Change In Number Of Atoms Research Articles

Diffusion Rates of Components in Metal-Silicides Depending on Atomic Number of Refractory Metal Component

Interdiffusion studies conducted in group IVB, VB and VIB metal-silicon systems are discussed in detail to show a pattern in the change of diffusion coefficients with the change in atomic number of the refractory metal (M) component. MSi2and M5Si3phases are considered for these discussions. It is shown that integrated diffusion coefficients increase with the increase in atomic number of the refractory component when the data are plotted with respect to the melting point normalized annealing temperature. This indicates the increase in overall defect concentration facilitating the diffusion of components. This is found to be true in both the phases. Additionally, the estimated ratios of tracer diffusion coefficients indicate the change in concentration of antisite defects in certain manner with the change in atomic number of the refractory components.

Diffusion pattern in MSi2 and M5Si3 silicides in group VB (M = V, Nb, Ta) and VIB (M = Mo, W) refractory metal-silicon systems

Group VB and VIB M–Si systems are considered to show an interesting pattern in the diffusion of components with the change in atomic number in a particular group (M = V, Nb, Ta or M = Mo, W, respectively). Mainly two phases, MSi2 and M5Si3 are considered for this discussion. Except for Ta–silicides, the activation energy for the integrated diffusion of MSi2 is always lower than M5Si3. In both phases, the relative mobilities measured by the ratio of the tracer diffusion coefficients, , decrease with an increasing atomic number in the given group. If determined at the same homologous temperature, the interdiffusion coefficients increase with the atomic number of the refractory metal in the MSi2 phases and decrease in the M5Si3 ones. This behaviour features the basic changes in the defect concentrations on different sublattices with a change in the atomic number of the refractory components.

Impregnation synthesis of a novel macroporous silica-based TODGA polymeric composite and its application in the adsorption of rare earths in nitric acid solution containing diethylenetriaminepentaacetic acid

A macroporous silica-based N, N, N′, N′-tetraoctyl-3-oxapentane-1,5-diamide (TODGA) polymeric composite (TODGA/SiO 2-P) was synthesized. It was done through impregnation and immobilization of TODGA molecule into the pores of the SiO 2-P particles utilizing a vacuum sucking technique. The macroporous SiO 2-P particles were the silica-based organic/inorganic composite synthesized by immobilizing styrene–divinylbenzene copolymer inside SiO 2 through the complicated polymerization reaction. The adsorption of rare earth (RE(III)) elements onto TODGA/SiO 2-P was investigated in HNO 3 solution containing diethylenetriaminepentaacetic acid (DTPA), an acidic multi-dentate chelating agent. It was found that in the presence of 0.05 M DTPA, NO 3 - and H + had significant effect on the TODGA/SiO 2-P adsorption due to the competition reactions of RE(III) with different species, H 4DTPA − and H 2DTPA 3−. With an increase in the concentration of NO 3 - from 0.115 M to 3.015 M, the adsorption of RE(III) onto TODGA/SiO 2-P increased noticeably. On the other hand, RE(III) showed strong adsorption at 0.1 M H +, weak adsorption at around pH 2, and no adsorption in excess of pH 2.3. In a 0.1 M H +–0.115 M NO 3 - –0.05 M DTPA solution, a change of the distribution coefficient of RE(III) onto TODGA/SiO 2-P with an increase in atomic number of RE(III) from La(III) to Lu(III) was investigated. The silica-based TODGA/SiO 2-P polymeric composite showed strong adsorption for heavy RE(III) over the light one. In a 0.01 M H +–1.0 M NO 3 - –0.05 M DTPA solution, the effect of the ratio of solid phase to liquid one on the relationship of the distribution coefficient of RE(III) with the change in atomic number of RE(III) was also studied. Based on the complicated disassociation equilibrium of DTPA, the influence of the concentrations of NO 3 - and H + on the adsorption of TODGA/SiO 2-P for RE(III) was demonstrated. This makes the partitioning of RE(III) and MA(III) together from high level liquid waste (HLLW) by the polymeric composite TODGA/SiO 2-P promising.

