Polychromatic beam

The first requirement is a source of infrared radiation that emits all frequencies of the spectral range being studied. This polychromatic beam is analyzed by a monochromator, formerly a system of prisms, today diffraction gratings. The movement of the monochromator causes the spectrum from the source to scan across an exit slit onto the detector. This kind of spectrometer in which the range of wavelengths is swept as a function of time and monochromator movement is called the dispersive type. 

The two effects just described help to determine the x-ray distribution at the target. Before an x-ray beam strikes a sample to be analyzed, the beam is usually modified further. For example, there may be absorption (and filtering) by the window of the x-ray tube, by an air path between tube and sample, by the walls of a cell containing the sample, and finally by the sample itself. Analogous considerations govern the absorption (and, with polychromatic beams, the filtering) of the beam entering the detector from the sample. 

Roentgen17 himself established that the rays easiest to absorb ( soft rays k large) came from soft tubes (poor vacuum), and that hard tubes (good vacuum) gave hard rays (k smaller). In this qualitative language, soft rays are filtered out of a polychromatic beam that passes through an absorber and the beam necessarily becomes harder in the process. We know today that, the harder the ray, the shorter its wavelength. 

Figure 1-8 shows log-log curves calculated from Barkla s absorption-coefficient data. (A log-log plot shows most clearly what Barkla discovered.) For carbon, the wavelength distribution is virtually unchanged from that of the incident polychromatic beam, mainly scattered x-rays being detected the situation is reminiscent of Figure 1-5. The curve for calcium, on the other hand, begins with a straight line that shows the presence in the scattered beam of a relatively intense component for which k is large and sensibly constant. The curve for tin shows two such components. Barkla realized that these components are emitted, and he eventually called them K and L spectra.22 He...

Equation 1-7 may be considered always valid for absorptiometry with monochromatic x-rays, and usually valid for absorptiometry with polychromatic x-rays also. Difficulties sometimes arise with polychromatic beams. 

Fig. 1-15. The molybdenum spectrum excited by 35-kv electrons and by the polychromatic beam from a 35-kv x-ray tube. With x-ray excitation, most of the energy appears in the characteristic lines. With electron excitation, most of it is wasted in the continuous spectrum.

The phosphor-photoelectric detector is generally used with polychromatic beams the intensity of which is high enough to make the detector instantaneous. External amplification easily increases its otTtput currents to values that can be read on a micro- or milliammeter. Output currents thus amplified could be used through servo links to control operations such as blending. 

We shall introduce absorptiometry with polychromatic beams by examining briefly an important and successful control application,6 the thickness gaging of steel strip, which illustrates certain strong points of the technique and foreshadows its limitations in chemical analysis. 

Chemical analysis, however, is often another story. Here the limitations are serious enough that absorptiometry with polychromatic beams can generally be used only if the qualitative composition of the sample is known—often only if the matrix (by which is meant all of the sample except the element being determined) changes little from sample to sample. 

To illustrate these considerations, and to introduce a detector in which measured x-ray intensity is given by an electric current, we shall use experimental results obtained on the simple laboratory photometer described in Section 3.5. The general approach is broadly applicable in absorptiometry with polychromatic beams. 

The photometer of Figure 3-2 was designed originally10 to study fuze testing, but its simplicity and reliability make it useful in any analytical laboratory where the diversity of problems is great enough to include some in which absorptiometry with polychromatic beams can provide useful auxiliary information even though it cannot often furnish a complete answer.11... 

The effective wavelength provides a useful way of characterizing polychromatic beams that are not appreciably affected by the presence of an absorption edge. One suspects intuitively that the presence of an absorption edge critically located could cause complications in absorptiometry with polychromatic beams. That this does happen has been demonstrated,13 and it limits the usefulness of the effective wavelength. 

Fig. 3-5. Attenuation of a polychromatic beam by various materials. Note superiority of beryllium as a window material. (Liebhafsky, Smith, Tanis, and Winslow, Anal. Chem19, 861.)...

The fact that gases have a simple equation of state makes possible the use of absorptiometry with polychromatic beams to give information about the state of a gas under conditions (in detonation waves,16 boundary layers,17 or supersonic flow18) transient or difficult of access. Temperature measurements19 have also been made. The technique is a unique method for studying the fluidization of a finely divided solid by a gas. Bed density profiles, which reveal the character and effectiveness of fluidization, have been readily determined20 without disturbing the system as probes would inevitably do. 

Inserting aluminum foil (0.0254 cm thick) reduced transmittance to 83.8% of its value for cell 5 alone. With aid of published data for aluminum, one obtains 0.556 A for effective wavelength of polychromatic beam. 

X-ray absorptiometry can be used to establish not only the average composition of a material, but its lack of uniformity as well. Owing to the high intensity attainable in polychromatic beams, it is possible to... 

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