional means of obtaining radionuclide concentrations in soils over large areas are often time-consuming, cumbersome, expensive, and potentially non-representative. In an attempt to develop improved systems and new methodologies for the rapid and economical characterization of largescale uranium contamination, two disparate monitoring technologies were compared at a contaminated site within the Fernald facility near Cincinnati, Ohio, USA. The results of this preliminary study suggest that uncollimated in-situ y spectrometry and high-energy /3-scintillation sensing may represent viable alternatives to, and in many cases could mitigate the need for, the collection of myriad soil samples and subsequent laboratory analyses when characterizing large contaminated sites.
INTRODUCTION The past operations of uranium production and support facilities at several United States Department of Energy (USDOE) sites have occasionally resulted in the local contamination of some surface soils. Before any effective remedial protocols can be established, the spatial distribution of this contaminant must be adequately characterized. Unfortunately, traditional means of obtaining soil activities (e.g. grab sampling followed by laboratory analyses) are cumbersome, expensive, time-consuming, and often non-representative when very large areas are under consideration. 147
148
A. J. Schilk, K. H. Abel, R. W. Perkins
Hence, new technologies must be developed, or existing ones improved, to allow for the cheaper, safer, and faster characterization of uranium concentrations at these critical sites. The USDOE’s Fernald Environmental Management Project (FEMP) near Cincinnati, Ohio, is one specific site of concern. This facility, once the primary production site for uranium materials for defense-related projects, began operation in the early 1950s and was mainly responsible for the processing of uranium metal from concentrates. Production ceased in the late 1980s. Currently, approximately 50% of the surface soils in and around the general production areas are known to be contaminated by various uranium-bearing compounds in excess of 1.3 x lo3 Bq/kg (35 pCi/ g) of soil as a result of normal operations and unintentional spills or emissions. Pacific Northwest Laboratory (PNL) has been tasked with adapting and/or developing technologies to measure uranium in surface soils to assist in the establishment of remedial protocols; in partial completion of this effort, two separate methodologies were demonstrated at FEMP. First, uncollimated in-situ y-ray spectrometry was used for the measurement of 234mPa, which is in secular equilibrium with its parent, 238U, and gives rise to a prominent 1.001 MeV y-ray. The ‘wide-view’ capability associated with this methodology, allowing many hundreds of square meters of soil to be observed concurrently, made it particularly useful in the preliminary screening of the site (i.e. for the rapid identification of areas of elevated concentrations and the daily generation of surfaceactivity contour maps). A second surface-monitoring system, designed and built by PNL, was used to measure the 2.29 MeV /?-flux-also from the from shallower depths and smaller surface decay of 234mPa-emanating areas (approximately 0.15 m2). This sensor served to establish ‘local’ surface uranium contamination levels for comparison with the in-situ y results. The purpose of this paper is to summarize the overall results of the characterization effort at the FEMP site and to outline the features of the aforementioned technologies with the hope of facilitating future critical evaluations.
TECHNOLOGICAL
THEORY AND METHODOLOGY
Intrinsic germanium (IG) detector y-ray spectroscopy has many advantages as an analytical tool for surface environmental monitoring, including its selective high resolution (enabling precise, quantitative, multiradioisotopic characterization), its relatively easy transportability, and
Characterization of uranium contamination in surface soils
149
the capability of rapid data reduction to provide essentially real-time results in the field. In-situ y-ray spectrometry has enjoyed widespread use around the world for the surface characterization of a multitude of photon-emitting radionuclides. Its use with regard to the quantification of uranium content in soils has, on the other hand, been somewhat limited (see, e.g. Daling et al., 1990; Reiman, 1983; Fritzsche, 1983; Lovborg et al., 1979). The extended-surface-area y monitor demonstrated at FEMP employed an uncollimated, down-looking IG detector/cryostat assembly that was suspended 1 m above the soil. This detector is sensitive to surface (and shallow subsurface) activity over many hundreds of square meters, and effectively averages any horizontal heterogeneities that may exist within its field of view. Although in theory this system would monitor complete 2-71 space, it has been established empirically (Helfer & Miller, 1988) that approximately 80-90% of the total observed fluence originates within a 10-m radius from the detector due to geometry factors and attenuation in the soil and air (for most y energies and soil distributions). Following calibration, a non-trivial process that leads to an energyspecific list of conversion factors for each radionuclide of interest, the concentration (activity/g) or surface inventory (activity/m*) of