Production of monodisperse uranium oxide particles and their characterization by scanning electron microscopy and secondary ion mass spectrometry

Spectrochimica Acta Part B 55 Ž2000. 1565᎐1575

Production of monodisperse uranium oxide particles and their characterization by scanning electron microscopy and secondary ion mass spectrometry N. Erdmanna , M. Betti a,U , O. Stetzer a,b, G. Tamborini a , J.V. Kratz b, N. Trautmann b, J. van Geel a a

European Commission, General Directorate Joint Research Center, Institute for Transuranium Elements, P.O. Box 2340, D-76125 Karlsruhe, Germany b Institut fur ¨ Kernchemie, Johannes Gutenberg-Uni¨ ersitat ¨ Mainz, D-55099 Mainz, Germany Received 8 April 2000; accepted 11 July 2000

Abstract Secondary ion mass spectrometry ŽSIMS. can be confidently used to measure uranium isotopic ratios in single particles. Dense particles of known isotopic composition and size allow the precision and the accuracy of the applied procedure to be estimated. These particles can be obtained by dissolving standard reference uranium materials, nebulizing the solution in droplets of proper diameter and collecting the particles after the desolvation and calcination of the droplets. A new instrumental set up, based on a commercial vibrating orifice aerosol generator to generate monodisperse droplets of the solutions from four uranium oxide reference materials, is described. The droplets were dried and calcined in a sequence of three furnaces. The morphology of the monodisperse uranium oxide particles was studied by scanning electron microscopy. It was observed that the particles were nearly spherical and consisted of dense material. Their diameter distribution evidenced the presence of two populations mainly, the first showing a narrow distribution with a maximum centered at approximately 1 ␮m. The first statistical moment ratios between the two populations remained practically constant at 1.24" 0.01. This demonstrated that the second population was due to the formation of one particle from two droplets of solution Žtheoretical double mass ' diameter 3

ratio of '2 s 1.26.. Secondary ion mass spectrometry was used to verify the isotopic composition of the produced particles. Typical accuracies of better than 0.4% for 235 Ur 238 U and a few percent for the minor isotopes have been

U

Corresponding author. Tel.: q49-7247-951-363; fax: q49-7247-951-186. E-mail address: [email protected] ŽM. Betti.. 0584-8547r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 0 0 . 0 0 2 6 2 - 7

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achieved. For the determination of the 236 U content, the signal at mass Ms 239 Ždue to 238 UHq . was used to correct the 235 UHq contribution to 236 U at mass Ms 236, greatly improving the accuracy of the 236r238 ratio with increasing enrichment of the 235 U isotope. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: Uranium oxide particles; Monodisperse; Secondary ion mass spectrometry; Scanning electron microscopy; Isotopic standard; Nuclear forensic analyses; Nuclear safeguards

1. Introduction International governmental organizations for the control of nuclear material, such as the Euratom Safeguards Office ŽESO. and the International Atomic Energy Agency ŽIAEA., have implemented new safeguard programs in order to strengthen the control compliance of nuclear facilities declarations. One of the first phases of these programs concerned the development of reliable analytical methods for bulk analysis as well as very sensitive and cost effective techniques for particle analysis w1x. Environmental sampling, together with the characterization of individual radioactive particles, was found to be an important means for detecting undeclared nuclear activities w2,3x. Instruments for on-line single-particle mass spectrometry have been developed by different research groups w4᎐9x, and a comprehensive review of these techniques has been recently published w10x. Advances in aerosol time-of-flight mass spectrometry make it possible to obtain simultaneous mass spectra from both positive and negative ions in a single ambient aerosol particle in real time w11x. As an off-line technique, secondary ion mass spectrometry ŽSIMS. has been applied for the detection of impurities and the isotopic composition of uranium- and plutonium-containing particles w12᎐17x. In some cases, scanning electron microscopy ŽSEM. has been used to study the morphology of the particles w15x. In order to reduce the measurement uncertainty and to improve the accuracy of these techniques, standard particles with a well known content of uranium are needed. These particles can allow the detection efficiency to be determined. Furthermore, reference particles with a welldefined isotopic composition are required for the

quality control of SIMS analyses. Since no such particles are commercially available, we decided to make them ourselves, and, accordingly, monodisperse uranium oxide particles were produced, starting from certified standard reference materials. In this paper, the results achieved by using a new experimental setup developed in our laboratory for the production of such particles have been described. The particles, produced from reference materials of different uranium isotopic composition using a commercial vibrating orifice aerosol generator, were afterwards characterized by SEM for their morphology and by SIMS for their isotopic composition.

