Feasibility Study of Isotope Ratio Analysis of Individual Uranium-Plutonium Mixed Oxide Particles with SIMS and ICP-MS

: Isotope ratio analysis of nuclear materials in individual particles is of great importance for nuclear safeguards. Although secondary ion mass spectrometry (SIMS) and thermal ionization mass spectrometry (TIMS) are utilized for the analysis of individual uranium particles, few studies were conducted for the analysis of individual uranium-plutonium mixed oxide particles. In this study, we applied SIMS and inductively coupled plasma mass spectrometry (ICP-MS) to the isotope ratio analysis of individual U-Pu mixed oxide particles. In the analysis of individual U-Pu particles prepared from mixed solution of uranium and plutonium standard reference materials, accurate 235 U/ 238 U, 240 Pu/ 239 Pu and 242 Pu/ 239 Pu isotope ratios were obtained with both methods. However, accurate analysis of 241 Pu/ 239 Pu isotope ratio was impossible, due to the interference of the 241 Am peak to the 241 Pu peak. In addition, it was indicated that the interference of the 238 UH peak to the 239 Pu peak has a possibility to prevent accurate analysis of plutonium isotope ratios. These problems would be avoided by a combination of ICP-MS and chemical separation of uranium, plutonium and americium in individual U-Pu particles.


Introduction
Mass spectrometry has a crucial role in nuclear safeguards. Isotope ratio analysis of individual uranium particles in environmental samples taken at nuclear facilities is performed to verify the absence of undeclared nuclear activities related to uranium enrichment and production of nuclear weapons. [1][2] For this purpose, two kinds of analytical techniques are mainly used, i.e., secondary ion mass spectrometry (SIMS) and thermal ionization mass spectrometry (TIMS) combined with a fission track method. [3][4][5][6][7][8][9][10][11][12] The SIMS allows us to analyze samples easily and rapidly. A main drawback in SIMS is molecular ion interferences by other elements in other particles. The interferences give inaccurate analytical results, especially for uranium minor isotopes ( 234 U and 236 U). In order to overcome this problem, a large geometry SIMS instrument with high mass resolution was used to separate peaks of uranium ions and molecular ions. 7 As the result, accurate isotope ratio results were acquired in the analysis of swipe samples taken at nuclear facilities by using the instrument. As an alternative method, we developed a technique by a combination of particle manipulation under scanning electron microscope (SEM) and isotope ratio analysis with SIMS. 5 The particle manipulation allowed us to transfer only uranium particles for subsequent SIMS analysis, giving accurate isotope ratio results.
As well as uranium, plutonium is increasingly used as a uranium-plutonium mixed oxide fuel in commercial nuclear reactors. Although undeclared nuclear activities using plutonium should also be revealed, only a few studies were conducted to analyze isotope ratios of individual plutonium and uraniumplutonium mixed oxide particles. [13][14][15] One of the reasons is the isobaric interference of the 241 Am peak to the 241 Pu peak in mass spectrometry. Americium-241 is formed by the decay of 241 Pu with the half life of 14.325 years, 16 meaning that 241 Am and 241 Pu usually coexist in a single plutonium or uranium-plutonium mixed oxide particle. In a previous study, 14 we developed an experimental technique for individual plutonium particles, which included particle manipulation, dissolution and chemical separation of plutonium and americium for subsequent isotope ratio analysis with inductively coupled plasma mass spectrometry (ICP-MS). Consequently, we could analyze isotope ratios accurately for individual plutonium particles.
In the present work, isotope ratio analysis of individual uranium-plutonium mixed oxide particles was performed by using SIMS and ICP-MS, and the applicability of these methods was discussed.

Samples
Uranium-plutonium mixed oxide particles were prepared from *Reprint requests to Dr. Fumitaka Esaka E-mail: esaka.fumitaka@jaea.go.jp mixed solution of standard reference materials (SRM 947, National Bureau of Standards (NBS), USA and CRM U-500, New Brunswick Laboratory (NBL), USA). Purification for the SRM 947 plutonium solution was carried out to remove americium by using an anion exchange method on July 14, 2008. The 10 µL of the mixed solution was pipetted onto a quartz glass and heated at 800 o C to produce U-Pu mixed oxide particles. Then, the obtained particles were suspended in n-dodecane and pipetted onto a Si planchet with a diameter of 25 mm (Nihon Exceed Co. Ltd., Japan).