the radionuclide(s) of concern may be determined for uniform or non-uniform (e.g. planar or exponential) sources, respectively. A uniformly distributed source was assumed at each sample location despite empirical evidence to the contrary (i.e. much of the local contamination has been shown to exist within the upper few centimeters of soil). Such an assumption leads to the condition in which the activity levels within the upper 15-20 cm (the ‘effective’ depth of view for the 1.001 MeV 234mPa y) are essentially averaged, thereby slightly overestimating the uranium content at depth while often underestimating the surface levels directly below the detector. Nevertheless, such averaged results are still viable with regard to largescale site cleanup efforts. Based on the sensor’s effective field of view as discussed above, a staggered map grid was established with measurement points on 20-m centers (see below). Individual count times were restricted to 10 minutes at each location. Calibration checks of the system’s efficiency and stability were performed by counting a standard source (241Am, 13’Cs, and 6oCo) at periodic intervals during each working day, and the data were reduced in the field utilizing an in-house, application-specific software package. The high-energy /Y?-scintillationsensor consists of three stacked ‘ribbons’ of l-mm* plastic scintillating fibers and is optimized for the detection of /3particles that are emitted from the decay of near-surface uranium. The 2.29 MeV (maximum energy) p-particle from 234mPa, a daughter of 238U
A. J. Schilk. K. H. Abel, R. W. Perkins
150
Unattenuated Gamma Ray High-Ener BetiB (e.g., from Byor SIr)
I
I
I A
3rClm
I
R
I
TypMfkgd. Attenuated Gamma Ray
Fig. 1. Simplified diagram of the PNL prototype B sensor showing the discrimination capabilities for high vs low b energies and ionizing vs non-ionizing radiation. The double lines are intended to represent those regions where ionization and excitation occur in the dopant material, leading to subsequent scintillations (e- denotes a Compton electron, which is indistinguishable from an incident p-particle). Only normally incident radiation is shown for clarity; penetration depths for non-normal incidence will be proportionally smaller.
and in secular equilibrium therewith, is ‘selectively’ measured due to its relatively high energy. A unique feature of this sensor is the stacked configuration (Fig. 1) that allows the discrimination between (a) unattenuated 234mPa @s that, when traveling at their ‘most probable’ energies (roughly 0.8 MeV), will penetrate nearly 3 mm of plastic, and (b) lowerenergy /?-particles arising from natural sources, viz. 40K, 232Th (plus daughters), and 238U (plus daughters); the maximum /? energies from these radionuclides or their progeny rarely exceed 1 MeV (or their associated abundances are relatively insignificant compared with (a) above) and the most probable penetration depth of an unattenuated, normally incident ‘background’ /I?will therefore be less than 1 to 2 mm. Hence, by observing the total number of times that all three layers scintillate concurrently per unit time, one may effectively quantify the 238U activity at the soil surface following an appropriate calibration (see below). This sensor is also somewhat sensitive to y-rays, which may produce interfering signals (Fig. 1). The multi-layer design affords some discrimination against y emission by observing, and partially canceling the contribution from, this ubiquitous phenomenon. Such selective measurement is possible since a P-particle (or other charged particle) will cause continuous excitations and consequent scintillation events along its entire path through the detector stack, while a y-ray might produce a Compton
Characterization of uranium contamination in swface soils
151
electron in only one (indeed, if any) of the individual layers, thereby generating a unique signal. Hence, by establishing an electronic requisite for coincident signals from all layers, one can primarily detect high-energy p events and either observe or effectively ignore much of the y contribution to the overall detector response. The signal-to-noise ratio is further enhanced by requiring intralayer coincidences as well (i.e. photomultiplier tubes at opposite ends of each fiber ribbon must receive concurrent pulses in order to be acknowledged), thereby mitigating the effect of thermionic-induced signals from the indiAnother important consideration in the vidual photomultipliers. construction of such a sensor is the need for electronic components that are sensitive enough to handle the rapid (< lo-ns) scintillation pulses originating from the excited fibers. As a consequence, PNL developed its own high-speed amplifiers, signal discriminators, and logic circuitry suitable for the specific technical requirements (see Fig. 2). All grassy areas were cropped before surface characterization, the sensor was set directly on the ground during data acquisition, and five 100-s counts were performed at each sample location. Calibration of this system consisted of monitoring a series of standard soil samples (viz. medium-grained river sands) that were ‘spiked’ with 238U02(N03)2. In order to ensure system stability and optimal operation, background and calibration counts were performed periodically throughout each working day.
