PART I.

Comparison of the Radiations.

=71. The Three Types of Radiation.= All the radio-active substances possess in common the power of acting on a photographic plate and of ionizing the gas in their immediate neighbourhood. The intensity of the radiations may be compared by means of their photographic or electrical action; and, in the case of the strongly radio-active substances, by the power they possess of lighting up a phosphorescent screen. Such comparisons, however, do not throw any light on the question whether the radiations are of the same or of different kinds, for it is well known that such different types of radiations as the short waves of ultra-violet light, Röntgen and cathode rays, all possess the property of producing ions throughout the volume of a gas, lighting up a fluorescent screen, and acting on a photographic plate. Neither can the ordinary optical methods be employed to examine the radiations under consideration, as they show no trace of regular reflection, refraction, or polarization.

Two general methods can be used to distinguish the types of the radiations given out by the same body, and also to compare the radiations from the different active substances. These methods are as follows:

(1) By observing whether the rays are appreciably deflected in a magnetic field.

(2) By comparing the relative absorption of the rays by solids and gases.

Examined in these ways, it has been found that there are three different types of radiation emitted from radio-active bodies, which for brevity and convenience have been termed by the writer the α, β, and γ rays.

(i) The α rays are very readily absorbed by thin metal foil and by a few centimetres of air. They have been shown to consist of positively charged bodies projected with a velocity of about ⅒ the velocity of light. They are deflected by intense magnetic and electric fields, but the amount of deviation is minute in comparison with the deviation, under the same conditions, of the cathode rays produced in a vacuum tube.

(ii) The β rays are far more penetrating in character than the α rays, and consist of negatively charged bodies projected with velocities of the same order as the velocity of light. They are far more readily deflected than the α rays, and are in fact identical with the cathode rays produced in a vacuum tube.

(iii) The γ rays are extremely penetrating, and non-deviable by a magnetic field. Their true nature is not definitely settled, but they are analogous in most respects to very penetrating Röntgen rays.

The three best known radio-active substances, uranium, thorium, and radium, all give out these three types of rays, each in an amount approximately proportional to its relative activity measured by the α rays. Polonium stands alone in giving only the α or easily absorbed rays[111].

=72. Deflection of the rays.= The rays emitted from the active bodies thus present a very close analogy with the rays which are produced in a highly exhausted vacuum tube when an electric discharge passes through it. The α rays correspond to the canal rays, discovered by Goldstein, which have been shown by Wien to consist of positively charged bodies projected with great velocity (see section 51). The β rays are the same as the cathode rays, while the γ rays resemble the Röntgen rays. In a vacuum tube, a large amount of electric energy is expended in producing the rays, but, in the radio-active bodies, the rays are emitted spontaneously, and at a rate uninfluenced by any chemical or physical agency. The α and β rays from the active bodies are projected with much greater velocity than the corresponding rays in a vacuum tube, while the γ rays are of much greater penetrating power than Röntgen rays.

The effect of a magnetic field on a pencil of rays from a radio-active substance giving out the three kinds of rays is very well illustrated in Fig. 22[112].

Some radium is placed in the bottom of a narrow cylindrical lead vessel _R_. A narrow pencil of rays consisting of α, β, and γ rays escapes from the opening. If a strong uniform magnetic field is applied at right angles to the plane of the paper, and directed towards the paper, the three types of rays are separated from one another. The γ rays continue in a straight line without any deviation. The β rays are deflected to the right, describing circular orbits the radii of which vary within wide limits. If the photographic plate _AC_ is placed under the radium vessel, the β rays produce a diffuse photographic impression on the right of the vessel _R_. The α rays are bent in the direction opposite to that of the β rays, and describe a portion of the arc of a circle of large radius, but they are rapidly absorbed after traversing a distance of a few centimetres from the vessel _R_. The amount of the deviation of the α rays compared with that of the β rays is much exaggerated in the figure.

=73. Ionizing and penetrating power of the rays.= Of the three kinds of rays, the α rays produce most of the ionization in the gas and the γ rays the least. With a thin layer of unscreened active material spread on the lower of two parallel plates 5 cms. apart, the amount of ionization due to the α, β, and γ rays is of the relative order 10,000, 100, and 1. These numbers are only rough approximations, and the differences become less marked as the thickness of the radio-active layer increases.

The average penetrating power of the rays is shown below. In the first column is given the thickness of the aluminium, which cuts each radiation down to half its value, and in the second the relative power of penetration of the rays.

Radiation Thickness of Relative Aluminium in cms. power of which cuts off half penetration the radiation

α rays 0·0005 cms. 1

β „ 0·05 cms. 100

γ „ 8 cms. 10000

The relative power of penetration is thus approximately inversely proportional to the relative ionization. These numbers, however, only indicate the order of relative penetrating power. This power varies considerably for the different active bodies.

The α rays from uranium and polonium are the least penetrating, and those from thorium the most. The β radiations from thorium and radium are very complex, and consist of rays widely different in penetrating power. Some of the β rays from these substances are much less and others much more penetrating than those from uranium, which gives out fairly homogeneous rays.

=74. Difficulties of comparative measurements.= It is difficult to make quantitative or even qualitative measurements of the relative intensity of the three types of rays from active substances. The three general methods employed depend upon the action of the rays in ionizing the gas, in acting on a photographic plate, and in causing phosphorescent or fluorescent effects in certain substances. In each of these methods the fraction of the rays which is absorbed and transformed into another form of energy is different for each type of ray. Even when one specific kind of ray is under observation, comparative measurements are rendered difficult by the complexity of that type of rays. For example, the β rays from radium consist of negatively charged particles projected with a wide range of velocity, and, in consequence, they are absorbed in different amounts in passing through a definite thickness of matter. In each case, only a fraction of the energy absorbed is transformed into the particular type of energy, whether ionic, chemical, or luminous, which serves as a means of measurement.

The rays which are the most active electrically are the least active photographically. Under ordinary conditions, most of the photographic action of uranium, thorium, and radium, is due to the β or cathodic rays. The α rays from uranium and thorium, on account of their weak action, have not yet been detected photographically. With active substances like radium and polonium, the α rays readily produce a photographic impression. So far the γ rays have been detected photographically from radium only. That no photographic action of these rays has yet been established for uranium and thorium is probably due merely to the fact that the effect sought for is very small, and during exposures for long intervals it is very difficult to avoid fogging of the plates owing to other causes. Considering the similarity of the radiations in other respects, there can be little doubt that the γ rays do produce some photographic action, though it is too small to observe with certainty.

These differences in the photographic and ionizing properties of the radiations must always be taken into account in comparing results obtained by the two methods. The apparent contradiction of results obtained by different observers using these two methods is found to be due to their differences in relative photographic and ionizing action. For example, with the unscreened active material, the ionization observed by the electrical method is due almost entirely to α rays, while the photographic action under the same condition is due almost entirely to the β rays.

It is often convenient to know what thickness of matter is sufficient to absorb a specific type of radiation. A thickness of aluminium or mica of ·01 cms. or a sheet of ordinary writing-paper is sufficient to absorb completely all the α rays. With such a screen over the active material, the effects are due only to the β and γ rays, which pass through with a very slight absorption. Most of the β rays are absorbed in 5 mms. of aluminium or 2 mms. of lead. The radiation passing through such screens consists very largely of the γ rays. As a rough working rule, it may be taken that a thickness of matter required to absorb any type of rays is inversely proportional to the density of the substance, _i.e._ the absorption is proportional to the density. This rule holds approximately for light substances, but, in heavy substances like mercury and lead, the radiations are about twice as readily absorbed as the density rule would lead us to expect.