What Is XRF?

Aim of the Article

The aim of this article is to offer the reader an overview of XRF as a technique, the instruments and components available and the industries that use them. Ultimately the article is designed to impartially guide the reader into making an informed decision about what type of XRF is available to them. Additionally, suggestions are offered on the legislation regarding the acquisition of XRF instruments.

Table of Contents

Introduction

X-ray Fluorescence (XRF) is a spectrometric technique used to perform elemental analysis and coating thickness measurements non-destructively on samples. XRF relies on the fundamental principle that when an energy source excites individual atoms, the atoms emit energy or a wavelength of light, characteristic of the atom it originated from. By analysing the number of photons of each energy, samples may be segregated into their composition elements. For example, similar steel components can be graded by differentiating them based on their constituent elements (Table 1). The data could additionally be viewed in a graphical representation, which makes identifying unknown elements easier (Figure 1). 

Table 1: Differentiation of steel alloys based on their elemental composition using XRF. The percent of Iron (Fe), Nickel (Ni), Manganese (Mn) and Chrome (Cr) in the alloy show clear differences between the alloys  

Figure 1: Graphical representation of a Stainless-Steel alloy (A) and Hastelloy (B). The composing elements of each alloy have been added for clarity and to distinguish one alloy from the other. The spectrum is displayed as signal to the detector (counts per second) versus the fluorescent energies (channel).

XRF Fundamentals

XRF instruments typically all function in a similar manner. Primary X-ray radiation is directed towards a target sample which is to be examined. The primary X-rays, upon reaching the target material, dislodge electrons from the innermost (K) shell of an atom. The resulting absence of electron in the K shell forces one from higher energy shells (L or M) to fill the void, thus returning the atom to the lowest energy state. As the electron drops to the innermost shell it releases a photon equivalent to the energy difference between the two electron shells. The diagram below shows a depiction of this process (Figure 2).

It is certainly true that atoms have more than one electron shell, each hosting electrons which are able to fill the empty space left by the displaced electron in the innermost shell. As the atomic number rises in the periodic table of elements so do the numbers of electrons that an element can hold in its orbits at a certain distance. Thus, when the electron from the K shell is displaced, by one from the L or M or any subsequent shell can take its place and result in a fluorescent signal. Electrons that fall from L shell to K shell result in energies designated Kα, while those that fall from the M shell are referred to as Kβ (Figure 2). It additionally is possible for electrons to be displaced from the L shell resulting in a signal and is denoted as Lα, Lβ etc.

Figure 2: Schematic diagram depicting the principle by which XRF instruments function. Primary X radiation (1.) dislodges an electron from the inner shell of an atom. The escaping electron (2.) leaves a void which other electrons can drop into to reach a more stable, lower energy state. By dropping to the innermost shell, energy (in the form of an X-ray photon) is released, equivalent to the difference in energy levels between the shells.

Instrument Set Up

Most XRF instruments on the market today belong to two distinct categories: energy-dispersive (ED) and wavelength-dispersive (WD) XRF. Within these categories exist numerous configurations of XRF instrument with differing X-ray sources, detectors, optics and chamber/system orientations. To give an overview of the topic both ED-XRF and WD-XRF will be explored though ED-XRF will be the focus of this article.

WD-XRF

First, WD-XRF systems operate on Bragg’s Law (Equation 1), which states:

When an X-ray meets the surface of a crystal at an incident angle (θ), it will also be reflected at the same angle (θ) off the crystal surface. And, when the path difference (d) is a whole number (n) of the wavelength, only then would constructive interference occur between the scattered X-ray beams (Bragg & Bragg, 1914).

The law can be summarised as:

Equation 1: Bragg’s Law equation. Λ is the wavelength of the X-ray, d is the spacing of the crystal layers (path difference), θ is the incident angle (angle between the incident ray and the scattered plane and n is an integer.

