Radioactivity standards for environmental monitoring. II

Environment International, Vol. 10, pp. 91-97, 1984

0160-4120/84 $3.00 + .00 Copyright©1984Pergamon Press Ltd.

Printed in the USA. All rights reserved.

RADIOACTIVITY STANDARDS FOR ENVIRONMENTAL MONITORING. II K. G. W. Inn, P. A. Mullen and J. M. R. Hutchinson National Bureau of Standards, Washington D.C., 20234, USA (Received 9 December 1983; Accepted 10 June 1984)

The current environmental-monitoring-standardsefforts of the tow-level-radioactivitylaboratory in the Radioactivity Group of the National Bureau of Standards are described. The calibration efforts include traceability programs, natural-matrix Standard Reference Materials, alpha-particle-emittingstandards, and international radioactivityintercomparisons.New radiometrologyefforts under developmentinclude a prototype radon-222 in water standard, 3,-ray emission-rate measurement techniques in the 60-keV region, and atom-counting techniques for radioactivity measurements.

INTRODUCTION

NBS has supported the evaluation of a proposed radio-bioassay accreditation program, initiated by the Nuclear Regulatory Commission, by providing calibrated radioactivity solutions and quality control audits. The accreditation program could eventually be a mechanism that will link measurements by radio-bioassay laboratories to the national radioactivity standards. Some preliminary but encouraging work with resonance-ionization mass spectrometry (RIMS) has also been initiated in collaboration with a number o f other groups inside and outside NBS.

The efforts by the National Bureau o f Standards (NBS) in the field of low-level radioactivity measurements can be divided into three areas: (1) standards development, including certified concentrations o f radiochemical tracers, (2) the establishment of traceability of measurements to NBS, and (3) the dissemination o f information on techniques through committees and organized meetings. During the early 1970's, the group was able to serve a large spectrum o f environmentally related communities by furnishing data and research, as well as standards tailored to particular user needs. However, since the late 1970's, support for environmental radioactivity work has diminished significantly. Nevertheless, since the last such report (Coursey e t al., 1978), there have been a number of developments which can be reported. The most significant o f these were achieved in collaboration with outside agencies and groups that donated their time and resources in order to maintain a repository of standards of good, publicly accepted, measurements at NBS. Of particular interest is the production o f seven natural-matrix standards (NMSs): Rocky Flats Soil, River Sediment, Lake Sediment, H u m a n Lung, H u m a n Liver, Irish Sea Sediment, and a low activity soil from Peru (Inn et al., 1984). During the development of the NMSs, it was noted that there were disturbing initial disagreements between laboratories in what some consider routine assays. A number o f these disagreements have been resolved and the remainder are under investigation.

CURRENT STANDARDS EFFORT

The NBS Low Level Laboratory has already been described (Coursey et al., 1978); it includes a wide variety of detection systems and a number of radiochemistry laboratories, including a "white lab" used for the radiochemical work at extremely low radioactivity levels. Nearly all of the projects of the low-level group involve an outside agency in a participative way, e.g., the natural-matrix standards are being produced in collaboration with seven public and private laboratories; the traceability programs are with the Environmental Monitoring Systems Laboratory in Las Vegas (EMSLLV) of the U.S. Environmental Protection Agency (U.S. EPA); and the Department o f Energy's Idaho Falls Radiological and Environmental Sciences Laboratory, under contract to the Nuclear Regulatory Commission, and the bioassay program are with Battelle Northwest Richland (BNW). 91

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K.G.W. Inn, P. A. Mullen, and J. M. R. Hutchinson

CALIBRATIONS Traceability Measurements (with EPA and RESL). The NBS environmental-level-radioactivity traceability program has been discussed in several previous publications (Cavallo et al. 1973, Mann, 1973; Mann, 1976; Mann et al., 1981; Inn and Noyce, 1982). Traceability in radioactivity must be based in part on periodic demonstrations of the reliability of radiochemical procedures, correct calibration of measuring instruments and the performance of the laboratory with test samples. To this end, NBS prepares a number of classes of test samples to check the calibrations of instruments, test sources with chemical and radioactivity interferences to check the adequacy of radiochemical procedures and the competence of the technicians, and natural-matrix materials to test sample-handling and dissolution techniques. The traceability programs with the EPA and the Radiological and Environment Sciences Laboratory, Idaho Falls, ID (RESL) are continuing to operate. Figures 1 and 2 display the results from the traceability exercise through mid-1983. Since March 1980 (Inn and

