Anal. Chem. 1999, 71, 2616-2622
Use of Secondary Ion Mass Spectrometry in Nuclear Forensic Analysis for the Characterization of Plutonium and Highly Enriched Uranium Particles Maria Betti,* Gabriele Tamborini, and Lothar Koch
European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, 76125 Karlsruhe, Germany
The application of secondary ion mass spectrometry (SIMS) analysis is described for the characterization of plutonium and highly enriched uranium (HEU) particles with a diameter to 10 µm. Applying a method previously described, particles of HEU could be detected in a scrap material, together with natural uranium. The isotopic composition of the particles was measured with a typical accuracy and precision of 0.5%. The spectrum of the trace elements in the uranium particles was also recorded. From the results it was possible to deduce that the uranium oxide, as UO2, was produced via a pyrochemical process. In a sample consisting of a mixture of three different species of particles, two of these were identified as plutonium particles. They were characterized according to their isotopic ratio 239/240 as well as to their dimension and shape. The results obtained by SIMS for the isotopic ratio were compared with those obtained analyzing the particles by Thermal Ionization Mass Spectrometry (TIMS). The shape and dimensions were confirmed by the analysis with Scanning Electron Microscopy (SEM). In both the cases the results obtained by SIMS were in good agreement with those from TIMS and SEM. In the past few years the demand for the determination of trace elements and for the identification and characterization of microparticles to reveal the existence of unidentified bulk nuclear materials or of undeclared nuclear activities has increased considerably. The Institute for Transuranium Elements (ITU) has been actively involved in the identification of nuclear materials from unknown activities for some time, having the facilities to handle and to analyze active and/or contaminated samples.1 Several years ago the capability of analytical chemistry to reveal ongoing undeclared nuclear activities by the analysis of environmental and swipe samples was demonstrated.2 In particular, the determination of the isotopic composition of single particles has * Corresponding author: (fax) 49-2747-951-595; (e-mail)
[email protected]. (1) Koch, L.; Betti, M.; Dolgov, J.; Mayer, K.; Ray, I. L.; Schubert, A.; Stalios, A. D.; Wallenius, M. Nuclear Forensics in Nuclear Material Safeguards. Proc. 19th Annual ESARDA Symp. On Safeguards and Nuclear Material Management, Montpellier, EUR 17665 EN; CEC Joint Research Centre: Ispra, 1997; pp 43-46.
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been recognized to be of the utmost importance in environmental monitoring for nonproliferation in nuclear safeguards and forensic investigations.3 From the several physical analytical techniques available at ITU, secondary ion mass spectrometry (SIMS) has been applied for the identification and the characterization of microparticles stemming from nuclear activities.4,5 In particular, it has been used in combination with electron probe microanalysis (EPMA) and glow discharge mass spectrometry (GDMS) for the characterization of environmental samples contaminated with traces of radioisotopes.6,7 Other authors have used SIMS for single particles of uranium and plutonium. In particular, the efficiency, accuracy, and precision of SIMS for the isotopic determination of uranium and plutonium in glass microparticles9 and in clay microspheres10 have been demonstrated. Moreover, an automated search technique using SIMS analysis has been recently presented.11 (2) Donohue, D. L.; Ziesler, R. Anal. Chem. 1993, 65(7), 359A-368A. (3) Cooley, J. N.; Kuhn, E.; Donohue, D. L. Current Status of Environmental Sampling for IAEA Safeguards. Proc. 19th Annual ESARDA Symp. On Safeguards and Nuclear Material Management, Montpellier, 1997, EUR 17665 EN; CEC Joint Research Centre: Ispra, 1997; pp 31-35. (4) Betti, M. Mass Spectrometric Techniques Applied for the Determination of Radionuclides Traces. Proc. Int. Workshop on the status of measurement techniques for the identification of nuclear signatures, Geel, Belgium, February 25-27, 1997, EUR 17312 EN, ISBN 92-828-0227-2; CEC Joint Research Centre: Ispra, 1997; pp 125-127. (5) Tamborini, G.; Betti, M.; Forcina, V.; Hiernaut, T.; Giovannone, B.; Koch, L. Spectrochim. Acta, Part B 1998, 53(9), 1289-1302. (6) Betti, M.; Giannarelli, S.; Lefevre, O.; Walker, C. T.; Koch, L. Detection