This is a desktop scanning electron microscope
or S. E. M.
The box on the left is the SEM,
the box on the right is the P. C.
It runs the software that interprets the data
from the SEM.
Light microscopes use visible light
to illuminate a sample
and a set of lenses to focus an image
either into our eyes or onto the sensor of a camera.
The big limitation is the wavelength of the light.
Details smaller than the wavelength will not be visible.
Electrons have both a particle nature and a wavelength
much shorter than visible light
so can see smaller details.
The SEM does not work like a camera.
The whole chamber inside the microscope
has had the air removed to create an almost perfect vacuum.
You can hear the pump working in the background.
Here is a cut-away diagram of what is inside the box.
Electrons will only move in a vacuum.
At the top is a hot tungsten filament
that drives off a cloud of electrons.
An electric field accelerates the electrons
to between 5 keV,
that is thousands of electron volts
and 15 keV.
Magnetic fields focus the electrons
onto a very small area of the sample.
When the beam hits the specimen
some electrons bounce back.
We call these back-scattered electrons.
These back-scattered electrons
are collected and analysed.
Higher atomic number elements are more efficient
at back-scattering electrons
so more hit the detector;
the software adds them up.
Bigger electron counts show as paler shades
of grey in the screen image.
The electron beam covers the area of the slide
by scanning across it in lines
to build up a complete picture.
Focussing and brightness in the picture
on the monitor screen
are handled automatically by the software.
We can examine different areas of the slide
without losing the vacuum
by using the two controls on the front of the SEM.
Software controls allow us to zoom in for a closer look at
the sample.
Here is a sequence for starting to look at a sample.
In this example, a thin polished section of rock
is mounted on a glass slide,
the slide is placed in the holder
and the height is adjusted using the rig.
The slide has to just go under the nylon roller.
This is done so that the slide does not damage the SEM
and is at the ideal working distance from the detectors.
The holder is mounted
at the base of the vacuum chamber.
The slide is centred by using the two controls
until the cross is under the notch.
The beam will now hit the middle of the slide.
The door is closed and held shut
until the vacuum secures a seal around the door edge.
The white button switches on the vacuum pump
and the blue light flashes
as the air is being pumped out.
Software monitors how good the vacuum is.
When a solid blue light is on the SEM is ready.
As well as analysing the output from the SEM
the software is also used to control it.
Pushing the on-screen start button
focuses the electron beam
and adjusts the brightness of the image. focuses the electron beam
and adjusts the brightness of the image.
Remember that what we are looking at
is not like a black and white picture from a camera
the image is a back-scatter intensity map.
Two areas of the slide
with identical material densities
will be the same shade of grey.
Having found the important feature on the slide
we can save an image to file.
In the cut-away diagram
you may have noticed another detector on the left.
This is an X ray detector.
When the 15keV electrons
hit the atoms, some energy
is temporarily stored in the orbiting electrons.
This energy is given out in the form of X rays.
The energy of these X rays
is determined by the composition
of the atom that they came from.
Each element has its own set of frequencies.
By examining the spectrum of X rays from the specimen
we can work out what elements are present
and where they are.
We call this an element map.
The technique is called
energy dispersive X ray spectroscopy
or E D S
We use a different App to do EDS.
This runs alongside the SEM software.
The image button transfers the SEM data to the new App.
You can choose to scan a particular area of the slide
by pulling the corner handles on the pale blue rectangle.
Pressing the EDS button
starts the analysis.
During scanning the beam dwells for a short time
on each point on the slide to collect data
that is data about the electrons and X rays.
These data become a pixel in the image.
Each pixel has multiple properties associated with it.
In this example the X ray count rate
is about 8000 per second.
The energy spectrum is displayed at the bottom of the App.
As long as enough X rays
from a particular element are detected
the software can automatically work out
the name of the elements.
To make it easier to see
each element has been colour coded.
Where different colours are next to each other
it can be difficult to resolve them at this stage.
This scan has been set to run for a fixed time.
The data can be saved for later re-analysis by the software.
The analysis also produces a Word document,
A copy of the SEM image,
a copy of the energy spectrum,
labelled with the element ids,
a copy of the element map
with colours to represent the individual elements.
A set of element maps
showing each element separately.
These separation maps allow you to see
where more than one element is present in a slide,
so for example,
oxygen and silicon in these 2 images
occur in the same space
so this is likely to be silica, in other words
a quartz grain.
And again in these 2 maps
iron and sulphur are present in these crystals
so this would be iron sulphide,
in other words pyrite.
Pyrite density is approximately twice that of quartz
and we can clearly see the brightness
difference in the SEM image.
Samples do not have to be thin sections,
3 dimensional samples can be attached
to a stub and mounted in the holder.
They still have to pass under the nylon roller on the rig.
This example has small fragments of ash from Stromboli.
These have been mounted on an adhesive pad.
There are also shells of micro-organisms
called foraminifera present
which must have been collected with the ash.
One difference with 3 dimensional objects
is that there may be shadows in both SEM
and EDS images
where no information reaches the sensor.
This is true especially for EDS
because the detector is offset to one side of the chamber.
Only the coloured areas in the image have been analysed.
This X ray shadowing
is like the shadow effects when the sun is low in the sky.