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Noyce, 1982) new radionuclides added to the histograms 1 and 2 include 46Sc, s7Co, 12sSb, ~27Xe, laaXe, la4Cs, ~SSEu, 2°7Bi, 23°Th, and 232Th. Additional solution test sources provided to RESL include chemical, radiochemical, and radiospectrometric interferences of 89Sr, '°Sr, "3Ba, and stable calcium. Both laboratories have demonstrated their continuing capabilities for analysis of a large number of radionuclides to within 5°70 of the NBS determinations. These results indicate that the traceability links between NBS and two quality-control laboratories in this country are, for the most part, under control. Radiobioassay Program (with Battelle Northwest Laboratories for the Nuclear Regulatory Commission). An American National Standards Institute (ANSI) committee was recently formed to write a standard that will set the minimum acceptable performance criteria to accredit a laboratory involved in radiobioassay measurements, such as uranium in excreta, in occupationally exposed individuals. It was a natural process to include a traceability mechanism in the draft of the ANSI stan-

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Fig. 1. Results of the Environmental Protection Agency-National Bureau of Standards (EPA-NBS) Radioactivity-Measurements Traceability Program from its inception through March 1982.

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dard, to link the radiobioassay laboratory to NBS through a testing laboratory. It would be necessary to complete the link to NBS by having the testing laboratory involved in a traceability program either with NBS or a quality control (QC) laboratory which is traceable to NBS. Battelle Northwest Laboratories, Richland, WA (BNW) has contracted to evaluate the pass-fail bias and precision criteria of the proposed ANSI standard by conducting two round-robin tests with bioassay laboratories to establish (1) the current state of radiobioassay competence, and (2) the usefulness of the proposed evaluation criteria for accreditation. During 1982, the first in vitro round-robin tests were performed with an simulated urine composed of its chemically stable constituents which had been spiked with known amounts of certified radionuclide solution sources (3H,

9°Sr, 137Cs, ~3Spu, 241Am, and natural uranium). The scheme used for establishing traceability of the urinetest samples to NBS is depicted in Fig. 3. The test solutions were assayed by Environmental Analysis Laboratory Corporation (EAL) which establishes its traceability link by participating in the U.S. EPA Environmental Radioactivity Laboratory Intercomparison Studies Program (Crosscheck Program), and, as mentioned above, U.S. EPA has established its link to NBS through its traceability program. BNW also had phantoms constructed for in vivo tests. NBS provided certified radionuclide solution sources of 6°C0, 9°Sr, laTCs, 144Ce, 23sU, and 24tAm for spiking the BNW phantom components. In addition to providing BNW with certified solution sources, NBS has also participated in development of traceability pathways for the radiobioassay laboratories

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K.G.W. Inn, P. A. Mullen,and J. M. R. Hutchinson

-CHECK

Fig. 3. Traceability of the Battelle Northwest Richland (BNW) radiobioassayin vitro test samplesto NBS.

by making recommendations on the testing procedures and the proposed ANSI standard. We will also review the results from the first round-robin test to evaluate the appropriateness of the proposed ANSI standard and the testing procedure. Natural-Matrix S R M s ( N M S )

Since 1978, NBS has produced a number of Standard Reference Environmentally Quasi-equilibrated Natural Matrix Materials (Bowen, 1978; Hutchinson and Mann, 1978; Noyce, 1980; Inn and Noyce, 1982) to test sample preparation and analytical techniques for radionuclides in soil, sediments, and human organs. The NMSs will be used to a large extent for tests of traceability of monitoring laboratories for which certification to ± 5070-15070 is adequate. To date, five matrices have been certified as SRMs: Rocky Flats Soil Number 1, River Sediment, Human Lung, Human Liver, and Peruvian Soil. Two other matrices are in the process of certification: Freshwater Lake Sediment and Ocean Sediment. Each of the certified NMSs was assayed by at least two, and as many as five, collaborating laboratories for radionuclides including 4°K, SSFe, ~°Co, 9°Sr, ~25Sb, 137Cs, ,S2Eu, lS,Eu, 226Ra' ~28Ac, 22STh, 230Th, Z32Th, 2a4U, 2asU, 23sU, 23sPu, 2a9pu+24°pu, and 24~Am. Laboratories participating in the certifying assays were the Environmental Measurements Laboratory, Los Alamos National Laboratory (University of California); NBS; Oregon State University School of Oceanography, Radiological and Environmental Sciences Laboratory; and Woods Hole Oceanographic Institution. The data provided by the collaborating laboratories have demon-