Photon dose calculation incorporating explicit electron transport

Significant advances have been made in recent years to improve photon dose calculation. However, accurate prediction of dose perturbation effects near the interfaces of different media, where charged particle equilibrium is not established, remain unsolved. Furthermore, changes in atomic number, which affect the multiple Coulomb scattering of the secondary electrons, are not accounted for by current photon dose calculation algorithms. As local interface effects are mainly due to the perturbation of secondary electrons, a photon-electron cascade model is proposed which incorporates explicit electron transport in the calculation of the primary photon dose component in heterogeneous media. The primary photon beam is treated as the source of many electron pencil beams. The latter are transported using the Fermi-Eyges theory. The scattered photon dose contribution is calculated with the dose spread array [T.R. Mackie, J.W. Scrimger, and J.J. Battista, Med. Phys. 12, 188-196 (1985)] approach. Comparisons of the calculation with Monte Carlo simulation and TLD measurements show good agreement for positions near the polystyrene-aluminum interfaces.

Analogous filters for beam shaping in diagnostic radiology

A computer simulation based on photon transport calculation was used to evaluate the properties of a wide variety of materials that could be used for the filtration of X-ray beams. A single filter was added to the inherent filtration. For comparison, the concept of analogous filters was introduced: two filters were considered analogous if both produced the same ratio of entrance exposure at the patient to the energy absorbed in the detector consisting of a pair of intensifying screens positioned immediately behind the water phantom. The efficiency, contrast and integral dose were calculated for filters of odd atomic numbers from 1 to 91 with a large number of different parameters (48600 different combinations). They include essentially all major diagnostic radiology procedures, except mammography. Only extreme cases are presented here. No magic filter was found which would produce increased contrasts or a decreased integral dose, while maintaining efficiency close to that of aluminium. Filters of some atomic numbers produced increased contrast, but had negligible efficiency. Analogous filters of atomic numbers between 7 and 37 and also between 71 and 91, had almost identical properties but varied efficiencies. Unpredictable behaviour was obtained with filters of atomic numbers 38 to 70, where small changes in atomic number may produce large changes in all the properties due to the presence of K-edge discontinuities.

Phase-shift calculations for extended X-ray absorption fine structure (EXAFS) and the fine structure of As2O3

Abstract Theoretical calculations are presented of three factors involved in Extended X-ray Absorption Fine Structure (EXAFS). It is found that multiple scattering within the atom gives rise to characteristic structure of the back-scattering amplitude. The back-scattering is not sensitive to charge and alters little with small changes in atomic number (Z), but exhibits a wide variety of shapes with large changes of 2. The back-scattering phase-shifts also show a trend similar to that of the amplitude. A comparison of the ionized atom and neutral atom Z=1 phase-shift shows a constant shift of the phase and little change in its k- dependence. Experimental measurements of the fine-structure of cubic and glassy As, O, are reported and are discussed in terms of the preceding calculations. It is found that As, O, molecules do not exist as a major constituent of the glass.

The generation, absorption and anisotropy of thick‐target bremsstrahlung and implications for quantitative energy dispersive analysis

AbstractThe bremsstrahlung spectrum from a thick specimen bombarded with 20 ke V electrons is studied with emphasis on the shape rather than the absolute intensity in the energy range 1–10 keV. Monte Carlo calculations are described in detail and used to compare the absorption corrections for characteristic and continuous (bremsstrahlung) radiation from a thick specimen and to determine the extent to which anisotropy modifies the intensity distribution of the continuum. The absorption corrections are found to differ by roughly 5% for f(χ) = 0.5 and 10% for f(χ) = 0.2, but it is shown that the ratio of the corrections can be predicted by using the ratio of two Philibert‐type formulae. Anisotropy has little influence on the absorption correction but changes in atomic number or geometry may result in the intensity distribution being altered by typically 5‐10% across the energy range 1–10keV. Numerical integration gives values for the generated brems‐strahlung intensity which are used in constructing an expression to represent the functional form of the background. Although experimental spectra confirm this to be superior to the combination of Kramers' law and a characteristic absorption expression, several sources of error at low energies, including bremsstrahlung from the Be window, are pointed out and improvements suggested. In order to eliminate the principal uncertainty, that of the overall efficiency curve for a Si(Li) detector and pulse‐processor, it is recommended that a stored spectrum be used as a ‘background standard’ for explicit background corrections. Errors are then reduced to the 0.1% absolute concentration level, which is suitable for quantitative analysis.

