Anal. Chem. 1980, 52, 1957-1958
LITERATURE
(8) Youden, W. J.; Steiner, E. H. “Statistical Manual of the Association of Official Analytical Chemists”; Association of Official Analytical Chemists: Washington, DC, 1975; pp 33-36.
CITED
(1) Fed. Reglst. 1977, 42, 62971. (2) Annu. Book ASTM Stand. 1975, Part 31, 362. (3) “Methods for Chemical Analysis of Water and Wastes”. ERA Report No. 600/4-79-020, March 1979, Method 353.3. (4) Connors, J. J.; Beland, J. J.-Am. Water Works Assoc. 1976, 68, 55-56. (5) Gales, Morris E., Jr., personal communication (Feb 7, 1980, ERA Methods Development and Quality Assurance Laboratory, Cincinnati, OH). (6) Wood, E. D.; Armstrong, F. A. J.; Richards, F. A. J. Mar. Biol. Assoc.
John H. Margeson* Jack C. Suggs M. Rodney Midgett Environmental Monitoring System Laboratory U.S. Environmental Protection Agency Research Triangle Park, North Carolina 27711
(7) Skoog, D. A.; West, D. M. "Fundamentals of Analtlcal Chemistry", 3rd ed.; Holt, Rinehart and Winston: New York, 1976; p 274.
Received for review April 28, 1980. Accepted July 15, 1980.
U.K. 1967, 47, 23-31.
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1957
Determination of Lead-210 in Environmental Samples by Gamma Spectrometry with High-Purity Germanium Detectors of larger effective sample size due to less self-absorption in the sample. In practice the above estimate is quite liberal and can only provide a bound on the sensitivity. Actual sensitivity will be much less due to the presence of radiation background, potentially poorer counting geometry, losses in the detector’s dead layer, and ineffectual collisions in the germanium. Nevertheless, the potential feasibility of direct assay of moderate-sized environmental samples of 210Pb without need of chemical processing or time delay is suggested. This technique has been experimentally tested with a surface soil sample and a high-purity germanium detector. The sample was surface soil from Boulder, CO, and the detector possessed a 65-cm2 active surface area. Optimum shielding for such a test was not available, so the detector was surrounded by an oversized soil sample approximately 10 cm in depth where the soil beyond the effective self-absorption In this conlayer acted as a shield from external sources. figuration, shielding from external radiations is adequate, but the higher energy photons in the outer layers of the soil penetrate the inner layer and add some low-energy background. Figure 1 shows the result. Figure la is from a sealed 226Ra source which contained daughter radiations (4, 5) including 210Pb. This source was not at equilibrium, and the ratio 214Pb/210Pb is enhanced over that for a source at equilibrium. Figure lb is the result of a 30-h count with the soil sample. The 210Pb photons are clearly present although the background is considerable. The isotope 214Pb is not visible since the sample is surface soil (top 1 cm) and the 210Pb comes predominantly from atmospheric 210Pb fallout (from 22ZRn decay) and not decay of 226Ra in the soil (7), and the emission intensity of the 214Pb 53-keV photon is less than that of the 210Pb photon (5). Figure lc was taken with the soil sample removed, and the absence of the 210Pb peak indicates that the 210Pb photon was not due to sources external to the soil. On the basis of calibration with 226Ra and 241Am sources, analysis of the soil spectrum indicated an approximate concentration of 210Pb of 0.2 dps/g. Counting error (predominantly due to the background) was ±14%. Although the sensitivity is significantly worse than the bound estimate, it is still quite usable. Soil and sediment concentrations of 210Pb typically range from 0.01 to 1.0 dps/g (3, 7) and counting times of a
Sir: Measurement of low-level environmental 210Pb (of the order of 0.1 dps/g) is important in many fields including health physics, geochronology, and environmental science. Presently, the standard procedures for environmental assay involve chemical separation and electrodeposition of 210Pb followed by a or ß spectrometry (1-3). Although these procedures are capable of high sensitivity with gram-size samples, many processing steps are involved, as well as time delays of days and longer for ingrowth of daughter activity. Four percent of the decays of 210Pb are accompanied by emission of a 47-keV photon (4, 5), but self-absorption in the sample and low sensitivity of standard Ge(Li) detectors to this energy photon have previously prohibited practical assay by direct photon measurement. Recently, a new generation of high-purity N-type germanium detectors has become available (6). These detectors are characterized by improved sensitivity to low-energy photons (due to thin diode blocking contacts) and increased active surface area. They offer the possibility of direct assay of 210Pb in moderate-sized environmental samples (of the order of 100 g) by measurement of the 47-keV photon. Good parameters for a high-purity germanium detector might be 70 cm2 active area and energy resolution of 1 keV for 47-keV photons. Due to the short range of 47-keV photons in germanium, essentially all photons striking the germanium crystal will be stopped. Consider a representative environmental surface soil sample of 0.1 dps/g and density 1.5 g/cm3. An effective thickness for absorption of 47-keV photons might typically be 2 cm. On the assumption sufficient sample is available, 210 g can be effectively utilized adjacent to the 70-cm2 surface. Assuming 25% detection efficiency (x geometry) and the 4% photon emission, one obtains