2. Experimental 2.1. Set-up for particle generation A scheme of the experimental setup for particle generation is shown in Fig. 1. Starting from a hydro-alcoholic solution of uranily-nitrate from the dissolution of reference materials, a spray procedure, together with the technique of solvent evaporation, was used to generate droplets. A commercially available aerosol generator ŽModel 3450 Vibrating Orifice Aerosol Generator, TSI Inc., 500 Cardigan Road, St. Paul, MN 55164, USA. was employed w18x. The droplets where then desolvated to form solid particles. A drying column consisting of Plexiglas, where a stream of pressurized air is passed, was installed between the orifice and the first furnace. A series of three furnaces was used for the calcination process. Then after cooling, the particles were collected on a Nucleopore filter. Typically, the particles were collected for 1 h per filter.

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Fig. 1. Experimental set-up for particle generation.

2.2. Materials used The uranium is added in the form of a uranylnitrate solution to a mixture of 50% MilliQ-water and 50% isopropanol Žanalytical grade.. By changing the concentration of the uranyl nitrate in this mixture, the mean diameter of the final aerosol particles can be varied. In order to generate particles of defined isotopic composition, a certified standard reference uranium oxide ŽU3 O 8 . powder, was used, namely CRM U005, CRM U010, CRM U030 and CRM U100 with 0.5%, 1%, 3% and 10% 235 U, respectively, obtained from the National Institute for Standard and Technology ŽNIST, USA.. These materials were certified by the US Department of Energy, New Brunswick Laboratory, ŽArgonne, IL, USA. with respect to 234 U, 235 U, 236 U and 238 U abundance. The powder was dissolved in 8 M HNO3 and the resulting solution was adjusted to a concentration of 40 mg Urg in a 1 M HNO3 solution. The final solution for the aerosol generation Ž62.5 mgrl uranium in a 50:50 mixture of isopropanol and water. was adjusted to generate particles of 1 ␮m. High purity carbon planchets ŽGrade A Carbon Planchets No. 17680, E.F. Fullam, Inc., USA.

with a diameter of 25 mm were used for particle analysis by SIMS. 2.3. Scanning electron microscopy A Philips XL40 scanning electron microscope ŽSEM. was used to determine the particle morphology, such as their shape and size w15,16,19x. The SEM was equipped with a detector for energy-dispersive X-ray analysis ŽEDX., consisting of a 10-mm2 Si᎐Li detector with a 10-␮m Be window, and with a detector for back-scattered electrons ŽBSE.. The particles could be analyzed directly on the Nucleopore filters after covering them with a thin layer of sputtered gold or carbon to make the sample electrically conducting. A voltage of 30 kV is typically applied when recording SEM images. The elemental content of a single particle could be analyzed by recording an X-ray spectrum with the EDX detector. This spectrum was evaluated by comparing the peak positions in the spectrum with those in a source library. 2.4. Secondary ion mass spectrometry At the Institute for Transuranium Elements ŽITU., SIMS has been optimized for the charac-