O---,
Storage
Fig. 2. Circuitry schematic for the /I sensor showing the requirements for both interlayer and intralayer coincidences to measure the total number of incident high-energy P-particles and to mitigate photomultiplier tube noise, respectively.
152
A. J. Schilk, K. H. Abel, R. W. Perkins
RESULTS AND DISCUSSION Surveys were conducted at FEMP’s former Drum Baling Area (DBA), a 9100-m2 site within the original production area that served as a staging area for clean and contaminated scrap materials. Soils within the DBA contain production-related uranium-bearing compounds to various depths as a result of airborne deposition and direct contact with radioactive fluids and/or contaminated scrap items. Figure 3 indicates the general boundaries for this site, as well as the, individual measurement locations utilized in this characterization study. Tabulated data results may be found in the associated PNL technical report by Schilk et al. (1993). Qualitatively, the p and y results correlate quite well in terms of ‘hotspot’ identification despite the extreme differences in the fields of view and observation depths associated with these inherently disparate methodologies (Figs 4 and 5). The quantitative results generated by these two techniques, however, are rarely found to be equivalent. Indeed, such agreement would occur only in the case of a vertically (and horizontally)
Fig. 3. Map view of the FEMP DBA showing sample locations (denoted by asterisks).
Individual sample sites are separated by 20 m in North-South
and East-West directions.
Characterization of uranium contamination in surface soils
153
Fig. 4. Uranium-238
surface-activity contour map for the FEMP DBA based on the insitu y-ray spectrometry. The contour interval is 2 000 Bq/kg.
uniform uranium distribution. Conversely, any contaminant distribution that decreases rapidly with depth (as at FEMP) would be manifested by a P-response which exceeds that of the in-situ y-ray spectrometer ‘expecting’ a uniformly distributed source, since the /I detection unit interrogates surface soils exclusively while the IG crystal obtains information down to depths exceeding 15 cm and essentially averages the overall y activity throughout this depth. Furthermore, unless the vertical distributions are particularly variable throughout the surveyed area, a significant correlation between the /I and y sensor responses would be expected (despite the inherent disparity in terms of sensor methodologies and depths of view) if these technologies represent mutually supportive and viable alternatives to traditional soil sampling. Such behavior is evident in Fig. 6, in which the fi and y results are seen to be well correlated, thereby lending support to the above premise. Based on these preliminary results, it is recommended that high-energy fl-scintillation detection and in-situ y-ray spectrometry be considered for further study as potential alternatives to extensive soil sampling with
154
A. J. Schilk, K. H. Abel. R. W. Perkins
Fig. 5. Uranium-238 surface-activity contour map for the FEMP DBA based on the highenergy fi-scintillation sensor. The contour interval is 10 000 Bq/kg.
subsequent laboratory analyses. Although these techniques may never fully supplant traditional sampling methods, their strengths (ease of use, field manoxverability, rapid data output, etc.) should be seriously considered and carefully weighed against the overall cost of current methodologies for the characterization of radionuclide concentrations at large contaminated sites. Recent improvements to the PNL in-situ y-ray spectrometry package include the development of a user-friendly graphical interface and data acquisition/reduction software package and the addition of a lightweight, battery powered, miniature multichannel analyzer unit. Such modifications facilitate rapid data output and eliminate cumbersome lengths of power/ signal cable and the need for bulky Geld electronics, computers, etc., thereby enhancing system safety and field manceuverability. The second generation p sensor incorporates an overlying anti-coincidence shield and thin (O-5 mm) bottom layer that enhance the signal-to-noise level by decreasing the detector sensitivity to cosmic-induced events and y radiation originating from the underlying soil, respectiveIy. This sensor is also being
Characterization
of uranium contamination in surface soils
155
11 8
Y
amv
r = 0.85 10
:
4
5
6
I
I
1
7
6
9
Natural Log of Gamma Results @q/kg) Fig. 6. Correlation
diagram comparing the p and y results from the FEMP DBA. These results are well correlated, suggesting that the data integrity is high over a significant range of surface activities.
utilized currently for the rapid and precise characterization of 90Sr in soils via the 2.28 MeV p emitted from its equilibrium daughter, 9oY.
