CHARACTERIZATION OF Pu-CONTAINING PARTICLES BY X-RAY MICROFLUORESCENCE

Transcription

1 Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol CHARACTERIZATION OF Pu-CONTAINING PARTICLES BY X-RAY MICROFLUORESCENCE Marco Mattiuzzi, Andrzej Markowicz, Piero Danesi IAEA Laboratories Seibersdorf A-2444 Seibersdorf, Austria Abstract Pu-containing particles were collected in the Mururoa Atoll at "Safety Trial Test" sites by staff of the International Atomic Energy Agency (IAEA) and other experts. Several particles in the diameter range of µm were first characterized by optical microscopy followed by qualitative and semi-quantitative analysis performed by X-ray microfluorescence technique (µxrf). The elemental distributions for selected elements are also presented. The composition varies greatly not only from particle to particle but also within the same particle indicating a high degree of heterogeneity. The concentrations of Pu range from a few percent to some ppm while other actinide elements, like U and Np, are present at lower concentrations. Other elements, mainly metallic, were also identified at the concentration level of ppm. The elemental maps presented in the paper show the heterogeneous distributions of the elements and correlations between different elemental distributions are observed. The semi-quantitative results presented can be combined with a statistical classification procedure to identify the emitting sources when a limited number of samples are available for analysis. Introduction In June 1995 the IAEA was requested to assess the radiological consequences of the nuclear experiments carried out in the atolls of Mururoa and Fangataufa in French Polynesia. The results of the overall study have been reported in detail in reference [1]; the results of the terrestrial campaign have been described in vol. 1. In this work more detailed information on Pu containing hot particles collected at the safety trial test sites of the Mururoa atoll (the Colette region ) are reported. A safety trial is a test where the nuclear device core is detonated under simulated faulty conditions and little or no fission process takes place. The core is destroyed by the conventional explosive with the production of fine particles and fragments containing plutonium, which are widely dispersed if the test is not confined. Five out of 15 safety trials were conducted in the atmosphere between 1966 and 1974 at the northern tip of the Mururoa atoll. In these areas the soil consists of exposed bedrock, covered in some places by a thin layer of sand or coral debris. The areas are occasionally flooded by seawater. After these five tests the site was partially decontaminated with the aim of reducing 239 Pu activity to levels below 10 6 Bq/m 2, averaged over a 20x20 m 2 area. However, local higher Pu activities could still be found in the form of hot particles. Several sub-mm particles were isolated at the IAEA Seibersdorf Laboratories from samples of fine coral debris and loose coral pieces. Two single hot particles were identified locally and collected at the test sites. ten out of 20 hot particles separated from the collected materials were next characterized by using a µxrf technique with a spatial resolution of 15 µm. Data on the 10 particles are reported in Table 1.

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol The characterization of individual radioactive particles is of great interest due to their importance in radiological studies as well as in the identification of emitting sources. The health risk associated to the presence of radioactive particles is related to the particle size and its bioavailabity. Information on the primary event that generated the particles such as fuel type, container alloy type and marine or terrestrial environment, can also be extracted. Due to their small sizes the identification, isolation and handling of these particles is rather difficult and usually only a limited number of samples can be available. Part. Code Size 239 Pu 241 Am 2Q IMG Part. Code Size 239 Pu 241 Am 2Q IMG x N Y x N Y x Y N x N Y 5 s/b x2/ Y N x N Y x Y Y x Y N x Y Y x Y N Table 1 Sub mm hot-particles from Mururoa Atoll isolated at the IAEA Laboratories, Seibersdorf. Columns 1 and 2 include the particle number and the sample code, see [1] for details. In column 3 approximate size (x100 µm) is shown. Columns 4 and 5 report the activities in kbq of respective radionuclides. In the sixth and seventh columns the type of analysis is reported (2Q = Qualit/Quanti-tative, IMG = elemental distribution, Y=yes, N=no). #4 #5s #5b #6 #8 #17 #18 Figure 1 Optical microscopic images of selected hot particles from the Mururoa Atoll. A different scale for displaying the particle bitmap images is used. Samples and measurements The particles were identified by a NaI probe after having distributed the collected material (coral soil) on a tray as a thin layer. The particles were then collected on the sticky side of an adhesive tape. The samples were divided into two sets, including single or multiple particles, respectively. The single particles were covered with a Mylar foil preventing the samples from falling off and next mounted on a sample holder. Some optical microscopy images acquired with a x5 magnification are reported in Figure 1. It is seen that particles #4, #8 and #18 have a relatively smooth surface and seem to be solid entities whereas particles #5, #6 and #17 are rough and morphologically heterogeneous. The µxrf measurements were performed by using the set-up available at the Seibersdorf IAEA Laboratories [2]. A Si-Li detector with 165 ev resolution (FWHM) at 5.9 kev was used to detect the characteristic X-rays. The beam detector angle was set at 35 o and the sample-detector distance was 15 mm. The X-ray source consisted of a Mo-anode X-ray tube operated at 50 kv and 30 ma combined with a 10 µm focusing monocapillary. The beam size was 15 µm at a sample-end capillary distance of approximately 2 mm. In order to obtain quali/quanti-tative information (2Q) and accurate imaging (IMG) of the elemental distribution two different measurements were performed.

