Particle Analysis of Environmental Swipe Samples

Transcription

1 IAEA-SM-367/10/07 Particle Analysis of Environmental Swipe Samples D. DONOHUE, S. VOGT, A. CIURAPINSKI, F. RUEDENAUER, M. HEDBERG Safeguards Analytical Laboratory International Atomic Energy Agency Vienna, Austria Abstract Particle analysis is a uniquely powerful collection of analytical methods which can be used to measure environmental samples for safeguards. Scanning Electron Microscopy can be used to measure the elemental composition and morphology of particles and Secondary Ion Mass Spectrometry is used to measure the isotopic content of uranium and plutonium in particles. These methods are applied in the IAEA s Safeguards Analytical Laboratory in Seibersdorf, as well as in the Network of Analytical Laboratories in the Member States. 1. INTRODUCTION Environmental sampling for safeguards (ESS) has been routinely applied by the IAEA since 1996, as a strengthening measure which provides additional assurance of the absence of undeclared nuclear materials or activities in States covered by comprehensive safeguards agreements. Sophisticated analytical techniques are applied to the environmental samples to detect chemical or isotopic signatures of the materials handled or the activities carried out at an inspected location. Particle analysis refers to an ensemble of powerful analytical methods which can characterize microscopic particles present in the samples. The analysis of individual particles has several distinct advantages compared to bulk analysis in which the entire sample is dissolved and analyzed. Particles of man-made materials coming from a nuclear process are normally present as pure compounds, thus greatly reducing the effects of dilution from naturally-occurring elements such as uranium. Particles in the range of several micrometers in diameter can travel large distances from the point of formation, thus increasing the detection probability for samples taken in the general vicinity of a nuclear process. Unlike bulk analysis, which gives only information about the average concentration or isotopic composition in a sample, particles are more representative of the range of elemental or isotopic information present at the inspected location. This makes evaluation of the analytical results vis-a-vis the declared activities more effective in detecting a possible anomaly. This paper will describe two particle analysis methods which are implemented by the IAEA in its Safeguards Analytical Laboratory (SAL) in Seibersdorf. Similar methods are also used in the Network of Analytical Laboratories (NWAL) which supports the IAEA s ESS programme. The first method is Scanning Electron Microscopy, combined with X-ray Fluorescence Spectrometry (SEM/XRF). This method is used to locate particles containing elements of interest - primarily U and Pu - to study their physical characteristics and elemental composition. The second analytical technique described here is Secondary Ion Mass Spectrometry (SIMS) which permits the measurement of U isotope ratios in particles. Special operating software allows large areas of the sample planchet to be searched and automatic measurements of up to several thousand particles to be performed in a single analysis.

2 2. SCANNING ELECTRON MICROSCOPY 2.1. Description of the SEM Method Scanning electron microscopy relies on the production of a micro-focused electron beam which is then scanned (rastered) over the surface of the specimen in vacuum. The energy of the electrons can be as high as 30keV and the diameter of the beam is typically 100 nm. When this beam of electrons strikes the sample, several physical processes take place: 1) scattering of the beam by topographic features of the specimen which can be detected and used to make a magnified image of the sample surface (secondary electron mode), 2) elastic scattering of the beam at high angles of reflection (close to 180 o ) giving an image which is sensitive to heavy (high atomic weight) elements (backscattered electron mode) and 3) stimulation of X- ray emission by atoms in the sample giving an elemental map of the surface or the elemental composition of a single spot on the sample surface (X-ray fluorescence mode). The detection of fluorescent X-rays coming from the sample can be accomplished with a solid-state detector which measures X-rays of a wide range of energies (energy-dispersive or EDX detection) or by using a spectrometer which disperses the X-rays according to their wavelength and can only measure one wavelength (energy) at a time (wavelength-dispersive or WDX detection). The SEM operated by the Clean Laboratory Unit of SAL is shown in Figure 1 with both the EDX and WDX spectrometers visible. Automated particle searching can be performed with this instrument using special software originally designed to detect gun-shot residue for police departments. In SAL, the search is directed at finding U-containing particles over several mm 2 of the planchet surface using first the backscattered electron signal to locate heavy particles, then the EDX system to measure the XRF spectrum of each particle found. The result is a data file containing up to several thousand particles; the particle data can be sorted to find those with the highest U content, or various other user-selected parameters (eg. U associated with F). After examining this FIG. 1. Scanning Electron Microscope in Clean Laboratory for Safeguards. 2