ACKNOWLEDGMENTS The authors would like to express their appreciation to the USDOE, Office of Technology Development, for its support of this project. In addition, the authors are grateful to Mr R. T. Reiman (EG&G Rocky Flats, Golden, Colorado) for the use of his NuMAl (Nuclear Materials Analysis) data reduction program, and to Dr D. C. Stromswold (PNL) and two anonymous reviewers for their editorial comments. Pacific Northwest Laboratory is operated for the US Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76RL0 1830. REFERENCES Daling, L., Chunxiang, Z., Zujie, G., Xian, L. & Guorong, H. (1990). Gammaspectrometric measurements of natural radionuclide contents in soil and gamma dose rates in Yangjiang, PR China. Nucl. Znstrum. Methods (A), 299, 687-9.
156
A. J. Schilk, K. H. Abel, R. W. Perkins
Fritzsche, A. E. (1983). Surface and Subsurface Gamma Survey of the Kellex Site, Jersey City, New Jersey. EGG-I 183-1795, EG&G, Las Vegas, Nevada. Helfer, I. K. & Miller, K. M. (1988). Calibration factors for Ge detectors used for field spectrometry. Health Phys., 55, 15-29. Lovborg, L., Better-Jensen, L., Kirkegaard, P. & Christiansen, E. M. (1979). Monitoring of natural soil radioactivity with portable gamma-ray spectrometers. Nucl. Instrum. Methods, 167, 341-8. Reiman, R. T. (1983). In-situ Gamma Analysis Support for Phase I, Middlesex Cleanup Project, Middlesex, New Jersey. EGG-10282-1003, EG&G, Las Vegas, Nevada. Schilk, A. J., Perkins, R. W., Abel, K. H. & Brodzinski, R. L. (1993). Surface and Subsurface Characterization of Uranium Contamination at the Fernald Environmental Management Site. PNL-86 17 Pacific Northwest Laboratory, Richland, Washington.
All rights reserved 0265-931X/95/$9.50
0265-931X(94)00007-7
Characterization of Uranium Contamination in Surface Soils
A. J. Schilk, K. H. Abel & R. W. Perkins Pacific Northwest (Received
Laboratory,
5 August
Richland,
Washington
99352, USA
1993; revised version received 5 April 1994; accepted 15 April 1994)
ABSTRACT Traditional means of obtaining radionuclide concentrations in soils over large areas are often time-consuming, cumbersome, expensive, and potentially non-representative. In an attempt to develop improved systems and new methodologies for the rapid and economical characterization of largescale uranium contamination, two disparate monitoring technologies were compared at a contaminated site within the Fernald facility near Cincinnati, Ohio, USA. The results of this preliminary study suggest that uncollimated in-situ y spectrometry and high-energy /3-scintillation sensing may represent viable alternatives to, and in many cases could mitigate the need for, the collection of myriad soil samples and subsequent laboratory analyses when characterizing large contaminated sites.
INTRODUCTION The past operations of uranium production and support facilities at several United States Department of Energy (USDOE) sites have occasionally resulted in the local contamination of some surface soils. Before any effective remedial protocols can be established, the spatial distribution of this contaminant must be adequately characterized. Unfortunately, traditional means of obtaining soil activities (e.g. grab sampling followed by laboratory analyses) are cumbersome, expensive, time-consuming, and often non-representative when very large areas are under consideration. 147
148
A. J. Schilk, K. H. Abel, R. W. Perkins
Hence, new technologies must be developed, or existing ones improved, to allow for the cheaper, safer, and faster characterization of uranium concentrations at these critical sites. The USDOE’s Fernald Environmental Management Project (FEMP) near Cincinnati, Ohio, is one specific site of concern. This facility, once the primary production site for uranium materials for defense-related projects, began operation in the early 1950s and was mainly responsible for the processing of uranium metal from concentrates. Production ceased in the late 1980s. Currently, approximately 50% of the surface soils in and around the general production areas are known to be contaminated by various uranium-bearing compounds in excess of 1.3 x lo3 Bq/kg (35 pCi/ g) of soil as a result of normal operations and unintentional spills or emissions. Pacific Northwest Laboratory (PNL) has been tasked with adapting and/or developing technologies to measure uranium in surface soils to assist in the establishment of remedial protocols; in partial completion of this effort, two separate methodologies were demonstrated at FEMP. First, uncollimated in-situ y-ray spectrometry was used for the measurement of 234mPa, which is in secular equilibrium with its parent, 238U, and gives rise to a prominent 1.001 MeV y-ray. The ‘wide-view’ capability associated with this methodology, allowing many hundreds of square meters of soil to be observed concurrently, made it particularly useful in the preliminary screening of the site (i.e. for the rapid identification of areas of elevated concentrations and the daily generation of surfaceactivity contour maps). A second surface-monitoring system, designed and built by PNL, was used to measure the 2.29 MeV /?-flux-also from the from shallower depths and smaller surface decay of 234mPa-emanating areas (approximately 0.15 m2). This sensor served to establish ‘local’ surface uranium contamination levels for comparison with the in-situ y results. The purpose of this paper is to summarize the overall results of the characterization effort at the FEMP site and to outline the features of the aforementioned technologies with the hope of facilitating future critical evaluations.
