Similar Atomic Numbers Research Articles

Structure and valence from complementary anomalous X-ray and neutron powder diffraction

We introduce a new type of crystallography whose major aim is to study the crystal chemistry and the electronic structure of constituent atoms or ions in crystals, rather than the crystal structure itself. These results are obtained from recent experiments that take advantage of the complementary nature of anomalous X-ray and neutron diffraction data, and of the fact that anomalous scattering can provide substantial changes in the scattering factors for atoms or ions with similar atomic number Z or with the same Z but different valence. Examples presented include: the determination of constituent ordering among several sites in Fe/Co/V alloys and in certain high temperature superconductors; the study of ordering that arises via valence fluctuation in metallic cerium; the determination of anomalous X-ray scattering factor variation for copper with oxidation state; and finally, the determination of site-specific valence of the Cu ions in the chain and plane sites in the high temperature superconductor YBa 2Cu 3O 6+ x as a function of oxygen stoichiometry.

X-ray image analysis: An application to petrographic modal analysis

Image analysis is a fast and accurate technique for measuring the relative amounts of various mineral components of a rock. In a routine scanning electron microscope (SEM) based modal analysis, backscattered electrons (BSE) are used to generate average atomic number contrast to determine the presence of the various minerals. The minerals are then identified with the use of an energy dispersive (EDS) or a wavelength dispersive spectrometer (WDS). Once a mineral is identified, its modal percent can be calculated by comparing the area of pixels which the mineral occupies with the total area scanned. This BSE/EDS method works well in most cases except where samples contain minerals with similar average atomic numbers or backscattering coefficients, thus producing ambiguity in mineral identification. In an attempt to eliminate this ambiguity and to enhance the analytical capabilities of image analysis, the present study will evaluate a superimposed x-ray map method which utilizes the logical operation of conjunction (AND) to combine multiple x-ray maps of characteristic elements to complete the modal analysis.

Radiative properties of puffed-gas mixtures: The case of optically thick plasmas composed of two elements with similar atomic numbers

Using detailed atomic and radiative transfer calculations, we consider the K-shell radiative properties of optically thick plasmas composed of two different elements with nearly the same atomic number. A sodium-neon mixture is chosen for specificity, and the plasma conditions considered are similar to those achieved in puffed-gas or exploding-wire Z pinches. The emissivity of an individual K-shell line is parameterized in terms of an ‘‘ultimate’’ photon escape probability which is a function of both the single-flight photon escape probability and the collisional quenching probability. Holding the total ion density (sodium plus neon) constant, we have evaluated the total K-shell emissivity as a function of sodium-to-neon ratio. In contrast to the optically thin case the emitted K-shell power peaks at values of this ratio intermediate between pure neon and pure sodium. The opacity effects responsible for this phenomenon are analyzed using the escape parameterization.

High-resolution structure determination by ALCHEMI

ALCHEMI (Atom Location by CHanneling Enhanced Microanalysis) enables the site occupancy of atoms in single crystals to be determined. In this article the fundamentals of the method for both EDS and EELS will be discussed. Unlike HRTEM, ALCHEMI does not place stringent resolution requirements on the microscope and, because EDS clearly distinguishes between elements of similar atomic number, it can offer some advantages over HRTEM. It does however, place certain constraints on the crystal. These constraints are: a) the sites of interest must lie on alternate crystallographic planes, b) the projected charge density on the alternate planes must be significantly different, and c) there must be at least one atomic species that lies solely on one of the planes.An electron beam incident on a crystal undergoes elastic scattering; in reciprocal space this is seen as a diffraction pattern and in real space this is a modulation of the electron current across the unit cell. When diffraction is strong (i.e., when the crystal is oriented near to the Bragg angle of a low-order reflection) the electron current at one point in the unit cell will differ significantly from that at another point.

The energy spectra and the quantum efficiencies of electrons emitted from the metallic elements irradiated by 60Co gamma rays

The energy spectra, angular distributions, and the absolute quantum efficiencies of forward emission electrons from 60Co γ-ray irradiated metallic elements have been measured using a double focusing β-ray spectrometer. Samples measured were C, Al, Cu, Mo, Sn, W, and Pb. Their thicknesses were chosen about equal to the maximum range of a 1.33 Mev photoelectron so as to ensure electron equilibrium. Similar spectral shapes were observed for samples with similar atomic numbers, but energy spectrum for the large atomic number sample was different from the small atomic number one. Photoelectron peaks were observed in the spectra for W and Pb. An empirical equation which gives the quantum efficiency as the function of atomic number was obtained. The efficiencies computed from this equation were in good agreement with experimental results and numerical calculations.

A Library of Electron Energy Loss Spectra at leV Resolution

The need for a reference "library" of EELS edges has recently been recognized by several workers, and a partial EELS library has become available for the most common elements at about 3eV resolution. Our work aims at expanding this pool of known spectra to less common elements, and improving the energy resolution to ≤ leV. It is hoped that the resultant library will be useful in several ways: as a source of experimental spectra for testing cross-section calculations and spectra quantitation programs, as a data bank of high-resolution "fingerprint" spectra for elemental identification of unknown samples, and as an educational tool showing the trends and features appropriate to whole classes of elements of similar atomic number.

Atomic site determination using the channeling effect in electron-induced x-ray emission

The variation of the characteristic X-ray emission from a thin crystal with crystallographic direction of the incident electron beam has proved to be a powerful effect for the determination of the crystallographic position of atoms with similar atomic numbers and low concentrations of atoms. The studies of a Al 2ZnO 4 spinel containing 3% Fe and 0.3% Mn, i.e. small concentrations of neighbours in the periodic system, will be presented. It is concluded that 62±2% of the Fe atoms and 97±10% of the Mn atoms are in tetrahedral positions. The high sensitivity of the method suggests that it is possible to determine the position of a monolayer or less of adatoms on a crystal surface. This possibility is discussed.