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terization of single uranium and plutonium microparticles w12᎐14,16x. A CAMECA IMS6F ŽParis, France. SIMS was used for this purpose. The instrument consists of a double-focusing mass spectrometer, which has been engineered for fast switching between the masses to be analyzed, and two microfocus ion sources, a cesium and a duoplasmatron Žwith oxygen or argon gas. source. For the analysis of uranium particles, an Oq 2 primary beam of 15 keV impact energy and having a current between 1 and 2 nA with a spot size of a few micrometers was used. Positive secondary ions were accelerated through 5 keV, and their intensity was controlled by tuning the primary-beam current. The energy band-pass of the mass spectrometer was set between 40 and 50 eV to reduce molecular ions. The instrument can typically operate with a mass resolution power ŽMRP. up to 25 000 MRP. However, for measurements of uranium isotopes, a resolution of 1000 MRP is sufficient. At this resolution, flat top peaks are obtained which greatly improve the accuracy of the measurement. To search for uranium particles, initially the microprobe beam is rastered over a large area, often 500 = 500 ␮m2 . For this purpose, the mass spectrometer was set to mass 238 to obtain a mapped distribution of the particles of interest. Once these particles were detected, the primary beam was scanned on a small area of the sample and high magnification images of individual particles could be taken. Finally, the isotopic composition of the particles was determined. For this purpose, the primary beam was focused on a single uranium particle and the isotopic ratios were calculated from the measured count rates of the different uranium isotopes. The mass calibration was checked each time before acquiring data on the isotopic ratios.

3. Results and discussion 3.1. Formation of a monodisperse aerosol and uranium oxide particles In order to obtain particles with a well-defined diameter, the Cm concentration of the feed

hydro-alcoholic uranyl-nitrate solution was adjusted according to: ⭋p s

ž

Cm qI d

1r3

/

⭋d

Ž1.

where ⭋p is the nominal diameter of the final particle, d is the density of the solid uranium oxide, I is the volumetric fraction of non-volatile impurities in the solvent, and ⭋d is the diameter of the droplet, in our case 40 ␮m. For particles approximately 1 ␮m in diameter, the contribution of I is negligible using high purity chemicals, as in the present investigation. Then the uranyl nitrate solution is fed into the aerosol generator with constant speed through a small orifice placed on a piezo-electric ceramic producing a liquid jet. Such a jet is unstable and breaks up into droplets w20, 21x. When left uncontrolled, non-uniform droplets are generated. However, applying an appropriate signal from a frequency generator to the piezo-electric ceramic, and thereby inducing a constant vibration, will cause a periodic disturbance to the liquid jet, which controls the breakup process to produce very uniform droplets. Furthermore, as only one droplet is produced per cycle of disturbance, the diameter of a droplet Ž⭋d . can be precisely calculated from the liquidfeed rate Ž ␯⬘. and the frequency of the disturbance Ž f . according to: ⭋d s

6␯⬘ ␲f

ž /

1r3

Ž2.

The optimum liquid flow rate-to-frequency of the disturbance ratio Ž ␯Xrf . depends on the diameter of the liquid jet, which is determined by the inner diameter of the orifice used. An orifice with a diameter of 20 ␮m ᎏ as used during the experiments ᎏ leads to a droplet diameter of 40 ␮m, the size of the dry particle is then obtained according to Eq. Ž1.. The final particle size distribution is only dependent on the droplet size distribution from the aerosol generator. To form a monodisperse aerosol, the droplets must be dispersed and diluted before significant coagulation occurs. This is achieved by driving the droplets with two streams of pressurized air Ždis-

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persion air and dilution air. through the drying column. In the drying column, the solvent evaporates, and the uranyl nitrate precipitates with decreasing droplet size. Finally, solvent-free particles of uranyl nitrate are obtained. These particles were heated up to approximately 800⬚C in a sequence of three furnaces to convert uranium nitrate into uranium oxide. In order to produce dense particles, the velocity of the calcination process was controlled by adjusting the airflow through the furnace sequence. The hot air stream was then cooled and the uranium oxide particles were collected on a Nucleopore filter at the end of the cooler. The monodisperse aerosol droplets’ diameter population was normally distributed. Whenever two or more droplets undergo mixing along the pathway to reach the target, the plot of the frequency vs. the droplet diameter shows two or more minor additional gaussian peaks in the right tail of the histograms. Assuming that the spherical form is still maintained after mixing, the gaussian maxima relevant to the population of two or three mixed drops are located at 3

3

1.26Žs '2 . and 1.44Žs '3 . times the diameter relevant to the normal distribution of single droplets, respectively. Once the droplets are dried, the size distribution of the particles is expected to represent the droplets’ behavior, provided that the observed departures from the spherical model of the measured diameters and the volume variation due to occasional cavities inside the particles are both normally distributed.