TECHNOLOGICAL
THEORY AND METHODOLOGY
Intrinsic germanium (IG) detector y-ray spectroscopy has many advantages as an analytical tool for surface environmental monitoring, including its selective high resolution (enabling precise, quantitative, multiradioisotopic characterization), its relatively easy transportability, and
Characterization of uranium contamination in surface soils
149
the capability of rapid data reduction to provide essentially real-time results in the field. In-situ y-ray spectrometry has enjoyed widespread use around the world for the surface characterization of a multitude of photon-emitting radionuclides. Its use with regard to the quantification of uranium content in soils has, on the other hand, been somewhat limited (see, e.g. Daling et al., 1990; Reiman, 1983; Fritzsche, 1983; Lovborg et al., 1979). The extended-surface-area y monitor demonstrated at FEMP employed an uncollimated, down-looking IG detector/cryostat assembly that was suspended 1 m above the soil. This detector is sensitive to surface (and shallow subsurface) activity over many hundreds of square meters, and effectively averages any horizontal heterogeneities that may exist within its field of view. Although in theory this system would monitor complete 2-71 space, it has been established empirically (Helfer & Miller, 1988) that approximately 80-90% of the total observed fluence originates within a 10-m radius from the detector due to geometry factors and attenuation in the soil and air (for most y energies and soil distributions). Following calibration, a non-trivial process that leads to an energyspecific list of conversion factors for each radionuclide of interest, the concentration (activity/g) or surface inventory (activity/m*) of the radionuclide(s) of concern may be determined for uniform or non-uniform (e.g. planar or exponential) sources, respectively. A uniformly distributed source was assumed at each sample location despite empirical evidence to the contrary (i.e. much of the local contamination has been shown to exist within the upper few centimeters of soil). Such an assumption leads to the condition in which the activity levels within the upper 15-20 cm (the ‘effective’ depth of view for the 1.001 MeV 234mPa y) are essentially averaged, thereby slightly overestimating the uranium content at depth while often underestimating the surface levels directly below the detector. Nevertheless, such averaged results are still viable with regard to largescale site cleanup efforts. Based on the sensor’s effective field of view as discussed above, a staggered map grid was established with measurement points on 20-m centers (see below). Individual count times were restricted to 10 minutes at each location. Calibration checks of the system’s efficiency and stability were performed by counting a standard source (241Am, 13’Cs, and 6oCo) at periodic intervals during each working day, and the data were reduced in the field utilizing an in-house, application-specific software package. The high-energy /Y?-scintillationsensor consists of three stacked ‘ribbons’ of l-mm* plastic scintillating fibers and is optimized for the detection of /3particles that are emitted from the decay of near-surface uranium. The 2.29 MeV (maximum energy) p-particle from 234mPa, a daughter of 238U
A. J. Schilk. K. H. Abel, R. W. Perkins
150
Unattenuated Gamma Ray High-Ener BetiB (e.g., from Byor SIr)
I
I
I A
3rClm
I
R
I
TypMfkgd. Attenuated Gamma Ray
Fig. 1. Simplified diagram of the PNL prototype B sensor showing the discrimination capabilities for high vs low b energies and ionizing vs non-ionizing radiation. The double lines are intended to represent those regions where ionization and excitation occur in the dopant material, leading to subsequent scintillations (e- denotes a Compton electron, which is indistinguishable from an incident p-particle). Only normally incident radiation is shown for clarity; penetration depths for non-normal incidence will be proportionally smaller.
