This is an introductory review of the physics and applications of low energy x-rays and electrons in the 10-1000 ev region. The basic interactions of these radiations within matter are discussed and typical de-excitation spectra, fluorescent x-ray and photoAuger electron, are presented. Specific examples of spectrographic methods and instruments for the low energy region are described as “based upon the use of long-spaced, Langmuir-Blodgett type of multilayers for ultrasoft x-ray analysis and the use of the hemispherical electrostatic analyzer for photo-Auger electron spectroscopy. Some examples of spectrographic signal, signal/background, and resolution are presented for applications to light element fluorescence, valence emission band, and photo-Auger electron analysis. The special aspects of the low energy x-ray analysis of high temperature plasmas and of x-ray astronomical sources in general are described.

Qualitative and quantitative analyses of elements below atomic number 20, and extending to atomic number 4, have been made practical and reasonably routine only in the past five to ten years by advances in: 1) excitation sources; 2) dispersive spectrometers; 3) detection devices; and 4) reductions of optic path absorption. At present agreement is lacking on the best combination of parameters for light element analysis. The principal contrasts in opinion concern excitation.

Direct electron excitation, particularly as employed in microprobe analysis (but not limited to such instruments), provides relatively high emission intensities of all soft X-rays, but also generates a high continuum, requires the sample to be at essentially electron gun vacuum, and introduces practical calibration problems (“matrix effects“). X-ray excitation of soft X-rays overcomes some of the latter three disadvantages, and has its own limitations. Sealed X-ray sources of conventional or semi-conventional design can provide useful (if not optimum) light element emission intensities down to atomic number 9, hut with serious loss of efficiency in many applications below atomic number 15 largely because of window-thinness limitations under electron bombardment.

Instrumentation for spectrometry in the long wavelength X-ray region has been improved significantly in the last several years. In the area of excitation of soft X-rays, many changes have taken place. Secondary excitation source windows have been made much thinner and high power windowless and ultra thin windowed continuously pumped X-ray tubes have been developed. One source has been described which allows a choice of either primary or secondary excitation. Cold cathode techniques have been developed in which pressure control is achieved using an automatic pressure controller. Primary electron excitation has been revived and has become very important mainly due to the popularity of electron microbeam probe analysis. In the area of spectral dispersion, many improvements and innovations have been seen, recently in both grating and crystal spectrometers. Organic single crystals have been grown having 2d spacings up to about 100Ǻ and soap films have been fabricated with 2d spacing as large as 165Ǻ, allowing crystal spectrometers to be used to about 160Ǻ.

A solution of the intensity equation of X-ray Fluorescence analysis is possible either by the assumption of an effective wave length or by a variation of the take-off angle. When the intensity of the fluorescence radiation is measured as a function of the take-off angle, the extrapolation of this curve against take-off parallel to the flat sample surface offers a simple evaluation of the intensity equation. Since the influence of the secondary excitation is also taken in to account the composition and the mass per unit area of samples can be investigated as well as mass absorption coefficients and mean wavelengths of the polychromatic primary radiation.

In the research described, the use of scattered x-rays has been successfully applied as an internal standard for the analysis of calcium in aqueous specimens containing a wide range of matrix components. In addition to the demonstration of the utility of scattered x-rays for light element analysis, some comments are offered on the fundamental aspects of this technique, since to date the method has not been explained thoroughly. The present research represents a continued effort to determine the fundamental importance of various parameters intrinsic to any collection of atoms undergoing scattering, such as the Rayleigh-Compton ratio, the scattering angle, the wave length utilized, and the presence or absence of discontinuities in the matrix absorption coefficient. It has been concluded that large values of the scattering angle coupled with short wavelength tend to yield improved internal compensation. The results also indicate that for light matrices the Compton component of the scattered continuum is of particular importance in achieving good internal standardization for matrix effects.