The schematic diagram below shows the layout of a typical WD-XRF instrument (Figure 3). In principle WD-XRF instruments require an X-ray generator, filter, sample stage, collimator, crystal and detector. The function of many components is similar in ED-XRF and WD-XRF instruments, as such in this section only the different components will be mentioned while the common components will be discussed in the ED-XRF section. The type of detector used in WD-XRF instruments may be of the proportional counter variety or silicon-based detectors for light elements up to iron (Fe), while scintillation type detectors may be used for heavier elements; the main difference between the ED-XRF and WD-XRF system detectors is that the WD-XRF detectors only analyse single wavelengths directed from the crystal, while ED-XRF detectors analyse the whole spectrum of energies from the sample. ED-XRF instruments use proportional counter type detectors or silicon-based detectors and will be discussed further below.

Figure 3: Schematic diagram of a typical WD-XRF instrument. X-rays first pass through a primary filter and excite the electrons in the sample. The secondary X-rays pass through a collimator before reaching the crystal which selectively (according to Bragg’s Law) sends signals to the detector.

The other major component that is different between the two types of XRF instrument is the crystal that is used in WD-XRF to selectively present the detector with wavelengths emerging from the sample. It is of note that two types of crystal/detector orientations can be used to detect signals. The first is a fixed crystal/detector geometry, where the signal is presented to crystals on a rotating turret. Each crystal in the turret can reflect a number of specific wavelengths to the detector. Although this set-up is designed for speed it is usually limited in the number of wavelengths it can transmit. The second set-up involves mounting the detector on a goniometer which allows it to rotate around the crystal to interrogate each wavelength sequentially by modifying the incidence angle between the crystal and the detector. This however makes instruments with this set-up quite slow in analysing a sample. 

ED-XRF

For ED-XRF instruments the orientation is quite straight forward as the schematic diagram below shows (Figure 4). The primary radiation used to elicit a signal from the sample is generated in an XRF generator.

Figure 4: Schematic diagram of a typical ED-XRF instrument setup. X-rays generated in the X-ray generator pass through a primary filter and a mirror providing an optical image, through a collimator and onto the sample. The fluorescence signal emitted by the sample is detected by the detector which is positioned at a critical angle to the primary excitation beam.

X-ray Generator

XRF generators are normally designed as a glass chamber housing the cathode and the target material, from which the X-rays will be generated, as the anode. Typically, tungsten is used as the target material as it offers sufficient excitation across a wide range of wavelengths, but specialist anodes can also be used to elicit excitation at specific wavelengths. These may include chromium, gold, silver, aluminium, molybdenum or even rhodium. The glass chamber is mounted in an oil-filled shield to prevent escape of ionising radiation, to dissipate the heat generated inside and to maintain stable temperatures even under heavy use, thus prolonging instrument longevity. The X-ray required to excite the sample are directed out through a window usually made from beryllium. ED-XRF instruments usually require between 10 and 50kV to generate the primary X-ray beam.

Primary Filters

The primary X-ray beam first passes through a primary filter, which modifies the beam to maximise the signal to noise ratio. Filters of nickel, aluminium and molybdenum are typically used in XRF but copper or titanium filters at varying thicknesses can also be used for specialist applications. As an example, aluminium filters are used in trace analysis of heavy metals, as they subdue background noise over which the trace amounts of heavy metals may not be detected. Some instruments possess a filter turret which houses multiple filters to give a wider range of applications the instrument can address. As an example, a 10 μm nickel filter is used to measure thicknesses of electrodeposited layers on various substrates, while an aluminium (500 μm or 1000 μm) filter may be used for precious metal analysis.

