Characterisation of hot particles remaining in soils from Palomares (Spain) using a nuclear microprobe

Accepted Manuscript Characterisation of hot particles remaining in soils from Palomares (Spain) using a nuclear microprobe J. García López, M.C. Jiménez-Ramos, M. García-León, R. García-Tenorio PII: S0168-583X(07)00419-3 DOI: 10.1016/j.nimb.2007.02.044 Reference: NIMB 54363 To appear in: Nucl. Instr. and Meth. in Phys. Res. B Please cite this article as: J. García López, M.C. Jiménez-Ramos, M. García-León, R. García-Tenorio, Characterisation of hot particles remaining in soils from Palomares (Spain) using a nuclear microprobe, Nucl. Instr. and Meth. in Phys. Res. B (2007), doi: 10.1016/j.nimb.2007.02.044 This is a PDF fi l e of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could aff ect the content, and all legal disclaimers that apply to the journal pertain. ACCEPTED MANUSCRIPT 1 Characterisation of hot particles remaining in soils from Palomares (Spain) 1 using a nuclear microprobe. 2 J. García López*1,2, M.C. Jiménez-Ramos2, M. García-León1,2 and R. García-Tenorio2 3 1Centro Nacional de Aceleradores, Av. Thomas A. Edison. Isla de la Cartuja. 41092 Sevilla, 4 Spain. 5 2Applied Nuclear Physics Research Group, University of Seville, P.O. Box 1065, 41080 6 Sevilla, Spain. 7 8 More than 40 years ago, an aircraft accident took place in Palomares (Spain) which involved 9 the destruction of two nuclear weapons. A portion of the remaining transuranic contamination 10 in the affected soils is present in the form of small high activity concentration particles (10-100 11 µm), also called “hot particles”, which contain plutonium and uranium. 12 Several hot particles have been isolated and identified from the superficial soils recently 13 collected in the zone affected by the accident. The isolation was carried out by screening the 14 soil using gamma-ray spectrometry, through discrimination of the high activity concentrations 15 of 241Am in the samples which indicates the presence of plutonium. The hot particles, 16 composed of several elements with very high atomic number, could be easily identified by 17 Scanning Electron Microscopy (SEM) in Backscattering Electron Image (BSE) mode. 18 Moreover, their morphology and size were also studied using SEM in Secondary Electron (SE) 19 mode. 20 * Corresponding author: Phone:+34954460553; Fax: +34954460145; E-mail: [email protected] ACCEPTED MANUSCRIPT 2 In this work, the hot particles have been investigated with the nuclear microprobe of the 1 National Accelerator Centre (CNA) in Seville. Compositional analysis, mapping and depth 2 distribution of different elements have been performed by a simultaneous combination of 3 Particle Induced X-ray Emission (PIXE) and Rutherford Backscattering Spectrometry (RBS). 4 Uranium and Plutonium have been identified in the PIXE spectra as the main components of 5 the particles, whereas the concentration of Americium is two orders of magnitude smaller. In 6 addition, an estimation of the particles density has been obtained by comparison of the RBS 7 results with the particles thickness directly determined by SEM. 8 PACS codes: 89.60.–k; 81.70.–q; 82.80.Yc; 29.27.–a 9 Keywords: Hot particles; Transuranic contamination; Nuclear weapon accident; Microprobe, 10 PIXE, RBS. 11 12 13 14 1. Introduction 15 On the morning of 17 January of 1966 a US B-52 plane, carrying four thermonuclear weapons, 16 collided in mid air with a US KC-135 tanker during a refueling operation over Spain. As a 17 result of this accident two bombs experienced non-nuclear detonations after impacting on land 18 over the surroundings of Palomares village (Spain), spreading several kilograms of plutonium 19 over an area of approximately 500 ha [1]. Although the affected area was subjected to a clean-20 up operation, it has been estimated that the total residual Pu inventory is in the order of 0.1 21 TBq [2]. A portion of the remaining transuranic contamination in the affected soils is present in 22 the form of agglomerates of high activity material which have sizes of about 10-100 µm, so-23 called “hot particles”. In order to assess the environmental impact of particle contaminated 24 ecosystems, detailed information is required on particle characteristics such a size, 25 composition, morphology, density and oxidation state of main elements. The use of non-26 destructive methods to characterize hot particles represents an analytical challenge; however, 27 ACCEPTED MANUSCRIPT 3 by introducing these advanced technologies to radioecology major improvements within this 1 field can be achieved [3]. Several non-destructive spectrochemical techniques such as gamma-2 ray and X-ray spectrometry, scanning electron microscopy (SEM) with X-ray microanalysis, 3 microscopic X-ray absorption near-edge structure (µ-XANES) and microscopic X-ray 4 fluorescence (µ-XRF) have been recently employed to characterize radioactive particles 5 stemming from the Thule accident [4, 5]. Nevertheless, to our knowledge, the high capability 6 of Ion Beam Analytical (IBA) techniques has seldom been applied to the study of this kind of 7 samples. Burns et al. have reported the analysis, using a nuclear microprobe, of particles 8 formed during the nuclear weapons trials at Maralinga [6], but no information about the 9 measuring conditions or the data treatment is contained in their work. 10 In the present paper two hot particles from Palomares have been characterized with the nuclear 11 microprobe of the National Accelerator Centre (CNA) in Seville. Compositional analysis, 12 mappings and depth distribution of main elements were obtained by a simultaneous 13 combination of Particle Induced X-ray Emission (PIXE) and Rutherford Backscattering 14 Spectrometry (RBS). Prior to the analysis of the PIXE spectra, the ionization cross sections and 15 relative intensities of L1, L2 and L3 x-rays lines have been obtained for Pu and Am, due to the 16 lack of experimental data for these two elements. In addition, by comparison of the microprobe 17 results with the particles thickness directly determined by SEM an estimation of the particles 18 density has been calculated. 19 2. Experimental 20 The sampling campaign, hot particles isolation, identification and characterization by SEM are 21 fully described in [7]. In brief, several soil samples were collected by our group in autumn 22 2001, in the vicinity of the location where one of the bombs exploded. In order to isolate the 23 hot particles from the bulk soil a sample splitting technique was applied. The technique is 24 based on the measurement by gamma-ray spectrometry of the 59.54 keV photon emitted from 25 ACCEPTED MANUSCRIPT 4 the decay of the 241Am isotope (T1/2=432.2 y). This radionuclide is present in the weapons 1 material as a daughter of the beta emitter 241Pu (T1/2=14.35 y). Once the 241Am activity is 2 identified, a procedure based in the successive splitting of the sample and the measurement of 3 the radioactive fraction by gamma-ray spectrometry is employed. After successive divisions of 4 the sample, starting with about 50 g of soil less than a few mg remain. The samples were then 5 attached to an adhesive carbon tape and examined by SEM in the backscattered electron (BSE) 6 and secondary electron (SE) modes. The BSE mode was used to identify the hot particles due 7 to the high contrast found between elements with high and low atomic number, whereas the SE 8 mode gave information about the topography and size of the particles. 9 The elemental analysis was carried out using the 3 MV tandem accelerator of the CNA, 10 described in detail elsewhere [8]. The microprobe focusing system and the data acquisition 11 setup are based on an Oxford Microbeams endstation OM2000 and on the Oxford Microbeams 12 DAQ system [9], respectively. The samples were irradiated with a 3 MeV proton beam of size 13 4x4 µm2 and a beam current of ~800 pA. A retractable Gresham Si(Li) detector (active area 80 14 mm2, resolution 145 eV) placed at 135º was used, together with a Titan amplifier, for PIXE 15 analysis. A light tight Ametek Si detector (Bu-CAM-300) placed at 143º in Cornell geometry 16 was employed for the simultaneous RBS measurements. The integrated ion charge was 17 controlled from the backscattered signal of protons from the carbon atoms in the carbon tape. 18 Elemental maps were done in scanning mode using a field 100 x 100 µm2 for the A15 particle 19 and 75x75 µm2 for the HP3 sample. In the first set of measurements a 50 µm thick mylar filter 20 was used, which allows the detection of low energy X-rays from the light elements (Si, Ca, Ti, 21 Fe, etc.) that form the soil. In order to reduce the background signal due to the pile-up from 22 these light elements and from the M line signals of U and Pu, and also to eliminate the noise 23 created by the backscattered protons that reach the Si(Li) detector, a second set of 24 measurements was performed using a 1 mm thick mylar filter. The elemental maps for U and 25 ACCEPTED MANUSCRIPT 5 Pu were determined from the intensities of the Lα lines of these two elements. Point 1 measurements were also carried out for the two particles. 