of Nuclear Signatures in Soil and Sediment Samples. Proc. 17th Annual ESARDA Symp. On Safeguards and Nuclear Material Management, Aachen, 1995, EUR 16290 EN; CEC Joint Research Centre: Ispra, 1995; pp 481-485. (7) Lefevre, O.; Betti, M.; Koch, L.; Walker, C. T. EMPA and Mass Spectrometry of Soil and Grass Containing Radioactivity from the Nuclear Accident of Chernobyl. Mikrochim. Acta 1996, 13, 399-408. (8) Lagerwaard, A.; Volkers, K. J.; Camelot, D.; Kooyman, P.; van Geel, J. Preparation and Characterization of Micrometer Size Nuclear Material Particles for Analytical Studies: A Need for Definition. Proc. Int. Workshop on the Status of Measurement Techniques for the Identification of Nuclear Signatures, Geel, Belgium, February 25-27, 1997, EUR 17312 EN, ISBN 92-828-0227-2; CEC Joint Research Centre: Ispra, 1997; pp 115-119. (9) Simons, D. S. Single particle standards for Isotopic measurements of Uranium by Secondary Ion Mass Spectrometry. J. Trace Microprobe Tech. 1986, 4, 185-195. (10) Stoffels, J. J.; Briant, J. K.; Simons, D. S. A Particulate Isotopic Standard of Uranium and Plutonium in an Aluminosilicate Matrix. J. Am. Soc. Mass Spectrom. 1994, 5, 852-858. 10.1021/ac981184r CCC: $18.00
© 1999 American Chemical Society Published on Web 06/10/1999
This paper describes the application of SIMS in nuclear forensic investigations to characterize and identify the origin of particles of plutonium and highly enriched uranium. SIMS data has been used to complement that from thermal ionization mass spectrometry (TIMS) and scanning electron microscopy (SEM). The isotopic ratios obtained by SIMS for the uranium and plutonium particles have been compared with those obtained by TIMS. As for the dimension and shape, the plutonium particles, from their SIMS image, were found to be consistent with those observed by SEM. Furthermore, SIMS has been used to obtain a complete spectrum of the trace elements contained in a microparticle. From the trace elements present and their concentration in microparticles of plutonium and uranium, the history of the sample can be revealed, namely the physical/chemical and/or industrial processes the sample has undergone. EXPERIMENTAL SECTION 1. Instrumentation. A CAMECA IMS 6f secondary ion mass spectrometer (SIMS) was used. This instrument consists of a double-focusing mass spectrometer that allows fast switching between the masses. In addition, it has microfocus ion sources (cesium and duoplasmatron with oxygen or argon gas) that can be used either in the microscope or microprobe mode. The instrument was also equipped with a spatially resolved pulsecounting resistive anode encoder (RAE) used for mapping the entire sample surface. The SIMS was employed both in the microscope and microprobe modes. A resolution up to 10 000 is achievable, but for the measurements of the isotopes of uranium and plutonium a resolution of 2000 is sufficient. At this resolution, flat-topped peaks were obtained which greatly improve the accuracy of the measurement. A detection limit in the ng/g - pg/g range is achieved by optimizing different instrumental parameters, such as acquisition time. As for the determination of the uranium isotopic composition, the mass calibration on the masses 234, 235, and 238 was performed before starting a new sample or a new cycle of measurements. As for plutonium, to span all the region of the masses of the plutonium isotopes, the mass calibration on the masses corresponding to the uranium oxide species, UO+ (250, 251, and 254) and UO2+ (266, 267 and 270), was also carried out. It was observed that during the course of the day the mass calibration did not change as much as in other mass spectrometric techniques, such as glow discharge mass spectrometry and thermal ionization mass spectrometry. 2. Sample Preparation. Manipulation and loading of samples was performed in a clean glovebox having the characteristics of a class 100 clean air hood, due to the fact that U and Pu are hazardous. The particles were stuck on special adhesive supports (LeitTabs, Plano W. Planet GmbH, Weszlar, Germany) and then fixed on a metallic disk with a diameter of 25 mm that fits into the SIMS sample holder. Together with the samples, a blank consisting of the adhesive support coated with carbon was also prepared and analyzed. (11) Simons, D. S.; Gillen, G.; Zeissler, C. J.; Fleming, R. H.; McNitt, P. J. Automated SIMS for determining Isotopic Distributions in Particle Populations. Secondary Ion Mass Spectrometry SIMS XI, in press 1998.
