Mode neutron activation

The apphed pretreatment techniques were digestion with a combination of acids in the pressurized or atmospheric mode, programmed dry ashing, microwave digestion and irradiation with thermal neutrons. The analytical methods of final determination, at least four different for each element, covered all modern plasma techniques, various AAS modes, voltammetry, instrumental and radiochemical neutron activation analysis and isotope dilution MS. Each participating laboratory was requested to make a minimum of five independent rephcate determinations of each element on at least two different bottles on different days. Moreover, a series of different steps was undertaken in order to ensure that no substantial systematic errors were left undetected. 

There are two basic ways to look for explosive material. They differ in their point of focus. Some sensors seek the mass of explosive material within a device. These are particularly useful when the device is well sealed and its surface is well cleaned of stray explosive molecules, or when the explosive being used is nonaromatic, that is, it does not readily release molecules from its bulk. We will refer to these as bulk sensors. They include X-ray techniques, both transmission and backscatter neutron activation in several techniques y -ray excitation, in either transmission or backscatter modes and nuclear resonance techniques, either nuclear magnetic resonance (NMR) or nuclear quadrupole resonance (NQR). Bruschini [1] has described these thoroughly. They are also described by the staff of the Jet Propulsion Laboratory [2], The following forms a very brief synopsis. 

Fig. 3 Experimental heat capacities of benzene [11], Cv is obtained from observed Cp after subtracting the expansion work, computed using the experimentally determined bulk modulus. The Cv estimated from molecular translational and librational lattice modes (obtained from neutron diffraction ADP s) is also plotted. Note that these external modes well reproduce the observed Cv up to ca. 100 K. Above this temperature the internal modes are active and Cv exceeds the classical limit of 3 k T...

Fig. 5. Use of a 4096-channel analyzer in the multiscaler mode for the determination of oxygen via 14 MeV neutron activation analysis...

In this section only the most recent methods (post 1990) for extraction and quantitative determination of PCAs in commercial products and environmental samples are discussed. While extraction and isolation techniques have relied mainly on techniques already developed for POPs, there have been major advances in the quantification of PCAs using gas chromatography mass spectrometry in electron capture negative ionization mode (GC-ECNIMS). No attempt will be made to discuss older methods of analysis such as thin-layer chromatography [13,50], and neutron activation methods [51]. 

The following possible modes of neutron activation can be distinguished ... 

Instrumental Methods of Analysis. The problems encountered in fusing and digesting oil shale make instrumental methods attractive. Instrumental neutron activation analysis (INAA) and x-ray fluorescence (XRF) analysis either in the wavelength dispersive or energy dispersive mode are especially attractive since they are capable of accurate multielement analyses. 

Earlier methods used to determine mercury in biological tissue and fluids were mainly colorimetric, using dithizone as the com-plexing agent. However, during the past two to three decades, AAS methods - predominantly the cold vapor principle with atomic absorption or atomic fluorescence detection - have become widely used due to their simplicity, sensitivity, and relatively low price. Neutron activation analysis (NAA), either in the instrumental or radiochemical mode, is still frequently used where nuclear reactors are available. Inductively coupled plasma mass spectrometry (ICP-MS) has become a valuable tool in mercury speciation. Gas and liquid chromatography, coupled with various detectors have also gained much importance for separa-tion/detection of mercury compounds (Table 17.1). 

In activation analysis advantage is taken of the fact that the decay properties such as the half-life and the mode and energy of radioactive decay of a particular nuclide serve to identify uniquely that nuclide. The analysis is achieved by the formation of radioactivity through irradiation of the sample either by neutrons or charged particles. Neutron irradiation is by far the more common technique, and hence this method is often referred to as neutron activation analysis, NAA. A major advantage in activation analysis is that it can be used for the simultaneous determination of a number of elements and complex samples. If the counting analysis of the sample is conducted with a Ge-detector and a multichannel analyzer, as many as a dozen or more elements can be measured quantitatively and simultaneously (instrumental NAA, or INAA). 

