MAGE platform

MAGE : micro-analyses and imagery geomaterials and environment

Platform of MMAE subnet of REGEF french network

Principle of EPMA technique (Electron Probe Micro Analyser)

Presentation EPMA Cameca SX100

EPMA for Electron Probe Micro Analysis is also known as Castaing microprobe. It is a non-destructive in situ analysis technique allowing the detection of all elements from Beryllium in a volume of about a cubic micrometer with a sensitivity of approximately 100ppm.

Principle
Electronic microprobe analyzes X-ray emission produced by interaction between incidents electrons and elements constituting material to be analyzed. Other products of this interaction can be exploited in the same device.

Interaction des électrons avec la matière

Interaction of electrons with matter

X-ray emission from material is analyzed in the case of EPMA CAMECA SX100 by five wavelength dispersive spectrometers (WDS: Wavelength Dispersion Spectrometer). This type of spectrometer consists of a monochromator crystal, a proportional gas flow counter and a counting chain. For a given wavelength, X-ray source (an analysis point on surface of sample), monochromator crystal and counter are located at three points on the focusing circle or Rowland circle.

Positions on Rowland circle

Under these ideal conditions there is constructive interference in crystal lattice for X-ray radiation reaching crystal in Bragg position. In this case it can be diffracted towards the detector.

Bragg law : 2dsinӨ = nλ

It is necessary to use several crystals : each having in fact an inter-reticular distance adapted to a range of wavelengths characteristic of a limited number of elements. In the particular case of EPMA CAMECA SX100, the use of several spectrometers, each equipped with up to three crystals, makes it possible to collect and measure X-rays from several elements at same time.

Schematic of a WDS spectrometer

Carrying out the X analysis consists of positioning the 5 spectrometers on the characteristic wavelengths of 5 elements and making a measurement on the RX peaks em
emerging. This measurement is accompanied by that of the continuous background (braking radiation or Bremsstrahlüng) on either side of each peak. By subtracting this background we obtain an intensity which will be used to calculate the concentration by comparison with the intensity obtained under the same conditions on a standard.

Example of an X-ray spectrum

Applications
Major and minor elements can be measured in most materials at nanogram scale.

Given the importance of positioning on Rowland’s circle, equipment is equipped with a precise device for three-axis positioning of sample, accompanied by a classic microscope with zoom, enabled through a color camera, identification of areas to be analyzed. In addition, transparent samples benefit from transmission and polarization lighting.

Specifications
• Elementary X-ray spectrum allowing qualitative or semi-quantitative analysis of material.
• Elemental X-intensity table from which composition of major elements can be calculated with a relative error bar of typically +/-1%.

In addition to X-ray emission, other radiations are also used by this microprobe, so that it can also produce Scanning Electron Microscopy images :
• We can of course produce chemical contrast images obtained from local X-ray EMISSION. By moving sample, imaged field can be located within millimeters.
• Topographic contrast images using SECONDARY ELECTRONS.
• Atomic number (or “phase”) contrast images generated by BACKSCATTERED ELECTRONS.

In-depth presentation of Electronic Microprobe :

EPMA