Sample preparation for SIMS
The Si planchet containing U-Pu particles and a glassy carbon planchet (25 mmφ, Hitachi Chemical Co. Ltd., Japan) were introduced into the main chamber of a scanning electron microscope (JSM-6700F, Jeol Co. Ltd., Japan). Each U-Pu particle was picked up with a glass needle attached to a manipulator and transferred onto the glassy carbon planchet. The glass needles were fabricated from glass rods with a diameter of 1 mm using a micropipette puller (PC-10, Narishige Co. Ltd., Japan) and thinly coated with gold.

Sample preparation for ICP-MS
Si chips with a dimension of 5 × 5 mm (Semitec Co. Ltd., Japan) were cleaned, in sequence, with 40% HNO 3 , 2% HNO 3 (TAMA-pure, Tama Chemicals Co. Ltd., Japan) and deionized Milli-Q water (resistivity : 18 MΩ·cm). The Si chips and the Si planchet containing U-Pu particles were introduced into a chamber of the SEM instrument. Individual U-Pu particles on the Si planchet were manipulated with the glass needle attached to the manipulator and transferred onto the center of each Si chip. Teflon bottles with a volume of 8 mL were cleaned with 4% HNO 3 and deionized Milli-Q water. Each Si chip containing a single U-Pu particle was put into the bottle and 2 mL of deionized Milli-Q water was added. After ultrasoneration for 5 min., the Si chip was removed and the water was evaporated. The 2 mL of HNO 3 solution was then added to the bottle and evaporated to dryness to dissolve the particle. This dissolution process was repeated twice. The sample was finally adjusted to 5% HNO 3 solution.

Instrumentation
A secondary ion mass spectrometer (IMS-6f, Cameca, France) was used for measuring isotope ratios in individual U-Pu particles. In SIMS analysis, an O 2 + focused beam of 15 keV with a current of 4-15 nA was rastered over a 50 × 50 µm area. Positive secondary ions emitted from a particle were accelerated through 5 keV. Transfer optics of 150 µm and a field aperture of 1,800 µm were used. An energy slit was adjusted for a band pass of 50 eV. The mass resolution was set at 300. Secondary ions were counted in an automatic peak-jumping mode. An electron multiplier was operated in an ion counting mode with a dead time of 27 ns.
A high resolution ICP-MS model ELEMENT 1 (Thermo Fisher Scientific, USA), having an ICP ion source with Guard Electrode and a double-focusing mass spectrometer, was used for isotope ratios analysis. The instrument was operated in electric scanning (E-scan) modes with a mass resolution (M/∆M) of 300. An Apex-Q desolvation system (Elemental Scientific Inc., USA) was used for sample introduction to the ICP-MS instrument. This system consisted of a heated cyclonic spray chamber, a Peltier cooled condenser and an ACM Naflon fluoropolymer membrane desolvation module, allowing high efficient sample introduction. The operating condition of the ICP-MS instrument is listed in Table 1. The mass bias factor assuming the following equation was determined using 5.14 pg/mL NBS SRM 947 standard solution: where R t is the certified isotope ratio, R m is the measured isotope ratio, ∆M is a difference in mass and C is a mass discrimination factor. Fig. 1 shows the SEM image and the energy dispersive X-ray (EDX) spectrum of a U-Pu particle on a Si planchet. In the EDX spectrum, U-Mα  are due to surface contaminants and the planchet material, respectively. Fig. 2 displays the mass spectrum of a U-Pu particle measured with SIMS. The peaks related to uranium ions were observed on the masses of 234, 235, 236, 238 and 239. The 235 UH peak overlapped with the 236 U peak and the 238 U peak was interfered by the 238 Pu peak. The peaks related to plutonium ions were observed on the masses of 238, 239, 240, 241 and 242. The 241 Pu peak was overlapped with the 241 Am peak. The formation of 241 Am is due to the decay of 241 Pu. The 239 Pu peak was interfered by the 238 UH peak. According to the previous studies, 6,15 the hydride formation rate ( 238 UH/ 238 U) in SIMS analysis was in the range from 3 × 10 −3 to 4 × 10 −4 . In the analysis of uranium particles, the hydride formation rate is determined for each particle by measuring 238 UH/ 238 U ratio. However, this can not be applied to the analysis of U-Pu particles because of the presence of the 239 Pu peak. Thus, it is difficult to estimate the contribution of the 238 UH peak to the 239 Pu peak in SIMS analysis.

