Radioactivity

The emanations are produced from the parent matter and escape into the air under some conditions. Rutherford and Soddy made a systematic examination of the emanating power of thorium com pounds under different conditions. The hydroxide emanates most freely, while in thorium nitrate, practically none of the emanation escapes into the air. Most of the compounds of actinium emanate very freely. Radium compounds, except in very thin films, retain most of the emanation in the compound. The occluded emanation can in all cases be released by solution or by heating. On account of its very slow period of decay, the emanation of radium can be collected like a gas arid stored, when it retains its characteristic properties for a month or more. A more detailed account of the properties of the radium emanation, which has proved of great practical importance, will be given later.

Active Deposits.

The radioactive emanations possess another very interesting and important property. The surface of any body exposed in the presence of an emanation acquires a temporary activity. Like the emanations this induced activity, as it was first termed, decays with the time though at quite a different rate from the emanation which causes it. The bodies made active be have as if they were covered by a very thin film of intensely active matter. The active substance can be removed partly by rubbing on the surface and can be dissolved by strong acids. On evaporating the acid, the active matter remains behind unchanged in amount. The active deposit obtained on a platinum wire or plate can be volatilized at a white heat and is again deposited on the cooler bodies in its neighbourhood. The activity can be concentrated to some extent on the negative electrode in a strong electric field, in dicating that the radioactive matter carries a positive charge. We shall see later that this activity produced on bodies is due to a de posit of non-gaseous matter derived from the transformation of the emanations. Each emanation gives a distinctive active deposit which decays at different rates. The active deposits of radium, thorium and actinium are very complex, and consist of several types of matter. Several hours after removal from the emanation the active deposit from radium decays to half-value in 26 minutes, from actinium half-value in 34 minutes, from thorium half-value in ro.6 hours.

Radiations from Radioactive Substances.

All the radio active substances possess in common the property of emitting radiations which darken a photographic plate and cause a dis charge of electrified bodies. Very active preparations also possess the property of causing marked phosphorescence in some sub stances. Bodies which phosphoresce under X-rays usually do so under the rays from radioactive matter. Barium platinocyanide, the mineral willemite (zinc silicate) and zinc sulphide are the best known examples.

There are in general three types of radiation emitted by the radioactive bodies, called the a-, 0- and 7-rays. Rutherford in 1899 showed that the radiation from uranium was complex and con sisted of (a) an easily absorbed radiation stopped by a sheet of paper or a few centimetres of air which he called the a-rays and (b) a far more penetrating radiation capable of passing through several millimetres of aluminium, called the j3-rays. Later Villard

found that radium emitted a very penetrating kind of radia tion, called the y-rays, capable of passing before absorption through twenty centimetres of iron and several centimetres of lead. A large amount of work, of which a more detailed account will be given later, has been carried out to determine the nature of these radiations.

The Alpha-rays.—The a-rays consist of a stream of material particles which are projected at high speed from the radioactive matter. The a-particles from all types of radioactive matter are identical in mass and consist of charged atoms of helium pro jected with velocities of about io,000 miles a second. The a-par ticles are expelled with a characteristic speed from each radioactive substance and have a definite distance of travel or "range" in mat ter before they are stopped. The range of the a-particles for different radioactive substances varies between about 3 cm. and II cm. in air at atmospheric pressure and temperature. Most of the energy emitted from radioactive bodies is in the form of « rays.

The Beta-rays consist of a stream of electrons which are pro jected with high velocities, approaching in some cases that of light. Unlike the emission of a-particles, a radioactive body emits 0-particles over a considerable range of velocity.

The Gamma-rays, which are of a very penetrating character, have been shown to be a type of X-rays of very high frequency. Usually the y-rays accompany the emission of 0-rays. Unlike the a- and 8-particles, the 7-rays are undeflected by a magnetic or electric field. It will be shown that in general the y-rays from a radioactive body consist of groups of electromagnetic radiations of widely different frequencies. The three types of radiation from active bodies are thus analogous in many respects to the radiations produced by the passing of an electric discharge through a vacuum tube at low pressure, but the individual intensity is on a much higher scale. In order to produce electrons of speed correspond ing to those emitted by radium C, a voltage of three million would have to be applied to the tube, and to excite X-rays of the fre quency of the y-rays at least two million volts would be neces sary. Similarly the energy of the a-particle from radium C cor responds to nearly eight million electron volts. The a-rays from a deposit of radium C equivalent in activity to ten milligrams of radium, are of very great intensity; they blacken a photographic plate in a fraction of a second and produce intense luminosity in phosphorescent substances like zinc sulphide and willemite. Sim ilarly the 0 and y-rays from such a source cause an evident lumi nosity in willemite and barium platinocyanide.