and in secular equilibrium therewith, is ‘selectively’ measured due to its relatively high energy. A unique feature of this sensor is the stacked configuration (Fig. 1) that allows the discrimination between (a) unattenuated 234mPa @s that, when traveling at their ‘most probable’ energies (roughly 0.8 MeV), will penetrate nearly 3 mm of plastic, and (b) lowerenergy /?-particles arising from natural sources, viz. 40K, 232Th (plus daughters), and 238U (plus daughters); the maximum /? energies from these radionuclides or their progeny rarely exceed 1 MeV (or their associated abundances are relatively insignificant compared with (a) above) and the most probable penetration depth of an unattenuated, normally incident ‘background’ /I?will therefore be less than 1 to 2 mm. Hence, by observing the total number of times that all three layers scintillate concurrently per unit time, one may effectively quantify the 238U activity at the soil surface following an appropriate calibration (see below). This sensor is also somewhat sensitive to y-rays, which may produce interfering signals (Fig. 1). The multi-layer design affords some discrimination against y emission by observing, and partially canceling the contribution from, this ubiquitous phenomenon. Such selective measurement is possible since a P-particle (or other charged particle) will cause continuous excitations and consequent scintillation events along its entire path through the detector stack, while a y-ray might produce a Compton
Characterization of uranium contamination in swface soils
151
electron in only one (indeed, if any) of the individual layers, thereby generating a unique signal. Hence, by establishing an electronic requisite for coincident signals from all layers, one can primarily detect high-energy p events and either observe or effectively ignore much of the y contribution to the overall detector response. The signal-to-noise ratio is further enhanced by requiring intralayer coincidences as well (i.e. photomultiplier tubes at opposite ends of each fiber ribbon must receive concurrent pulses in order to be acknowledged), thereby mitigating the effect of thermionic-induced signals from the indiAnother important consideration in the vidual photomultipliers. construction of such a sensor is the need for electronic components that are sensitive enough to handle the rapid (< lo-ns) scintillation pulses originating from the excited fibers. As a consequence, PNL developed its own high-speed amplifiers, signal discriminators, and logic circuitry suitable for the specific technical requirements (see Fig. 2). All grassy areas were cropped before surface characterization, the sensor was set directly on the ground during data acquisition, and five 100-s counts were performed at each sample location. Calibration of this system consisted of monitoring a series of standard soil samples (viz. medium-grained river sands) that were ‘spiked’ with 238U02(N03)2. In order to ensure system stability and optimal operation, background and calibration counts were performed periodically throughout each working day.
O---,
Storage
Fig. 2. Circuitry schematic for the /I sensor showing the requirements for both interlayer and intralayer coincidences to measure the total number of incident high-energy P-particles and to mitigate photomultiplier tube noise, respectively.
152
A. J. Schilk, K. H. Abel, R. W. Perkins
RESULTS AND DISCUSSION Surveys were conducted at FEMP’s former Drum Baling Area (DBA), a 9100-m2 site within the original production area that served as a staging area for clean and contaminated scrap materials. Soils within the DBA contain production-related uranium-bearing compounds to various depths as a result of airborne deposition and direct contact with radioactive fluids and/or contaminated scrap items. Figure 3 indicates the general boundaries for this site, as well as the, individual measurement locations utilized in this characterization study. Tabulated data results may be found in the associated PNL technical report by Schilk et al. (1993). Qualitatively, the p and y results correlate quite well in terms of ‘hotspot’ identification despite the extreme differences in the fields of view and observation depths associated with these inherently disparate methodologies (Figs 4 and 5). The quantitative results generated by these two techniques, however, are rarely found to be equivalent. Indeed, such agreement would occur only in the case of a vertically (and horizontally)
Fig. 3. Map view of the FEMP DBA showing sample locations (denoted by asterisks).
Individual sample sites are separated by 20 m in North-South
and East-West directions.
