ABSTRACT
Crystallographic orientation dependence deteriorates the performance of surface analysis methods such as secondary ion mass spectrometry (SIMS) and focused ion beam (FIB). This study explores the corresponding potential challenges of laser ablation (LA) as a powerful sampling tool for inductively coupled plasma-mass spectrometry (ICP-MS). To this end, three UO2 single crystals of different orientation as well as polycrystalline UO2 were produced and characterized. Subsequently, a ns-laser ablation system was employed to study laser-matter interaction in detail. Firing the laser continuously at 1 Hz with various single shot fluence (2, 4, 6, 8, 12 J cm-2) for diverse periods created LA craters impacted by cumulative fluence between 50 and 650 J cm-2. Repeated LA experiments on the (100) plane of a UO2 single crystal at the beginning and end of the entire study revealed highly reproducible (<3%) LA rates, only limited by the fluctuation of the laser energy output of the ns-LA system. After thorough cleaning of the ablated samples, surface roughness and average depth of LA craters were determined using confocal laser scanning profilometry. Both LA rate and average depth of craters decreased exponentially with increasing single shot fluence independently of the crystal orientation. Surface roughness increased linearly with increasing cumulative fluence having largest intensification for lowest single shot fluence. Scanning electron microscope (SEM) images not only revealed the conical silhouette of LA craters, but also identified a convex meniscus at its bottom. This particular shape of the crater bottom with a deeper ring surrounding the central region is a result of melted and re-solidified UO2 generated during the LA process and the main limiting factor for the achievable depth resolution. The rapid re-solidification of the liquid phase after each single laser shot created tiles of different shape and orientation, depending on UO2 crystal orientation. Three different types of ejected particles radially distributed around the LA craters were identified by SEM, providing profound insights into laser-UO2 interaction.
ABSTRACT
The precise and accurate determination of the radionuclide inventory in radioactive waste streams, including those generated during nuclear decommissioning, is a key aspect in establishing the best-suited nuclear waste management and disposal options. Radiocarbon ([Formula: see text]) is playing a crucial role in this scenario because it is one of the so-called difficult to measure isotopes; currently, [Formula: see text] analysis requires complex systems, such as accelerator mass spectrometry (AMS) or liquid scintillation counting (LSC). AMS has an outstanding limit of detection, but only a few facilities are available worldwide; LSC, which can have similar performance, is more widespread, but sample preparation can be nontrivial. In this paper, we demonstrate that the laser-based saturated-absorption cavity ring-down (SCAR) spectroscopic technique has several distinct advantages and represents a mature and accurate alternative for [Formula: see text] content determination in nuclear waste. As a proof-of-principle experiment, we show consistent results of AMS and SCAR for samples of concrete and graphite originating from nuclear installations. In particular, we determined mole fractions of 1.312(9) F[Formula: see text] and 30.951(7) F[Formula: see text] corresponding to â¼1.5 and 36.2 parts per trillion (ppt), respectively, for two different graphite samples originating from different regions of the Adiabatic Resonance Crossing activator prototype installed on one irradiation line of an MC40 Scanditronix cyclotron. Moreover, we measure a mole fraction of 0.593(8) F[Formula: see text] ([Formula: see text] ppt) from a concrete sample originating from an external wall of the Ispra-1 nuclear research reactor currently in the decommissioning phase.