Altering the energy dependence of LiF TLDs by pre-irradiation.

Energy dependences (thermoluminescent response to 30–130 keVeffx rays compared to that from 60Co gammas) were measured for normal LiF dosimeters and for ones which had been sensitized by pre‐irradiation (103–106R) followed by annealing near 300°C. Unsensitized LiF (Harshaw's TLD‐100 and TLD‐700) responds slightly higher (5–10%) to the x rays than expected from changes in dose. When LiF is radiation sensitized, the relative response decreases in the 30–135 keVeff range as much as 40% for pre‐exposures ≳ 105 R. A commercial dosimeter (Radiation Detection Co. “throwaway powder”) displays this altered energy dependence because the LiF is pre‐exposed by the manufacturer. To emphasize the major point: Sensitization alters the energy dependence of LiF without any change in atomic number. Possible uses for this effect are discussed.

Electronic Densities of States from X-Ray Photoelectron Spectroscopy.

In x-ray photoelectron spectroscopy (XPS), a sample is exposed to low energy x rays (approximately 1 keV), and the resultant photoelectrons are analyzed with high precision for kinetic energy. After correction for inelastic scattering, the measured photoelectron spectrum should reflect the valence band density of states, as well as the binding energies of several core electronic levels. All features in this spectrum will be modulated by appropriate photoelectric cross sections, and there are several types of final-state effects which could complicate the interpretation further. In comparison with ultraviolet photoelectron spectroscopy (UPS), XPS has the following advantages: (1) the effects of inelastic scattering are less pronounced and can be corrected for by using a core reference level, (2) core levels can also be used to monitor the chemical state of the sample, (3) the free electron states in the photoemission process do not introduce significant distortion of the photoelectron spectrum, and (4) the surface condition of the sample does not appear to be as critical as in UPS. XPS seems to be capable of giving a very good description of the general shape of the density-of-states function. A decided advantage of UPS at the present time, however, is approximately a fourfold higher resolution. We have used XPS to study the densities of states of the metals Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au, and also the compounds ZnS, CdCl2, and HgO. The d bands of these solids are observed to have systematic behavior with changes in atomic number, and to agree qualitatively with the results of theory and other experiments. A rigid band model is found to work reasonably well for Ir, Pt and Au. The d bands of Ag, Ir, Pt, Au and HgO are found to have a similar two-component shape.

The Radioactive Disintegration Constant, Lambda

To Dr. Howard P. Doub Editor of Radiology Dear Dr. Doub There is a misconception regarding the disintegration constant, lambda (λ) which seems wide-Spread among radiologists, radiological residents, and not a few radiation physicists. In most texts the disintegration constant is defined as “the fractional change in number of atoms which occurs in unit time.” This definition is correct only in the limiting case where the unit time interval is infinitely short. In all other cases, lambda may be a fraction or it may be a number greater than one; it is never the fractional change in number of atoms as given in the above definition. Consider radon, whose half-life is 3.825 days. Its disintegration constant is 0.693/3.825 = 0.18 per day. In this case lambda is a fraction. However, it is equally justifiable to consider the disintegration constant per month. In this case the half-life of radon is 0.1275 months, and the disintegration constant is 0.693/0.1275 = 5.43 per month, a number which is certainly not a fraction. Lambda will thus be either a fraction or a whole number depending on the half-life and the units in which it is expressed. I have asked many radiologists, and a few radiation physicists, this question: “The decay constant of radon is 0.18 day−1. What per cent of the atoms disintegrate per day?” The answer is invariably “18 per cent.” This is not correct. The per cent of radon atoms disintegrating per day must be obtained from the equation N = N0e-lambda t If N0 is taken as 100, lambda as 0.18 day−1, T as 1 day, then N equals 84, and 16 atoms out of 100 have decayed per day, or 16 per cent per day, not 18 per cent, as would be concluded from the common definition of the constant. It is possible that this misconception has gone along so many years because in the above classical example, using radon, the correct and incorrect answers are so nearly alike that most persons have assumed a small arithmetic error somewhere along the line. It is not an arithmetic error at all; it is a philosophical error. When the disintegration constant of radon is taken per day, the per cent disintegrating per day is about 11 per cent less than the constant. As the unit of time decreases, so does the error. The per cent of atoms decaying per hour is only about 0.5 per cent less than the constant. However, it must be realized that there will always be a difference until the unit of time becomes zero. This misconception, which occurs even in the early radium literature, probably stems from a misuse of the basic differential equation dN/N = -lambda dT. The left-hand term of this equation cannot be treated as an ordinary arithmetic fraction, which is done when lambda is defined as the fractional change per unit time.

