2.  The high-energy end of the spectrum – the particle range

Here we deal with changes in the atomic nucleus, or with changes in electronic energy levels relatively ‘deep’ within atoms. The radiation is treated more as high-energy particles than as waves. In order to use ‘spectro-‘ methods, there must be a way to distinguish the particles by their energies. While there are instruments which can act like prisms in part of this range, most measurements use a solid-state detector made of silicon doped (‘drifted’) with lithium. Germanium is used for particularly high energies (wavelengths < about 0.3 Å). A voltage is applied to the detector. An X-ray or gamma ray absorbed in the detector produces electrons (and ‘holes’) which give a pulse of current proportional to the particle energy, and a pulse height analyzer (PHA) can give a spectrum which is a plot of the number of pulses versus particle energy.

Elements that are naturally radioactive, or which have been radioactive by exposure to neutrons (as in neutron activation analysis, NAA) have well-known half-lives and emit characteristic gamma rays. Gamma ray spectra, especially if they are recorded at different times, identify and quantitate many elements. The need for a strong neutron source limits application of NAA.

X-rays can be used in emission or fluorescence. X-rays are emitted when atoms are struck by fast-moving electrons, and their wavelengths are characteristic of the element. This process is most often used in the electron microprobe. A finely focused beam of electrons is directed onto a point, or scanned across a sample. Some of the X-rays produced are caught in a solid-state detector, and produce a spectrum, or generate an image showing the distribution of a particular element. The detector-analyzer combination does XES (X-ray Energy Spectrometry) or EDXRA (Energy-dispersive X-ray analysis). These systems are not sensitive to the very lightest atoms.

In X-ray fluorescence (XRF), a beam of X-rays is directed onto a sample. They should be ‘monochromatic’, i. e., all should have the same energy. That can be arranged by selecting the material for the X-ray tube and the tube voltage, and by removing less energetic X-rays with ‘filters’. The X-rays absorbed by the sample raise inner electrons of atoms to higher energy levels, and when they ‘relax’ back to the ‘ground state’ they emit other X-rays. Those give spectra characteristic of the elements present and their concentrations.




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