An Overview of XRF Basics
1. Fundamental Principles
1.1 Electromagnetic Radiation, Quanta
From a physical point of view, X-rays are of the same nature as visible light. Visible light can be described as electromagnetic wave radiation whose variety of colors (e.g. the colors of the rainbow) we interpret as different wavelengths. The wavelengths of electromagnetic radiation reach from the kilometer range of radio waves up to the picometer range (10-12 m) of hard gamma radiation (Table 1).
| Energy range (keV) | Wavelength range | Name |
|---|---|---|
| < 10-7 | cm to km | Radio waves (short, medium, long waves) |
| < 10-3 | µm to cm | Microwaves |
| < 10-3 | µm to mm | Infra-red |
| 0.0017 - 0.0033 | 380 to 750 nm | Visible light |
| 0.0033 - 0.1 | 10 to 380 nm | Ultra-violet |
| 0.11 - 100 | 0.01 to 12 nm | X-rays |
| 10 - 5000 | 0.0002 to 0.12 nm | Gamma radiation |
In the following text, the unit nanometer (nm = 10-9 m) is used for the wavelength, λ (= Lambda), and the unit kiloelectronvolts (keV) for energy, E.
Comment
In literature the unit Angström (Å) is often stated for the wavelength:
1 Å = 0.1 nm = 10-10 m
The following relationship (conversion formula) exists between the units E (keV) and λ (nm):
| E(keV) = | 1.24 | or | λ(nm) = | 1.24 |
| λ(nm) | E(keV) |
The X-ray fluorescence analysis records the following range of energy or wavelengths:
E = 0.11 – 60 keV
λ = 11.3 – 0.02 nm
Apart from the wave properties, light also has the properties of particles. This is expressed by the term "photon". In the following we will be using the term quanta or X-ray quanta for this. The radiation intensity is the number of X-ray quanta that are emitted or measured per unit of time. We use the number of X-ray quanta measured per second, cps (= counts per second) or kcps (= kilocounts per second) as the unit of intensity.
1.1.1 The Origin of X-rays
Electromagnetic radiation can occur whenever electrically charged particles, particularly electrons, lose energy as a result of a change in their state of motion, e.g. upon deceleration, changing direction or moving to a lower energy level in the atomic shell. The deceleration of electrons and the transition from an energy level in the atomic shell to a lower one play an important part in the creation of X-rays in the field of X-ray analysis. To understand the processes in the atomic shell we must take a look at the Bohr's atomic model.
1.1.2 Bohr's Atomic Model
Bohr's atomic model describes the structure of an atom as an atomic nucleus surrounded by electron shells (Fig. 1):
Fig. 1: Bohr's atomic model, shell model
The positively charged nucleus is surrounded by electrons that move within defined areas ("shells"). The differences in the strength of the electrons' bonds to the atomic nucleus are very clear depending on the area or level they occupy, i.e. they vary in their energy. When we talk about this we refer to energy levels or energy shells. This means that a clearly defined minimum amount of energy is required to release an electron of the innermost shell from the atom. To release an electron of the second innermost shell from the atom, a clearly defined minimum amount of energy is required that is lower than that needed to release an innermost electron. An electron's bond within an atom is weaker the farther away it is from the atom's nucleus. The minimum amount of energy required to release an electron from the atom, and thus the energy with which it is bound in the atom, is also referred to as the binding energy of the electron in the atom.
The binding energy of an electron in an atom is established mainly by determining the incident. It is for this reason that the term absorption edge is very often found in literature:
Energy level = binding energy = absorption edge
The individual shells are labelled with the letters K, L, M, N, ... , the innermost shell being the K-shell, the second innermost the L-shell, etc. The K-shell is occupied by 2 electrons. The L-shell has three sub-levels and can contain up to 8 electrons. The M-shell has five sub-levels and can contain up to 18 electrons.
1.1.3 Characteristic Radiation
Every element is clearly defined by its atomic number Z in the periodic table of elements or by the number of its electrons in a neutral state. The binding energies or the energy levels in every element are different and characteristic for every element as a result of the varying number of electrons (negative charges) or the number Z of the positive charges in the atomic nucleus (= atomic number).
If an electron of an inner shell is now separated from the atom by the irradiation of energy, an electron from a higher shell falls into this resultant "hole" which releases an amount of energy equivalent to the difference between the energy levels involved.
The energy being released can be either emitted in the form of an X-ray or transferred to another atomic shell electron (Auger effect). The probability of an X-ray resulting from this process is called the fluorescence yield ω. This depends on the element's atomic number and the shell in which the "hole" occurred. ω is very low for light elements (approx. 10-4 for boron) and almost reaches a value of 1 for the K-shell of heavier elements (e.g. uranium).
However, since the energy or wavelength of the X-ray is very characteristic for the element from which it is emitted; such radiation is called characteristic X-rays.
This provides the basis for determining chemical elements with the aid of X-ray fluorescence analysis.