Activation analysis photon sources

With analytical methods such as x-ray fluorescence (XRF), proton-induced x-ray emission (PIXE), and instrumental neutron activation analysis (INAA), many metals can be simultaneously analyzed without destroying the sample matrix. Of these, XRF and PEXE have good sensitivity and are frequently used to analyze nickel in environmental samples containing low levels of nickel such as rain, snow, and air (Hansson et al. 1988 Landsberger et al. 1983 Schroeder et al. 1987 Wiersema et al. 1984). The Texas Air Control Board, which uses XRF in its network of air monitors, reported a mean minimum detectable value of 6 ng nickel/m (Wiersema et al. 1984). A detection limit of 30 ng/L was obtained using PIXE with a nonselective preconcentration step (Hansson et al. 1988). In these techniques, the sample (e.g., air particulates collected on a filter) is irradiated with a source of x-ray photons or protons. The excited atoms emit their own characteristic energy spectrum, which is detected with an x-ray detector and multichannel analyzer. INAA and neutron activation analysis (NAA) with prior nickel separation and concentration have poor sensitivity and are rarely used (Schroeder et al. 1987 Stoeppler 1984). 

PAA Photon activation analysis involves irradiation with high energy photons (produced by conversion of electron energy into bremsstrahlung serving as the source of the photons). Photons emitted in the delayed decay are detected and used as the analytical signal. 

The first photonuclear activation for analytical purposes was performed with radionuclides as the activating radiation source. These applications were reported in the early 1950s, although apparently the first beryllium determinations by photodisintegration were performed in the late 1930s in the Soviet Union. The analytical detection power of photon activation analysis using radionuclide sources is poor and restricted to the analysis of deuterium, beryllium, several fissile nuclides, and a few nuclides that have low-lying isomeric states. Nonetheless, nuclide excitation is still in use. 

The achievable product activity depends on the photon energy and flux density of the activating source (Figure 1). Generally, the difference in detection limits among the elements is nowhere near as large as in neutron activation analysis. Typically, the detection limits, assuming purely instrumental analysis, lie between 0.001 and 1 pg, whereas in thermal... 

In radionuclide photoactivation, y-ray sources have mostly been used, but others have also been reported, e.g., " Na or °Co. The use of isotopic sources for photon activation analysis is of limited value and is restricted to a few appropriate cases. 

Of these accelerators, two types have mostly been used for photon activation analysis, namely the linear accelerator (also called linac) and the microtron. These and other accelerators will not be described in detail here since normally the analyst is engaged in the sample handling, activity measurement, and data processing rather than in the operation of the radiation sources, which is usually done by separate operating personnel. 

Activation analysis has become an important technique for the environmental biologist as well as for others. By means of bombardment with neutrons, high-energy photons, or charged particles, stable elements can be transformed to radionuclides. These radionuclides can be measured relatively easily, and the results can be interpreted in terms of die type and quantity of stable elements present in the sample of interest. Although several types of bombardment may be used to transform stable elements to radionuclides, slow neutrons are usually employed. Details concerning the procedure are described in many sources (e.g., Schulze, 1969 Hendee, 1973b). In addition, publications of direct interest to the environmental scientist are fairly numerous (Leddicotte, 1969 Byrne et aL, 1971 Pillay and Thomas, 1971 Filby and Shah, 1974). 

Analysis by activation with y-radiation derived from isotopic photon sources is limited, by the low energy of the photons available, to the use of (y,y ) reactions giving rise to isomeric states of stable nuclides. Studies of the analytical uses of an 80 kCi Co source have been reported by Veres and Parlicsek and of 5 and 50kCi Co sources by Law and Iddings. The method is limited to a few elements and sensitivity is poor. However, large samples can be used and the method may have some application for routine instrumental analysis of impurities in industrial products. 

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