a thin film of radioactive matter of one kind is taken, the particles which escape without absorption are found to be homogeneous and consist of particles projected at an identical speed. Observations of the velocity and mass of the particle have been made by Rutherford. The general method employed for this purpose is similar to that used for the determination of the velocity and mass of the electron in a vacuum tube. The deflection of a pencil of rays in a vacuum is determined for both a magnetic and electric field. From these observations the velocity and value e/m (the ratio of the charge carried by the particle to its mass) are determined. The value of e/m has been found to be the same for the particles from all the types of radioactive matter that have been examined, indicating that the α particles from all radioactive substances are identical in mass. The value of e/m found for the α particle is 5.07 X 103. Now the value of e/m for the hydrogen atom set free in the electrolysis of water is 9660. On the assumption that the value of the charge e is the same for the α particle as for the hydrogen atom, the value would indicate that the α particle has about twice the mass of the hydrogen atom, i.e. has the same mass as the hydrogen molecule. If the charge on the α particle is twice that on the hydrogen atom, the value of e/m indicates that the α particle is a helium atom, for the latter has an atomic weight of four times that of hydrogen. It was difficult at first to decide between these and other hypotheses, but we shall show later that there is now no doubt that the α particle is in reality a helium atom carrying two elementary charges. We may consequently regard the α rays as a stream of helium atoms which are projected from a radioactive substance with a high velocity. The maximum velocity of the α particle from radium is 2 X 109 cms. per second, or one fifteenth of the velocity of light. Although the α rays are the least penetrating of the radiations, it will be seen that they play an extremely important part in radioactive phenomena. They are responsible for the greater part of the ionization and heating effects of radioactive matter and are closely connected with the transformations occurring in them.
Under ordinary experimental conditions the greater part of the ionization observed in a gas is due to the α particles. This ionization due to the α rays does not extend in air at atmospheric pressure for more than 7 cms. from radium, and 8.6 cms. from thorium. If a screen of aluminium about .01 cms. thick is placed over the active material, the α rays are completely absorbed, and the ionization above the screen is then due to the β and γ rays alone. If a layer of lead about 2 mms. thick is placed over the active material, the β rays are stopped, and the ionization is then due almost entirely to the penetrating γ rays. By the use of screens of suitable thickness we are thus able to sift out the various types of rays. These three types of radiations all set up secondary radiations in passing through matter. A pencil of β rays falling on matter is widely scattered in all directions. This scattered radiation is sometimes called the secondary β rays. The γ rays give rise to secondary rays which consist in part of scattered γ rays and in part electrons moving with a high velocity. These secondary rays in turn produce tertiary rays and so on. The impact of the α rays on matter sets free a number of slow moving electrons which are very easily deflected by a magnetic or electric field. This type of radiation was first observed by J. J. Thomson, and has been called by him the δ rays.
Emanations or Radioactive Gases.—In addition to their power of emitting penetrating radiations, the substances thorium, actinium and radium possess another very striking and important property. Rutherford (15) in 1900 showed that thorium compounds (especially the oxide) continuously emitted a radioactive emanation or gas. This emanation can be carried away by a current of air and its properties tested apart from the substance which produces it. A little later Dorn showed that radium possesses a similar property, while Giesel and Debierne observed a similar effect with actinium. These emanations all possess the property of ionizing a gas and, if sufficiently intense, of producing marked photographic and phosphorescent action. The activity of the radioactive gases is not permanent but disappears according to a definite law with the time, viz. the activity falls off in a geometric progression with the time. The emanations are distinguished by the different rates at which they lose their activity. The emanation of actinium is very short lived, the time for the activity to fall to half value, i.e. the period of the emanation, being 3.7 seconds. The period of the thorium emanation is 54 seconds and of the radium emanation 3.9 days. This property of emitting an emanation is shown in a very striking manner by actinium. A compound of actinium is wrapped in a sheet of thin paper and laid on a screen of phosphorescent zinc sulphide. In a dark room the phosphorescence, marked by the characteristic scintillation, is seen to extend on all sides from the active body. A puff of air is seen to remove the emanation and with it the greater part of the phosphorescence. Fresh emanation immediately diffuses out and the experiment may be repeated indefinitely. The emanations have all the properties of radioactive gases. They can be transferred from point to point by currents of air. The emanations can be separated from the air or other gas with which they are mixed by the action of extreme cold. Rutherford and Soddy (16) showed that under ordinary conditions the temperature of condensation of the radium emanation mixed was -150° C.
The emanations are produced from the parent matter and escape into the air under some conditions. Rutherford and Soddy (17) made a systematic examination of the emanating power of thorium compounds 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 and stored, when it retains its characteristic properties for a month or more.
Induced Activity.—Curie (18) showed that radium possessed another remarkable property. The surface of any body placed near radium, or still better, immersed in the emanation from it, acquires a new property. The surface after removal is found to be strongly active. Like the emanations, this induced activity in a body decays with the time, though at quite a different rate from the emanation itself. Rutherford (19) independently showed that thorium possessed a like property. He showed that the bodies made active behaved as if a thin film of intensely active matter were deposited on their surface. The active matter could be partly removed by rubbing, and could be dissolved off by strong acids. When the acid was evaporated the active matter remained behind. It was shown that induced activity was due to the emanations, and could not be produced if no emanation was present. We shall see that induced activity on bodies is due to a deposit 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—26 minutes, for actinium half-value—34 minutes, for thorium half-value— 10.5 hours; The active deposits obtained on a platinum wire or plate are volatilized before a white heat, and are again deposited on the cooler bodies in the neighbourhood. Rutherford showed that the induced activity could be concentrated on the negative electrode in a strong electric field, indicating that the radioactive carriers had a positive charge. The distribution of the active deposit in a gas at low pressure has been investigated in detail by Makower and Russ.
Theory of Radioactive Transformations.—We have seen that the radioactive bodies spontaneously and continuously emit a great number of α and β particles. In addition, new types of radioactive matter like the emanations and active deposits