I1 for which values of the standard deviations were available. The reduced x2 value for the modified Rasberry-Heinrich equation was 0.77, while the reduced x2 value for the Rasberry-Heinrich equation was 42.3, indicating considerable improvement when the terms accounting for the influence between the coexistent elements of the matrix are included. Analyses of the experimental data for the NBS samples obtained from these two empirical coefficients methods applied in an iterative scheme are given in Table IV. As expected, the overall results obtained from the modified RasberryHeinrich method are better, although some of the Fe values are closer when the modification is not used. However, these Fe values are artificially low because the Cr values are significantly underestimated. When better Cr values are obtained, the iterated Fe values without modification no longer provide the better analysis. The use of the modified Rasberry-Heinrich equation yields better results for samples with a wide range of ternary compositions and correctly reduces to the unmodified Rasberry-Heinrich equation when the sample of interest contains only two elements.
typical wide-angle, radioisotope-excited XRF systems. Thus, data obtained from these systems can be accurately corrected for interelement effects using properly weighted mean angles in the mathematical model developed for fixed-angle systems. This approach eliminates the need for a large number of reference standards for every type of sample being analyzed as required by the empirical coefficients method. Finally, it has been demonstrated here that in some systems (for example, the Ni-Fe-Cr system excited by a lo9Cdsource) the tertiary fluorescence component cannot be neglected without significantly affecting the analysis. The tertiary fluorescence contribution to the total response increases as the energy difference between the absorption edges of the ternary system and the exciting energy increases. When the tertiary component is significant, it can be accounted for by the fundamental parameters method or by properly modifying existing empirical coefficients methods. In either case a knowledge of the tertiary fluorescence component is required and can be obtained by using the Monte Carlo simulation.
CONCLUSIONS
E. Gillam and H. T. Heal, Brit. J. Appl. fhys., 3 , 353 (1952). J. Sherman, Spectrochim. Acta, 7, 283 (1955). J. Sherman, Spectrochim. Acta, 15, 466 (1959). T. Shiraiwa and N. Fujino, Jpn. J. Appl. fhys., 5 , 868 (1966). J. W. Criss and L. S. Birks. Anal. Chem., 40, 1080 (1968). D. A. Stephenson, Anal. Chem., 43, 310 (1971). R. W. Gould and S. R. Bates, %-Ray Spectrom., 1, 29 (1972). (8)B. W. Budesinsky, %Ray Spectrom., 4, 166 (1975). (9) R. P. Gardner and A. R. Hawthorne, X-Ray Spectrom., 4, 138 (1975). (IO) A. R. Hawthorne and R . P. Gardner, Anal. Chem., 47, 2220 (1975). (11) S. D. Rasberry and K. F. J. Heinrich, Anal. Chem., 46, 81 (1974). (12) R. Tertian, X-Ray Spectrom., 2, 95 (1973). (13) W. J. Veigele, AtomicData Tables, 5, 51 (1973). (14) W. Bambynek et at., Rev. Mod. fttys., 44, 716 (1972). (15) F. Arinc, Unpublished Ph.D. Thesis, North Carolina State University,Raleigh, N.C., 1976.
Although much of the preceding discussion was centered on the Ni-Fe-Cr ternary alloy, a system in which the interelement effects are particularly strong, the methods used are quite general and can as easily be applied to other analysis problems. The Monte Carlo simulation program, in addition to accounting for wide-angle geometry, is flexible enough to account for sample thicknesses less than the saturation thickness and for heterogeneous samples, such as layered specimens. These cases, which are especially difficult to treat analytically, may in some instances warrant the use of the Monte Carlo model directly in the inverse determination of elemental weight fractions. The present work demonstrates that the use of response weighted mean angles in the fixed-angle analytical relationships provides an accurate approximation to the response of
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7)
RECEIVEDfor review April 19, 1976. Accepted August 19, 1976. The authors wish to acknowledge partial financial assistance from the Environmental Protection Agency under Grant No. R-802759.
Detection and Characterization of Aerosols Containing Transuranic Elements with the Nuclear Track Technique Bruce Center' and Frank
H. Ruddy"
Department of Chemistry and Nuclear Radiation Center, Washington State University, Pullman, Wash. 99 163
Dielectrlc track detector autoradiography of alpha particles combined with neutron Induced fissionfragment radiography provides a technique for detectlon, Isotope identlflcatlon, and measurement of sire characteristics of transuranlc elements present In airborne particulate form. Isotope identlflcation Is based on the observed ratio of decay constant to fisslon cross section for each particle. Shes are calculated from the number of tracks observed per partlcle. The method has been applied successfully to uncontamlnatedair samples as well as samples taken from a plutonium reprocessing laboratory.
leases of these elements to the environment during fuel reprocessing operations ( I , 2). Several of the actinides, and in particular plutonium, are extremely toxic in amounts as low as 1wg. Due mainly to its low solubility, plutonium if released as the oxide and inhaled by man has a 250-300 day biological half-life in the lungs ( 3 ) ,the critical organ for this element. The dose delivered by the short range a particles is highly localized around insoluble hot spots in the lungs. Although the procedures for calculation of doses from a emitters in the lungs are presently a subject of debate (4-6),clearly, detection methods for airborne plutonium should supply information on particle sizes in order to evaluate inhalation hazards.
