An introduction to the science of radio-activity

Preface

In the following pages I have endeavoured to give a concise and popular account of the properties of the radio-active elements and the theoretical conceptions which are introduced by the study of radio-active phenomena. This work, as its title indicates, does not claim to be more than an introduction to the subject, and no attempt has been made at exhaustive treatment. Following what has appeared to me to be the most lucid method of presentation, I have divided the subject into three parts: descriptive, theoretical, and practical; the last being added in the hope that the comparatively simple experiments described may be of interest to those wishing to investigate for themselves some of the remarkable properties of the radio-active elements.

Frequent reference has been made to Professor Rutherford’s treatise, and I am indebted to this valuable work for several numerical determinations. I also wish to express my thanks to Mr. F. H. Glew for the radiographs illustrating reversal by radium rays and secondary radiation.

CHARLES W. RAFFETY.

PURLEY,- SURREY, August 29, 1908.

Contents

PAE it PAGES Introductory note—Spectrum of radium—Chemical properties of radium—Polonium—Actinium . ...... . . 41-10 II Rays emitted by radium—Appearance and luminosity of radium Senn Sane e eters se eRe se gt Oe AL Ii Nature of the rays emitted by the radio-active elements . . 15-26 eV Methods of investigation—Complexity of the rays—Deviation of the gays in a.magnetic field “2. « «fe 6 « » 27-33 Vv

The a rays—Magnetic and electrostatic deviation—Properties of the a rays—lIonisation range—Power to penetrate matter— Photographic action—Action of the a rays on fluorescent substances—The spinthariscope—F luorescopic method of testing for a radiation—Kinetic energy of the a particles—Heat emissiom by radium compounds 4-59 .. 20. + os». B4-46

VI

II

6. Rays emitted by Radium.—So far as the purely chemical properties of radium are concerned, there is nothing to distinguish its behaviour from that of a normal element. It is when the physical properties of radium are considered that the marvel becomes most impressive, for phenomena are encountered hitherto unsuspected and unimagined, and on a scale of intensity which appears enormous in comparison with the amount of matter involved in their manifestation.

It should first be clearly stated that the rays emitted by the radio-active elements are absolutely invisible and can only be made evident and investigated by their effects. On account of the high activity of radium and the consequent intensity of the rays, effects are observed which are not so evident with the elements of feebler radio-active properties. ‘The difference is, however, one of degree rather than of kind, and this will be clear when the theory of radio-activity is discussed.

The first point to notice is the one mentioned above namely, that the absorption of the rays by matter is a property depending upon density and independent of the nature and chemical properties of the absorbing medium. In this there is a marked similarity to the X rays, but the rays from radium are characterised by a power of penetration which is extraordinary. The most penetrating radiation which can be obtained from an X-ray tube is

12 THEOSCIE NCE OF MRALIO-4.Cl iia ie stopped by a thin sheet of metal—indeed, except in very thin sheets, all the ordinary metals but aluminium are opaque to the rays—yet the rays emitted from a very small quantity of a pure radium salt can easily be detected after having passed through a centimetre of lead.

A few milligrams are sufficient to demonstrate this power of penetration and give a very distinct impression on a photographic plate after the rays have passed through very considerable thicknesses of various metals. A sealed glass tube containing this small quantity of radium will produce excellent radiographs of coins or keys etc. in a purse with an exposure of an hour or so, by merely supporting the tube for the requisite time a few inches above the plate with the purse laid upon it.

Like the X rays, the rays from radium cause bright fluorescence in certain minerals and crystals and various chemical compounds. Diamonds and some other precious stones glow in the dark on the approach of a tube containing a small quantity of radium. Barium-platino-cyanide is particularly sensitive to radium rays, and crystals of this compound will glow brightly even when the radium is held several inches away, or after the rays have passed through a considerable thickness of wood or even several coins. The mineral willemite is also sensitive to the rays and so are certain others in varying degrees. In some compounds there is also a distinct phosphorescence for a short time after the removal of the radium.

The electrical effects of radium rays are very evident, as the rays cause an intense ionisation of the air through which they pass. The mere approach of a tube containing a few milligrams of radium will cause a rapid fall of the leaves of a charged electroscope, and even if the tube be enclosed in an outer metal cylinder, the movement of the

RAYS EMITTED BY RADIUM 13 leaves is still quite perceptible provided the metal is not very thick. A piece of silk ribbon, folded in two and electrified by friction so that the two lengths repel one another, will be speedily discharged if held above the tube, and an electrified silk tassel will likewise quickly collapse.