4 Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol First, 121 pixels in an 11x11 grid were scanned with 600 s measuring time per pixel. The spectra were saved for the individual pixels for off-line analysis. By running the AXIL software package the characteristic X-ray intensities for various elements were calculated on a pixel basis. The following two criteria were then applied to eliminate the net intensities which were statistically insignificant: CTS < 3 BCK and Err CTS > 0.3CTS, where CTS and BCK are the intensities (in counts) of the characteristic X-ray peak and the background respectively, Err CTS is the uncertainty of the net intensity CTS associated with counting statistics and fitting procedure. The rectangular scan area containing the particle was determined by a pre-scan run. The pixels were then defined to be on the particle if more than two elements were identified. To generate high resolution elemental distributions maps up to128x128 pixels (depending on particle size) were investigated with a measuring time per pixel as low as 1 s. During these measurements the characteristic X-ray intensities were mapped, by using the SPECTOR software recently developed for the elemental micro X-ray imaging [3]. The software stores on-line the integral counts I ROI for the selected spectral regions (ROI). For each ROI a 2D (bit-) map is sequentially updated with I ROI for different X and Y coordinate on the sample. The map color scale is adjusted for each ROI to the maximum I ROI accordingly. Part. Quanti Quali 4 Cl-Ca-Fe-Ga-Sr-Pb-U-Np-Pu Ti-Cu-Zn-Am-Ni 5s Cl-Ca-Ti-Mn-Fe-Ni-Cu-Zn-Ga-Sr-Pb-U-Np-Pu Am-Pt 5b Cl-Ca-Ti-Cr-Mn-Fe-Ni-Cu-Zn-Ga-Sr-Pb-U-Np-Pu Si-Am-Th 6 Ca-Ti-Cr-Mn-Fe-Co-Ni-Cu-Zn-Ga-Sr-Pt-Pb-U-Np-Pu 8 Ca-Fe-Sr-Ra-U-Np-Pu Ba-Bi-Po-Pb 17 Cl-Ca-Cr-Mn-Fe-Ni-Zn-Ga-U-Pu Np-Th-Pt-Pb 18 Cl-Ca-Cr-Mn-Fe-Co-Ni-Ga-Br-Sr-U-Np-Pu Zn-Ti-Cu-Pb Table 2 Elements identified in at least one pixel for the particles specified in first column (see text for details) Qualitative & (semi-) quantitative results The results of the elemental identification for the elements with atomic number Z > 12 are reported in Table 2. The Quanti and Quali columns include all the elements that were identified in at least one pixel on the particle and for which quantitative and qualitative results were obtained. The calibration for the quantitative analysis, for both K and L characteristic lines, was performed by measuring certified pure element standards (9 pixels measured, 60 s measuring time per pixels). The sensitivity factors were determined for each element similarly to conventional XRF analysis. A usual correction for the matrix absorption effect (based on the geometry of the measurements, excitation energy and tabulated mass absorption coefficients) for the thick calibration samples was applied. The average sensitivity S avg was then calculated for each calibration element, and a relationship between S avg and the atomic number Z was established to perform quantitative analysis for other elements which are not included in the calibration process. The X-ray intensities I Z for the hot particles, stored on a pixel basis, were converted into the concentration C Z by using CZ = IZ FZ Savg (Z) where F Z is the absorption correction factor calculated by using AXIL tools with the assumption that the particle is a thick sample consisting of a uniform matrix of CaCO 3 (98%) + Fe(1%) + Pu(1%). Corrections for sample roughness and sample geometry were not taken into account. The quantitative results are summarized in Table 3.