3 Sample U at % O at % O/U Ratio Probable Identification Na 2 U 2 O Na 2 UO Na 2 UO 4 or MgUO UO 2 TABLE I. Measurement of Uranium Compounds by SEM/XRF. information, the operator may choose to revisit selected particles for the more time-consuming WDX measurement which is the most precise for measuring elemental composition. Table I shows the results of measuring the oxygen to uranium ratios in particles found in a number of test samples. It can be seen that uranium dioxide (UO 2 ) can be easily distinguished from uranates and di-uranates, although it is more difficult to specify the cation involved in the latter compounds. WDX analysis of elemental ratios is especially useful in measuring the age of plutonium materials collected on special swipe samples from inside hot cells. A particle containing mostly Pu may also contain measurable amounts of U (from the original fuel) as well as Am coming from the decay of the 241 Pu isotope. SEM/XRF is capable of measuring the U and Am content of a Pu particle at concentrations down to 0.1%. Alternately, the measurement of U/Pu ratios in hot cell swipes may indicate whether spent fuel has been chemically treated to recover the Pu Sample Preparation for SEM Typical samples for SEM are 10 x 10 cm cotton swipes and cellulose hot cell swipes. There are 2 basic sample preparation methods which can be used, depending on the situation. The most simple method is to use a self-adhesive carbon disc which is 1 cm in diameter attached to an aluminum SEM stub. The surface of this disc is coated with an adhesive which is used to pick up particles directly from the surface of the swipe. This method is primarily used for hot cell swipes because of the radiation hazard associated with more time-consuming methods. Hot cell swipes are smaller in diameter than cotton swipes and therefore the particulate material is more concentrated in a small area. The fraction of material from the swipe which ends up on the disc is usually quite small (1-10%), but it is expected to be reasonably representative of the sample. For cotton swipes having a much larger surface area, a more representative sampling of the particles can be accomplished by cutting up the swipe into pieces of about 1 cm 2 and placing them into an organic solvent such as heptane in a small glass vial. The vial is then placed into an ultrasonic bath to release the particles from the swipe and suspend them in the solvent. Many pieces of a swipe can be treated in this way and the heptane fractions combined to give a final suspension which is more representative of the material collected on the swipe. The suspension can be centrifuged to concentrate the particles which are then pipetted onto a SEM stub and dried. 3

4 2.3 Typical SEM Results The IAEA, in collaboration with the Laboratory for Microparticle Analysis in Moscow, has established an Atlas of U-containing particles from various nuclear processes. Particles were recovered from environmental samples taken near U mining operations, centrifuge enrichment facilities, fuel fabrication and hot cell facilities. The Atlas contains photomicrographs of typical particles, their size and morphology and information on their elemental composition from EDX measurements. This information may prove useful in identifying suspicious particles in environmental samples strictly by their morphology, thus streamlining the measurement of large particle ensembles. Especially for particles coming from the hydrolysis, in moist air, of UF 6 gas, the morphology and fluorine content may give a clue to the age of the particles, thus helping to discriminate between recent activities and historical ones. A recent application of automated particle searching with the SEM was the study of soil samples coming from a location where depleted uranium munitions were used. The samples consisted of soil which was dried, sieved to collect particles less than 45 micrometers in diameter and then pressed onto adhesive carbon discs attached to SEM stubs. The searching was accomplished in the backscattered electron mode with EDX spectra taken of each heavy particle found. In this way, up to a thousand U-containing particles were found in each of 2 soil samples. Figure 2 shows a typical DU particle having a diameter of approximately 7 micrometers. The EDX spectrum of such particles also revealed a Ti peak at a concentration of about 1% in the uranium, something which is expected for DU used in armor-piercing ammunition. The study of Pu-containing particles from inside hot cells can yield important information about the activities which have been carried out there. In particular, the handling of irradiated reactor fuel should produce particles in which the U/Pu ratio is high (U/Pu = depending on the irradiation conditions). Particles which contain more Pu than expected could indicate that chemical separation activities were carried out. In addition, the amount of Am in a particle in comparison to the Pu can give an indication of the age of the material since it FIG. 2. SEM photomicrograph of DU particle from a soil sample. 4