3.2. Morphology and size distribution of the particles

SEM analyses of the uranium oxide particles on the filters showed that the particles were nearly spherical and agglomeration occurred to a very low extent. A SEM image of several uranium oxide particles on a Nucleopore filter is shown in Fig. 2. The elemental analysis by EDX confirmed uranium as the main constituent. Oxygen, or possibly remaining nitrogen, could not be measured

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Fig. 2. SEM image of four monodisperse uranium oxide particles on a Nucleopore filter.

since the window of the EDX detector did not transmit these low energy X-rays. In order to test whether the particles consisted of solid material or were hollow inside, some particles were crushed with the help of the sharp edge of tweezers. In Fig. 3 a SEM image of several particles broken into pieces is shown. It can be clearly observed that the pieces consisted of mainly solid material, except for some small cavities. Since these cavities were very small compared to the rest of the solid particle they were considered negligible. The size distribution of the particles was determined by taking five images at different locations on the sample with the SEM at 800 = magnification. The area contained a few thousand particles in total. They were evaluated with image analysis software ŽZeiss Vision, KS 300. using a program for automated analysis containing a measurement algorithm for the determination of the average diameter of objects. All steps of the program were controlled by observing the images on the screen. This evaluation was done for particles produced from the uranium solutions of the four different reference materials. Fig. 4 shows the particle diameter distribution as obtained for the four uranium standard solutions. Two diameter populations were always present, regardless of the uranium isotopic composition. The data obtained for the particles’ diameter deconvolution for the four different standards are given in Table 1. The first population had a

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Table 1 Significant findings obtained by particles’ diameter deconvolution dataa,b,c CRM code U005 U010 U030 U100

235

U%

0.5 1 3 10

Ž m1 .1

Ž m1 .2

Ž m1 .2 rŽ m1 .1

1.03 0.95 0.94 1.07

1.31 1.15 1.18 1.30

1.27 1.21 1.26 1.21

1 Area % 75.7 83.0 82.1 74.5

a For each normally distributed population Ž1: single particle population; 2: double particle population. the first order statistical moment Ž m1 .1 and Ž m1 . 2 Žexpressed in ␮m. and their ratios are given. The area of the first population in % of total area is also reported. b The area of the second population is the complement to 100 except in the case where a third population was observed. c The area of the third population, observed in two cases, was always lower than 2% of the total area. In these two cases Ž m1 . 3 rŽ m1 .1 was always equal to 1.47.

narrow distribution at approximately 1 ␮m. This diameter agreed very well with the value computed according to Eq. Ž1.. The ratio of the first statistical moment of the two populations remained practically constant at 1.24. This was in agreement with the value computed when the coagulation of two droplets behind the orifice was considered: the first maximum was due to single droplets, the second due to the formation of one particle from two droplets Žtheoretical: double 3 mass ' diameter ratio of '2 s 1.26.. In two cases, Žsee Fig. 4a,d. a third population is observed, relevant to mixing of three droplets, the first statistical moment ratio Ž m1 . 3rŽ m1 .1 being 1.47 3 Žtheoretical: triple mass ' diameter ratio of '3 s 1.44.. From the mean diameter of a single particle Ž1 ␮m. and of the droplets at the nebulizer orifice exit Ž40 ␮m., the following could be drawn: Ži. the initial volume of a droplet Ž6 = 10y5 mm3 . decreased along the desolvation pathway down to 5 = 10y1 0 mm3 Žvolume of a particle.; the probability of mixing particle᎐particle was, therefore, negligible compared with that of the droplets; and Žii. by considering the concentration Ž62.5 mgrl. of uranium in the solution to be nebulized, the uranium content in a single droplet of mean size, and consequently in one particle of 1 ␮m, was only 4 pg. For further analysis of the particles, it had to be noted that, for example, only 0.13 fg of 234 U was present in a single particle prepared from CRMU005. This amount was close to or even below the detection limit of most analytical methods.

