shells of progressively decreasing binding energy, and it is seen that (with the exception of the K shell) each shell consists of several sub-shells of slightly different energy. We find that during this time the overall spectral peak is found at the characteristic frequency of the reverse shock. 3.1 shows how the orbital electrons may be grouped together into the K, L, M etc. The standard model of gamma-ray burst afterglows is based on synchrotron radiation from a blast wave produced when the relativistic ejecta encounters the surrounding medium. The classification of the orbital electrons into shells and the designation of individual electrons by means of quantum numbers are well-known features of the Bohr theory. The development of wave mechanics added to this understanding and enabled a detailed interpretation of the fine structure of spectra to be built up. This ‘characteristic’ radiation has been the subject of much study, and can be understood in considerable detail from the Bohr theory of the atom as elaborated by Sommerfeld and others. Crystal diffraction had not yet been discovered, but by studying the absorption of the radiation by successive layers of material it was established that the radiation consisted of a continuous spectrum (discussed in the previous chapter), and also a monoenergetic component, or ‘line’ spectrum (soon to be found to consist of several lines) the penetrating power of which depended markedly on the target element. Soon after the discovery of X-rays, attempts were made to investigate the spectral characteristics of the radiation. Energy levels and X-ray spectra from singly-ionized atoms
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