IV. Induced Radioactivity and Radioactive Emanations.
Induced Radioactivity.—Radium, thorium, and actinium have the property of acting externally, apart from the Becquerel rays that they emit. They communicate little by little their radioactive properties to substances in their neighborhood, and the latter emit in turn Becquerel rays. The activity can be transmitted to gases, liquids, and solids. The phenomenon is known as induced radioactivity.
Induced radioactivity propagates itself in gases step by step by a sort of conduction, and is not at all due to direct radiation from the body which causes it.
When the activated substance is removed from the radioactive body, the induced radioactivity on it persists for a certain time. It diminishes, nevertheless, little by little and finally disappears.
Emanation.—To explain these phenomena, Rutherford supposes that radium and thorium are constantly giving off an unstable, radioactive, gaseous material, which he calls an emanation. The emanation spreads into the gas surrounding the radioactive substance. It destroys itself little by little by giving off Becquerel rays and by producing other unstable radioactive substances that are not volatile. These new substances attach themselves to the surface of solid bodies and render them radioactive.
Without stating so many hypotheses, we may adopt the name emanation to designate the radioactive energy in the form which it has when it spreads into the gas surrounding the radioactive substance. It may be supposed also, that this energy disappears in creating the energy of induced radioactivity in solids.
The Radioactivity Induced by Radium and the Emanation from Radium.—When a solid salt of radium is placed in a closed space filled with air, the interior walls of the closed space and all solids in it become radioactive. We may, for example, place inside the space a plate of any solid, leave it there a certain time, and then remove it to study its activity. It is found that the activity of the plate increases at first according to the length of its stay in the space, but that it reaches a limiting value after a sufficiently long time. When the excited plate is removed from the space it loses its activity according to an exponential law, the radiation falling to one-half its value in about half an hour. In a general way all solid substances under the same conditions acquire and lose their activity in the same manner.
The phenomena are much more intense (about twenty times), if instead of the solid salt of radium, we place a solution of it inside the closed space in an open vessel.
The nature and the pressure of the gas contained in the closed space have no influence upon the observed phenomena.
The induced activity in a closed space is proportional to the quantity of radium present.