Particle isolation for analysis of uranium minor isotopes in individual particles by secondary ion mass spectrometry

Comparison of AMS, TIMS, and SIMS techniques for determining uranium isotope ratios in individual particles

The determination of isotope ratios in individual uranium particles is very important for nuclear safeguards. In this work, accelerator mass spectrometry (AMS), thermal ionization mass spectrometry (TIMS), and secondary ion mass spectrometry (SIMS) were applied to isotope ratio analysis of individual uranium particles and compared in terms of background, measurement accuracy, and efficiency. Several individual uranium particles (1-7 μm) from certified reference materials were used as samples. The results show that the average values of blank counting rate of ²³⁵ U for AMS, FT-TIMS (FT: fission track), SEM-TIMS (SEM: scanning electron microscope), and SIMS were 7.3, 7.8, 2.7 and 2.2 cps, respectively. The relative error of ²³⁴ U/²³⁵ U and ²³⁴ U/²³⁶ U isotope ratios of the particles from U200 for AMS were within 10% and 20%, whereas the results of FT-TIMS and SIMS were within 5% and 10%, respectively. The relative error and external precision of ²³⁴ U/²³⁸ U and ²³⁵ U/²³⁸ U of the particles from U850 for the method of AMS, SEM-TIMS, and SIMS were within 10% and 5%, respectively. For ²³⁶ U/²³⁸ U, the average values of the relative error and external precision measured by AMS were within 5%, which measured by SEM-TIMS and SIMS were all within 10%. AMS has advantages in measuring ²³⁶ U/²³⁸ U. The measurement time of AMS and SEM-TIMS was shorter than that of FT-TIMS and longer than that of SIMS. It is considered that AMS and SEM-TIMS have a certain development prospect, and it is necessary to research deeply.

New horizons in microparticle forensics: Actinide imaging and detection of 238Pu and 242mAm in hot particles

Micrometer-sized pollutant particles are of highest concern in environmental and life sciences, cosmochemistry, and forensics. From their composition, detailed information on origin and potential risks to human health or environment is obtained. We combine secondary ion mass spectrometry with resonant laser ionization to selectively examine elemental and isotopic composition of individual particles at submicrometer spatial resolution. Avoiding any chemical sample preparation, isobaric interferences are suppressed by five orders of magnitude. In contrast to most mass spectrometric techniques, only negligible mass is consumed, leaving the particle intact for further studies. Identification of actinide elements and their isotopes on a Chernobyl hot particle, including ^242mAm at ultratrace levels, proved the performance. Beyond that, the technique is applicable to almost all elements and opens up previously unexplored scientific applications.

Determination of minor isotope ratios of individual uranium particles by accelerator mass spectrometry

The determination of uranium isotope ratios in uranium particle is an essential technique in the analysis of environmental samples for nuclear safeguards. At present, mass spectrometry has been widely used to measure isotope ratios of uranium particles. We developed a new analytical method for measuring minor isotope ratios of individual uranium particles directly. The technique includes single particle identification by energy dispersive X-ray spectrometer (EDX), transfer by scanning electron microscope (SEM) combined with micromanipulator and isotope ratios analysis with accelerator mass spectrometry (AMS). The minor isotope ratios of individual uranium particles with four isotopic abundances and different sizes from certified reference materials were measured by AMS. The results show that the relative error (RE, i.e., the deviation of measured value from the certified value) of ²³⁴ U/²³⁵ U and ²³⁴ U/²³⁶ U isotope ratios was 9.0% and 19.3%, respectively, and the relative standard deviation (RSD) was within 9.9% and 16%, respectively. The minimum particle determined was about 2 μm. The method was found to be an alternative analytical means for nuclear safeguards.

Determination of the isotopic composition of single sub-micrometer-sized uranium particles by laser ablation coupled with multi-collector inductively coupled plasma mass spectrometry

RATIONALE

A multi-collector inductively coupled plasma (MC-ICP) mass spectrometer coupled to a UV ns-laser ablation (LA) system was used to measure uranium isotopic ratios (²³⁴ U/²³⁸ U, ²³⁵ U/²³⁸ U and ²³⁶ U/²³⁸ U) in single uranium particles of various sizes and isotopic compositions, including home-made sub-micrometric natural uranium particles of narrow size distribution (415 ± 60 nm).