Collimators

Next the primary X-ray beam is collimated to a specific spot size by passing through a collimator. The application of the ED-XRF system is used for usually dictates the type of collimator that is used. To measure coatings such as electroless nickel immersion gold (ENIG) in electronics applications, a small collimator must be used to ensure only the feature of interest is excited to produce a signal. A compromise must be made between the size of the beam and the signal (count rate), as decreasing the size of the collimator (and effective primary beam size), the signal becomes difficult to differentiate from the background signal. One way to overcome this issue is to use capillary optics to reduce the primary beam size. Capillaries are normally glass cylinders which utilize internal reflections to direct the beam down the length of the capillary into the sample. Thus, the energy imparted onto the sample is greater even at sizes down to 10 μm.

Figure 5: Different types of capillaries which use uses Total External Reflection to direct the primary X-ray beam on to the focal plane of the sample. A monocapillary (left) is a glass tube which focuses the X-ray beam onto the sample surface. A Polycapillary (right) is a bundle of glass capillaries, tapered at each end which deliver the X-ray beam to the sample.

Detectors

In order to detect the signal from the samples, ED-XRF systems use one of two types of detectors: Proportional Counter or Silicon based detectors.

Proportional counter detectors are characteristically gas-filled chambers (housing a cathode at the periphery) with a wire filament at their core (anode), see below (Figure 6). The detector is typically filled with a noble gas (neon, argon, krypton or xenon) and can be mixed with a secondary gas to provide a quench once the signal is detected.

The proportional counter functions by allowing the X-ray fluorescence signal (ionised particles) to enter the detector and collide with atoms of the inert gas inside. The inert gas atoms are themselves ionised to produce electrons, which migrate towards the anode, and a positively charged ion which migrate to the cathode.

Collectively the electrons and positive ions are referred to as “ion pairs.” As the electrons get closer to the anode they tend to get accelerated by the electric potential of the anode and collide with further atoms of gas and multiply the signal. These multiplications are referred to as Townsend Avalanches, which are detected as changes in voltage across the anode. These signals are then interpreted by the software that is supplied with the instruments from various manufacturers.

Figure 6: Schematic Diagram of a proportional counter detector (left) and the method of signal detection within a proportional counter (expanded).

Proportional counter detectors have relatively large apertures for signals to enter, therefore an advantage of such a detector is that the signal received is relatively abundant. For relatively simple applications (for example measuring thicknesses of general electrodeposited coatings), short measurement times can be expected while still maintaining a high degree of accuracy. Typical resolution of approximately 900eV can be expected for this type of detector. For further information on these detectors please see the review by G.F. Knoll (Knoll, 2000).

The Silicon Drift Detector (SDD) is currently the pinnacle of detector technology. They are silicon semiconductors doped with p-type acceptors and n-type donors to allow the controlled propagation of the X-ray signal to the anode. It acts much like the proportional counter detector in that is measures the incoming ionised particle “cloud” by attracting them to the anode by applying a differential charge between the anode and cathode. The charges at the anode are converted to a signal by a Field Effect Transistor (FET) which are then amplified by the preamplifier and measured by the pulse processor. The pulse processor not only has to differentiate between different X-ray energies but also between different events arriving at very short intervals. For more information on SDD please see Strüder et al. (Strüder, et al., 1998).

Silicon Drift Detectors due to their small size tend to acquire lower count rates (rate of incoming signals). However, due to their design they are faster, have lower noise, no requirement for external cooling (e.g. liquid nitrogen) and they have much better resolution (120eV – 200eV) when compared to proportional counter detectors. As a result, they are better able to resolve similar alloys when performing elemental analysis, much thinner coatings can be measured, trace analysis is more accurate, elements of lower atomic weight and measurement of multiple layers is far better than with proportional counter detectors.

The above components are essential for correct functioning of the instruments; however, some instruments can possess multiples of the same component. For example, instruments do exist where multiple X-ray generators have been fitted to use different primary excitation beams to preferentially excite different atoms in a sample. Multiple detectors can also be used to speed up measurements or isolate specific responses from atoms in a sample.