strated that their radioanalytical capabilities are traceable to NBS to within 5°7o for the certified radionuclide assays in which they participated. Additional information provided with the NMSs includes: particle-size distributions, semiquantitative emission-spectrometric elemental analysis, semiquantitative mineralogical analysis, mass-spectrometric plutonium isotopic analysis, analytical methodology, gamma-ray spectrum of the material taken with a 60-cm 3 Ge(Li) detector, and homogeneity data. River Sediment (SRM 4350B) is a freshwater, lowcalcium, and low-organic-content, river sediment matrix obtained from the Columbia River, downriver from the Hanford Works Reservation in the state of Washington. The radionuclides introduced into the matrix from atmospheric fallout and historical releases from the Hanford facilities are acid leachable and are distributed homogeneously throughout the matrix. Human Lung (SRM 4351) and Human Liver (SRM 4352) are intended for use in the measurement of radionuclides under study by members of the Uranium and Transuranium Registries and other laboratories studying the movement of heavy alpha-particle-emitting radionuclides within the human body and the effects of such contaminants. Plutonium-contaminated organs from two chronically exposed individuals were obtained from the Transuranium Registry and blended with diluent organs to produce final products with plutonium activity of approximately 0.03 Bq g-1 (Volchok et al., 1980). (The Transuranium Registry is an organization composed of a number of major laboratories that keep records of radioactivity contamination in occupationally exposed individuals, and perform analytical work and draw scientific conclusions concerning, for example, the movement of radioactivity within the human body.) Although the distribution of transuranic radionuclides is homogeneous throughout the Human Liver material, there is significant inhomogeneity of the 2a9pu+24°pu, 2aspu, and 24'Am in the Human Lung material due to the micronsize hot particulates containing plutonium. The Human Lung and Human Liver materials can be used by analysts to check radioanalytical methods for 23'pu + 2'°Pu to an accuracy better than a factor of 2 and better than 40070, respectively, for a single measurement (Inn and McInroy, 1982). Rocky Flats Soil Number 1 (SRM 4353) was obtained from Rocky Flats, CO, and represents a low-calcium, low-organic-content soil (Volchok et al., 1980; Inn et al., 1984). The origin of the transuranic radionuclides are from atmospheric fallout and dated releases from the Rocky Flats Plant. A significant fraction of the transuranic radionuclides is resistant to leaching with strong acids. Furthermore, the transuranic radionuclide content is affected by an occasional hot particle. A statistical model has been developed to identify data affected by hot particles for rejection from the data set.

Radioactivity standards

Peruvian Soil (SRM 4355) was obtained by the International Atomic Energy Agency (IAEA) from La Molina, Lima, Peru. There are nonmeasurable radioactivity concentrations for many fallout radionuclides, and the material is intended as a blank or a low-level standard in tests of measurements of environmental radioactivity. This material is at such a low level of activity in the artificially produced radionuclides that some unusual interferences from natural radionuclides appear. For example, the concentration of '37Cs is so low that 4°K and STRb interferes in the beta-particle counting even after radiochemical purification. Included within the SRM 4355 certificate are results from an elemental analysis intercomparison study conducted on another batch of the same material by the IAEA (Dybczynski et al., 1978). Freshwater Lake Sediment (Gyttja) is a high-organic, low-calcium content (approximately 50 wt 070), freshwater lake sediment (Merritt, 1980). The sediment was obtained from Perch Lake on the Chalk River facilities, Canada. The radionuclides originate from leachates and are transported to the lake by streams and shallow aquifers. Ocean Sediment (Irish Sea Sediment) represents a low-calcium content, oceanic continental-shelf matrix. The origins of the radionuclides in the sediment are from atmospheric fallout and the Windscale reprocessing plant. The radionuclides in the Irish Sea Sediment were diluted with Chesapeake Bay Sediment at a ratio of 1:200 to reduce the plutonium concentration in the final material to approximately 0.01 Bq g-1. We anticipate completion of certification of the Gyttja by late 1984 and the Irish Sea Sediment by the following year. Furthermore, we anticipate certifying 21°Pb and transamericium radionuclides for the above mentioned NMSs in the future.

