X-rays and applications

Dr HH Mate *

X-rays or Roentgen X-rays, are electromagnetic waves in which periodically variable electric and magnetic fields are perpendicular to each other and to the direction of propagation. Thus they are identical in nature with visible light and all the other types of radiation that constitute the electromagnetic spectrum such as ultraviolet, infrared, gamma rays form radioactive atomic disintegrations, microwaves, and radio or hertzian waves.

In general, X-rays are generated as the result of energy transistors of atomic electrons caused by the bombardment of a material of high atomic weight by high energy electrons.

Roentgen’s findings: The unequivocal establishment of the nature of these rays was not made in W.C. Roentgen’s experiments following the discovery of a new kind of ray” in 1985.

In his first communication, Roentgen described the properties of these rays as follows :
They were invisible; and moved in straight lines,
were unaffected by electric or magnetic fields, and hence not electrically charged;
passed through matter opaque to ordinary light since they penetrated through the black cardboard around his cathode ray tube;
were differentially absorbed by matter of different densities or of different atomic weights;
affected photographic plated;
produced fluorescence in certain chemicals, such as in the barium platinocyanide screen with which the initial discovery was made and in the wall of his glass tube opposite the cathode;
produced ionization in gases;
and were evidently produced by the stoppage at the anode of the beam of rays, as identified by J.J. Thomson in 1877 as electrons, issuing from the cathode in his vacuum tube.

Along with all these definitive characteristics of the X-rays, however, other crucial experiments designed to establish similarity or differences from ordinary light were clearly called for.

The fundamental optical properties of light were well established in 1895; reflection from mirrors, refraction in prisms (i.e, change in direction in passing from air into glass, for example), by means of which a beam of white light could be spread out into a rainbow or spectrum of colours; diffraction by narrow slits or ruled gratings, also a method of producing spectra; and polarization, or constraint of the transverse vibrations to a single direction.

In spite of the best efforts of Roentgen, no indubitable evidence of any of these four optical phenomena could be found. Hence the designation “ X “- unknown was assigned by W.C. Roentgen.

Many theories were proposed to account for the apparently unique quality of x-rays, which seemed to be so closely similar and yet so greatly different from light; some suggested that they were vortex rings in the ether, and waves with longitudinal vibrations, that is, vibrations parallel to the direction of propagation as in small waves, instead of transverse as with light.

Later discoveries: Inevitably, other scientists studying the enigma found the essential experimental conditions to prove that X-rays can be polarized, refracted in prisms and in crystals; reflected by mirrors, and diffracted by ruled gratings.

Instead of being refracted in passing from a less dense medium (air) to a more same direction as light so that the index of refraction is always greater than 1, X-rays are deviated in the opposite direction by a very small amount, so that the index of refraction is less than 1 by an amount as small as 10-6.

Thus total reflection from mirror is observed only when the beam impinges at a very small grazing angle, a necessary condition understandably missed by W.C. Roentgen. Similarly, the beam must graze a ruled diffraction grating if a spectrum is to be observed.

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.