1 Present address, Westinghouse Hanford company, Hanford, Washington.
tion On particle Sizes is lost in the first Step Of such procedures. Similarly, direct detection methods (9) using a or y spec-
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Figure 1. (a)Alpha autoradiograph of a particle containing plutonium. (b) Radiograph of neutron induced fission fragments from a particle containing plutonium. T h e magnification is 400X and the grid size is 12.9 pm in both photographs
Table I. Ratios of Decay Constant to Fission Cross Section for Selected Transuranics and Transuranic Mixtures Isotope hq(cm-%-l) 1.51 x 1013 238Pu(isotopically pure) 1.48 X 10l2 238Pu (typical isotope mixture)a U (natural abundance) 1.17 X lo6 Pu (low irradiation) 1.36 x 109 Pu (recycle in LWR) 1.91 x 1010 8.68 X lo4 U (20%enriched) 5.39 x 104 U (95%enriched) Isotope mixture from'Amersham/Searle Corporation. b Isotopic abundances from J. W. Healy (LA-5483-ms).
Table 11. Experimental and Expected Decay Constant to Fission Cross Section Ratios Experiment (cm-2-s-1) X/of(cm-2-s-1) 238Pu (1.55 f 0.18) X 10l2 (1.48 & 0.01) X 10l2 239Pu (9.56 f 2.16) X lo8 (12.2 f 0.1) x 108 241Am (1.64 f 0.13) X 1013 (1.60 f 0.03) X 1013
trometry provide no information on particle sizes. The present method uses dielectric track recorders for autoradiographic detection and characterization of particulate actinide elements. A recent book by Fleischer, Price, and Walker (IO) reviews the applications of dielectric track detectors in analytical chemistry.
EXPERIMENTAL Reagents. Samples. Air samples are taken by conventional methods, and the filters are covered with a thin plastic foil (0.75 mg/cm2Makrofol). The covering prevents the particles on the filter from changing position during the subsequent radiographs. For a autoradiographs, the plastic-covered filter is placed adjacent to a 100-pmthick sheet of Kodak Path6 cellulose nitrate. The a particles emitted during decay of the element pass through the plastic covering and register as tracks in the cellulose nitrate. A particle containing 2136
Figure 2. Diameter distribution obtained by fission fragment radiography of an air filter obtained from a plutonium reprocessing plant a concentration of a transuranic element will result in the formation of a cluster of tracks with a common origin at the location of the particle. The same covered air filter is then placed in contact with a sheet of mica and irradiated with neutrons in the W.S.U. TRIGA Mark IV reactor. Using suitable reference points, the same particles may be located and clusters of fission fragments are observed. Standards. The neutron dose is measured using NBS SRM glass wafers (If) placed next to mica sheets during the irradiation. Irradiations. Sampleswere irradiated in the W.S.U. TRIGA Mark IV reactor with integrated fluxes of 1013-1017neutrons/cm2.The mica track detectors were allowed to cool several days before scanning. Exposure times for the a radiographs were from 1h to 1 day. Processing of Track Detectors. The cellulose nitrate track detectors were etched in 6.25 N NaOH at 25 "C for 24 h. The mica track detectors were etched in 40% hydrofluoric acid for 30 min. The detectors were scanned on an optical microscope using a magnification of 4OOX.
RESULTS AND DISCUSSION A typical track cluster is shown in Figure la. The number of 01 tracks observed is directly proportional to the number of atoms of actinide present and the decay constant for a decay. Corrections are made for those a particles which because of their low angle of incidence do not penetrate the Makrofol spacer. The greater slowing of those 01 particles with smaller angles of incidence increases their registration factors in the cellulose nitrate to values closer to unity. (Etching efficiency in cellulose nitrate is less complete for high energies and low
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
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Figure 3.