Instrument Types

There is a wide variety of XRF instruments on the market currently and they generally fall into 4 categories:

Chambered – Beam down systems: These are typical XRF systems on the market. The X-ray beam is generated above the sample; thus, these instruments tend to have larger chambers, tables for sample support and potentially XY motorised stages which permit automated measurements of many samples at once, increasing throughput. Focusing of the instrument can be manual or automatic and they can accommodate complex component geometries.

Chambered – Beam up systems: These instruments offer a smaller footprint than the “beam down” systems, but normally have a smaller chamber. They unfortunately do not handle complex geometries as their focus is fixed to a window onto which a sample is placed. This limits the type of sample that can be tested to components with flat sides. The benefits of having a system of this type is that any sample that needs to be tested is immediately in focus once placed on the window.

Portable instruments: These instruments are miniaturised versions of the above instruments. They are primarily used for large components and/or Positive Material Identification (PMI) however, certain manufacturers have designed these instruments to be able to measure multiple layered coatings. This makes these instruments highly versatile, however, because they lack a chamber, they require extra engineering and operator vigilance to be operated safely. Some manufacturers offer these instruments with a chamber for added versatility and safety.

Special projects: These instruments tend to be more complex than the off the shelf instruments above. They can include, but are not limited to, instruments which operate under a vacuum or helium purge [for the analysis of extremely light elements], reel-to-reel systems which automatically analyse strip plated components and offer feedback control to the plating line [for thickness testing of coatings on strip plated connectors], Wafer handling systems which incorporate into clean room environments for automated measurements [ used for semiconductor wafer manufacture]. These systems tend to allow more automated control of the systems that they are integrated with.

Sample Preparation and Parameters Affecting Readings

XRF tends not to require much sample preparation at all. Once a component is manufactured to its final specification or indeed any intermediate steps, the sample can be analysed by XRF. This makes XRF an extremely versatile method for testing that samples meet manufacturers specifications. Not only are solid samples able to be tested but XRF can be utilised for testing liquids for metal content, powders and slurries for trace element analysis, plastics or rubber for RoHS applications to name but a few.

There are six factors that influence the trueness and precision of X-ray fluorescence measurements:

Measuring Distance:

The measuring distance is one of the first parameters that needs to be addressed when a sample needs to be tested. The signal at the detector (count rate) can be summarised by the equation

Thus, as the measuring distance increases, the count rate drops as a function of the equation above. Additionally, the repeatability of the precision can be written as:

This suggests that to increase the count rate at the detector and the precision repeatability of the readings, one must decrease the distance from the sample surface to be measured. This may not be possible with complex component geometries however, pioneering methods such as the patented Distance Controlled Measurements (DCM) method offered by Helmut Fischer GmbH seek to overcome this obstacle by correcting the readings in relation to the measuring distance. This allows one to measure precisely and truly at suboptimal distances. Without DCM, acquisition times would need to be much longer than what is practical for everyday use.

Collimator size:

The size of the collimator selected for the application is the next factor that influences the trueness and precision of the readings. The diameter of the collimator and thus the size of the X-ray beam (spot) directly affects the count rate.

This suggests that the more X-ray energy that is delivered to the sample the more that the detector can detect. In practice, a smaller measurement spot allows the best determination of the inhomogeneities in a sample. Additionally, the measurement times tend to be longer to improve trueness and precision. Conversely, a larger spot size is influenced less by the inhomogeneity within the sample; the acquisition times tend to be lower with larger collimators.

Measurement Time:

The measurement times tend to increase repeatability as more signal is offered to the detector and the instrument can perform statistical calculation on more data.

By this relationship a four time longer measuring time improves the repeatability precision by a factor of 2. Often a very long measurement time (> 4 minutes) is not feasible, thus it is recommended to perform several shorter measurements and average them.

Sample thickness:

The thickness of a coating influences the measurement uncertainty. Typically, all elements have a maximum thickness at which the uncertainty of the readings is far higher than the possible thickness of the sample. This is dictated by the density of the coating that is being measured and beyond this thickness, coatings tend to absorb the fluorescent signal themselves again increasing the uncertainty.