Mixed-Radionuclide Standards Coursey et ai. (1982) have developed a long-lived mixed-radionuclide standard containing '2sSb, lS4Eu, and 'SSEu which has a functional half-life of over 10 yr, with useful photon emissions at over 18 energies between 25 and 1600 keV. This standard will replace the original NBS mixed radionuclide standard containing sTCo, ~oCo' SSSr' say, 1o9Cd ' 113Sn ' laTCs ' 139Ce ' and 2°aI-Ig. The mixture was chosen to minimize correlated summing corrections and to maximize the half-life, while providing many usable lines. Alpha-Particle-Emitting Standards Radium-228 solution standards, calibrated by liquidscintillation counting, were produced in 1978 (Noyce et al., 1982), and another set is now being reissued. The 22SRa was purified from aged 232Th and counting was begun immediately. The ingrowth of the progeny was followed as a check for impurities and malfunctioning of the instrumentation. An extrapolation back to the

95

time of purification was made, and the solution was certified for 228Ra at the time. Recently, a second batch of 22SRa sources was calibrated and issued. As a result of an improved calibration, the certified value on the first batch was changed by approximately 8070. Thorium-230 and plutonium-239 solution standards were prepared and calibrated by alpha-particle counting in 0.1~r and 0.8~r defined-solid-angle scintillation detectors. Americium-243 solution standards at low radioactivity concentration, to be used as a tracer, were prepared and calibrated using the 0.8~" alpha-particle counter with confirmation by 41r alpha-particle liquid-scintillation counting. Beta particles from the decay of the 239Np daughter interfered in the 0.17r counter but not in the 0.8r counter, which has a thin CsI(T1) detector that is basically insensitive to all radiations other than alpha particles. (The 0.17r detector has a relatively thick CaF scintillator that is sensitive to beta particles.) The 23°Th and the 243Am point sources used in the calibrations were made with both Ludox, a seeding agent, and Catanac, a wetting agent. No measurable difference in the efficiency was observed in the case of the ~3°Th sources, but the Ludox appeared somewhat superior in the case of 243Am. Uranium-232 solution standards to be used as tracers are planned and will be issued through U.S. EPA in the coming year.

International Radioactivity Intercomparisons The development of new techniques in the Radioactivity Group has led to a significant improvement in the measurement of the photo-emission or disintegration rates of three important radionuclide standards: SSFe, 12sI (measurements performed by D. Golas at NBS), and 222Rn. Typically, measurement uncertainties in radioactivity measurements by national metrology laboratories are in the range of 0.3%-1.0070. For two of these three nuclides, the agreement of NBS with the mean values of 22 other laboratories in the United States, Canada, and Europe is 0.1070. With the third radionuclide, 222Rn, the agreement is 2.4070, with a relatively wide spread in the measurements from other laboratories. The calibration of these materials required a wide variety of measurement techniques, including defined-solid-angle counting (55Fe), sum-peak counting ('2sI), and pulse-ionizationchamber counting (222Rn).