5 Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol Part. El. Avg. Max. Min. Part. El. Avg. Max. Min. 4 Cl 1E+1 2E+1 3E+0 17 Zn 7E-1 6E+0 2E-1 5s Cl 9E+0 3E+1 2E+0 5s Zn 5E-1 3E+0 3E-3 17 Cl 6E+0 1E+1 3E+0 5b Zn 3E-1 2E+0 3E-3 5b Cl 5E+0 2E+1 2E+0 6 Zn 1E-1 4E-1 6E-2 18 Cl 2E-1 4E-1 8E-2 4 Ga 1E-1 3E-1 4E-3 18 Ca 1E+1 5E+1 1E-1 5s Ga 5E-2 2E-1 4E-3 4 Ca 1E+1 5E+1 4E-1 17 Ga 3E-2 1E-1 3E-3 5b Ca 1E+1 6E+1 5E-1 5b Ga 2E-2 7E-2 3E-3 5s Ca 1E+1 3E+1 5E-1 18 Ga 2E-2 6E-2 8E-3 6 Ca 9E+0 5E+1 6E-1 6 Ga 2E-2 4E-2 2E-3 17 Ca 7E+0 2E+1 5E-1 18 Br 2E-2 3E-2 8E-3 8 Ca 2E+0 4E+0 3E-1 18 Sr 5E+0 1E+1 7E-2 5s Ti 4E-1 7E-1 1E-1 4 Sr 1E+0 3E+0 1E-1 6 Ti 4E-1 1E+0 1E-1 8 Sr 5E-1 4E+0 1E-2 5b Ti 3E-1 5E-1 1E-1 5s Sr 2E-1 2E+0 4E-3 18 Cr 4E-1 2E+0 1E-1 6 Sr 2E-1 1E+0 1E-2 17 Cr 4E-1 9E-1 6E-2 5b Sr 8E-2 5E-1 4E-3 5b Cr 2E-1 6E-1 3E-2 4 Pb 2E-2 3E-2 6E-3 6 Cr 1E-1 3E-1 4E-2 5s Pb 4E-3 1E-2 2E-3 18 Mn 2E-1 5E-1 1E-1 5b Pb 3E-3 5E-3 1E-3 17 Mn 1E-1 3E-1 2E-2 8 Ra 1E-2 7E-2 2E-3 6 Mn 9E-2 3E-1 3E-2 17 U 7E-2 2E-1 2E-3 5s Mn 8E-2 2E-1 3E-2 6 U 5E-2 3E-1 1E-3 5b Mn 8E-2 2E-1 4E-2 18 U 3E-2 7E-2 8E-3 18 Fe 5E+0 3E+1 6E-2 4 U 2E-2 3E-2 3E-3 6 Fe 4E+0 4E+1 2E-2 5s U 2E-2 4E-2 2E-3 17 Fe 2E+0 1E+1 4E-2 5b U 1E-2 2E-2 2E-3 5s Fe 2E+0 5E+0 9E-3 8 U 9E-3 2E-2 2E-3 5b Fe 1E+0 5E+0 1E-2 4 Np 6E-2 1E-1 4E-3 4 Fe 4E-1 9E-1 6E-2 5s Np 2E-2 1E-1 4E-3 8 Fe 3E-2 7E-2 2E-2 18 Np 2E-2 4E-2 6E-3 18 Co 6E-2 1E-1 4E-2 8 Np 2E-2 1E-1 2E-3 6 Co 4E-2 1E-1 1E-2 5b Np 1E-2 4E-2 4E-3 18 Ni 1E-1 4E-1 2E-2 6 Np 5E-3 9E-3 2E-3 17 Ni 7E-2 2E-1 6E-3 4 Pu 1E+0 4E+0 9E-3 5s Ni 4E-2 9E-2 7E-3 5s Pu 4E-1 3E+0 7E-3 5b Ni 4E-2 1E-1 5E-3 17 Pu 4E-1 2E+0 5E-3 6 Ni 4E-2 1E-1 5E-3 8 Pu 3E-1 2E+0 2E-3 6 Cu 2E-1 7E-1 1E-1 18 Pu 2E-1 1E+0 6E-3 5s Cu 6E-2 2E-1 4E-3 5b Pu 1E-1 1E+0 5E-3 5b Cu 5E-2 2E-1 3E-3 6 Pu 1E-1 4E-1 1E-3 Table 3 Average, maximum and minimum concentrations in % (columns Avg., Max., Min ) for different elements ( El ) in the different particles Figure 2 Elemental distributions of selected elements for particle #6. The scan area corresponds to 1.4x1.4 mm 2 with a 128x128 pixel resolution.