A method is described for the x-ray fluorescence analysis of small samples, taken from museum objects, to determine alloy composition. The samples are dissolved in an appropriate reagent and absorbed on cellulose powder. The resulting powder is formed into a film of less than critical thickness and the effective absorption of the sample for the characteristic wavelength of the element being measured is determined. The effective absorption coefficient is used to correct the observed intensities in order to obtain quantitative results.

Recently on-stream analysis by various methods and instruments has been greatly and rightfully emphasized. This is particularly true in such research as air and stream pollutions, desalting of ocean water and ore processing.

Among the various methods, instruments, and procedures adopted, the X-ray fluorescence method is perhaps the best among them. This is because the x-ray fluorescence method neither consumes nor destroys the samples taken for analysis. It also has the merits of simplicity and specificity. However, on-stream analysis of liquid samples containing elements having an atomic number below 20 by the X-ray fluorescence method is not an easy task particularly when vacuum spectrograph is being used.

In this study liquid cells of different dimensions for the on-stream analysis operated in air, helium and in vacuum have been successfully constructed. The cells have been tested for several months.

Phosphate rock from different sources varies widely in composition. This results in a large interelement effect. A mathematical model was developed to correct for matrix variations in each sample. This model utilizes the X-ray intensity from each of the six elements considered and the mass absorption coefficients of the elements to calculate concentrations. Accurate analyses for P, Si, Ca, Fe, Al, and Mg are obtained using this model. The X-ray instrument is interfaced with a small computer that performs the calculations on each sample and prints the results.

Sample preparation, standardization and the computer programs used for sample calculation and standardization will be discussed.

Investigation of the NiLIII band at various excitation voltages and the use of Ni and Ti thin films in the path of the NiLII,III and TiLII,III radiation, has shown that with proper electrode geometry and using target voltages of about 4 kv the effect of self absorption on emission edge shift and band shape is negligible for these elements. However, the LII/LIII intensity ratio of titanium is strongly effected by self absorption, but it is not effected for nickel and iron by self absorption.

A survey of several investigators’ soft x-ray band spectra from alloys and conducting compounds showed that the observed band changes were related to the electronegativity difference of the combining elements or the ionic character of the band. Peaks appearing on the low energy side of transition metal bands when combined with 2nd period elements appear to be a cross transition between the 2s or 2p band of the non-metal and inner level of the metal.

The LIII x-ray emission bands from some 3d transition metal oxides and the K emission bands from some second and third period metal oxides and nitrides are shown and discussed in relation to the energy band structure of these compounds. It is demonstrated that the LIII emission bands can be grossly distorted by excitation conditions. Many of the compound spectra show strong evidence of a crossover transition which originates in the anion and terminates in the metal ion. For the 3d compounds this crossover transition appears to be related to the elsctrical and magnetic properties. In oxides of some second and third period elements it appears that the crossover transitions are responsible for the entire K band spectrum.

X -ray spectra arising from the valence bands of solids are useful for basic studies of the electronic structure of most materials and for practical measurements of unknowns to obtain information on local atomic structure and material properties as well as chemical composition. Understanding the characteristics of valence band spectra is prerequisite to their fullest use. One-electron and many-body aspects of the x-ray emission process, and the effects of experimental conditions, must be understood and are reviewed. Interpretation of spectral features and determination of electronic structure are complementary parts of one procedure which is based on the use of bonding theory. The various band and bond theories which are finding use for spectral interpretation are briefly reviewed. Calculation of the electronic structure of aluminum metal and quartz, and interpretation of their x-ray spectra, are examples which illustrate basic work with valence band spectra.

Soft X-ray spectroscopy promises to be one of the most powerful techniques for solving a wide variety of materials characterization problems. Although chemical analysis of the light elements is commonly done by the electron microprobe and X-ray fluorescence, the “chemical effect on X-ray spectra” has been utilized in relatively few cases. This appears to result from a general lack of appreciation for the applicability of chemical effect measurements and perhaps to a greater extent it is due to the unavailability of appropriate soft X-ray spectrometers. Most studies have been carried out using instruments that were not particularly well suited to chemical effect studies. The purpose of this paper is to describe a versatile new soft X-ray spectrometer and auxiliary equipment for the study of the chemical effect. The spectrometer is housed in a versatile, high vacuum chamber 24 inches in diameter by 4 5/16 inch vertical clearance.