Characteristic K X-ray production in heavy ion-atom collisions in solids

We report on some of the factors influencing the characteristic K X-ray yields which result from heavy ( Z 1 ∼ 35) ion bombardment of similar atomic number targets ( Z 2). All targets are in the form of narrow, shallow distributions implanted into solid hosts. At the beam energies used, viz. 2 to 4 MeV, K-vacancy production is presumed to arise from a collision sequence, the first producing a projectile L-shell vacancy in a projectile-host atom encounter and the second, a projectile-target atom encounter, transferring the L-vacancy via transient MO orbitals to the K-shell of one of the heavy collision partners. Evidence for a two-step process is presented. First, Kr K X-ray excitation functions measured for Kr ++ bombardment of Si(Kr) and Be(Kr) targets are identical in shape. The different magnitudes are attributed to different efficiencies for Kr 2p-shell ionization in KrSi and KrBe collisions. Second, systematic variation of the host material for fixed ( Z 1, Z 2) demonstrates clearly the role of level matching effects between the projectile-2p and host-1s energy levels in creating projectile 2p-vacancies.

Ionization of air by beta rays from point sources.

With the rapidly increasing use of radioisotopes in research, therapy, and radiation protection studies, the problem of accurately determining the distribution of the energy imparted to chemical systems and to biological materials has greatly increased in importance. In the presence of beta radiation, the problem is simple only at points in a medium where the concentration is uniform over distances equal to or greater than the range of the emitted particles. Unfortunately this situation is of rare occurrence. In the more usual case, the problem requires: (a) evaluation of the dose as a function of the distance from a point source and (b) integration at a given point of the energy contributed by beta particles originating within the range of the particles. Since knowledge of (a) is required in order to proceed with (b), the investigations reported here were undertaken using air as a homogeneous medium. This choice allowed the use of large-dimensioned apparatus, with its relative ease of construction, and permits direct application to denser media of similar atomic numbers, such as tissues. Since the objective of these measurements was an accurate determination of the ionization distribution, the principal efforts were directed toward the demonstration that the observed ionization currents were due exclusively to the emitted particles in their undisturbed journey through the air and that all the effects due to solid scatterers or absorbers were properly accounted for. Owing to the special construction of one of our chambers, it was also necessary to prove that all the ions formed in the detecting volume were collected. Because of limitations of space, the many experiments performed will only be outlined here. The first step was to select a large room in which to make the measurements (Fig. 1). The source, S, was moved along the indicated diagonal of the room on which the ionization chamber, I. C., was placed. L represents the locus of points beyond which a p32 beta particle of maximum energy, elastically scattered, once in its path, could not possibly reach the center of the chamber. Thus it is beyond question that the size of the room was adequate. The possible effects of the earth's magnetic field (indicated by H in the figure) were eliminated by theoretical considerations and by experimental study. The source was supported by means of a large L-shaped frame dimensioned so that it subtended a solid angie at the source of less than 1.0 per cent, reducing scattering effects to negligible proportions. The source was suspended by means of thin (10 mil) wires and prevented from swinging by similar wires. It was mounted on a thin aluminum washer (Fig. 2) of 6 inches outside diameter and 2 inches inside diameter.

Surface dose.

To the rather complicated subject of the measurement of tissue doses of ionizing radiations, the surface dose adds its own problems. This present summary will touch upon the theory of dose determinations, give the methods and results of some measurements, and indicate a few of their practical applications. Calculations will be made of the energy absorbed for beams of x-radiation produced at a number of different kilovoltages and for elements of higher atomic number in the skin. The specific ionization will also be estimated for the 100 kilovolt to 1 million volt range, to see if these factors show any correlation with threshold erythema dose. There are important difficulties in obtaining a measure of some physical quantity which will give an indication of the biological, effect to be expected when the superficial tissues of the body are receiving radiation. If x-rays or gamma rays are to be measured in roentgens at a given point, then the associated corpuscular emission produced by the radiation at that point should be made to expend all its energy in the production of ionization in a known air volume. The secondary electrons set in motion by the original radiation travel distances in air which are much longer than the dimensions of the usual ionization chamber. Therefore, most of those electrons produced in the chamber volume have left it before using up all their energy in the formation of ion pairs. If, however, enough air or material of similar atomic number surrounds the measuring air volume, and if the flux of primary radiation is constant, then as many electrons will be produced to enter that volume as are leaving it. If the surrounding substance is thicker than the minimum necessary, there will not be an over-supply of electrons, as those coming from a thickness of material greater then their range will be absorbed within that material. In treating patients at high energies, where the secondary particles travel greater distances, this electronic equilibrium becomes impossible of exact achievement. Some secondary electrons produced by the 22-mev betatron, for instance, will travel a distance in tissue of the order of centimeters. When such a thickness of material is in front of the air cavity, there will be a change in flux of primary radiation due to the inverse-square law and to absorption within the material itself. Thus the best that can usually be done is to employ an “equilibrium wall thickness,” which will produce the maximum ionization (1). This should also be air wall, that is, it should have the same effective atomic number as air at the wave length of the radiation being used. If a sufficiently thick air wall chamber is being used, the question next arises as to the exact position at which the measurement is valid. At high energies most of the electrons causing ionization in the air volume have been produced some distance in front of it.