Focusing:

Samples must be focused such that the software can calculate the amount of energy that is delivered to the sample being tested. If the sample is not on the focal plane suboptimal amounts of energy are delivered leading to erroneous results. Manufacturers have introduced cameras to their equipment to allow focusing of the samples while companies such as Helmut Fischer GmbH offer an additional ’Ratio Mode’ function that measures the relative drop in a reference material signal as the test signal rises.

Trigonometry:

Samples with surfaces being tested that are not horizontal to the primary excitation beam can cause erroneous readings. A non-perpendicular relationship between the primary x-ray beam and the sample surface can cause shielding of the detector, or a variation in the secondary x-ray emission path length through a coating, either of which can change the detected intensity of fluorescent radiation from the sample and therefore the calculated thickness and / or composition.

Industries That Utilize XRF

XRF instruments have seen prominence in scientific and industrial applications, with many companies performing electrodeposition of metals or alloys on manufactured components relying on XRF to meet specifications.

Typical industries include the electronics industry, which uses XRF to measure coatings and perform elemental analysis on ever decreasing sized components. Applications include measuring the Pb content of solder, ENIG or ENEPIG and semiconductor manufacture. The precious metals recycling, manufacturing and assaying world utilise XRF to analyse the metal content of alloys such as jewellery. The aerospace industry utilises XRF for research and development of novel coatings for components such as turbine blades. Suppliers into any of these industries are required to comply with specifications set by their customers, as such XRF is a fast and reliable method for ensuring compliance.

Regulating the use of XRF Instruments

Currently the Health Safety Executive (HSE) has set out the Ionising Radiation Regulations 2017 (IRR17) which all companies and operators must follow for the safe use of XRF instruments. IRR17 states that any instrument capable of producing ionising radiation with components operating at a potential difference of more than 5kV, must be registered. Full rules and regulations can be found on the government website (gov.co.uk, 2017). Briefly, a few rules have to be followed when acquiring an XRF instrument:

  • The HSE has to be notified of an XRF instrument when it has been acquired.
  • Prior to the system being delivered onsite, the company that bought the system must employ a Radiation Protection Adviser (RPA) to advise on the safe use of the equipment (including but not limited to, setting up exclusion zones, designating operators, safety of any pregnant employees etc).
  • Companies must designate a Radiation Protection Supervisor (RPS) from within the company who are responsible for the instrument and operators to enforce safe operation procedures and in case of incidents.

In the Republic of Ireland, the Environmental Protection Agency (EPA) is tasked with enforcing Ionising Radiation Regulations 2019 (IRR19). For further information on the rules and regulations in Ireland regarding IRR19 please visit the EPA website (EPA, 2020).

For more information on how Fischer Instrumentation (GB) Ltd. can assist with your XRF requirements, please follow the links below.

References

Bragg, W. H. & Bragg, L., 1914. The Diffraction of Short Electromagnetic Waves by a Crystal. In: Proceedings of the Cambride Philosphical Society VOL XVII. s.l.:Cambrdge: at the University Press, pp. 34-57.

EPA, 2020. Environmental Protection Agency. [Online]
Available at: https://www.epa.ie/radiation/regulation/irr2019/

gov.co.uk, 2017. Ionising Radiation Regulations 2017. [Online]
Available at: http://www.legislation.gov.uk/uksi/2017/1075/pdfs/uksi_20171075_en.pdf

Knoll, G. F., 2000. Radiation detectors for X-ray and gamma-ray spectroscopy. Journal of Radioanalytical and Nuclear Chemistry, 243(1), pp. 125-131.

Strüder, L., Leutenegger, P. & Lechner, P., 1998. Silicon drift detector – The key to new experiments. The Science of Nature, 85(11), pp. 539-543.

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