2 3 2.1 Data treatment 4 The RBS and PIXE fitting codes known by the authors lack for data such a cross-sections, 5 atomic branchings, and stopping powers for elements with Z > 92, so that the treatment of our 6 RBS and PIXE spectra involves additional difficulties. Although we can not expect to attain an 7 exact solution for the samples composition, values have been estimated employing some 8 reasonable approximations and adding some specific data to the fitting codes used in this work. 9 The RBS spectra were evaluated with the SIMNRA code [10]. The fitting was done using only 10 U and lighter elements. For our matrix, the RBS results found for U represent, nearly, the 11 actual U+Pu concentration of the particles. Indeed, for these two elements the RBS signals 12 overlap, their stopping powers must be reasonably close and their Rutherford cross sections are 13 very similar (σR(Pu)/σR(U)) = 1.04). Parallel problems regarding the absence of data for 14 transuranic elements were found during the analysis of the PIXE spectra. The energies and 15 relative intensities of L line series for Pu and Am were determined using the Evaluated Atomic 16 Data Library (EADL) and the RELAX code [11], which takes into account all radiative 17 transitions, for each atomic L sub-shell, together with the fluorescence yields and Coster-18 Kronig probabilities to calculate the emitted spectrum. The 26 more intense L lines for Pu and 19 Am were added to the X-ray library of the WINQXAS program package [12], which was used 20 to deconvolute the PIXE spectra and to determine the area of the L peaks. In order to transform 21 peak areas into concentrations, the values of the L sub-shell ionization cross, σ(L), are needed. 22 The σ(Lα) values for 3 MeV protons were directly calculated for U and Pu from ref [13], while, 23 the σ(Lα) value for Am was obtained from interpolation of the cross sections for elements with 24 Z= 92, 94 and 96, as shown in Fig. 1. The relative concentration U/Pu and U/Am was 25 ACCEPTED MANUSCRIPT 6 calculated from the Lα peak areas corrected by the corresponding cross sections. It should be 1 emphasized that although self-absorption of photons inside the hot particle and attenuation 2 through the mylar filter are not negligible, these transmission values are very similar for the 3 three X-ray energies of interest (Lα(U) = 13.612 keV, Lα(Pu) = 14.276 keV and Lα(Am) = 4 14.615 keV), and therefore they cancel out when calculating the relative concentrations. 5 6 3. Results and discussion 7 Fig. 2 shows the elemental maps for Fe, Pu and U of particle HP3. The iron, one of the 8 constituents of the soil, is present throughout the map, which indicates that the particle is 9 surrounded and also partially coated with soil material. The U and Pu show a rather uniform 10 spatial distribution within the particle, which has a rounded shape with a diameter of ~20 µm. 11 We did not find any trace of these two radioactive elements outside the particle. Similar maps 12 were found for the sample A15. In order to obtain quantitative results, several spots were 13 analyzed on each particle with PIXE and RBS. In Fig. 3 we can see the effect of the X-ray 14 filter thickness on the PIXE spectra for the particle A15. The spectrum recorded with a 50 µm 15 thick mylar filter (Fig. 3a) is dominated by the peak at 3-4 keV, due both to the K lines of the 16 potassium and calcium present on the particles surface and to the M lines from U and Pu. A 17 rough analysis of the K peaks areas of these elements compared to the L peak areas of U and 18 Pu, and corrected by the corresponding transmission factors and ionization cross sections 19 ( σ(Kα)Si = 21500 b; σ(Kα)Ca = 2950 b; σ(Kα)Fe = 500 b; σ(Lα)Pu = 13 b), indicates that the 20 atomic percentage of Si, Ca and Fe into the particles is, at the most, a few percent. Therefore, 21 the PIXE signal of the light elements very likely comes from a thin layer of soil material that 22 partially coats the hot particles. Although in Fig 3a the main L lines of the high atomic number 23 elements are also clearly observed, their quantitative analysis is hindered by the background 24 due to the pile-up of the more intense signals and by the noise introduced by backscattered 25 ACCEPTED MANUSCRIPT 7 protons. By increasing the mylar thickness to 1 mm (Fig. 3b) we drastically reduce all the X-1 ray signals below 5 keV and avoid the protons reaching the Si(Li) detector, and in consequence 2 the background in the region of interest (12 – 22 keV) is decreased by a factor of 20. This low 3 background is essential not only for a proper analysis of the U and Pu L-peaks, but it also 4 allows the detection and quantification of Am, whose Lα signal appears in the spectrum as a 5 small shoulder of the much more intense Pu-Lα peak. In Table 1, Lα peak areas and relative 6 concentrations for U, Pu and Am at different points of particles A15 and HP3 are presented. 