We calculate the extended X-ray absorption fine structure by treating the ejected photoelectron as a spherical wave which expands in the lattice and is partially scattered by neighbors of the absorbing atom. The neighboring atoms are treated as point scattered and the total scattered wave is summed from the waves scattered by each atom. The fine structure is determined from the dipole transition matrix between the initial K-state and the final photoelectron state. Calculations compare favorably with experimental data.

Studies of the fine structure of absorption edges in the soft x-ray region are becoming increasingly important as a tool for materials characterization. Examples of application includes determination of chemical state of elements, bonding and band structure studies. Intensity and resolving power of the x-ray spectrometer are important experimental considerations. As a rule adjustment of instrumental parameters, such as collimation, to give increased intensity adversely affect resolving power. Optimization of intensity and resolving power must therefore he achieved.

A newly designed double crystal spectrometer has been constructed for high-resolution absorption edge studies in the wavelength region of 5Ǻ to 70Ǻ.

The entire system is enclosed in a vacuum chamber ion pumped to the 10-7 torr range. The second crystal motion is obtained by means of an ULTRADEX 360-sided polygon and sine arm that is automatically step-scanned. The spectrometer functions equally well in the (l, + l) and (l, - l) orientation and as a precise single crystal spectrometer.

Although searches so far have been restricted to a few small rockets and balloons, some 40 discrete x-ray sources have already been resolved against a diffuse, nearly isotropic background radiation. The strongest source is about 2000 times as bright as the weakest detectable with present rocket instruments. Nearly all of the discrete sources lie close to the galactic plane and most likely are members of the spiral arms of the Milky Way. One x-ray source at high galactic latitude is identifiable with a distant radio galaxy, Virgo A, and its x-ray luminosity is 70 times its radio power. The diffuse background radiation seems to be resolvable into at least two components: one may be associated with the interaction of cosmic rays and the microwave photons of the cosmological 3 K background; the other with bremsstrahlung from hot, intergalactic gas.

This paper will discuss some of the techniques used for soft x-ray astronomy (10 kev - 0.5 kev). A more extensive treatment will be found in review papers by Giacconi, Reidy, Vaiana, Van Speybroeck and Zehnpfennig and by Giacconi, Gursky and Van Speybroeck. The earliest experiments in x-ray astronomy were performed using mechanically collimated small area ( ∼ 10 cm2) geiger tubes flown in open stabilized rockets. Subsequent improvements in sensitivity have resulted from the use of much larger ( ∼ 1000 cm2) counters and improved background rejection techniques. Important spectral information has been obtained by using gas counters which are operated as proportional counters rather than as geiger tubes and the development of the modulation collimator has significantly improved the obtainable angular resolution. The most promising new technique for x-ray astronomy is the development of grazing incidence x-ray optics .

A non-dispersive X-ray detection system consisting of proporticnal counters plus filters is being prepared for the Command-Service Module of the Apollo spacecraft as part of a “geochemistry“ package. It will detect solar induced characteristic X-rays from the abundant elements on the lunar surface during the orbiting phases of the mission. The objective will be a compilation of a map of the lunar chemical composition and to detect regional differences. The system and its theoretical performance are described.

The soft x - ray slitless spectrograph is described, and its usefulness in making rapid surveys of wide spectral regions is discussed Fabrication of the 1μ thick gold-shadowed transmission gratings used in the instrument is described. Spectra taken in the laboratory and during a solar sounding rocket flight are presented. Because a simple absorbing strip model is inadequate to explain the distribution of intensities among the spectral orders, a phase-grating model must be adopted.