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Cathodoluminescence System


evaluating the economic potential of deposits [ 18 ]. In particular, CL spectroscopy is useful in distin- guishing plutonic quartz (that is, the high-quality detrital grains) from lower-grade quartz cements by their distinct CL signatures.


A typical CL workflow for sandstone is shown in Figure 7 . Figure 7a shows a panchromatic photomultiplier (PMT) image that exhibits an interesting CL contrast on a fractured grain. Subsequently, a ROI is selected from this image to perform a more in-depth spectral analysis. To visualize the contrast in the hyperspectral CL data we have made false-color RGB images where the spectral region from 390 to 650 nm is separated in 3 sections that are binned such that the total intensity in these spectral regions defi nes an RGB code. Figures 7 c and 7 d show such images for 5 and 15 kV acceleration voltage. In both cases there is strong contrast revealing regions with strong “blue” and “red” emission that are completely invisible in the SE image ( Figure 7b ). We also note that the colors are not entirely the same for the two acceleration voltages and that the image for 15 kV is more blurry and uniform, reflecting a larger excitation volume. In (e) and (f) we show spectra for the two distinctly colored regions as indicated in (d) at 5 and 15 kV, respectively. Consistent with the color coding in (c, d) we fi nd that the blue peak at λ = 410 nm is dominant at region 1 whereas the red peak at λ = 640 nm dominates in region 2. T ese peaks are associated with intrinsic defects in the SiO 2 crystal matrix [ 19 ]. T eir diff erent relative magnitude indicates that these are two different quartz types: the blue emitting quartz corresponds to the larger cracked detrital crystal grain whereas the red emitting quartz corresponds to cement that has infi lled the cracks [ 18 ].


Figure 8 : Quartz analysis. Single-wavelength PMT measurements taken with a (a) 450 nm and a (b) 600 nm band pass fi lter with a 40 nm bandwidth. (c) SEM image of a sandstone sample corresponding to the region inspected with the PMT. (d) Red-blue color image in which the PMT images in (a) and (b) are placed in the blue and red color channel, respectively. For these measurements the conditions were 40 nA beam current, 15 kV acceleration voltage, and a 40 μ s dwell time.


they are also used by the petroleum industry for assessment of reservoir rocks.


Discussion


Knowledge about the distinct spectral properties of diff erent quartz types can be used to perform fast segmentation of larger areas by performing color-fi ltered PMT imaging. T e images in Figures 8 a and 8 b correspond to CL images from the same area but taken with diff erent color fi lters that are roughly matched to the two peak positions of Figure 7f . Such color-fi ltered images can be combined into a multicolor image as shown in Figure 8d (in this case the images in (a) and (b) are mapped onto the blue and red color channels, respectively). Such false coloring can be used for any combination of color fi lters and helps to visualize CL features. These segmentation studies provide a means to quanti- tatively map the quartz composition of a rock sample. By combining such results with in-depth analysis of the observed textures such as fractures and grain contacts, the geological history as well as the porosity or permeability of the rock can be established. While such studies are of fundamental interest,


2016 May • www.microscopy-today.com


The motorized parabolic mirror system in the SPARC allows precise alignment of the optical system, thus greatly boosting the collection effi ency and enabling measurements on low-light-emitting samples such as the metallic nanostructures mentioned above. Additionally, in cases where the incoherent form of CL is studied, it allows one to employ lower acceler- ation voltages, in which the CL yield per incoming electron is reduced, but the spatial resolution improves because of the smaller interaction volume [ 3 ] (see Figures 7 c and 7 d). Because the alignment procedure in the SPARC can be done determin- istically using the 2D imaging camera, it is highly reproducible. Hence, light output can be quantitatively compared between samples, allowing for growth-process optimization and optical quality assurance. By studying a material with a known CL emission like aluminum and measuring the probe current, the absolute system response can be recorded, enabling comparison of the CL emission in diff erent materials in absolute terms (that is, the absolute number of photons emitted per electron) [ 20 ]. The ability to perform angle-resolved measurements, including a full study of CL emission polarization, adds a whole new dimension to CL research enabling a large number of CL applications. T e angular profi les and polarization sensititivity can be used to study directionality of nanostructures [ 6 , 8 , 12 ], dispersion in (a)periodic photonic crystals [ 5 ], and chiral (handed) structures [ 10 ], and to rigorously separate coherent and incoherent CL contributions [ 10 , 20 ].


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