Analysis of U-particles by fission track-etch method and quadrupole ICP-MS

Track-etch method was developed at VTT to identify uranium containing particles of interest from swipe material. The extracted uranium containing particles were analysed with ICP-MS after track-etch analysis. For extraction both ashing and ultrasonification were used. The particles were collected in Collodion and the mixture was spread on Makrofol detectors, where it formed a thin film. Irradiation was carried out in Triga Mk II reactor in Otaniemi in the neutron flux of 1.2 x 1012 ncm-2s-1 and irradiation time was 1 h. After irradiation Collodion film was separated from the Makrofol in hot water after marking the specimens. The Makrofols were etched in 6,5 M KOH for 15 min and glued to the microscope slides. The fission tracks were examined under a microscope and the Collodion piece with the uranium particle was cut out from the Collodion film with a razor knife. The Collodion pieces were picked up into 0.5 ml polyethylene vials. The vial was filled with acetone to dissolve the Collodion. Acetone was evaporated so much that 250 μl of 5% HNO3 could be added. The rest of the acetone was evaporated under an infrared lamp. After that samples were analysed with quadrupole ICP-MS (VG Plasma Quad 2+). The small amount of sample solution with low uranium concentration makes the task specially demanding. The use of microconcentric nebulizer (Cetac MCN-100) is obligatory. By using the natural uptake the sample flow rate is 50 μl/min. In this work sample volume 250 μl was enough for 3 parallel measurements for each sample. The track-etch prints of one particle and the particle in the Collodion film shown in the Figure 2. The mass spectrum of a 3 pg particle of 20% enriched uranium in 250 μl is shown in the Figure 3. The three parallel measurements of 3 pg particle of 20 % enriched uranium (NIST) gave isotopic ratio 0.22 ± 0.04 and a bigger particle, 7.5 pg of the same enrichment give 0.28 ± 0.06 isotope ratio 235/238 when given value is 0.25125. The analysed 3 % enriched particles were bigger, 21 and 38 pg and isotope ratios calculated from three parallel measurements were 0.016 ± 0.006 and 0.028 ± 0.001 and given value 0.03143. The small amount of counts in 235 mass-peak is the limiting factor in the analysis. This procedure was used for testing the separation and analysis of uranium containing particles from test swipes, the Track-etch method for extracting uranium containing particles from swipe material and ICP-MS for analysis of single particles. The theoretical size of the analysed particles varied from <1 μm (3pg) to 2 μm (38 pg). The low background in the mass peak 235, only <5 counts/s, makes it possible to separate rather small particles with different enrichment from each others. For natural uranium, the total amount of uranium in the analysis should be at least 200-250 pg for the isotope ratio measurement. According to these measurements the precision of ± 10 % could be possible in that case. The theoretical diameter of the particle is about 4 μm. With the precision reached with quadrupole ICP-MS it is possible to separate particles of NU, LEU and HU. The need for more experience in particle separation is obvious. The NU contamination in the samples can be avoided by performing the sample preparation and dissolution of separated particles in a clean room. Also the microscopic work including cutting of the Collodion film should be done in a cleaner environment than in normal laboratory. The results are anyway encouraging and when more particles from each sample were analysed it is possible that the isotopic ratios would be nearer the expected ones