ANALYTICAL FACILITIES AT THE DEPARTMENT OF GEOLOGICAL MAPPING
Computer Controlled Scanning Electron Microscope (CCSEM) Laboratory

Dirk Frei and Jørgen Kystol

Basic information on the nature and origin of both geological and non-geological materials can often be derived from grain size, shape, texture and surface properties. This type of information can be readily obtained by examination using the Scanning Electron Microscope (SEM). Like any microscope, the SEM is used to magnify small objects. While a standard microscope uses light and glass lenses to magnify, the SEM uses a finely focused beam of electrons and electronic lenses, leading to much enhanced magnification capabilities. Our SEM laboratory is based on a PHILIPS XL 40 SEM equipped with a ThermoNoran energy dispersive X-ray detection system (EDX), providing useful magnifications from 20 to 300,000 times. The system is capable of detecting secondary electrons (SE), back-scattered electrons (BSE), and the energy of characteristic photoelectron X-rays. These three types of signals are emitted from the sample as a result of the bombardment by the finely focused electron beam (also called primary electrons) and are the most commonly used in SEM.

Secondary electrons are produced when a primary electron excites an electron in the sample. The excited electron undergoes elastic and inelastic collisions as it moves towards the surface of the sample, where it can escape if it still has sufficient energy. Although electrons are generated throughout the region excited by the primary electron beam, only those electrons that originate from less than approximately 10 Å depth in the sample escape to be detected as secondary electrons. This shallow depth of production makes secondary electrons very sensitive to topography, and hence the image obtained from secondary electrons is perfectly suited to characterise sample morphology.

Back-scattered electrons (BSE) are primary electrons that have undergone a number of collisions in the sample and have subsequently re-emerged from the sample surface. The number of BSE that re-emerge is controlled by the mean atomic number of the elements composing the sample: the greater the mean atomic number of the sample, the greater the back-scattered electron yield. Therefore, the image obtained from back-scattered electrons is an atomic number contrast image and hence allows resolution of variations in chemical composition.

Characteristic photoelectron X-rays are generated when the primary electrons of the beam interact with the sample atoms. These X-rays are characteristic for the elements present in the sample. For example, if calcium is present in the sample, then in the X-ray spectrum (a plot of X-ray energy versus intensity) an X-ray emission line ("peak") for calcium will appear. By identifying the emission lines in an acquired spectrum using an energy dispersive X-ray detector (EDX) it is possible to determine which elements are present in the sample. By normalizing the intensities of the X-ray peaks indentified to a major peak or the total number of counts it is possible to quantify the proportions of each element present.

SEM and EDX analyses are capable of providing unparalleled information on particulate samples when operating in tandem under the control of imaging and spectral analysis software. Our laboratory uses the Vantage image and spectral analysis software package for fully computer-controlled operation of the SEM.
Computer Controlled SEM (CCSEM) allows fully automated data collection, making it possible to acquire information such as grain size, shape, composition, and abundance of particles of a sample. Because the analysis is automated, a large number of particles can be analysed quickly and consistently. CCSEM is currently one of the most powerful tools for the fast and precise determination of grain size distributions and modal abundances in particulate samples.

For all inquiries and for additional information about the CCSEM laboratory contact Dirk Frei (email: df@geus.dk; Tel: 0045-38142263) or Jørgen Kystol (email: jk@geus.dk; Tel: 0045-38142675).