Page:EB1911 - Volume 22.djvu/818

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801
RADIOACTIVITY


very thin glass walls of the containing tube. The collected particle gave the spectrum of helium, showing, without doubt, that the α particle must be a helium atom.

Since the α particle is an atom of helium, all radioactive matter which expels α particles must give rise to helium. In agreement with this, Debierne and Giesel have shown that actinium as well as radium produces helium. Observations of the production of helium by radium have been made by Ramsay and Soddy, Curie and Dewar, Himstedt and others. The rate of production of helium per gram of radium was first definitely measured by Dewar (43). His preliminary measurements gave a value of 134 cubic mms. of helium per year per gram of radium and its products. Later observations extending over a larger interval give a rate of production about 168 cubic mms. per year. As a result of preliminary measurements, Boltwood and Rutherford (44) have found a growth of 163 cubic mms. per year. It is of interest to note that the rate of production of helium by radium is in excellent agreement with the value calculated theoretically. From their work of counting the particles and measuring their charge, Rutherford and Geiger showed that the rate of production of helium should be 158 cubic mms. per year.

Properties of the α Rays.—We have seen that the rays are positively charged atoms of helium projected at a high velocity, which are capable of penetrating through thin metal sheets and several centimetres of air. Early observations indicated that the ionization due to a layer of radioactive matter decreased approximately according to an exponential law with the thickness of the absorbing matter placed over the active matter. The true nature of the absorption of the α rays was first shown by Bragg and by Bragg and Kleeman (45). The active particles projected from a thin film of active matter of one kind have identical velocities, and are able to ionize the air for a definite distance, termed the “ range ” of the α particle. It was found that the ionization per centimetre of path due to a narrow pencil of α rays increases with the distance from the active matter, at first slowly, then more rapidly, near the end of the range. After passing through a maximum value the ionization falls off rapidly to zero. The range of an α particle in air has a definite value which can be accurately measured. If a uniform screen of matter is placed in the path of the pencil of rays the range is reduced by a definite amount proportional to the thickness of the screen. All the α particles have their velocity reduced by the same amount in their passage through the screen. The ranges in air of the α rays from the various products of the radioelements have been measured. The ranges for the different products vary between 2.8 cms. and 8.6 cms.

Bragg has shown that the range of an α particle in different elements is nearly proportional to the square roots of their atomic weights. Using the photographic method, Rutherford (46) showed that the velocity V of an α particle of range R cms. in air is given by V2=K(R+1.25), where K is a constant. In his experiments he was unable to detect particles which had a velocity lower than 8.8 X 108 cms. per second. Geiger (47), using the scintillation method, has recently found that a particles of still lower velocity can be detected under suitable conditions by the scintillations produced on a zinc sulphide screen. He has found that the connexion between velocity and range can be closely expressed by V3=KR, where K is a constant.

On account of the great energy of motion of the α particle, it was at first thought that it pursued a rectilinear path in the gas without appreciable defection due to its encounters with the molecules. Geiger (48) has, however, shown by the scintillation method that the α particles are scattered to a marked extent in passing through matter. The scattering increases with the atomic weight of the substance traversed, and becomes more marked with decreasing velocity of the α particle. A small fraction of the α particles falling on a thick screen are deflected through more than a right angle, and emerge again on the side of incidence.

XXII. 26

Rutherford and Geiger (49) have devised an electrical method of counting the α particles expelled from radioactive matter. The α particle enters through a small opening into a metal tube containing a gas at a reduced pressure. The ionization produced by the a particle in its passage through the gas is magnified several thousand times by the movement of the ions in a strong electric field. In this way, the entrance of an α particle into the detecting vessel is shown by a sudden and large deflection of the measuring instrument. By this method, they determined that 3.4 X 1010 α particles are ejected per second from one gram of radium itself and from each of its α ray products in equilibrium with it. By measuring the charge on a counted number of a particles, it was found that the α particle carries a positive charge of 9.3 X 10-10 electrostatic units. From other evidence, it is known that this must be twice the fundamental unit of charge carried by the hydrogen atom. It follows that this unit charge is 4.65 X 10-10 units. This value is in good agreement with numerous recent determinations of this fundamental quantity by other methods. With this data, it is possible to calculate directly the values of some important radioactive data. The calculated and observed values are given below:—

Calculated. Observed.
Volume of the emanation in cubic millimetres per gram of radium .585 .6
Volume of helium in cubic millimetres produced per year per gram of radium 158 169
Heating effect of radium per gram per hour in gram calories 113 118
Half-period of transformation of radium in year 1760 2000

The calculated values are in all cases in good agreement with the experimental numbers.

It is well known from the experiments of Sir William Crookes (50) that the α rays produce visible scintillations when they fall on a screen of phosphorescent zinc sulphide. This is shown in the instrument called the spinthariscope. By means of a suitable microscope, the number of these scintillations on a given area in a given time can be counted. The number so obtained is practically identical with the number of α particles incident on the screen, determined by the electrical method of counting. This shows that each α particle produces a visible flash of light when it falls on a suitable zinc sulphide screen. The scintillations produced by α rays are observed in certain diamonds, and their number has been counted by Regener (51) and the charge on each particle has been deduced. The latter was the first to employ the scintillation method for actual counting of α particles. Kinoshita has shown that the number of α particles can also be counted by the photographic method, and that each particle must produce a detectable effect.

Absorption of β Rays.—We have seen that the β particles, which are emitted from a number of radioactive products, carry a negative charge and have the same small mass as the particles constituting the cathode rays. The velocity of expulsion and penetrating power of the β rays varies widely for different products. For example, the rays from radium B are very easily absorbed, while some of the rays from radium C are of a very penetrating type. It has been found that for a single β ray product, the particles are absorbed according to an exponential law with the thickness of matter traversed, and Hahn has made use of this fact to isolate a number of new products. It has been generally assumed that the exponential law of absorption is a criterion that the β rays are all expelled at the same speed. In addition, it has been supposed that the β particles do not decrease much in velocity in passing through matter. Wilson has recently made experiments upon homogeneous β rays, and finds that the intensity of the radiation falls off in some cases according to a linear rather than to an exponential law, and that there is undoubted evidence that the β particles decrease in velocity in traversing matter. Experiments upon the absorption of β rays are greatly complicated by the scattering of the 5 rays in their encounters with the molecules. For example, if a pencil of β rays falls on a metal, a large fraction of the rays are scattered