METHODS

The LA-ICP mass spectrometer was operated in wet plasma conditions thanks to simultaneous injection of the laser aerosol and water vapor through a desolvating nebulizer. The isotopic ratios were corrected for mass bias and gain factors between detectors. The ²³⁶ U/²³⁸ U ratios were also corrected for the presence of ²³⁵ U hydrides and tailing of the ²³⁸ U⁺ peak.

RESULTS

²³⁶ U/²³⁸ U ratios were successfully measured in micrometer-sized particles from the NBS U050 certified standard material with a ²³⁶ U/²³⁸ U ratio of ~5 × 10^-4 . The analysis of 77 natural uranium sub-μm-sized particles yielded a very good trueness with respect to the expected ²³⁴ U/²³⁸ U and ²³⁵ U/²³⁸ U ratios, while the measurement errors for single particles ranged from -2.7% to +2.1% for ²³⁵ U/²³⁸ U and from -17% to +33% for the ²³⁴ U/²³⁸ U ratios. Their relative combined standard uncertainties ranged from 3.3% to 32.8% and from 0.4% to 4.0% for ²³⁴ U/²³⁸ U and ²³⁵ U/²³⁸ U ratios, respectively. In addition, extremely low detection limits, in the attogram range, were achieved.

CONCLUSIONS

This study demonstrates that coupling of a ns-laser ablation system with a MC-ICP mass spectrometer allows measurements of the isotopic composition in natural uranium particles of a few hundreds of nm with very good trueness, average combined standard uncertainties of ~1% for ²³⁵ U/²³⁸ U ratios and 12% for ²³⁴ U/²³⁸ U ratios, and detections limits of a few ag for minor isotopes.

Considerations for uranium isotope ratio analysis by atmospheric pressure ionization mass spectrometry

The accurate measurement of uranium isotope ratios from trace samples lies at the foundation of achieving nuclear nonproliferation. These challenging measurements necessitate both the continued characterization and evaluation of evolving mass spectrometric technologies as well as the propagation of sound measurement approaches. For the first time in this work, we present the analysis of uranium isotope ratio measurements from discrete liquid injections with an ultra-high-resolution hybrid quadrupole time-of-flight mass spectrometer. Also presented are important measurement considerations for evaluating the performance of this type and other atmospheric pressure and ambient ionization mass spectrometers for uranium isotope analysis. Specifically, as the goal of achieving isotope ratios from as little as a single picogram of solid material is approached, factors such as mass spectral sampling rate, collision induced dissociation (CID) potentials, and mass resolution can dramatically alter the measured isotope ratio as a function of mass loading. We present the ability to accurately measure 235UO2+/238UO2+ down to 10s of picograms of solubilized uranium oxide through a proper consideration of mass spectral parameters while identifying limitations and opportunities for pushing this limit further.

Fully convolutional neural network for removing background in noisy images of uranium bearing particles

A fully convolutional neural network (FCN) was developed to supersede automatic or manual thresholding algorithms used for tabulating SIMS particle search data. The FCN was designed to perform a binary classification of pixels in each image belonging to a particle or not, thereby effectively removing background signal without manually or automatically determining an intensity threshold. Using 8000 images from 28 different particle screening analyses, the FCN was trained to accurately predict pixels belonging to a particle with near 99% accuracy. Background eliminated images were then segmented using a watershed technique in order to determine isotopic ratios of identified particles. A comparison of the isotopic distributions of an independent data set segmented using the neural network with a commercially available automated particle measurement (APM) program developed by CAMECA was performed. This comparison highlighted the necessity for effective background removal to ensure that resulting particle identification is not only accurate, but preserves valuable signal that could be lost due to improper segmentation. The FCN approach improves the robustness of current state-of-the-art particle searching algorithms by reducing user input biases, resulting in an improved absolute signal per particle and decreased uncertainty of the determined isotope ratios.

Revisiting the fission track method for the analysis of particles in safeguards environmental samples

This paper details an improved approach to environmental particle analysis for safeguards by means of a combination of an upgraded version of the so-called fission track method with state-of-the-art microscope and microprobe techniques. Improvements to the fission track method comprise a novel sample assembly, the automation of several of its steps and the extensive use of correlative microscopy. This is followed by an automated isolation of particles-of-interest by means of laser micro-dissection (LMD) and their collection onto a harvester for transfer to other micro-analytical instruments for further analysis. The samples examined in this contribution were analysed for their nuclear material signatures, in particular the presence of uranium isotopes. The length of a single analysis cycle herewith was reduced to 12 days.