THE EFFECT OF BETA-DECAY ON THE EXCHANGE PROPERTIES OF THE RARE EARTH–EDTA COMPLEX IONS

It is shown that the rare earth ion Yb3+ is very resistant towards ordinary thermal exchange when it is complexed with the chelating agent EDTA in aqueous solution. However, when the complexed rare earth atom, as the 1.8-h Yb-177, emits a beta-particle, the daughter atom Lu-177 escapes readily from the chelate structure. Nuclear recoil arising from the beta-particle emission is shown not to be the cause of the escape of the daughter atom. It is suggested that the observed lability of the daughter atom is a result of a high degree of chemical reactivity of the chelate ion arising from the sudden change in atomic number of the central metal ion of the chelate structure.

CHROMOSOME BREAKAGE INDUCED BY ABSORBED RADIOACTIVE PHOSPHORUS, P32

Radioactive phosphorus (P32) was made available to individual germinating seeds and seedlings of Triticum vulgare, T. durum, T. monococcum, and Hordeum distichon. Each treated plant was provided with either 0.18 or 0.018 micro-curies of P32. Both concentrations were effective in causing chromosome breakage and rearrangements. Many aberrant chromosome configurations occurred in microsporocytes of treated tetraploid and hexaploid plants. Only one chromosome aberration was found in T. monococcum and none in barley. Structural changes occurred also in chromosomes of vulgare wheat grown in soil to which fertilizer containing P32 was added. It is possible that the effectiveness of small amounts of P32 in inducing mutations is increased by its inclusion in chromosome molecules. Emitted beta particles would then be shot off very close to the sensitive target. Further, the recoiling atomic nucleus is almost certain to have enough energy to break any chemical bond. In any case since there is a change in atomic number (from 15 to 16) and in valence (P valence 5, S valence 2), molecular bonds must be released in the process.

The Single Scattering of Electrons in Gases

The single scattering of electrons was measured by means of a cloud chamber. Electrons having energies from 0.9 to 12 Mev were used in air, argon, krypton, and xenon. A total of 2173 meters of track yielded 801 deflections between 15\ifmmode^\circ\else\textdegree\fi{} and 90\ifmmode^\circ\else\textdegree\fi{}. The projected angles of scattering were compared with the theoretical values obtained by making a plane projection of the Mott scattering distribution. The errors inherent in the cloud chamber method and the corrections to be applied were treated in detail. From a combination of the data for all the energies and gases, the ratio of the experimental to the theoretical scattering was found to be 1.02. The angle intervals 25\ifmmode^\circ\else\textdegree\fi{}-35\ifmmode^\circ\else\textdegree\fi{}, 35\ifmmode^\circ\else\textdegree\fi{}-45\ifmmode^\circ\else\textdegree\fi{}, and 45\ifmmode^\circ\else\textdegree\fi{}-55\ifmmode^\circ\else\textdegree\fi{}, taken individually, show a somewhat greater ratio: 1.20, 1.43, and 1.25, respectively. Throughout the data the variations of the scattering cross section with changes in atomic number, energy, and angle were as expected within the experimental accuracy. No support whatever was found for the large discrepancies reported by several other authors.