Diameter distributions obtained by fission fragment radiography of the stages of a cascade impactor air sampler
The diameters for greater than 50% collection are as follows: Stage 1: 9.4 pm: Stage 2: 3.8 pm; Stage 3: 2.5 jm; Stage 4: 1.8 Km; Stage 5: 0.01 of air were drawn through the impactor
angles of incidence (12)). A typical fission track cluster is shown in Figure lb. The number of fission tracks observed is directly proportional to the number of fissionable atoms present, the cross section for thermal neutron induced fission, and the flux of neutrons. If the integrated neutron flux is known and the registration geometry factors are calculated, the ratio of decay constant to fission cross section may be deduced from the observed ratio of a tracks to fission tracks for a particular particle. The ratio varies over a wide range for transuranic nuclides. In Table I, the expected ratios for several nuclide mixtures are shown. It can be seen that plutonium may be easily differentiated from natural uranium and enriched 235U.Individual isotopes of transuranic elements may be easily distinguished and the typical isotopic mixtures of plutonium isotopes may be differentiated. In order to calibrate the method, particulate mixtures of known transuranic elements have been autoradiographed, giving the results shown in Table 11.The agreement is excellent if reasonable isotopic compositions are used. Once the isotopic identity of a particulate sample is known, amounts of the isotope present per particle may be calculated from either the a or fission radiograph data using the appropriate nuclear constant (13,14).The crude assumption that the particles are spherical may be made and diameter distributions may be calculated such as that shown in Figure 2 for a filter taken from a plutonium reprocessing area. The plutonium was assumed to be in the form of the dioxide. When the two halves of the filter were radiographed separately, the two distributions were quite similar. The cutoff on the low diameter end of the distribution corresponds to the limit on the number of tracks accepted as a cluster. Several large particles with diameters approaching 1pm are observable and these particles, although accounting for less than 3% of the total number distribution, are responsible for 60% of the summed radioactivity. Such information is important to the evaluation of the dose hazard from inhalation of such a particle distribution. In the event that a complex nonhomo-
1000 m3
geneous mixture of isotopes is present, each particle may be identified separately and size distributions may be calculated separately for each component provided that the isotopes are on separate particles. In the case of 239Pu,particles containing as little as 108 atoms may be identified and sized. The limiting factor is the time required to obtain sufficient statistical accuracy in the a autoradiograph to identify the correct decay constant to cross section ratio. A maximum radiograph period of 30 days was assumed. The detection limit with identification is less for shorter half life isotopes. The distribution of particle sizes shown in Figure 2 is based on content of plutonium only and may not be related to the aerodynamic characteristics of the particles. In order to investigate the aerodynamic characteristics of fissionable materials, samples were taken with a five-stage cascade impactor in order to obtain the following aerodynamic fractions (in each case, the lower limit on the range is the aerodynamic diameter above which greater than 50% collection efficiency is obtained): Stage 1,9.4pm; Stage 2,3.8 pm; Stage 3,2.5 pm; Stage 4,1.8 pm; Stage 5,O.Ol pm (15).The ambient air of Pullman, Washington, was sampled since sampling a t a plutonium reprocessing plant could not be arranged. Each stage was fission radiographed and particle size distributions were calculated based on the assumption that the fissionable material present was natural isotopic abundance U3O8. The distributions are shown in Figure 3. The maxima in the distributions are the result of the lower cutoff on the number of tracks representing a cluster under the present radiograph conditions. The radiograph sizes are small compared to the aerodynamic sizes of the particles, but the radiograph size distributions seem to be the same on all stages and independent of aerodynamic characteristics. Although the dust collected consists of particles with widely varying uranium contents, the similarity of the distributions can best be explained in terms of small uranium rich particles that attach themselves to larger particles that are less rich in uranium. An alternative, but less likely, explanation is that
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
I
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DIAMETER, d ( p m )
DIAMETER, d ( p m )
Figure 4. Diameter distributions obtained by fission fragment radiography of environmental air samples (a) Pullman, Washington. (b) Richland, Washington. (c) Kennewick, Washington. (d) Pasco, Washington. 1000 m3 of air were drawn through the filters
the larger particles contain small, uranium rich phases which lead to similar radiograph distributions when the larger particles are aerodynamically separated. The total amount of fissionable material is largest on the middle stages and reflects the larger total amount of dust in these ranges. No fractionation of the fissionable particles according to the aerodynamic characteristics of the larger particles to which they attach is apparent in these data. Figure 4a presents the diameter distributions resulting from the summation of the five stages of the cascade impactor. It is, therefore, the diameter distribution of the total fissionable material collected by the impactor. Fission radiographs were also performed on high volume air filters from Richland, Kennewick, and Pasco, Washington, provided by the Washington State Department of Ecology. The diameter distributions are depicted in Figures 4b-4d. The Pasco sample showed the most fissionable material, corresponding to more dust loading on the filter (U308 was assumed). These distributions were qualitatively similar for the Richland, Kennewick, and Pasco samples, but the fissionable particles were larger in the case of the Pullman samples indicating a different environmental source of the fissionable material.
CONCLUSIONS In the event of an accidental release of actinide elements into the environment, a complete evaluation of the inhalation 2138
hazards would require information on the aerodynamic properties of the particles (or particles onto which the actinide particles are attached) and information on the radioactivity of the actinide particles. The latter information is obtained directly by CY autoradiography. In practice, the sensitivity of detection for a transuranic element is limited by the total amount of dust collected, since uranium in the dust will result in background in the fission radiographs. For 239Pu,an amount equal to about l/~b of the total uranium present in the dust would result in about a 50% background subtraction. The total loading of dust that would result in a reduction in sensitivity of the CY radiograph (due to self absorption) corresponds to a thickness of about 20 Fm, or approximately the range of an CY particle. In preliminary experiments with soil samples containing 239Pu, 237Np,or 241Am,particles and colloids could be detected, but difficulty was encountered in obtaining quantitative results due to the heterogeneous nature of the soil samples (14,16). In the case of water samples, identification would only be possible in the case of a colloidal suspension rather than a true solution, since the method requires the presence of concentrated particles containing the transuranic material. Other than natural uranium background, the most serious interference to be guarded against would be homogeneous mixtures of transuranic isotopes leading to misleading decay
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