detector count rate
=
~
0.04 photon/decay X
dps/g
x
^ count/photon 0.21
If
X 210 g
=
count/s
(1)
background present, this result would imply only 7.9 min would be required to obtain 100 counts and 10% counting precision (Poisson counting statistics). The sensitivity for water samples would be improved further because no
were
0003-2700/80/0352-1957501.00/0
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1980 American Chemical Society
1958
Anal. Chem. 1980, 52, 1958-1959 R a
The background is largely due to the Compton edge of higher energy photopeaks. These higher energy photons come from both external sources and the sample itself. Hence the sample size should be no larger than that which gives a thickness where self-absorption of the 47-keV photons becomes dominant. Larger samples add background from more penetrating high-energy photons without adding 210Pb counts. Environmental samples with large amounts of higher energy emitters will be more difficult to count than those with small amounts. Shielding from external sources of 210Pb should be relatively easy due to the short range of 47-keV photons. The greater challenge is to reduce the higher energy photons. Compton suppression systems (9) would be one effective, but expensive, way to reduce background from both sample and external
226
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_L
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3000
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200
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200 CHANNEL
300
Standard methods must still remain the choice when maximum sensitivity is required. Emission of a and ß radiation follows all the decays of 210Pb and its daughters (not just 4% as is the case for the 47-keV photon) and there is no fundamental limit to the size sample that can be processed. However, many environmental samples involve 210Pb concentrations and sample sizes suitable for photon measurement, and in these cases high-purity germanium detectors offer a technique that is significantly more time saving and convenient.
400
c )
400
ACKNOWLEDGMENT
NUMBER
The assistance of the staff of the Nuclear Physics Laboratory of the University of Colorado in providing facilities and carrying out these experimental measurements is gratefully acknowledged. LITERATURE CITED
Figure 1. Spectra with a high-purity germanium detector: (a) sealed 22®Ra source; (b) soil sample (30 h count); (c) soil sample removed (46 h count). Low-energy electronic cutoff in all spectra corresponds to approximately 20 keV. or longer are practical with germanium detector systems. The above results demonstrate the feasibility of assaying concentrations of 210Pb of the order of 0.1 dps/g in moderate-sized environmental samples with high-purity germanium detectors. They were substantiated by additional measurements. For environmental samples, identification of the 47keV photon from 210Pb in spectra should be relatively unambiguous. The nearest commonly observed 7 emissions due to natural radioactivity (53 keV, 214Pb; 42 keV, 231Th; 51 keV, ^U) are easily resolvable with a germanium detector (8). Due to the few steps involved, the procedure should be convenient and reliable. Significant improvements in sensitivity should be possible with shielding and detectors optimized for such
day
(1) Rama; Koide, M.; Goldberg, E. D. Science (Washington, D.C.) 1961, 134 98—99 (2) Flynn, W. W. Anal. Chim. Acta 1968, 43, 221-227. (3) Krlshnaswamy, S.; Lai, D.; Martin, J. M.; Meybeck, M. Earth Planet. Sci. Lett. 1971, 11, 407-414. (4) Lederer, C. M.; Shirley, V. S. "Table of Isotopes", 7th ed.; Wiley: New
York, 1978.
(5) "Radiological Health Handbook"; U.S. Department of Health, Education, and Welfare: Rockville, MD, 1970. (6) Pehl, R. H. Phys. Today 1977, 30, 11, 50-61. (7) Moore, . E.; Poet, S, E. J. Geophys. Res. 1976, 81, 1056-1058. (8) Perkins, R. W.; Thomas, C. W. “Workshop on Methods for Measuring
Radiation In and around Mills"; Atomic Industrial Forum: Washington, D.C., 1977; pp 255-274. (9) Camp, D. C.; Gatrousls, C.; Maynard, L. A. Nucí. Instrum. Methods 1973, 117, 189-211,
Stephen D. Schery
measurement.
Geophysical Research Center New Mexico Institute of Mining and Technology Socorro, New Mexico 87801
A major focus of design for future systems optimized for 210Pb measurement should be reduction in the low-energy background. For example, for the experimental measurements described above the sensitivity for the same counting time and error would be increased by over a factor of 10 if the background were not present. Several points are important about reduction of low-energy background in 210Pb measurements.
Received for review March 25, 1980. Accepted July 25, 1980. This research was supported in part by U.S. Department of Energy Contract No. DE-AS04-78ETO5364.
Comments on Experimental Studies on Spatial Distributions of Atoms Surrounding an Individual Solute Particle Vaporizing in an Analytical Flame ñames from solutions containing phosphate migrate toward the edges more slowly than do those from phosphate-free solutions”. We can find no statements in the two papers (2,
Sir: On page 1903 of the paper by Boss and Hieftje (1), the authors make the following statement: “West, Fassel, and Kniseley (16) determined that Ca atoms released in N20/C2H2 0003-2700/80/0352-1958S01.00/0
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1980 American Chemical Society
