Analytical Capabilities

Introduction

The primary function of the XRF Core Scanner facility is to enable scientists to collect high-resolution elemental intensities (not concentrations) from core section-halves. The data is typically used to identify zones of interest and to qualitatively identify features such as lithological boundaries, gradual changes in mineralogy, alteration halos (in hard rocks), etc.

Instrumentation Overview

Both of the Avaatech XRF Core Scanners in our lab have similar components and thus provide very similar data. The instruments are designed to measure discrete points along a user-defined grid. The typical spot size is ~1 square centimeter, but smaller spot sizes are possible, if desired. Normally, the spots are measured down the center-line of the section (or sample), but both instruments can be adjusted to allow off-center analyses. However, due to the geometry of the detector assembly, it may not be possible to scan very far from the center-line if the core material is below the liner.

The instruments measure characteristic x-rays using a technique called Energy Dispersive Spectroscopy (EDS). In EDS, photons of all energies are measured simultaneously, allowing for faster analyses at the expense of peak resolution (and therefore measurement accuracy and precision). Both of our instruments use Brightspec silicon drift detectors (SDD), providing relatively high resolution peaks (given the constraints) and high throughput. This enables us to keep counting times under 20 seconds, and in most cases, under 10 seconds per point.

Both instruments are also fitted with water-cooled, 100 W rhodium side-window x-ray tubes. The spot size is adjusted by cross-core slits and down-core slits, and the source spectrum can be modified by several different filters: (1) Al, (2) thin Pd, (3) thick Pd, and (4) Cu.

The gap between the detector and the sample can be filled with helium to purge air and to improve the signals of the lighter elements, such as Al, Si, K, and Ca (see quick start guide).

Analytical Capabilities

Our x-ray sources are capable of generating excitation voltages up to 50 kVp, meaning that, in theory, we can measure any element that has an x-ray absorption edge below 50 keV (absorption edges). In practice, however, there are many factors that limit our ability to measure very light elements (Z < 12) and very heavy elements (Z > 58).

At the low end of the energy spectrum, we can't see peaks below ~1.0 - 1.2 keV even if we can excite the elements in the sample. The x-rays that are generated are readily absorbed by materials in their path. That means that Al is the lightest element that we can reliably measure, and we can only barely detect Mg and Na. At the high end of the energy spectrum, we can only observe L-lines for elements heavier than Eu (Z=63). Given than L-lines are far lower in intensity than corresponding K-lines [1], we can only measure heavier elements (Z > 63) if they are present in very high concentrations. In most cases, the heaviest K-line that we expect to see is actually La or Ce (Z=58) if REEs are present in high concentrations.

It's also worth pointing out that, due to the use of a rhodium x-ray source, it's very difficult to reliably measure chlorine. The chlorine K-peak has a very strong overlap with the scattered Rh L-peaks.

In most cases, you'll want to measure your core sections using multiple excitation conditions. The reasons for this will be explained elsewhere in this documentation, but in short, you'll get far better results if you focus on measuring elements in groups rather than trying to measure all elements simultaneously. In practice, we shape the energy profile of our source beam to target each group and we make repeat measurements. Our typical laboratory measurement program is as follows:

• 10 kVp, no filter: Major and minor elements (Al, Si, K, Ca, Ti, Mn, Fe, Cr, P, S, possibly Mg)

• 30 kVp, thick Pd filter: Heavier major and minor elements plus most geologically relevant trace elements (Ca, Ti, Mn, Fe, Ni, Sr, Rb, Zr, Zn, and many others)

• 50 kVp, Cu filter: Heavier trace elements (Sr, Rb, Zr, Ba, possibly La and Ce)

The lists of elements provided above are not exhaustive, and you should work with the Lab Manager to decide the optimal measurement conditions for your cores. The detection limit for each element can vary widely due to peak interference and spectral artifacts, but you can find optimistic estimates here. Elements such as Al will appear as small peaks, even when present in several weight percent, whereas heavier elements may be detected when present in the parts-per-million range.

Time Required For Scans

When planning your data acquisition program, you'll want to be aware of how long it will take to scan each section. Table 1 shows the time required to scan a typical 1.5 meter section at different resolutions and different combinations of excitation conditions. You can see that scans can take anywhere from a few minutes (low resolution, one energy) to several hours (high resolution, multiple energies).

In reality, you won't be able to keep pace with the instrument if you choose to scan at low resolutions and/or fewer excitation conditions. It takes time to prepare core sections for analyses (see quick start guide). If you're efficient, and the core surfaces are in good shape, you might be able to prepare a section and choose the points in ~10-15 minutes. In many cases, it will take significantly longer. Thus, we typically advise users to choose their conditions such that runs last at least 30 minutes.

You'll also need to budget time during your visit for other activities, such as running the standards, swapping sections, and occasionally allowing the Lab Manger to perform basic maintenance such as film changes.

Quantification

We do not provide users with the means to generate element concentrations from the peak intensities. Though it is possible (in theory) to quantify the element concentrations in the core material, it is very difficult in practice due to the irregularities in the surfaces and matrices of the core materials.

The relative intensities of element peaks (what we measure) are not directly related to the relative element concentrations. The observed peak intensity for an element depends on: (1) the concentration of the element, (2) the concentration of all other elements that absorb or emit radiation at similar energies (due to secondary fluorescence and absorption), (3) the atomic number of the element, (4) the characteristic x-ray line being measured, (5) the excitation conditions of the x-ray source, (6) the size and intensity of the x-ray beam, (7) the angle of the surface relative to the source and the detector, and (8) the thicknesses and compositions of all materials in the path of the incoming or outgoing x-rays. The lack of ability to account for many of the above measurement variations is what leads us to stay away from quantification.

If you would like to learn more about quantification, please ask the Lab Manager during your visit.