Spectroscopic Instruments, X-Ray

apparatus in which X rays are excited in the substance under investigation, are decomposed into a spectrum, and are registered. Precision X-ray spectroscopic instruments are used to investigate the fine structure of X-ray spectra. Analytical instruments are used to determine the elemental composition of substances in X-ray spectrochemical analysis. Precision instruments should have a high resolving power, and analytical instruments should have a high transmission. Different types of instruments are used according to the purposes of the investigation, the conditions under which the investigation is carried out, and the nature of the object investigated.

Dispersive instruments. Dispersive instruments are based on the decomposition of X-radiation into a spectrum by means of diffraction. Such instruments consist of an X-ray tube, a power supply for the tube, a dispersing element (a crystal analyzer or diffraction grating), an X-ray detector, and electronic apparatus that supplies the detector with power and registers the detector pulses. Precision instruments use either crystal analyzers or diffraction gratings. The crystal analyzers are near-perfect crystals that are curved, or “bent,” along the surface of a circular cylinder or a sphere (Figure 1 ,a); the diffraction gratings are concave and have a spherical curvature (Figure l,b). Analytical instruments use either curved crystals or flat crystals. When a flat crystal is used, a Soller slit collimator, consisting of several equally spaced metal foils, is employed to limit to the angular range of the radiation incident on the crystal to between a few minutes and 1 degree of arc (Figure 1 ,c).

The X-ray detectors most frequently used are proportional, scintillation, or semiconductor photon counters; for soft X rays, they are photocathodes with a secondary open-type multiplier are used. If the instrument is intended for the study of primary X-ray spectra, then the test substance is deposited on the anode of a demountable X-ray tube, which is evacuated to a pressure of less than 10^-5 mm Hg. If the properties of the substance are being studied by means of fluorescent X rays, then a sealed X-ray tube is used, and the test substance is placed outside the tube as close as possible to its window.

An X-ray spectroscopic instrument that is designed for the simultaneous registration of one or two spectral lines is known as an X-ray spectrometer or, in the case of photographic recording, as an X-ray spectrograph. Instruments that simultaneously register many (up to 24) spectral lines are called X-ray quantometers. A quantometer has a separate small spectrometer for each spectral line. Each such spectrometer, together with its electronic registration system, constitutes a channel. The radiation from the

Figure 1. Optical diagrams of X-ray spectrometers: (a) focusing spectrometer with crystal analyzer, (b) focusing spectrometer with diffraction grating(c) spectrometer with flat crystal and Soller slit collimators; (A) crystal analyzer, (G) diffraction grating, (s) radiation source, (S₁) and (S₂) slits, (f) focusing circle, (0’) center of focusing circle, (0) center of circle along which curved crystal or concave grating lies, (D) detector, (P) photocathode, (M) electron multiplier, (C,) and (C₂) Soller slit collimators

object being analyzed enters all of the quantometer channels simultaneously. The number of detector pulses counted in a specified time interval is recorded by a digital printer. In many cases, the pulses are integrated in the spectrometers, and the results of continuous scanning of the spectrum are then recorded on an automatic chart recorder. The channel outputs can be fed to a computer for further processing.

Since the continuous recording of a spectrum in precision spectrometers introduces some distortion, automatic step scanning is sometimes used, wherein the number of detector pulses is recorded at many equidistant points of the spectrum. At these points, the spectrometer is stationary for a specified time; the shift from point to point is carried out rapidly. In analytical spectrometers, step scanning is sometimes used along the points of the spectrum where the analytical lines of the elements being determined are located. Such spectrometers operate according to a program that specifies the set of elements to be determined, the pulse-counting time at each of the corresponding points of the spectrum, the parameters required for the electronic recording system, and the type of crystal analyzer (the spectrometers have three or four interchangeable crystals). The spectrometer automatically executes the entire program and records the results.

In industrial enterprises, special-purpose X-ray spectroscopic instruments are often used as composition sensors for the determination of one or more elements. An example is the ARF-4M; it uses the standard-background method of analysis, which is based on the ratio of the intensities of the analytical and background lines. These lines are located close to one another and are registered by a single detector after passing through the two corresponding slits. An oscillating shutter alternately covers each slit; simultaneously, the two systems that register the detector pulses are switched. The registration system stops counting pulses after reaching a specified number of them for the background line. The number of pulses counted for the analytical line is proportional to the ratio of its intensity to the intensity of the background line. Such composition sensors are employed at ore-enrichment plants and nonferrous metallurgical plants. The ARF-4M permits 12 different elements to be determined.

Nondispersive instruments. Nondispersive X-ray spectroscopic instruments are used for X-ray spectrochemical analysis. In such instruments, the X rays from the test specimen are registered directly by scintillation, proportional, or semiconductor counters (Figure 2). The pulse amplitudes of the counters are proportional to the energies of the photons of the radiation being studied. The analytical lines are separated out by a single-channel or multichannel pulse-height analyzer. When the specimen is placed close to the window of the counter, the usable solid angle of radiation for each atom is very large, and the intensity registered is several orders of magnitude greater than that in dispersive instruments.

Figure 2. Schematic of nondispersive X-ray spectrum analyzer: (1) radioisotopic source, (2) protective screen, (3) specimen being analyzed, (4) filter, (5) detector

Consequently, an analysis can be made even with the very weak fluorescent X rays from a specimen that is excited by radioisotopic sources or by miniature X-ray tubes having an anode current no greater than several microamperes (µA).

The disadvantage of nondispersive X-ray spectroscopic instruments is the relatively poor resolution of the proportional detector. In order to eliminate interference from the lines adjacent to the analytical line, a pair of balanced filters for the two adjacent elements is usually used in tandem. The filters permit the region of the spectrum where the analytical line is located to be separated out, and they improve the resolution of the nondispersive instrument.

Because of their small size and weight, portable nondispersive analyzers can be used under field conditions in mineral prospecting and can be lowered into boreholes 40 mm or more in diameter down to depths as great as 100 m.

Microanalyzers. Microanalyzers are based on the microprobe method of analysis. Here, primary X rays are emitted from a specimen excited by a point-focus electron beam, or probe, about 1 micrometer in diameter; the radiation is dispersed into a spectrum, and the spectrum is registered. To produce a fine electron beam, an electron gun and focusing magnetic lenses are employed. By using high-transmission focusing spectrometers with curved crystals or concave diffraction gratings, the spectrum at a given point on the specimen can be obtained with a probe current of several µA. The point can be selected visually with an optical microscope.

If the specimen and probe are stationary and the spectrometer scans, the entire radiation spectrum of the specimen can be measured, and a complete analysis can be obtained of the specimen’s composition at the given point. If the probe and spectrometer are stationary and the specimen is moved, the distribution of the element to which the spectrometer is tuned can be recorded along the path traced out by the probe on the specimen. In the third possible case, the spectrometer and specimen are stationary, and the probe is made to scan by means of two pairs of deflection plates to which varying potentials are applied. If an area of the specimen’s surface that is, say, approximately 0.4 mm by 0.4 mm in size is scanned in synchronism with the line scanning of a cathode-ray oscilloscope tube whose input is supplied by the output potential of the spectrometer detector, then the oscilloscope screen will show a greatly enlarged image of the distribution, over the scanned surface, of the element to which the spectrometer is tuned.

In current microanalyzers, two X-ray spectrometers are often used, one with a crystal analyzer and the other with a diffraction grating. This approach permits a local analysis to be made for all elements, beginning with Li.