SIMS analysis
For two individual U-Pu mixed oxide particles, the isotope ratios of 235 U/ 238 U, 240 Pu/ 239 Pu, 241 Pu/ 239 Pu and 242 Pu/ 239 Pu were measured and are summarized in Table 2. The isotope ratios of 240 Pu/ 239 Pu, and 242 Pu/ 239 Pu were in good agreement with the certified values, respectively. The isotope ratio of 235 U/ 238 U measured for the particle No. 2 was slightly lower than the certified value, while that measured for the particle No.1 corresponded to the certified value. The isotope ratios of 241 Pu/ 239 Pu were higher than the certified value, which is due to the contribution of the 241 Am peak to the 241 Pu peak. In SIMS analysis, the separation of 241 Pu and 241 Am peaks is impossible, even if high mass resolution is used in the measurement. This is a limitation of SIMS for isotope ratio analysis of individual U-Pu particles.
As shown in Table 2, accurate 240 Pu/ 239 Pu isotope ratios were obtained, though no correction for the interference of the 238 UH peak to the 239 Pu peak was made. If the particle has higher 238 U/ 239 Pu ratio, the 239 Pu peak would be strongly interfered by the 238 UH peak, which makes it impossible to determine plutonium isotope ratios accurately. This is another limitation of SIMS for isotope ratio analysis of U-Pu particles.

ICP-MS analysis
The application of ICP-MS to the isotope ratios analysis of individual U-Pu particles was also examined in this study. The isotope ratios of 235 U/ 238 U, 240 Pu/ 239 Pu, 241 Pu/ 239 Pu and 242 Pu/ 239 Pu were measured and are summarized in Table 3. The isotope ratios of 241 Pu/ 239 Pu were higher than the certified value. This is due to the contribution of the 241 Am peak to the 241 Pu peak. The isotope ratios of 240 Pu/ 239 Pu were in good agreement with the certified value. As well as SIMS analysis, the 239 Pu peak is interfered by the 238 UH peak in ICP-MS analysis. In a previous study, 6 the hydride formation rate ( 238 UH/ 238 U) in ICP-MS analysis was estimated to be    about 5 × 10 −5 , which is at least one order of magnitude lower than that in SIMS analysis. Therefore, even if the particle has higher 238 U/ 239 Pu ratio, the interference of the 238 UH peak to the 239 Pu peak may be avoided to a certain extent. In contrast to SIMS, ICP-MS has a potential to avoid these spectral interferences caused by uranium hydride and americium ions. In a previous study, we performed chemical separation of plutonium and americium in individual plutonium particles and successfully determined the 241 Pu/ 239 Pu isotope ratios with ICP-MS. 14 The combination of chemical separation and ICP-MS analysis would overcome these problems and enable us to perform accurate isotope ratio analysis for individual U-Pu particles, though further study is necessary.

Conclusions
In the present work, analytical techniques using SIMS and ICP-MS were applied to the isotope ratio analysis of individual U-Pu mixed oxide particles prepared from the mixed solution of SRM 947 and CRM U-500 standard reference materials. The isotope ratios of 235 U/ 238 U, 240 Pu/ 239 Pu and 242 Pu/ 239 Pu were accurately determined with both methods. However, accurate analysis of 241 Pu/ 239 Pu isotope ratio was impossible in both techniques, due to the interference of the 241 Am peak to the 241 Pu peak. In order to overcome this problem, ICP-MS would be appropriate because the method can be combined with chemical separation of uranium, plutonium and americium in individual U-Pu particles.