FIGURE 4.5 Gammaray spectrum after excitation by epithermal neutron activation analysis (ENAA) for determination of the uranium content in Nigerian food products. Comparison of the gamma ray spectrum of uranium in normal mode and with Compton suppression. (From Kapsimalis, R. et al., Appl. Radiat. hot., 67, 2097, 2009. With permission.)... 

Inert sampling could be done when desired. Zr, W and Ni were determined by XRF, Ti and Cr by neutron activation analysis (NAA), Mg by AAS, C with a Leco carbon analyzer and Cl by potentiometric titration. FTIR in diffuse reflectance mode was used to follow the chemisorption and to detect possible decomposition of the reactant. Scanning electron microscopy with an energy dispersive spectrometer (SEM/EDS) was used to determine element concentrations through the particles. The specific surface area and pore volume were determined by means of nitrogen adsorption and condensation with Micromeritics ASAP 2400 equipment. Detailed experimental conditions used in the characterization are in Ref. 16. 

With respect to the time of measurement, NAA falls into two categories (1) prompt gamma-ray neutron activation analysis (PGNAA), where measurements take place during irradiation, or (2) delayed gamma-ray neutron activation analysis (DGNAA), where the measurements follow radioactive decay. The latter operational mode is more common. 

In this example, the excited nucleus that is formed decays with the emission of a 7 ray with a distinctive energy. Neutron-activated nuclei may decay by other modes, however, such as j8 decay. The activity of the radioisotope formed is measured. This measurement, together with such factors as the rate of neutron bombardment, the half-life of the radioisotope, and the efficiency of the radiation detector, can be used to calculate the quantity of the element in the sample. The method is especially attractive because (1) trace quantities of elements can be determined (sometimes in parts per billion or less) (2) a sample can be tested without destroying it and (3) the sample can be in any state of matter, including biological materials. Neutron activation analysis has been used, for instance, to determine the authenticity of old paintings. Old masters formulated their own paints. Differences between formulations are easily detected through the trace elements they contain. 

As a result of slow (thermal) neutron irradiation, a sample composed of stable atoms of a variety of elements will produce several radioactive isotopes of these activated elements. For a nuclear reaction to be useful analytically in the delayed NAA mode the element of interest must be capable of undergoing a nuclear reaction of some sort, the product of which must be radioactively unstable. The daughter nucleus must have a half-life of the order of days or months (so that it can be conveniently measured), and it should emit a particle which has a characteristic energy and is free from interference from other particles which may be produced by other elements within the sample. The induced radioactivity is complex as it comprises a summation of all the active species present. Individual species are identified by computer-aided de-convolution of the data. Parry (1991 42-9) and Glascock (1998) summarize the relevant decay schemes, and Alfassi (1990 3) and Glascock (1991 Table 3) list y ray energy spectra and percentage abundances for a number of isotopes useful in NAA. 

The diffusion coefficients for iodine are close to those measured previously in the same glass (12) at the experimental temperatures. However, the activation energies of these two measurements differ. The experiments differ to some degree in that iodine and other fission products were made from dissolved U02 in this experiment, while iodine only was made from Te02 under a neutron flux in the previous experiment. The latter mode of formation should lead to a greater excess of oxygen in the glass. 

Figure 2.1. Dispersion of phonons in a deuterated anthracene crystal (S. L. Chaplot et al., 1982). Upper part calculated spectra. Lower part dispersion measured by neutron scattering. The Raman-active modes at k = 0(F) are marked R, and have symmetry /4,(S) or Bt(A). Note the weak dispersion of the lower R mode along c. ... 

Neutron [102] and electron [70,103] irradiated TCNQ salts were investigated by IR spectroscopy. The spectra of irradiated samples gradually become weaker when the dose is increased the broad lines change shape and decrease in intensity some even vanish (Fig. 18). It was shown [103] that the vibrational features connected with the totally symmetric modes of the TCNQ are particularly sensitive to structural disorder. In some cases, for example, in MTPP(TCNQ)2 irradiated by electrons, the disappearance of distinct doublets of activated ag modes was noticed. Different mechanisms of the excitation of both the narrow and wide components explain their different dose and temperature dependences [70]. The changes of band intensity in the UV-VIS region imitate the electrical conductivity as a function of dose. 

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