Radium rays can cause certain chemical changes. Oxygen appears to take the form of ozone in the presence of radium ; while radium salts in solution in water slowly decompose it, liberating its component gases, oxygen and hydrogen. Glass tubes in which radium is sealed become discoloured in the course of time. With impure salts of a not very high activity, the colour assumed is a beautiful mauve or amethyst tint, and several weeks or even months are required before it becomes at all decided. With salts of greater purity, the discolouration is more rapid and is generally brown, growing darker with time until it is almost black. Some cases of change produced by radium rays appear to be rather of the nature of physical modifications.

The physiological effects of radium rays are most marked, and great care is necessary in handling and experimenting with radium on account of the harm resulting from excessive exposure to the rays. Many investigators suffered from ignorance of this in the early days before the properties of the rays were understood. Prof. Curie, having exposed his arm to the rays for some time, suffered from an intense and painful inflammation as the result. The carrying of tubes containing radium in the pocket is attended with risk and possibly serious results, unless precautions are taken to shield the body from the rays by enclosing the tubes in thick lead. The action is analogous to that of the X rays, but it is only when a fairly large quantity is employed that there is much necessity for protection unless the exposure is prolonged.

7. Appearance and Luminosity of Radium Salts:— There is nothing in the appearance of radium compounds that is in any way remarkable, nothing to give a hint of their marvellous physical properties. The forms most generally met with are the chloride and bromide salts, both of which appear as whitish crystals which, in the case of the bromide, darken with time and assume a brownish tint. Viewed in a darkened room, these radium compounds show a faint luminosity. This light must not be confused in any way with the rays referred to above; it is entirely a secondary effect which is much more pronounced with the impure salts than with the pure. Some specimens of impure salts show a very marked luminosity and it is in all cases necessary for the salt to be dry for the maximum effect to be observed. The action is apparently analogous to the fluorescence produced in certain substances referred to above, as the rays, which are emitted by the radium in all directions, also strike inwards, causing the substance of the compound to glow.

ijt

8. Nature of the Rays emitted by the Radio-active Elements.—In order to understand the nature of the radiation in question, it is best to first briefly consider the other forms of invisible radiation known to scientists before Prof. Becquerel discovered the radio- ~ activity of the element uranium. The classical experimental investigations of Sir William Crookes on the discharge of electricity through high vacua may be said to have prepared the way for the correct interpretation of the wonderful phenomena of radio-activity. The rays which were found to proceed from the kathode of an exhausted tube, called “radiant-matter” by their discoverer, and afterwards known as the kathode rays, have been investigated very thoroughly by Prof. J. J. Thomson and other physicists. The results have been not only to confirm to the full the idea that these rays are particulate in character, but also to show that these particles are not atoms but bodies roughly one-thousandth the mass of the hydrogen atom, which is the lightest atom known to science. Prof. J. J. Thomson called these particles “ corpuscles,’ and has demonstrated that they are always invariable in character no matter whether the gas in the bulb in which they are generated be elementary or compound, or whatever metal may be employed to form the kathode plate. From this it seemed evident that these corpuscles were common constituents of all atoms from

16 THEE SCIENCE SOLAKALDIO-AGCI Iie which, under suitable conditions, they could be detached. It was also discovered that the corpuscle carried a negative charge of ‘electricity which was of invariable amount whatever their source, and that this charge was equal to that carried by the hydrogen ion in electrolysis. Prof. Thomson was enabled to determine the velocity attained by these corpuscles within the vacuum tube by means of the deflection suffered by the rays in traversing a magnetic or electrostatic field, and from this he was also able to find both the charge carried by a corpuscle and its mass.

The determination of these facts concerning the corpuscles was a veritable triumph both from the theoretical

Fic. 1.—Kathode Ray Tube.