3.3. Isotopic composition determined by SIMS analysis For isotopic characterization, the particles were transferred from the filters to high purity carbon planchets. For this purpose, the filter containing the particles was submerged in ethanol and the particles were suspended from the filter into ethanol using an ultra-sonic bath. The same procedure was applied to each of the four batches with different isotopic composition. Then, for each batch, a fraction of the ethanol suspension was pipetted onto a single planchet and the ethanol was evaporated. By SEM it was confirmed that at least 100 particles were present at the center of each planchet. The four different planchets were measured with SIMS. In Fig. 5, a typical secondary-ion

Fig. 3. SEM image of crushed monodisperse uranium oxide particles on a Nucleopore filter.

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Fig. 4. Size distribution of the monodisperse uranium oxide particles. Ža. CRM U005; Žb. CRM U001; Žc. CRM U003; and Žd. CRM U010.

image of particles is shown. This image, observed with the mass spectrometer set to mass Ms 238, was obtained by scanning the primary beam over an area of 30 = 30 ␮m2 in size. Secondary ion beam intensities for the atomic masses 234, 235, 236, 238 and 239 were measured, and uranium isotopic ratios 234r238, 235r238 and 236r238 were calculated from the pertinent intensities. Ten different particles were analyzed for each isotopic composition with nine replicates for each particle. The number of determinations used for the analysis of these particles was chosen according to the stability of the signals of the uranium isotopes and the calculated uncertainty. Fig. 6 shows the mean 234r238 and 235r238 isotopic ratio values and the error bars relevant to nine measurements for the individual particles and for each isotopic composition. The certified values were plotted as horizontal lines for comparison. In the case of the 234r238 ratio, the error was larger due to the low signal intensity of 234 U, close to the detection limit of the method. The signal on mass 239 Ždue only to the formation of 238 UHq molecules since no plutonium

was present in the starting material. was used to correct the signal on mass 236 for the contribu-

Fig. 5. SIMS ion mapping image on the mass 238 of four monodisperse uranium oxide particles. The ␮-marker shown corresponds to 0.8 ␮m.

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tion of 235 UHq. From Fig. 7 the mean 236r238 isotopic ratios for the individual particles before Ža. and after Žb., correcting for the contribution of 235 UHq, can be compared. It can be clearly observed that the agreement with the certified values after the correction was improved, except in the case of the CRMU005, depleted in 235 U. In this case, the contribution of 235 UHq was negligible and, therefore, no correction needed to be applied. The mean isotope ratios derived from the measurement of 10 individual particles for each isotopic composition are given in Table 2, together with the certified values. The % R.S.D. Žrelative standard deviation. together with % bias measured value y certified value s = 100 valcertified value ues are also reported. As can be seen from these values, the SIMS results were in good agreement with the certified

ž

/

values. For 235 U, a typical precision lower than 1% was found and an accuracy better than 0.4% was achieved. For the minor isotopes, the precision greatly improved with increasing concentration. This effect was most evident for 234 U. Here, in the case of the U005 and U010, a bias of 20% and 7% from the certified value was also observed, which was due to the low signal intensities. In fact, the detection limit Ž3␴ . of the SIMS procedure, was reached for U005 Ž% R.S.D.: experimental s 36; theoretical calculated for the detection limit of 3␴ s 47.. The detection limit of the isotopic ratio was controlled by the quantity of the minor isotope in the particle. In the case of U005, the mass of 234 U in the particle was 0.13 fg. Applying a correction for 235 UHq formation, a typical accuracy of 5%, 3% and 0.6% was obtained for 236 U in U010, U030 and U100, re-

Fig. 6. Experimental values with error bars for the Ža. 234r238 and Žb. 235r238 ratios obtained from SIMS analysis of monodisperse uranium oxide particles with different isotopic composition. The horizontal line represents the certified values.

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Fig. 7. Experimental values with error bars for the ratio 236 Ur 238 U Ža. without correction for 235 UHq formation and Žb. with correction for 235 UHq formation, obtained from SIMS analysis of monodisperse uranium oxide particles with different isotopic composition. The horizontal line represents the certified values.

