Charged particle-induced X-ray emission (PIXE): theory

The use of charged particles as projectiles to induce X-ray fluorescence emission dates back to 1970 . Johansson used an intense proton beam of a few MeV, delivered by an electrostatic gas pedal.

By studying the spectrum of X-ray photons emitted by a Si(Li)-type detector, he laid the foundations for a highly sensitive multi-element analysis method: the PIXE technique (Particle Induced X-ray Emission). This sensitivity is essentially due to the intensity of the particle beams available, their ionizing power and, above all, the excellent signal-to-noise ratio of the background, far better at the time than that obtained with X-ray tubes or electron beams.

Today, this analytical technique has become a basic method for determining the presence of an elemental chemical species (Z-information) independently of any influence from its chemical environment. It provides absolute quantitative information in terms of mass per square centimeter of sample, and enables routine trace analysis in many fields of application: life and environmental sciences, physico-chemical characterization of materials, archaeometry... These applications are described in detail in Part 2 (article ).

The advantages more particularly associated with charged particle-induced X emission are as follows:

• very good sensitivity, in routine analysis, over most of the periodic table (Z > 10), with a lower relative detection limit of around 10 - ⁶ g/g and an absolute mass limit of 10- ¹⁶ g when used with a microscopic beam. This limit can be optimized over a particular zone of the periodic table by choosing the energy of the incident particles and the type of X-ray lines detected (K or L line). Under these conditions, this value varies by no more than an order of magnitude (between 10 - ⁵ and 10- ⁶ g/g) over the range 10 < Z < 92 ;

• a very high level of safety. X-ray line identification is unambiguous and spectral interference is generally limited;

• This is a non-destructive analysis technique for most materials;

• possible coupling with other ion-beam analysis techniques, such as Rutherford Backscattering Spectrometry (RBS) or Nuclear Reaction Analysis (NRA), which can provide...