For the SEM analysis the same sample preparation method was employed. In this case the carbon coating was not necessary. Plutonium particles for SIMS analysis were embedded in a foil of indium metal. A blank of the indium foil was also run to test isobaric interferences. 3. SIMS Experimental Conditions. The blank sample was used at the beginning of the analysis to measure the background intensity from mass 234 to 244. This study would also reveal interference, resulting from the adhesive support, the carbon coating, or the indium foil, by the formation of molecular peaks with the same nominal mass as the analytes. The level of interference from traces of heavy metals, as revealed by the analysis of the two blanks, reached a maximum value of 10 counts/s and was eliminated by applying an offset of 40 V to the extraction potential to discriminate against low-energy secondary ions. In fact, the distribution of the secondary ions changes according to ionic species, and the molecular ions present a narrower energy distribution than the atomic ones. Therefore, by applying an offset to the extraction potential it is possible to select an energy distribution that represents only the atomic ions’ contribution to the signal of the secondary ions and, excludes the contribution of interfering molecular ions. This value of 40 V was chosen upon considering the energy distribution curves of the analytes.12 In this way, a very low background was obtained, and all subsequent measurements were performed using the same offset voltage. In the case of the plutonium particles on the indium foil, the isotope at mass 113 forming In2O+ interfered on the mass 242 of plutonium. Consequently, only the masses 239 and 240 were measured for Pu. The mass 241 was not measured because of the isobaric interference of Am-241 that could not be resolved either in SIMS or in TIMS. Particles produced from certified reference materials (U005, U010, and U030 - NBS) with known 234/238 and 235/238 mass ratios,8 were employed to check the instrumental response during the analysis. 4. SIMS Instrumental Analysis. The samples were introduced into the source under a high vacuum of 5 × 10-10 Torr and bombarded with a primary O2+ beam between 10 and 15 keV having a current intensity between 100 and 200 nA for uranium particles. As for plutonium particles, a primary impact beam of 17 keV with a current between 1 and 2 nA was used for rastering. Positive secondary ions, accelerated through 5 keV, were then focused by a set of transfer lenses and deflected by a magnetic field (δM/M stability of about 10-6). The energy slit was adjusted for a band-pass between 40 and 50 eV. Ions were counted by an electron multiplier in the ion counting mode with a dead time of 25 × 10-9 s, involving a dynamic range from 5 × 105 to 1 counts/ s. The lateral resolution in the microprobe mode was in the range of 0.5-2 µm; in microscope mode the beam was completely defocused. Once the image was obtained, working in miscroscope mode or employing the resistive anode encoder detector, for the first evaluation, a computer program developed at the Institute was used to estimate the isotopic ratio from the intensity of the images obtained for the different isotopes. For a more precise analysis, (12) Tamborini, G. De´veloppement de la technique SIMS pour l’analyse de particules radioactives et ses applications a` diffe´rent e´chantillions. Ph.D. Thesis, Universite de Paris-Sud, December 15, 1998.
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Figure 2. Images acquired for the masses 238 and 235 of the uranium isotopes over a sample area of 250 × 250 µm. The colors are related to the intensity of the signal that increases from the blue through the red to white. Table 1. Application of the Method of the Image Intensity Ratio for the Determination of the Isotopic Ratio. (Tested on Reference Particles and Compared with the Method of the Isotopic Acquisition) Figure 1. Signal stability for the uranium isotopes and an acquisition time of 500 s.
