Neutron absorption correction

In some cases, thermal neutrons can also be used to measure the absolute abundances of other elements. Transforming the neutron spectrum into elemental abundances can be quite involved. For example, to determine the titanium abundances in lunar spectra, Elphic et at. (2002) first had to obtain FeO estimates from Clementine spectral reflectances and Th abundances from gamma-ray data, and then estimate the abundances of the rare earth elements gadolinium and samarium from their correlations with thorium. They then estimated the absorption of neutrons by major elements using the FeO data and further absorption effects by gadolinium and samarium, which have particularly large neutron cross-sections. After making these corrections, the residual neutron absorptions were inferred to be due to titanium alone. 

These INS studies of the alkali metal hydrides provide an excellent example of the careful analysis of INS experimental data. It includes the application of corrections for multiple scattering, neutron absorption and heavy-ion scattering the extraction of quantities related to the hydrogen dynamics (the hydrogen mean square displacement, mean kinetic energy and the hydrogen Einstein frequency) and provided the density of vibrational states for each type of atom, shown individually and ab initio modelling of the full INS spectra. 

Neutron absorption in fission products has a small effect on decay-heat power for r < 10 s and is treated by a correction factor G. The corrected total decay-heat power is given by the ANS Standard, in terms of thermal-neutron flux (in neutrons/cm s), reactor operating time T (in s), and cooling time t (in s) as... 

Mitchel et al. [235] have utilized the potential of neutron diffraction to further study the PPy-pTS system. The pulsed neutron source ISIS (Rutherford Appleton Laboratory) permits one to use time-of-flight detection with one or more static detectors [236]. Two detectors have been used simultaneously to collect the data for parallel and perpendicular directions, comparable to the x-ray data shown before. In this way the radiation is used very efficiently, and very thin samples can be examined (10-20 pm [229]). Additionally, angle-dependent absorption corrections are avoided. The requirement of a low hydrogen content of the... 

In the dawn of the history of nuclear science, the neutron flux density (/) of Ra-Be source was only 10 -10 n cm s The total activity produced by such neutron sources via (n,y) reaction was very low and so was the specific activity of the radionuclides. The Szilard-Chalmers process, however, could dramatically increase the specific activity the improvement could reach orders of magnitude. In the measurement of P radioactivity, which was a frequent task in early days of nuclear science, samples with low specific activity brought sometimes troublesome problems of self-absorption corrections. By the introduction of the Szilard-Chalmers process, however, this difficulty could be avoided, because the measurement could be performed within small statistical errors using a sample with high specific activity. Therefore, the Szilard-Chalmers process became one of the useful means of preparation of radioisotopes for measurement, as Szilard and Chalmers (1934b) recognized the importance of this technique in their early work. 

Neutron diffraction was carried out on the liquid samples on the D4c diffractometer [6] at the ILL, using neutron wavelengths of 0.5, 0.33, and 0.29 A. A Cu(220) monochromator was used for 0.5 A and a Cu(331) monochromator was used for both the 0.33 and 0.29 A neutrons. In order to avoid harmonic contamination a rhodium filter was inserted between the monochromator and the sample when measuring at 0.5 A. Diffraction measurements were taken at the three wavelengths on the empty furnace, the empty container, the liquid sample, nickel powder in a vanadium can for calibration, a vanadium bar for normalisation, and powder in a silica container for a self absorption correction. All measurements were taken at ambient temperature, except the sample measurement, where a vanadium furnace was used to heat the sample to 800 °C. The essential diffraction parameters are shown in Table 7.1. Significant care was taken in minimising the background to avoid the possibility of an A1 peak coincident with the InSe first peak [13]... 

Fig. 7.5 a The Lorch modified G(r) s for 0.5 A (red) and 0.33 A (green) compared with neutron diffraction data of Lague [13] also taken at 0.5 A. The broken black lines show the low r cutoff data. There is a small broadening ( 0.1 A) of the first peak in the 0.33 A data which is probably related to the diffractometer resolution function [23]. b The back Fourier transforms (broken black lines) of the cutoff data shown in (a) compared with the original F(q) s (solid coloured lines). There is a significant difference in the case of the 0.33 A data that is probably due to an incorrect absorption correction... 

Many of the published methods for the determination of metals in seawater are concerned with the determination of a single element. Single-element methods are discussed firstly in Sects. 5.2-5.73. However, much of the published work is concerned not only with the determination of a single element but with the determination of groups of elements (Sect. 5.74). This is particularly so in the case of techniques such as graphite furnace atomic absorption spectrometry, Zeeman background-corrected atomic absorption spectrometry, and inductively coupled plasma spectrometry. This also applies to other techniques, such as voltammetry, polarography, neutron activation analysis, X-ray fluroescence spectroscopy, and isotope dilution techniques. 

Copper. In the presence of sulfur dioxide, copper-protein cloudiness may develop in white wines. Only small amounts of copper (about 0.3 to 0.5 mg/liter) cause cloudiness. Widespread use of stainless steel in modern wineries has reduced copper pickup, but many wineries routinely test their wines for copper. Atomic absorption spectrophotometry is the method of choice (51) although reducing sugars and ethanol interfere, and correction tables must be used (107). To reduce this interference, chelating and extracting with ketone is recommended (108). Lacking this equipment colorimetric procedures can be used, especially with di-ethyldithiocarbamate (3, 4, 6, 9,10, 22,109). Neutron activation analysis has been used for determining copper in musts (110). 

Moisture analyzers include a large variety of designs listed here. The list includes their inaccuracies (1) electrolytic hygrometer (2-5% FS), (2) capacitance (3% FS), (3) impedance (3% FS), (4) piezoelectric (10% AR or 2 ppm by volume), (5) heat of adsorption, (6) infrared (0.5-1% FS), (7) microwave (for a 1-15% moisture range, error is within 0.5%, less if corrected for density), (8) Karl Fischer titrator (0.5-1% FS), (9) drying oven (0.5-1% FS), (10) dipole, (11) cavity ring down, (12) fast neutron (0.2% in solid s density corrected), and (13) radio-frequency absorption (5 ppm). 

FIGURE 8.2 Structure factors S(QZ) obtained from neutron-scattering patterns of butylam-monium vermiculite gels (upper panels) and from a 0.1 M protonated butylammonium salt solution with no clay (lowest panel). The upper panels show S(QZ) for gels prepared in a 0.1 M deuterated salt solution and in 0.1 and 0.01 M protonated salt solutions. The momentum transfer Q was perpendicular to the clay plates, and the structure factor S(QZ) has been normalized after correction for background scattering and absorption. 

Other systematic errors include absorption, extinction, and multiple reflection. Absorption effects and the required corrections are well understood. When a monochromatic beam of X rays or neutrons with incident intensity / passes through a crystal, the intensity is reduced exponentially... 

Total concentrations of trace elements were determined by neutron activation analyses and atomic absorption. Radium concentrations were determined by the emanation method and by the germanium-lithium counting of natural radioactivity, corrected by reference to a National Bureau of Standards (NBS) uranium ore. Chemical analyses were performed by standard methods. 

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