NEW TECHNIQUES This section describes new methods of radioactivity calibration that have been developed for two radionuclides of environmental interest: the activity of 222Rn dissolved in water, and the 60-keV -r-ray-emission rate in the decay of 241Am. The development of the 49V standard (with calibrated emission rate of 4.9-keV K-x rays)

96

(Hutchinson and Mullen, 1983a) is similar to a previously described procedure for the decay of SSFe, and is described above in the Standards section of this paper. Prototype Radon-222 in Water Standards NBS has developed a prototype standard that generates samples of radium-free ~ R n gas dissolved in distilled water (Hutchinson et al., 1984). This standard is intended to be used for quality control in the U.S. EPA's program to survey drinking water. It consists of a generator and delivery system that can dispense solutions of 222Rn with concentration known to ± 10070into, for example, a liquid-scintillation vial containing scintillation cocktail. The prototype consists of a source of 22~Ra which is deposited on an ion-exchange filter, sandwiched between two layers of thin polyethylene tape, and then immersed in water in a specially constructed accumulation chamber (Fig. 4). The chamber is then flushed and 222Rnis allowed to diffuse and accumulate for a measured time; then it is flushed again into a large syringe from which the standard solution is dispensed. From the measurements

K . G . W . Inn, P. A. Mullen, and J. M. R. Hutchinson

made at NBS over the past two years, the ~2Rn in the ion-exchange filter-polyethylene sandwich, and therefore the concentration of radon dissolved in the water, can be predicted accurately. 59.5-ke V 7-Ray Emission-Rate Measurement A specially constructed NaI(TI) thin-walled-well detector was used to measure the 59.5-keV v-ray-emission rate from a 2'1Am point source (Hutchinson and Mullen, 1983b). This "r-ray energy is in a region where Ge(Li)- and Si(Li)-detector efficiencies vary rapidly and where there is a paucity of standards. Previous measurements had depended upon interpolation of efficiency curves. The total counting rate in the 59.5-keV peak was corrected for: (1) wall absorption (+ 1.17°70), (2) escape of photons through the end of the well (+ 1.296070, (3) iodine K-x-ray escape (+ 3.52070), (4) photoelectron tail (+ 0.31070),

Fig. 4. Accumulation chamber of radon-in-water standard. The sandwiched 2Z6Rasource is located in the balljoint located to the right of the large syringe.

Radioactivity standards

(5) 43-keV counts included in the 59.5-keV peak (-0.31 07o), and (6) coincidence summing of 26-keV 3,-rays and L-xrays which resulted in pulses recorded in the 59.5keV peak (-0.28070). M A S S SPECTROMETRIC BASIS FOR RADIOA C T I V I T Y MEASUREMENTS In the past, mass spectrometry has not had the mass sensitivity to differentiate radioactive from stable species in the normal environmental-abundance ratios, which may be as low as one part in 1015. Two technological developments over the past five years have changed this situation so that there is the hope that, within the not-too-distant future, environmental-radioactivity measurements will be placed on a mass-spectrometric basis. Tandem-accelerator mass spectrometry (TAMS) has demonstrated the capability for significant increases in sensitivity over traditional mass-spectrometric techniques- making possible isotope-abundance ratio measurements in the 1 part in 1016 range. In addition, resonance-ionization spectrometry (RIMS) has demonstrated the remarkable potential for labeling a single atom in a background of 1019 atoms of different elements. TAMS has been used extensively in geophysical measurements, mainly on low-Z elements such as '°Be, 14C, and 36C1. Although RIMS is in its infancy, a great deal of work is being done to establish its position as a useful scientific and industrial tool. It is expected that it may be used to measure ppb concentrations of elements within approximately a year. Efforts are being made at NBS to make RIMS useful for isotopic analyses. Acknowledgements-The authors would like to acknowledge Dr. W.

B. Mann, Section Chief Emeritus, for his kind encouragement, and continued guidance of the Environmental Radioactivity Program at the National Bureau of Standards. The technical and financial support of the Department of Energy, Environmental Protection Agency, Nuclear Regulatory Commission, and the member laboratories of the ICRM are also gratefully acknowledged.

REFERENCES Bowen, V. T. (1978) Natural matrix standards, Environ. Int. 1, 35-39. Cavallo, L. M., Coursey, B. M., Garfinkel, S. B., Hutchinson, J. M. R., and Mann, W. B. (1973) Needs for radioactivity standards and measurements in different fields, Nucl. Instrum. Methods 112, 5-18. Coursey, B. M., Hutchinson, J. M. R., Lucas, L. L., Mann, W. B., Matsumura, T., and Noyce, J. R. (1978) Radioactivity standards for environmental monitoring, J. RadioanaL Chem. 43, 451-460.

