Ionization by X-rays and Rays from Radium. Radioactivity.
As has been said, the electrons in a vacuum tube cause its wall to emit a greenish light when they strike it. Upon meeting the glass wall or a piece of metal (the anticathode) placed in the tube the electrons cause also the emission of the peculiar, penetrating rays called Röntgen rays in honour of their discoverer, or more commonly X-rays. They may be described as ultra-violet rays with exceedingly small wave-lengths ([cf. p. 54]). When, further, the electrons meet gas molecules in the tube they break them to pieces, separating them into positive and negative ions (ionization). The positive ions are the ones which appear in the canal rays. The ions set in motion by electrical forces can break other gas molecules to pieces, thus assisting in the ionization process. At the same time the gas molecules and atoms are made to produce disturbances in the ether, and thus to cause the light phenomena which arise in a tube which is not too strongly exhausted.
The free air can be ionized in various ways; this ionization can be detected because the air becomes more or less conducting. In fact, electric forces will drive the positive and negative ions through the air in opposite directions, thus giving rise to an electric current. If the ionization process is not steadily continued, the air gradually loses its conductivity, since the positive and negative ions recombine into neutral atoms or molecules. Ionization can be produced by flames, since the air rising from a flame contains ions. A strong ionization can also be brought about by X-rays and by ultra-violet rays. In the higher strata of the atmosphere the ultra-violet rays of the sun exercise an ionizing influence. Most of all, however, the air is ionized by rays from the so-called radioactive substances which in very small quantities are distributed about the world.
The characteristic radiation from these substances was discovered in the last decade of the nineteenth century by the French physicist, Becquerel, and afterwards studied by M. and Mme. Curie. From the radioactive uranium mineral, pitchblende, the latter separated the many times more strongly radioactive element radium. The proper nature of the rays was later explained, particularly through the investigations of the English physicists, Rutherford, Soddy and Ramsay. These rays, which can produce heat effects, photographic effects and ionization, are of three quite different classes, and accordingly are known as α-rays, β-rays, and γ-rays. The last named, like the X-rays, are ultra-violet rays, but they have often even shorter wave-lengths and a much greater power of penetration than the usual X-rays. The β-rays are electrons which are ejected with much greater velocity than the cathode rays; in some cases their velocity goes up to 99·8 per cent. that of light. The α-rays are positive atomic ions, which move with a velocity varying according to the emitting radioactive element from ¹/₂₀ to almost ¹/₁₀ that of light. It has further been proved that the α-particles are atoms of the element helium, which has the atomic weight 4, and that they possess two positive charges, i.e., they must take up two electrons to produce a neutral helium atom.
There is no doubt that the process which takes place in the emission of radiation from the radioactive elements is a transformation of the element, an explosion of the atoms accompanied by the emission either of double-charged helium atoms or of electrons, and the forming of the atoms of a new element. The energy of the rays is an internal atomic energy, freed by these transformations. The element uranium, with the greatest of all known atomic weights (238), passes, by several intermediate steps, into radium with atomic weight 226; from radium there comes, after a series of steps, lead, or, in any case, an element which, in all its chemical properties, behaves like lead. We shall go no further into this subject, merely remarking that the transformations are quite independent of the chemical combinations into which the radioactive elements have entered, and of all external influences.
When α-particles from radium are sent against a screen with a coating of especially prepared zinc sulphide, on this screen, in the dark, there can be seen a characteristic light phenomenon, the so-called scintillation, which consists of many flashes of light. Each individual flash means that an α-particle, a helium atom, has hit the screen. In this bombardment by atoms the individual atom-projectiles are made visible in a manner similar to that in which the individual raindrops which fall on the surface of a body of water are made visible by the wave rings which spread from the places where the drops meet the water. This flash of light was the first effect of the individual atom to be available for investigation and observation. The incredibility of anything so small as an atom producing a visible effect is lessened when, instead of paying attention merely to the small size or mass of the atom, its kinetic energy is considered; this energy is proportional to the square of the velocity, which is here of overwhelming magnitude. For the most rapid α-particles the velocity is 2·26 × 10⁹ cm. per second; their kinetic energy is then about ⁴/₃₀ of the kinetic energy of a weight of one milligram of a substance at a velocity of one centimetre per second. This energy may seem very small, but, at least, it is not a magnitude of “inconceivable minuteness,” and it is sufficient under the conditions given above to produce a visible light effect. We must here also consider the extreme sensitiveness of the eye.
Fig. 19.—Photograph of paths described by
α-particles (positive helium ions) emitted
from a radioactive substance.
More practical methods of revealing the effects of the individual α-particles and of counting them are founded on their very strong ionization power. By strengthening the ionization power of α-particles, Rutherford and Geiger were able to make the air in a so-called ionization chamber so good a conductor that an individual α-particle caused a deflection in an electrical apparatus, an electrometre.
Fig. 20.—Photograph of the path of a
β-particle (an electron).
(Both 19 and 20 are photographs by C. T. R. Wilson.)
With a more direct method the English scientist C. T. R. Wilson has shown the paths of the α-particles by making use of the characteristic property of ions, that in damp air they attract the neutral water molecules which then form drops of water with the ions as nuclei. In air which is completely free of dust and ions the water vapour is not condensed, even if the temperature is decreased so as to give rise to supersaturation, but as soon as the air is ionized the vapour condenses into a fog. When Wilson sent α-particles through air, supersaturated with water vapour, the vapour condensed into small drops on the ions produced by the particles; the streaks of fog thus obtained could be photographed. [Fig. 19] shows such a photograph of the paths of a number of atoms. When a streak of fog ends abruptly it does not mean that the α-particles have suddenly halted, but that their velocity has decreased so that they can no longer break the molecules of air to pieces, producing ions. The paths of the β-particles have been photographed in the same way, although an electron of the β-particles has a mass about 7000 times smaller than that of a helium atom; the electron has, however, a far greater velocity than the helium atom. This velocity causes the ions to be farther apart, so that each drop of water formed around the individual ions can appear in the photograph by itself ([cf. Fig. 20]).