Characterization of uranium contamination in surface soils
153
Fig. 4. Uranium-238
surface-activity contour map for the FEMP DBA based on the insitu y-ray spectrometry. The contour interval is 2 000 Bq/kg.
uniform uranium distribution. Conversely, any contaminant distribution that decreases rapidly with depth (as at FEMP) would be manifested by a P-response which exceeds that of the in-situ y-ray spectrometer ‘expecting’ a uniformly distributed source, since the /I detection unit interrogates surface soils exclusively while the IG crystal obtains information down to depths exceeding 15 cm and essentially averages the overall y activity throughout this depth. Furthermore, unless the vertical distributions are particularly variable throughout the surveyed area, a significant correlation between the /I and y sensor responses would be expected (despite the inherent disparity in terms of sensor methodologies and depths of view) if these technologies represent mutually supportive and viable alternatives to traditional soil sampling. Such behavior is evident in Fig. 6, in which the fi and y results are seen to be well correlated, thereby lending support to the above premise. Based on these preliminary results, it is recommended that high-energy fl-scintillation detection and in-situ y-ray spectrometry be considered for further study as potential alternatives to extensive soil sampling with
154
A. J. Schilk, K. H. Abel. R. W. Perkins
Fig. 5. Uranium-238 surface-activity contour map for the FEMP DBA based on the highenergy fi-scintillation sensor. The contour interval is 10 000 Bq/kg.
subsequent laboratory analyses. Although these techniques may never fully supplant traditional sampling methods, their strengths (ease of use, field manoxverability, rapid data output, etc.) should be seriously considered and carefully weighed against the overall cost of current methodologies for the characterization of radionuclide concentrations at large contaminated sites. Recent improvements to the PNL in-situ y-ray spectrometry package include the development of a user-friendly graphical interface and data acquisition/reduction software package and the addition of a lightweight, battery powered, miniature multichannel analyzer unit. Such modifications facilitate rapid data output and eliminate cumbersome lengths of power/ signal cable and the need for bulky Geld electronics, computers, etc., thereby enhancing system safety and field manceuverability. The second generation p sensor incorporates an overlying anti-coincidence shield and thin (O-5 mm) bottom layer that enhance the signal-to-noise level by decreasing the detector sensitivity to cosmic-induced events and y radiation originating from the underlying soil, respectiveIy. This sensor is also being
Characterization
of uranium contamination in surface soils
155
11 8
Y
amv
r = 0.85 10
:
4
5
6
I
I
1
7
6
9
Natural Log of Gamma Results @q/kg) Fig. 6. Correlation
diagram comparing the p and y results from the FEMP DBA. These results are well correlated, suggesting that the data integrity is high over a significant range of surface activities.
utilized currently for the rapid and precise characterization of 90Sr in soils via the 2.28 MeV p emitted from its equilibrium daughter, 9oY.
ACKNOWLEDGMENTS The authors would like to express their appreciation to the USDOE, Office of Technology Development, for its support of this project. In addition, the authors are grateful to Mr R. T. Reiman (EG&G Rocky Flats, Golden, Colorado) for the use of his NuMAl (Nuclear Materials Analysis) data reduction program, and to Dr D. C. Stromswold (PNL) and two anonymous reviewers for their editorial comments. Pacific Northwest Laboratory is operated for the US Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76RL0 1830. REFERENCES Daling, L., Chunxiang, Z., Zujie, G., Xian, L. & Guorong, H. (1990). Gammaspectrometric measurements of natural radionuclide contents in soil and gamma dose rates in Yangjiang, PR China. Nucl. Znstrum. Methods (A), 299, 687-9.
156
A. J. Schilk, K. H. Abel, R. W. Perkins
Fritzsche, A. E. (1983). Surface and Subsurface Gamma Survey of the Kellex Site, Jersey City, New Jersey. EGG-I 183-1795, EG&G, Las Vegas, Nevada. Helfer, I. K. & Miller, K. M. (1988). Calibration factors for Ge detectors used for field spectrometry. Health Phys., 55, 15-29. Lovborg, L., Better-Jensen, L., Kirkegaard, P. & Christiansen, E. M. (1979). Monitoring of natural soil radioactivity with portable gamma-ray spectrometers. Nucl. Instrum. Methods, 167, 341-8. Reiman, R. T. (1983). In-situ Gamma Analysis Support for Phase I, Middlesex Cleanup Project, Middlesex, New Jersey. EGG-10282-1003, EG&G, Las Vegas, Nevada. Schilk, A. J., Perkins, R. W., Abel, K. H. & Brodzinski, R. L. (1993). Surface and Subsurface Characterization of Uranium Contamination at the Fernald Environmental Management Site. PNL-86 17 Pacific Northwest Laboratory, Richland, Washington.