1. Electron microprobe in Askja
Bergdís Björk Bæringsdóttir
Daníel Arnar Tómasson
Kristján Einar Guðmundsson

2. Theory of scanning electron microscope
Electrons are released from the filament of the SEM and focused towards the sample
A portion of the electrons from the beam miss the electrons in the sample and are deflected back by the magnetic field of the nucleus.
These are the backscattered electrons (BE) whose energy is largely unaffected in the process
Amount of BE deflection increases with atomic number (Z), creating a increase in signal of backscattered energy and a contrast to the image when there is a compound of high atomic number scattering (1)
(1)

3. Secondary electrons
A secondary electron (SE) is ejected when it is hit by an electron from the beam
In the collision it is ejected with an amount of kinetic energy equal to the difference between its ionization energy and the energy of the electron beam:
KE(SE) = E(BE) – IE(SE)

4. X-ray emission & auger electrons
Once ejected, there is a vacancy in the shell
A higher-shell electron lowers its energy by jumping down into the vacant slot
x-ray emission is emitted, proportional to the energy difference between the two shells
The x-ray energy also excites a higher shell electron and cause it to be ejected
During a constant release of electrons from the filament, all of these processes (and more) occur simultanously to some extent

5. The electron microprobe
High-definition imaging and
precise quantitative analysis.
Used on minerals, rocks, metals,
alloys, ceramics, glass, bones and shells.
It can analyze spots in samples down
to about 1 micrometer in diameter
Has a vacuum about 10-4 to 10-5 Pa
and a current about 15 kV.

6. How it works
A very thin electron beam that has an emission current about 12 microAmps is directed at a sample, which only gets around 20 nanoAmps of that beam, so electrons and X-rays are released so the X-rays transmit to a crystal that reflects the beam to a sensor. The crystal and the sensor move simultaneously by the element that is being viewed. Then the image of the surface of the sample can be viewed.

7. Sensors
Sensors which view backscattered electrons that indicate changes in the density of the sample.
Wavelength-dispersive spectrometry (WDS) that is used for accurate measurements of the amount of elements from beryllium to uranium.
EDS detector that senses X-rays by their energy.
CL detector that senses weak emission of visible light from the samples.

8. Sample holder
Before samples are placed in the sample holder they are sanded and covered with a layer of carbon.
The carbon layer is added by super heating a graphite rode until carbon „gas“ is made which covers the sample and ensures conductivity
The sample that was used was a part of an aluminium house covering

9. Analysis Here you can see a small piece of the sanded surface.
As is visible in the secondary electron photo the sample is vary scratched close up. But the backscattered electron analyses different densities.
Secondary electron photo
Backscattered electron photo

10. Backscattered electrons
A small area ~ 80 µm was selected to analyse, where all different „colours“ of the sample are in the selected area.
As backscattered electrons detect different density's it is possible to use them to analyse the chemical structure of the sample, high density = light phase, low density = dark phase.
A databank is used for the analysis.

11. Elemental analysis

12. Elemental composition of sample
Dark phase-001
Formula mass% Atom% Sigma Net K ratio Line
O K
Al K
Si K
P K
Ti K
Total
Light phase-002
Formula mass% Atom% Sigma Net K ratio Line
O K
Al K
Mn K
Fe K
Total
Main phase-003
Formula mass% Atom% Sigma Net K ratio Line
O K
Al K
Total

13. Second darkest phase-004
Formula mass% Atom% Sigma Net K ratio Line
O K
Al K
Si K
P K
Total

14. Elemental composition of total sample area
Formula mass% Atom% Sigma Net K ratio Line
O K
Al K
Si K
P* K
Mn K
Fe K
Total