231Pa
over
the primary beam was focused onto the particles, and the mass spectrum between mass 234 and 244 was recorded and the intensities of the uranium and/or plutonium isotopes accurately measured. After setting the resolution to obtain flat-topped peaks and checking the mass calibration, the acquisition for the determination of the isotopic ratio was started. The enrichment of the particles was detected by focusing the primary ion beam on the particle and acquiring ion beam intensities for the uranium isotopes, at the masses 234, 235, 236, and 238 according to a method already described.5 By means of this method, accuracy and precision of 0.5% for the ratio 235/238 and of 2.0% for the ratio 234/238 can be obtained. In Figure 1 the stability of the signals for the uranium isotopes and 231Pa over an acquisition time of about 500 s is shown. The signal corresponding to the mass 231 has been reported since it shows that, also for a few counts, the signals can be stable over the acquisition time. The particle had a dimension of 5.5 µm. RESULTS AND DISCUSSION 1. Highly Enriched Uranium Particles. In Figure 2 the images acquired for the masses 238 and 235 of the uranium isotopes over a sample area of 250 × 250 µm are shown. The different colors are related to the intensity of the signal, which increases from blue through red to white. Although the sample was rich in uranium particles, the highly enriched particles (particles containing more than 20% 235U) could be clearly identified (see Figure 2, left side). The dimensions of each particle detected were estimated from their images, and values between 2 and 10 µm were found. The enrichment of the particles was determined as described in the Experimental Section. In addition, the isotopic composition was estimated from the ratio of the images for the different isotopes. Before applying this method to real samples it was tested on reference particles of uranium 1-2 µm in size. In Table 1 the results obtained for the isotopic ratio 235/238 for five reference 2618 Analytical Chemistry, Vol. 71, No. 14, July 15, 1999
particle 235/238 isotopic 235/238 image particle identification acquisition (IA) ratio (IR) IA/IR size (µm) 1 2 3 4 5
0.03148 0.03155 0.03151 0.03172 0.03155
0.03143 0.03141 0.03153 0.03144 0.03159
1.002 1.004 0.999 1.009 0.999
1.1 1.4 1.8 1.4 1.5
Table 2. Comparison between the Isotopic Acquisition Obtained in the Microprobe Mode and Image Intensity Ratio for the Determination of the Uranium Enrichment in the Particles isotopic acquisition/image ratio
particle size (µm)
1.005 0.995 1.000 1.008 0.998 1.003 1.009 0.999 1.002
3.6 2.9 6.3 5.0 5.0 7.0 4.5 3.5 4.0
a The ratio of the results obtained with the two methods is reported along with the particle size.
particles of similar size with the method of isotopic acquisition and image acquisition are compared. For each particle the values obtained with both methods as well as the ratio of the values are given. As can be seen, the agreement is consistently good despite the small particle size, indicating that the software calculating the isotopic ratio from the image intensity can be used for small particles. The same comparison has also been performed for particles in real samples, and the results obtained are summarized in Table 2 as simply the ratio of the values obtained with the two methods. As can also be seen for real samples, a good agreement between the two methods was found, the ratio being always close to one. For the uranium particles, the mass spectra were recorded. In Figure 3 the secondary ion mass spectrum obtained for one of these particles over the mass range 230-238 (Figure 3a) is
Figure 3. Secondary ion mass spectrum obtained from a highly enriched uranium particle (Figure 3a) is compared with that recorded for one particle of slightly enriched uranium (Figure 3b). Mass range 230-238.
compared with that recorded for one particle obtained from standard material (NBS030), 3% enriched uranium (Figure 3b). As can be seen, the HEU particle (5.5 µm) is characterized by the appearance of peaks at mass 231 and 230 in the mass spectrum. They correspond to the Pa and Th isotopes that can be used to calculate the age of the material. Because the signals have different intensities, the one relevant to 231Pa was recorded by the electron multiplier and the 235U was registered using the Faraday cup as detector; however, both the signals were corrected for the background. The age of the particle could be calculated on the basis of the ratio 231Pa/235U considering the decay of the 235 uranium in 231 protactinium and applying the following equation:13
N235U(t) N
231
Pa(t)
)
e-λt )R 1 - e-λt
(1)
Here, N is the number of atoms at the time t, λ ) ln 2/T1/2 where T1/2 is the half-live time of 235 uranium, and t corresponds to the time elapsed from the last treatment of the sample (chemical separation, fabrication, or enrichment). (13) Lieser, K. H. Nuclear and Radiochemistry, fundamentals and applications; VCH: Weinheim, Germany, 1997.