6 Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol The concentration (Avg.) of an element was calculated as the average value of the results obtained for all the pixels where that element was identified. The maximum and minimum concentrations for the individual pixels are also reported. Due to the uncertainties in the calibration, matrix composition and the absorption correction factor for the real (non-flat) sample, the concentrations given in Table 3 can vary within a factor of 3. Figure 3 Elemental distributions for selected elements in particle #2. The scan area corresponds to 1.1x1.1 mm 2 with 75x75 pixel resolution. Imaging of the elemental distributions At microscopic scale the local composition of these particles varies due to their intrinsic heterogeneity. Scanning the sample with a microbeam results in the elemental distributions represented by the plots of the intensities of the characteristic X-rays as a function of the sample coordinate. Based on the elemental distributions the correlation between the elements can also be identified. Some examples are given in Figure 2 where selected elements are mapped and the correlation between them can be seen, as in the case of Ca-Sr, Cr-Ni or Zn-U-Pu. Other interesting features can be observed for Fe (highly concentrated in one spot), Cu (mainly coming as tiny clusters) or Pb (widely spread all over the particle). Similar features are seen in Figure 3 where selected elemental distributions for particle #2 are presented. Zn is present in one small spot only, U and Pu have totally different distributions (U mainly concentrated in one location at the bottom of the particle with Pu distributed rather homogeneously in the whole particle). It is worth mentioning that the Geometry Figure 4 Examples of Absorption, Matrix variation and Geometry patterns in sub-mm particle scanning (see text for details). different effects related to absorption, matrix variation and geometry influence the elemental images shown in Figs 2 and 3. Accounting for these effects would result in elemental distributions that are closer to the actual distributions and would provide a possibility to perform a more accurate quantitative analysis. The absorption effect can be accounted for by plotting the intensity ratio for the characteristic X-ray lines of the same element. This

7 Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol effect can be seen in Figure 4 (Abs) where the ratio of Pu-L β / Pu-M α is presented; in the absorption pattern, according to the color scale shown on the right, the places indicated by dark blue are characterized by the weakest absorption. The matrix variation effect is shown in Figure 4 (Matr.) where the coherently scattered primary photon beam is mapped (since the coherent scattering depends strongly on the atomic number, matrix variations are clearly seen). The geometry effect is related to the geometry of the µxrf set-up. For non-flat samples the characteristic radiation is locally self-shadowed by the sample. In Figure 4 (Geometry) the effect for the rather uniform (in terms of matrix) particle #8 is shown. The left part of the Pu-M α distribution is missing as the detector is positioned at right angles relative to the beam. The estimation of these (interrelated) effects would give the possibility to obtain more accurate elemental concentrations and their distributions. Discussion and Conclusions The characterization of the sub-mm particles was performed by using a µxrf set-up. Several particles were analyzed and both qualitative and (semi-) quantitative results were obtained for many elements (up to 18). Based on the results some information on: i) the environment where the particles were collected (marine, presence of Cl, Ca and Sr at % concentration level), ii) the composition of metal container used for exploding the device (presence of the elements of the container alloy such as Cr, Mn, Fe, Ni, Cu and Zn at concentration level below %), and iii) the original test material (Pu, U and Np at the concentration level from % down to some hundreds ppm in case of Pu and at lower concentration level for other elements), were obtained. The particle s composition varied very much not only for the different samples but also within the same sample; for some elements in an individual particle the concentration range was as wide as three orders of magnitude. The imaging of elemental distribution showed the correlation between the elements and confirmed a high degree of heterogeneity in the elemental composition. These results confirm that µxrf can provide valuable data for the characterization of sub-mm heterogeneous samples where the concentrations of the elements are as low as a few tens ppm. To perform a classification of the particles in terms of geographical origin and other characteristics the investigation at microscopic level can be combined with a statistical classification procedure, such as principal component or cluster analysis. Acknowledgments: The authors are grateful to Mr. M. Makarewicz and to the staff of Chemistry Unit, IAEA Seibersdorf Laboratories, for providing the samples. REFERENCES [1] The Radiological Situation at the Atoll of Mururoa and Fangataufa, Radiological Assessment Report Series, ISBN ISSN , volumes 1 to 6, STI/PUB 1028, IAEA, Vienna 1998 [2] G. Bernasconi, N. Haselberger, A. Markowicz and V. Valkovic, Nucl. Inst. and Meth. B 86(1994)333 [3] M. Bogovac, M. Mattiuzzi, in preparation, 1999