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Description
Molecular plating (MP) is a widely used technique for preparing thin targets for nuclear physics applications, including cross-section measurements [1-3] and muon spectroscopy [4]. Target characterization typically relies on areal density (mg/cm²), which gives the total amount of material but not its spatial distribution. In non-uniform films, the same total mass can correspond to different local thicknesses, affecting energy loss and energy straggling of charged parti-cles in cross-section measurements, as well as interaction probabilities in muon spectroscopy. Therefore, reliable esti-mation of the film thickness and its spatial variation is required for a more complete characterization of molecular-plated targets.
Thickness determination by indirect methods, such as mass-based or optical approaches, relies on key parameters such as density and refractive index. These parameters are not well defined for molecular-plated films, and must therefore be treated as effective values.
In this work, we explore optical interference as a simple and robust approach to estimate the thickness of molecular-plated gadolinium films. Thin films were deposited galvanostatically on carbon substrates from dimethyl acetamide-based solutions. Deposition yields were quantified by ICP-OES, and film thickness was independently verified by SEM. For films in the 50–200 nm thickness range, distinct interference colors were observed, originating from light reflected at the film surface and at the film–substrate interface.
Film thickness was determined by mapping experimentally observed colors to simulated interference color charts, using a hue-based comparison to minimize sensitivity to illumination conditions. The thickness values obtained from this optical analysis are in good agreement with those derived from deposition yields and SEM measurements. In this comparison, yield-based thicknesses were calculated assuming a representative density for an amorphous hydrox-ide/carbonate matrix, while the interference analysis relied on an effective refractive index. Although these assump-tions introduce systematic uncertainty, the agreement with independent measurements indicates that their impact is limited under the investigated conditions.
The smooth and monotonic evolution of interference colors with deposition time further suggests that the effective optical properties remain approximately constant during film growth. Overall, interference color analysis provides a rapid, non-destructive, and spatially resolved tool for thickness estimation of molecular-plated films. The method is well suited for routine target preparation and process monitoring, and offers indirect insight into the consistency of film growth under typical MP conditions.