From eq 1 t can be obtained:
1 1+R t ) - ln λ R
(2)
According to eq 2, on the basis of the SIMS results, a time of 15.30 ( 0.49 years was calculated. This means that the last separation took place about 15 years ago. The result is that the particle is 88% enriched. The 230Th was also detected, but its signal was too low to be used for the calculation of the age on the basis of the ratio 234U/230Th. It should be noted that the spectrum shown in Figure 3b, relevant to a 3% enriched uranium particle, was obtained in a previous experiment without applying any offset to the extraction potential. In this case, the formation of hydrides on the mass 239 is evident. In a previous investigation5 it has been demonstrated that the application of an offset of 40 V reduces the hydride contribution to a negligible extent. Moreover, the use of a cooling system by means of liquid nitrogen in the main chamber can further reduce the formation of hydrides. As for the determination of the 236U, in our laboratory the SIMS is calibrated with particles obtained from the NBS030 reference material. The ratio 236U/ 238U is also measured, and, after the correction for the contribution Analytical Chemistry, Vol. 71, No. 14, July 15, 1999
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Figure 4. Experimental precision for the ratio 231/235 as a function of the number of counts acquired on the mass 231 mass.
Figure 5. Secondary ion mass spectrum from a highly enriched uranium particle. Mass range 10-100.
of the hydride formation, the error over a 6-month period has become less than 3.5%. The instrumental precision for the measurement of the isotopic ratios between different elements, supposing that the dead time of the detector is stable and can be corrected, is a function of the number of ions counted in the detector and of the acquisition time. For two different isotopes, X and Y, the statistic error on the measurement of their ratio, that is, the instrumental precision, can be expressed as
Error % ) 100 × [(1/Ix) + (1/Iy)]1/2 where Ix and Iy are the intensities of the signals of the isotopes X and Y, given by the number of counts multiplied by the acquisition time, respectively. In Figure 4 the experimental precision for the ratio 231/235 as obtained for 15 consecutive analyses of the ratio by SIMS, versus the number of counts for the mass 231 is reported. 2620 Analytical Chemistry, Vol. 71, No. 14, July 15, 1999
Figure 6. Images acquired for the masses 239 and 240 of a plutonium particle. (a) Particle with a diameter of 3.5 µm. (b) Particle with a diameter of 15 µm.
As can be seen, the instrumental precision of the ratio increases with the number of counts of the minor isotope considered, and for 60 counts on the 231 mass an instrumental precision for the ratio of about 3.5% is obtained. The experimental curve can be represented by a logarithmic equation: y ) - 21.196 ln(x) + 27.06 (R2 ) 0.999). The particles of uranium were also detected by scanning electron microscopy (SEM) coupled with an energy disperse X-ray system (EDX). The X-ray distribution maps were recorded on
Table 3. Comparison of SIMS and TIMS Results for the Determination of the Ratio 240/239 in Two Different Types of Pu Particles particle type 1 SIMS/TIMSa,b
particle type 2 SIMS/TIMS
0.9811 (1.6%) 0.9991 (0.7%) 0.9863 (1.4%) 1.0068 (0.4%) 0.9906 (1.2%) 0.9949 (0.5%)
1.0203 (0.3%) 1.0350 (0.4%) 1.0252 (0.6%) 0.9821 (0.7%) 0.9992 (1.6%)
a The average of the TIMS measurements for each type of particles is used. b The rsd% on the 240/239 ratio measured by SIMS is given in parentheses.