5 FIG. 3. WDX spectrum of a Pu particle, showing U and Am impurities. was formed. For these measurements, it is necessary to utilize the higher resolution afforded by the WDX spectrometer attached to our SEM instrument. The Pu impurity in a primarily U particle can be quantified down to approximately 0.2 %, whereas the Am or U impurity in a primarily Pu particle can be measured at about 2 times lower concentration. The resolution and sensitivity of WDX measurements can be seen in Figure SECONDARY ION MASS SPECTROMETRY 3.1. Description of the SIMS Method The technique of secondary ion mass spectrometry begins with forming an energetic beam of ions - the primary ion beam - which is then used to bombard the sample, causing sputtering of atoms and secondary ions from the sample surface. The secondary ions are accelerated and separated according to their mass in a magnetic field and finally detected with one of several devices. SIMS instruments produce an image of the sample using secondary ions of a chosen mass; thus a sample containing uranium particles would form an image using 238 U + ions (this is called the ion microscope mode of operation). By storing an image using 238 U + and a similar image using 235 U + ions, it is possible to measure the enrichment of particles in a sample. Automatic scanning software (PSEARCH) allows us to scan a significant area of the sample planchet surface and to find and measure many thousand particles in a measurement session lasting 4-6 hours. 5

6 A second mode of SIMS data taking involves focusing the primary ion beam onto a particle and measuring its mass spectrum. This is called the ion microprobe mode of operation and provides the best quality isotopic information for the major isotopes as well as the minor isotopes such as 234 U and 236 U. The ion microprobe mode is much more time consuming because the measurement of a single particle may take minutes, compared to about 1 minute per field in the ion microscope mode SIMS Sample Preparation The sample preparation method described for SEM measurements (i.e. ultrasoneration in heptane) is also used to prepare sample planchets for SIMS. Samples which contain very small numbers of U particles require much more intensive sample preparation, including multiple ultrasonic treatments or ashing of the swipe substrate, which presents its own problems because of the relatively high ash content of the cotton used. It is always necessary to strike a balance between the number of particles of interest on the planchet and the number of uninteresting ones which can disturb the analysis through electrostatic charging effects Typical SIMS Results The PSEARCH software allows automatic searching of the planchet surface for U-containing particles and provides a measurement of their 235 U/ 238 U ratios. The primary ion beam is defocused to a diameter of 150 micrometers and the ion images from this field are collected with a position sensitive detector. These images are then processed to locate the particles and to calculate their enrichment. Figure 4 shows the ion images resulting from 235 U + and 238 U + in one field; a typical PSEARCH run would consist of fields and can take several hours. Figure 5 shows one typical form of data presentation resulting from a PSEARCH measurement. Hundreds of particles were detected and each particle is plotted against its 235 U/ 238 U ratio. Patterns can be easily seen in such data, including particles with natural isotopic composition (0.7% 235 U), low-enriched material with 235 U = 3 to 5 % and a number of particles at enrichments higher than 5 %. Therefore, it would be easy to detect the presence of high-enriched U in samples from a facility which declares the handling of only natural or low-enriched U. FIG. 4. PSEARCH-generated 235 U (left) and 238 U (right) ion images from the same field. Differences in relative intensity indicate different enrichments in the particles. 6

7 FIG. 5. Scatterplot of U particle data from a PSEARCH measurement. 4. CONCLUSIONS AND FUTURE WORK Particle analysis techniques are a powerful tool for detecting and measuring nuclear materials in environmental swipe samples. Scanning electron microscopy combined with X-ray fluorescence spectrometry can measure the elemental content of particles smaller than 1 micrometer in diameter. This represents a sensitivity in the femtogram (10-15 gram) range. The use of sophisticated software allows the analyst to detect and record data from many thousands of particles in a measurement session and these particles can be reliably relocated for more detailed examination. Secondary ion mass spectrometry provides the isotopic information which SEM/XRF does not. Thus, the 2 techniques complement each other to give the maximum of useful information for safeguards purposes. In the near future, SAL/CL will be able to unambiguously identify and locate particles of interest in a non-destructive way with the SEM and then find them again with SIMS for the isotopic measurement. Further improvements will be made in the sample preparation techniques used for each method. The goal is to recover as many useful particles as possible from environmental swipe samples which may contain only a few candidate particles. We will continue to develop fasttrack SIMS in which the analysis time for SIMS measurements is kept below 2 weeks in order to meet the Safeguards timeliness goal. 7