Ionising radiation emission

In terms of atomic spectrometry, NAA is a method combining excitation by nuclear reaction with delayed de-excitation of the radioactive atoms produced by emission of ionising radiation (fi, y, X-ray). Measurement of delayed particles or radiations from the decay of a radioactive product of a neutron-induced nuclear reaction is known as simple or delayed-gamma NAA, and may be purely instrumental (INAA). The y-ray energies are characteristic of specific indicator radionuclides, and their intensities are proportional to the amounts of the various target nuclides in the sample. NAA can thus... 

Radiation. Radiation (implying ionising radiation) describes both electromagnetic emission (X-rays and gamma rays) and particulate emission (alphas, betas and neutrons). 

Ionising radiation by alpha particles (= He Mons). In comparison with beta and gamma radiation, alpha radiation has the least penetrating power and the highest linear energy transfer. Radionuchde that decays to a more stable nuclide by emission of an alpha particle. 

Both emission and absorption spectra are affected in a complex way by variations in atomisation temperature. The means of excitation contributes to the complexity of the spectra. Thermal excitation by flames (1500-3000 K) only results in a limited number of lines and simple spectra. Higher temperatures increase the total atom population of the flame, and thus the sensitivity. With certain elements, however, the increase in atom population is more than offset by the loss of atoms as a result of ionisation. Temperature also determines the relative number of excited and unexcited atoms in a source. The number of unexcited atoms in a typical flame exceeds the number of excited ones by a factor of 103 to 1010 or more. At higher temperatures (up to 10 000 K), in plasmas and electrical discharges, more complex spectra result, owing to the excitation to more and higher levels, and contributions of ionised species. On the other hand, atomic absorption and atomic fluorescence spectrometry, which require excitation by absorption of UV/VIS radiation, mainly involve resonance transitions, and result in very simple spectra. 

Analysis by atomic (or optical) emission spectroscopy is based on the study of radiation emitted by atoms in their excited state, ionised by the effect of high temperature. All elements can be measured by this technique, in contrast to conventional flames that only allow the analysis of a limited number of elements. Emission spectra, which are obtained in an electron rich environment, are more complex than in flame emission. Therefore, the optical part of the spectrometer has to be of very high quality to resolve interferences and matrix effects.-... 

In atomic emission analysis, one or several specific spectral lines are monitored for each analyte. It is technically difficult and requires a high performance instrument because emission of radiation does not only occur from the analyte but also from any additional material introduced in the high temperature thermal source (e.g. matrix, solution). Because emission can occur from either excited or ionised atoms, thousands of different spectral lines can be observed. Several of these lines are much more intense than those due to the analyte, which can be present at ultratrace levels. 

Various methods can be used to measure the intensity of radioactive emissions. These exploit the ability of radiation from radioactive isotopes to cause ionisation (Geiger-Miiller counting), to excite fluorophores (scintillation counting), or to cause exposure of light-sensitive photographic emulsion (autoradiography) (Slater 1990 Rickwood et al. 1993). 

Carbon-14, the radioactive isotope of carbon, has been widely used in mechanistic studies with organic hydrocarbons but not in kinetic studies. decays to with the emission of a beta particle and has an extremely long half life (5568 30 years). The emitted radiation can be detected by an ionisation chamber,... 

From the above it can be concluded that there exists for each spectral line emitted by a radiation (plasma) source an optimum temperature at which its emission intensity reaches a maximum. This so-called standard temperature depends on the energy of ionisation and excitation of this element, and on the electron pressure and temperature in the plasma. The standard temperatures for many atom lines are around 4000 K, while the standard temperature of ionic lines is often around 10000 K. 

Deuterium lamps are commonly used for UV spectroscopy. They give a moderately intense continuum of UV radiation from 360 nm down to 160 nm, with a weaker pseudo-continuum with strong line emission superimposed throughout the visible (Fig. 14.4b). Deuterium lamps with quartz envelopes generate ozone, a toxic chemical hazard formed via ionisation of O2 molecules. 

In laser vaporisation experiments, generating a plume , the laser s frequency may be synchronised with the resonance line of the element (analyte) to be analysed. The basic principles are (i) absorption of the radiation by the analyte (LAAS laser atomic absorption spectrometry) (ii) fluorescence (LIE, laser-induced fluorescence LEAFS) or (Hi) production of ionisation products (ions and electrons). LIF is an analytical method of high precision that is suitable for the measurement of diatomic species in the plume. Excitation spectroscopy or laser-excited fluorescence is not concerned with the spectral composition of the fluorescence but with how the overall intensity of emission varies with the wavelength of excitation. 

Certain interactions with matter of the radiation accompanying the decay of unstable nuclides (a- and /9-particles, y rays) are the basis for the detection and measurement of radioactivity These include photochemical processes, by which a radioactive sample placed in close contact with photographic emulsion causes blackening of the latter upon development (autoradiography) gas ionisation and the deriving production of current pulses that can be analysed and measured by suitable devices excitation of orbital electrons of special molecules, either in a crystalline form or in solution, with subsequent emission of light pulses to be converted into electric current by a photoelectric detector (scintillation)... 

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