7 The concentration ratios are calculated taking into consideration the ionization cross section for 8 different elements, as stated in the Experimental section. The Pu/U ratio differs between the 9 two particles; for HP3 this magnitude is approximately constant with a value of 1.25 whereas 10 for A15 the ratios are considerable higher, ranging from 1.7 to 2.3. These results may reflect 11 that the two particles originated from different parts of the weapon. Local variations in the ratio 12 within the A15 particle may be due to preferential leaching or weathering of one element with 13 respect to the other one. It is worth to mention that our results are very different from the 14 results of radioactive particles stemming from a similar accident occurred at Thule (Greenland) 15 in 1968, whose Pu/U Lα intensity ratios (uncorrected from cross sections) determined by µ-16 XRF equals ~ 0.3 [4]. With regard to the americium content in our samples, the PIXE results 17 indicate that its concentration is about two orders of magnitude lower compared to plutonium. 18 It should be emphasized that the present PIXE results are the first results published for direct 19 determination of Am content in hot particles using IBA techniques. However, within the 20 accuracy of our measurements, it is difficult to conclude if the small variations found for the 21 Pu/Am ratio reflect a physical phenomenon or if they are due to statistical effects. 22 The global composition and thickness of the particles (in at/cm2) were determined by RBS. The 23 recorded RBS spectra for different points of sample HP3 are depicted in Fig. 4. The signal 24 corresponding to the radioactive particle is almost completely separated from a broader peak 25 ACCEPTED MANUSCRIPT 8 that appears at lower energy due to backscattered protons from the carbon tape. As stated 1 above, this low energy signal was used to control the integrated beam fluence and therefore it 2 was possible to normalize all the RBS and PIXE spectra. The simulations of the RBS spectra 3 indicate that the matrix of the hot particles is mainly formed of U, Pu, O and C, in agreement 4 with the PIXE results. The mass resolution is not good enough to separate the heavy mass 5 elements and therefore, by RBS we can only determine the total (U+Pu) atomic percentage. 6 However, by combining these results with the relative Pu/U concentration found by PIXE we 7 can find out the individual contribution of each element. The RBS spectra show that about 10 8 to 15% of the atoms that form the hot particles are U and Pu. The concentration of these 9 elements varies significantly at different points of the particles and also their depth distribution 10 was found to be inhomogeneous in several of the analyzed spots. As shown, in the spectrum 11 from HP3-3 (Fig. 4) the (U+Pu) atomic concentration equals 12% at the surface and increases 12 almost linearly with depth until reaching 14% at the depth of 4 x 1019 at/cm2 (~10 µm). From 13 here, the (U+Pu) concentration remains constant or slightly decreases until the bottom of the 14 particle (~8 x 1019 at/cm2). It is not possible to say from these measurements if the surface is 15 equally depleted of both elements, U and Pu. However, recent PIXE and RBS studies carried 16 out by our group using alpha particles indicate that the observed gradients are due 17 preferentially to Pu [14]. The finding that the surface of the hot particles is partially depleted of 18 radioactive elements with respect to the core could be of the utmost importance from the 19 radiological and environmental point of view. Indeed, a major fraction of the contamination in 20 Palomares exists as particles with large size (tens of microns) [15] and this considerably 21 prevents its resuspension by the winds in the area, which diminishes the possible risk due to 22 inhalation by the population living in the zone. However, if the superficial depletion does not 23 stem from the initial formation of the particles but, on the contrary, it is related to leaching and 24 weathering processes during the last 40 years, the potential hazard of these hot particles could 25 increase with time. 26 ACCEPTED MANUSCRIPT 15 Fi Fig.3 0 2 4 6 8 10 12 14 16 18 20 22 24 1 10 100 1000 10000 U and Pu M lines U and Pu L lines Ti Fe Ca K Filter: 50 µ µµ µm mylar Yield Energy (keV) Si (a) 2 4 6 8 10 12 14 16 18 20 22 24 1 10 100 1000 10000 (b) Filter: 1 mm mylar PuLγ γγ γ ULγ γγ γ ULβ ββ β PuLγ γγ γULγ γγ γ PuLγ γγ γ ULγ γγ γ PuLβ ββ β ULβ β β β + PuLβ ββ β ULβ ββ β PuLl ULl PuLα αα α ULα αα α Ba Ba Pb Ga Cu Fe Yield Energy (keV) Fe ACCEPTED MANUSCRIPT 16 20 40 60 80 100 120 140 160 180 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Oxygen U+Pu 14 at% U+Pu 8 at% Yield Channel 1 2 3 Carbon tape U+Pu 12 at% Fig.4 ACCEPTED MANUSCRIPT 17 Spot U (Lα αα α) Pu (Lα αα α) Am (Lα αα α) Pu/U (±1%) (σ σσ σ corrected) Pu/Am (±10%) (σ σσ σ corrected) A15-1 36479 53251 498 1.71 99 A15-2 23575 44603 328 2.21 126 A15-3 23815 47305 344 2.32 128 A15-4 29983 46094 419 1.80 102 A15-5 18226 28596 309 1.84 86 HP3-1 75325 80547 774 1.25 97 HP3-2 61516 68555 617 1.30 103 HP3-3 97805 102201 1215 1.22 78 Table 1