97 Coursey, B. M., Hoppes, D. D., and Schima, F. J. (1982) Determination of the photon-emission rates of the NBS long-lived mixedradionuclide standard, Nucl. Instrum. Methods 193, 1-8. Dybczynski, R., Tugsavul, A., and Suschny, O. (1978) Report on the intercomparison run soil-5 for the determination of trace elements in soil. IAEA IRLI46, International Atomic Energy Agency Laboratory, Seibersdorf, Germany. Hutchinson, J. M. R. and Mann, W. B., eds. (1978) Metrology needs in the measurement of environmental radioactivity. Proceedings of a seminar sponsored by the International Committee for Radionuclide Metroiogy, Environ. Int. 1, special issue. Hutchinson, J. M. R. and Mullen, P. A. (1983a) Calibration of k-xray-emission rates in the decay of 4W, Int. J. Appl. Radiat. Isot. 34, 539-542. Hutchinson, J. M. R., and Mullen, P. A. (1983h) Pin-well-Nal(Tl) counting of 59.5-keV v-rays in the decay of 241Am, Int. J. Appl. Radiat. Isot. 34, 543-546. Hutchinson, J. M. R., Mullen, P. A., and Colle, R. (1984) Development of a regenerativeradon-in-water radioactivitystandard. Proceedings of the I C R M Alpha-Particle Spectrometry and LowLevel Measurements Seminar, Harwell, May 1983, to be published in Nucl. Instrum. Methods. Inn, K. G. W. and Noyce, J. R. (1982) The National Bureau of Standards low-levelradioactivity-measurementsprogram. Proceedings of the Symposium on Traceability for Ionizing Radiation Measurements, NBS Special Publication 609, p. I17-127. National Bureau of Standards, Washington, DC. Inn, K. G. W. and Mclnroy, J. F. (1982) Natural matrix materials for low-level radioactivitymeasurements: lung and liver.Third World Congress of the World Federation of Nuclear Medicine and Biology, August 29-September 2, Paris, France. Inn, K. G. W., Hutchinson, J. M. R., and Liggett,W. S. (1984) The National Bureau of Standards Rocky FlatssoilStandard Reference Material, alpha particlespectrometry and low-levelmeasurement. InternationalCommittee for Radionuclide Metrology, M a y I0-13, 1983, Harwell, England, to be published in Nucl. Instrum. Methods.

Mann, W. B. (1975) Radionuclide metrology and quality assurance. Environmental Monitoring Series, EPA-670/4-75-006, Proceedings of Conference: Activities and Needs Related to Radioactivity Standards for Environmental Measurements, August 21, 1973. U.S. Environmental Protection Agency, Washington, DC. Mann, W. B. (1976) Reliability and traceability in radioactivity measurements, Proceedings of Southeastern Workshop on the Utilization and Interpretation of Environmental Radiation Data, March 1-3, Orlando, FL. Mann, W. B., Hutchinson, J. M. R., and Edgerly, D. E. (1981) National and international traceability in radioactivity measurements. IAEA-SM-252/21, p. 173, Proceedings of the International Atomic Energy International Symposium of Methods of LowLevel Counting and Spectrometry, West Berlin, Federal Republic of Germany. Merritt, W. F. (1980) Gyttja, a low-level, high-organic reference material, Environ. Int. 3, 399-400. Noyce, J. R. (1983) Standards for the assay of radionuclides in solid environmental samples. ASTM Spec. Tech. Publ. 698, Proceedings of the ASTM Conference on Effluent and Environmental Radiation Surveillance, July 9-14, 1978, Johnson, VT. p. 309-326. American Society for Testing and Materials, Washington, DC. Noyce, J. R., Hutchinson, J. M. R., and Kolb, W. A. (1983) Radiochemical isolation of radium-228 and its calibration by liquidscintillation counting and gamma-ray spectrometry, J. Radioanal. Chem. 79, 5-13. Volchok, H. L., Feiner, M., Inn, K. G. W., and Mclnroy, J. F. (1980) Development of some natural matrix standards-progress report, Environ. Int. 3, 395-398.