Dr HH Mate
Continued from prev issue
From 1895 to 1992 there seemed to be no analyzer capable of dispersing an x-ray beam into a spectrum. The spectacular Lane diffraction pattern of a zinc sulphide crystal in 1912 proved the electromagnetic wave nature of x-rays and the ordered structure of crystals with atoms lying on families of planes to constitute three- dimensional diffraction gratings, all governed by the simple Bragg law n8= 2dsin 2 (which must be corrected for refraction extremely accurate work).
Here n is an integer indicating the order of the spectrum, 8 the wavelength, d the crystal lattice spacing of one set of planes, and 2 the angle between the incident ray and this set of planes.
The range of x-rays in the electromagnetic spectrum, as excited in x-ray tubes by the bombardment of anode targets by cathode electrons under a high accelerating potential, overlaps the ultraviolet range on the order of 100 nanometers on the long-wavelength side, and the shortest wavelength limit moves downwards as voltages increase.
An accelerating potential of 109V, now readily generated, produces a 8 of 10-6 nanometer, or about 1/6000 the wavelength of yellow light.
Quantum theory: In the consideration of Roentgen rays as continuous electromagnetic waves, it must not be dismissed that they also appear to be propagated in discontinuous bundles, or quanta, in accordance with the laws first enunciated by plank and extended by Albert Einstein early in the twentieth century.
In diffraction, refraction, polarization, and interference phenomena, x-rays, together with all other related radiation, appear to act as waves and 8 has a real significance. Beams of corpuscular electrons and neutrons are diffracted so that they too have wavelengths. In other phenomena such as the appearance of sharp spectral lines, a definite short-wavelength limit 8o of the continuous white spectrum defined by 8o=bceV, where b is plank’s constant, c the velocity of electromagnetic radiation, including light and x-rays, e the charge of electron, and v the accelerating voltage, the shift in wavelength of x-rays scattered by electrons in atoms (Compton effect) and the photoelectric effect- the energy seems to be propagated and transferred in quanta, called photons, defined lay valued of bv, where the frequency is c/8.
Applications of X-rays: Important uses have been found for x-rays in many fields of scientific endeavour.
For example, roentgen spectrometry is the science of measuring values with a known crystal of lattice spacing of d, Roentgen diffractometry is the science of determining unknown values of d, and thereby crystal structures, with x-ray beams of known 8o.
In both cases, the experimental measurement is that of the angle 2.
Extensive tables of the wavelengths of x-ray emission lines in series (K,L,M and so on) and so – called absorption edges, characteristic of the chemical elements, afford the necessary information for chemical analyses, exactly as in the case of optical emission spectra and for derivation of theories of atomic structure to account for the origin of spectra.
(The writer is a researcher, educationist, academician and sociologist. He can be reached at [email protected]