Isotopic Heterogeneity Imaged in a Uranium Fuel Pellet with Extreme Ultraviolet Laser Ablation and Ionization Time-of-Flight Mass Spectrometry

Abstract

We use extreme ultraviolet laser ablation and ionization time-of-flight mass spectrometry (EUV TOF) to map uranium isotopic heterogeneity at the nanoscale (≤100 nm). Using low-enriched uranium fuel pellets that were made by blending two isotopically distinct feedstocks, we show that EUV TOF can map the 235U/238U content in 100 nm-sized pixels. The two-dimensional (2D) isotope maps reveal U ratio variations in sub-microscale to ≥1 μm areas of the pellet that had not been fully exposed by microscale or bulk mass spectrometry analyses. Compared to the ratio distribution measured in a homogeneous U reference material, the ratios in the enriched pellet follow a ∼3× wider distribution. These results indicate U heterogeneity in the fuel pellet from incomplete blending of the different source materials. EUV TOF results agree well with those obtained on the same enriched pellets by nanoscale secondary ionization mass spectrometry (NanoSIMS), which reveals a comparable U isotope ratio distribution at the same spatial scale. EUV TOF’s ability to assess and map isotopic heterogeneity at the nanoscale makes it a promising tool in fields such as nuclear forensics, geochemistry, and biology that could benefit from uncovering sub-microscale sources of chemical modifications.

Imaging isotopic content at the nanoscale using extreme ultraviolet laser ablation and ionization mass spectrometry

“…12,13 In a recent publication, we demonstrated EUV TOF's high spatial resolution capabilities to map isotopic heterogeneity in a uranium fuel pellet. 14 More specifically, we showed that EUV TOF can map the 235 U/ 238 U isotope ratio in 100 nm sized pixels. The results revealed microscale (>100 nm) heterogeneity with domains characteristic of the two starting materials (i.e., feedstock).…”

“…The EUV TOF mass spectrometer, developed and built at CSU, has been described in detail elsewhere. [12][13][14] In brief, the table-top Ne-like Ar 8+ EUV capillary discharge laser 16 (sized 0.4 m 2 or ~3 ft 2 ) operates at a wavelength of 46.9 nm with a maximum pulse energy of ~10 µJ and duration of ~1.5 ns. 17 The EUV laser beam illuminates a free-standing zone plate (NA = 0.12) that focuses the beam onto the sample.…”

“…12,13 EUV TOF and NanoSIMS mapped the 235 U/ 238 U ratio in a heterogenous low enriched uranium (LEU, ~2% 235 U and ~98% 238 U) fuel sample 22 and natural uranium (NU, 0.725% 235 U and ~99% 238 U) metal certified reference material (NBL, CRM 112-A) 23 prepared at Pacific Northwest National Laboratory using a focused ion beam/scanning electron microscope (FIB/SEM). 14 The sample dimensions were ~20 µm x 20 µm x 3 µm (Figure 1A). All samples were coated with conductive gold coating prior to analysis.…”

“…7,14 16 O + isotopes over the LEU and NU samples using 1 µm pixels (i.e., sample step size) at a laser fluence of ~1 J/cm 2 with mapping field sizes of ~25 µm x 25 µm (Figure 1B,C). The same isotopes were re-mapped over a small ~20 µm x 1 µm area of the LEU sample using 100 nm pixels at a laser fluence of <1 J/cm 2 (Figure 1D), 14 laser conditions previously used to ablate craters in resist with a diameter ≤400 nm and depth ≤40 nm. 12 NanoSIMS mapped the same isotope species on a similar LEU subsample using 98 nm pixels with a 2-4 pA Oprimary ion beam with a diameter of ~300 nm (Figure 1E).…”

“…3,5 Recently, we have shown that an extreme ultraviolet (EUV) laser coupled to a mass spectrometer can also offer unsurpassed three-dimensional (3D) nanoscale spatial resolution for chemical mapping. [12][13][14][15] Our group at Colorado State University (CSU) has built and developed an EUV laser ablation and ionization time-offlight mass spectrometer (EUV TOF) that was the first of its kind to achieve mass spectral imaging with 3D nanoscale spatial resolution. 12,13 The instrument uses a table-top EUV Ar 8+ capillary discharge laser, also developed at CSU, that operates at a wavelength of 46.9 nm (26.4 eV photon energy).…”