Figure 7. Scanning electron micrograph showing the three particle types in a forensic sample.
selected areas of the sample surface. Uranium particles were detected in a range of sizes from 2.5 to 9 µm and with a variety of different forms. The morphology of the individual particles indicated that they were crystalline and oxide rather than metal, revealing that they were not hydrolyzed UF6 but obtained by pyrochemical processes. SIMS could not give information about the chemical form of the uranium particles, metallic uranium or oxide, since an oxygen primary beam was used. In the investigation here described SIMS was used to detect UO2 particles enriched up to 90% in U-235, and fluorine was never detected. SIMS analysis of the particles in question revealed impurities typical for the pyrochemical process. In Figure 5 the secondary ion mass spectrum for the mass 10 to 100 is reported. The light elements characteristic of such a process, such as C, Na, Mg, Al, Si, K, Ca, Cr, and Fe, are highlighted. Thus, in this particular case, SIMS has been used to assess, by complementary and independent information, a result found with another technique.
2. Plutonium Particles. Microparticles of plutonium can also be handled in our laboratory and analyzed by SIMS to obtain their size, shape, and isotopic ratios. For instance, in one sample two different kinds of plutonium particles were found. One type was small, almost round, with a diameter ranging between 3 and 6 µm (Figure 6a). The other was a bigger rod-shaped particle with a length from 30 to 65 µm and a diameter of approximately 15 µm (Figure 6b). The same dimensions and shapes were also found by SEM. In fact, as can be seen in Figure 7 where a scanning electron micrograph is shown, three separate particle types were identified in the sample. The particle labeled 1, identified as platelets of PuO2, corresponds to the particle of plutonium in Figure 6a. The particle labeled 2, recognized as a fibrous PuO2 rod corresponds to the particle in Figure 6b. The third particle types were hexagonal rods of U3O8. SIMS determined the 240/239 isotopic ratio in several Pu particles of types 1 and 2. Analysis by thermal ionization mass spectrometry was also performed on the two types of plutonium particles. In Table 3 the ratio of the isotopic ratios 240/239 obtained by SIMS and TIMS is reported. For the two categories of particles these ratios are always close to one, indicating good agreement between the two independent methods. The figure in
Figure 8. Experimental precision for the SIMS measurements of the ratio 239/240.
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parentheses is the relative standard deviation obtained for the 240/ 239 SIMS ratio from running 10 acquisitions on the same particles. In Figure 8 the experimental precision, as previously defined, obtained by SIMS for the determination of the ratio 240/239 is reported against the experimental counts for the mass 240 (minor isotope of the ratio). The experimental values can be fitted by a power equation: y ) 3.3805x-1.1278 (R2 ) 0.8981). As can be seen, for only 120 counts of the minor isotope, the precision for the ratio has a value of 4.5%, and when the counts are increased to around 10 000 the precision improves to 0.5%. SUMMARY In nuclear forensic analysis, secondary ion mass spectrometry has been used for the identification of particles of highly enriched uranium in the presence of particles of natural and low-enriched uranium. Moreover, the isotopic composition of the highly enriched uranium particles could be determined from the ratio of the image intensity generated for the masses 235 and 238 using a software specifically developed in our group. The results obtained were in good agreement with the results from a procedure previously described based on the isotopic ratio acquisition. Furthermore, SIMS has been used to obtain a complete spectrum of the trace elements contained in the particle.
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From the trace elements present in the particles, the history of the sample can be revealed; that is, the physical/chemical and/ or industrial processes the sample had undergone can be discovered. In the microscope mode, SIMS also has the capability to provide information on the dimension of the particles. In the case of a sample containing a mixture of three different particles, two different categories of plutonium particles could be identified, and their dimensions measured by SIMS were in agreement with that revealed by SEM. SIMS could be used easily for the characterization also of plutonium particles with a good precision and accuracy. The isotopic ratio for the mass 239/240 was in perfect agreement with those obtained by TIMS. ACKNOWLEDGMENT The authors acknowledge Dr. Ian Ray and Mr. Helmut Thiele for the SEM measurements and Mr. Hans-Gunter Schneider for assistance in the preparation of plutonium particles for SIMS analysis.
Received for review October 29, 1998. Accepted April 16, 1999. AC981184R
