Surface World May 2020 Surface World May 2020 | Page 52

TESTING & MEASUREMENT
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.
Introduction
X-ray Fluorescence (XRF) is a spectrometric technique used to perform
elemental analysis and coating thickness measurements nondestructively
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
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 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).
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.
50 MAY 2020
twitter: @surfaceworldmag