Table 2 SIMS isotopic ratios obtained for uranium oxide particles from four different certified standard reference uranium materials: CRM U005, U010, U030, and U100 a,b U005

U010

U030

U100

234

Ur238 U

Certified value Measured value Precision ŽR.S.D.%. Accuracy ŽBias%.

2.19 E-5 Ž2.62" 0.95. E-5 36.4 19.41

5.46 E-5 Ž5.85" 0.57. E-5 9.7 6.92

1.96 E-4 Ž1.96" 0.07. E-4 3.8 y0.23

7.54 E-4 Ž7.54" 0.12. E-4 1.5 0.13

235

Ur238 U

Certified value Measured value Precision ŽR.S.D.%. Accuracy ŽBias%.

4.92 E-3 Ž4.90" 0.05. E-3 1.1 y0.39

1.01 E-2 Ž1.02" 0.01. E-2 1.0 0.15

3.15 E-2 Ž3.15" 0.01. E-2 0.3 0.13

1.14 E-1 Ž1.14" 0.01. E-1 0.8 0.06

236

Ur238 U

Certified value Measured value Precision ŽR.S.D.%. Accuracy ŽBias%.

4.68 E-5 Ž4.34" 0.32. E-5 22.0 y7.3

6.87 E-5 Ž6.49" 0.51. E-5 7.9 y5.53

2.10 E-4 Ž2.17" 0.20. E-4 9.1 3.09

4.23 E-4 Ž4.25" 0.27. E-4 6.2 0.62

a b

The certified and measured values for the 234r238, 235r238 and 236r238 ratios together with % bias and % R.S.D. are given. The 236r238 ratio was corrected for the contribution of 235 UHq using the signal on mass 239 Ž 238 UHq ., except for U005.

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spectively. In the case of the sample U005, an accuracy of 7% was obtained without any correction. Note that, owing to the great number of replicates per each isotopic composition, the mean isotopic ratio found was a good estimation of the statistical value with a narrow confidence interval. This means that the bias estimated represents very well the size of the systematic errors. As for 236 U, because a correction was applied for 235 UHq formation, a larger bias than expected for each isotopic ratio was found.

SIMS and other methods for particle analysis such as, for example, nuclear track methods. They are used for optimization of the sample preparation procedures of the routine analysis of swipe samples for safeguards. Uranium particles up to 90% 235 U enrichment are also foreseen and particles of smaller sizes will be produced. A plan to have a similar setup for the production of plutonium oxide particles and particles consisting of a mixture of uranium and thorium or uranium and plutonium, which are of interest in nuclear safeguards, is also under investigation.

4. Conclusions and outlook

Acknowledgements

Control particles, consisting of isotopically certified monodisperse uranium oxide microspheres of 1 ␮m in diameter, have been produced. The study of the morphology of these particles has demonstrated that they consisted of solid material. Their diameter distribution shows the presence of two populations mainly; the relevant particle mean diameter ratio was found to be 1.24 Žtheoretical s 1.26.. The two populations were attributed to single and double droplets with a volume of 3.3 = 10y5 and 6.6 = 10y5 mm3, respectively. The isotopic compositions have been verified by SIMS, obtaining results with a typical accuracy better than 0.4% for 235 U and a few percent for the minor isotopes. In this condition of high reproducibility and accuracy, the 238 U mass contained in a particle of mean diameter of 1 ␮m, was estimated to be of the order of 4 pg, and the lowest abundant isotope mass in the order of tenths of fg. A 3␴ detection limit was reached for the isotopic mass ratio when the mass of the less abundant isotope in the particle was 0.13 fg and 4 pg for the isotope 238 U. For the correct determination of the 236 U concentration, the 236 Ur 238 U ratio was corrected for the contribution of 235 UHq using the signal on mass Ms 239 Ž 238 UHq ., except for the particles with isotopically depleted uranium. This correction greatly improved the accuracy of the 236r238 ratio. The particles can be transferred to various substrates and then used for the calibration of SEM,

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