As described earlier, when an excited atom relaxes it releases the excess energy in the form of a photon. If the energy difference between the vacant electron energy state and that from which the electron filling it originates is large enough, then the photon emitted is an x-ray. The energies of the electron states depend on the charge of the nucleus and the arrangement of the other electrons in the atom, so the energies of the emitted photons are characteristic of the emitting atoms. This allows for the determination of two sample characteristics. Firstly, the peaks in the x-ray spectrum enable the microscopist to identify the elements that are present and, secondly, the relative intensities of the peaks allow for the quantitative analysis of the
proportions of each element present in the sample.
.For the atom to emit an x-ray photon the vacant energy state must lie in the K bands for elements lithium to
potassium, in the K or L bands for calcium to barium and in the K,L or M bands for elements lanthanum onwards. In this labelling convention the K shell refers to the two innermost
electron energy levels, the L shell to the 8 energy levels
outside this and the M shell to the 18 energy levels outside these. In the alternative, chemical, nomenclature the K
electrons are known as the Is, L electrons as 2s and 2p and the M electrons as the 3s,3p and 3d electrons.
It can be seen that as the number of electrons in the atom increase the number of possible electron transitions increases as well. In the case of Zinc, for instance, with thirty
electrons there are 900 combinations. Fortunately for x-ray analysis only a few of these transitions are possible, and of these few even fewer have a large enough energy difference to emit x-rays. Zinc for instance has only 5 detectable x-ray peaks.
2.9.1 Energy Dispersive X-Ray Analysis
When an x-ray photon enters a semiconductor such as silicon it creates electron-hole pairs. As the potential energy of each pair is 3.8eV and the energy of the x-ray photon is in the order of several keV a large number of such pairs are created. The precise number of pairs is a measure of the incident photon energy to quite a high resolution. If a
voltage is applied across the silicon then the electron-hole pairs will allow a current to flow, the magnitude of the
current being proportional to the number of pairs created and hence also proportional to the incident photon energy.
In practice the resistivity of pure silicon is too low, and the current flow induced by the electron-hole pairs is masked by the much higher steady state current. To avoid this problem the x-ray detector is made as a p-i-n diode, reverse biased with the active detector of the i region sandwiched between the much thinner p and n regions. The silicon is cooled to 77K (liquid nitrogen) to further decrease the conductivity. The cooling also decreases any noise due to thermal effects and prevents diffusion of the lithium dopant used to generate the diode structure. The voltage is applied through a coating of gold on the front and back surfaces of the detector. The
coating on the front surface is made as thin as possible to minimise x-ray absorption. The detector is protected behind a beryllium window to prevent contamination of the detector or degradation of detector vacuum. This window has the
undesirable effect of blocking the low energy electrons from light elements, and if one is fitted, as was the case for the detector used in this project, it is impossible to detect the presence of elements lighter then sodium. The window does, however, have the advantage of preventing secondary electrons entering the detector. Figure 9 shows the arrangement of a typical solid state detector.
There is presently a trend towards windowless detectors or detectors with an ultra-thin Mylar window. These both allow analysis of lighter elements up to boron. The ultra thin
window retains the ability to stop electrons but is not strong enough to withstand a pressure differential between the column and the detector. Such installations therefore require a
shutter arrangement to maintain detector vacuum if the microscope column is vented to air.
The current flow is transient, lasting less than a microsecond and is usually referred to as a pulse, these
pulses are amplified by an FET transistor mounted close to the detector and also cooled to reduce electrical noise and then transmitted, via a pulse processing unit, to a multi-channel analyser (MCA). This MCA increments the value of the channel corresponding to the particular x-ray energy and hence over a period of time a histogram of the emitted x-ray spectrum is
built up.