K = kathode plate. A = anode.

point of view, and as a wonderfully delicate piece of experimental work, and the importance of these investigations cannot be overestimated, for by means of them we have become acquainted with bodies little more than one-thousandth of the mass of the lightest known atom, and which can travel with an almost inconceivable velocity. The velocity of these particulate kathode rays is about 25,000 miles per second, although this is variable within

NATURE OF THE RAYS 17 fairly wide limits according to circumstances, the most important being the electric potential of the kathode; indeed, the velocity is directly and almost entirely dependent on the kathode potential, but this, in turn, is dependent on the degree of exhaustion of the tube. ‘

It appeared evident that the kathode rays could not penetrate the glass side of the tube, as there was no trace of them outside. The energy of motion of the corpuscles was evidently absorbed by the glass, which showed a bright spot of fluorescence where the rays impinged upon it. As the rays always travel at right angles to the surface of the kathode whence they originate, they form a parallel beam when proceeding from a kathode composed of a smooth flat-plate. By having the kathode formed of a concave surface, however, they can be brought to a “focus,” and at such a point of convergence, the heating effects produced by the stopping of the corpuscles is very evident.

Lenard was the first to use a tube with a “ window” of thin aluminium through which the beam of corpuscles. passed readily, and their effect outside the tube could be investigated. One of the most important effects produced by these “ Lenard rays” (as the kathode stream is called when it has passed out of the tube) is an intense ionisation of the air. Sir W. Crookes had demonstrated, by some of the most beautiful experiments imaginable, the wonderful power of the kathode rays to cause fluorescence and phosphorescence in various minerals and chemical compounds. A large number of minerals and crystals and a

1 Velocity is generally of the order of 2 x 10° cms. per second, but under exceptional conditions a velocity of about 10'° ems. per second is possible.

G wide range of chemical compounds exhibit marvellously beautiful hues when enclosed within the exhausted bulb and subjected to the bombardment of the kathode rays. “Lenard rays” also show this power of causing fluorescence and phosphorescence, and the effects are obviously far more easy to investigate.

It will be remembered that Prof. Rontgen first discovered that the fluorescing glass opposite to the kathode in a vacuum tube was the source of an unsuspected form _ of radiation which could penetrate objects quite opaque to ordinary light, and affect strongly a photographic plate; this being the discovery of the now well-known X rays. It was, however, some time before the method of development of this radiation was recognised. It appeared that wherever and whenever kathode rays were stopped by impact with solids, X rays were produced, and it was afterwards discovered that the impact of the corpuscles on dense solids, like the heavy metals, produced a far more powerful emission of X rays than when the stoppage was allowed to take place by the glass side of the tube. Besides their wonderful power of penetration and photographic action, the X rays are also able to produce brilliant fluorescent and phosphorescent effects, although they themselves are quite invisible. It was found possible to produce “shadow-pictures” of objects on fluorescent screens, but in the early types of tubes the definition was bad owing to the radiation proceeding from a considerable area. The great advance in the production of X-ray tubes was the insertion of a platinum anti-kathode in the path of the rays brought to a “focus” on its surface by a concave kathode. This gives intense radiation from a small area and the definition is vastly improved.

It soon became clear that the X rays, whatever their

NATURE OF THE RAYS 19 real nature, were not composed of particulate bodies carrying a negative electric charge like corpuscles, for they cannot be deviated in the strongest magnetic fields, and their wonderful power to penetrate matter seemed also against this view although, as will be seen later, corpuscles travelling at enormously high velocities possess a marvellously great power of penetration. It appeared, rather,

Fig. 2.—X-ray Focus Tube. : K = kathode. A=anode, P = anti-kathode.

that the X rays were some sort of etheric disturbance set up by the sudden stoppage of the flying corpuscles, Theoretical considerations point to the conclusion that whenever an electric charge in steady, uniform motion is stopped, a pulse is communicated to the surrounding ether, and it seemed probable that the impact of the corpuscles on the platinum anti-kathode set up a very penetrating etheric disturbance. That this form of radiation must be of a special and peculiar description,

20 THE SCIENCE (OL (RARTOACIAV IT quite unlike light or ultra-violet rays, was evident from the failure of all attempts to reflect or refract the wavepulses, and at the present time the mystery of the X rays can hardly be said to be fully solved.

In connection with the phenomena of vacuum tube discharges, another type of rays has been investigated by Prof. Thomson. Knowing that the atoms of the residual gas in the tube are broken down into ions in the process of conveying an electric discharge, it seemed reasonable to look for a stream of positively charged carriers (7.¢. positive ions) projected from the anode. Such “ positive rays”? have been found, although for their proper investigation a specially arranged tube is necessary. Unlike the kathode rays which travel in straight lines independent of the position of the anode terminal, the path followed by the positive rays from the anode appears to be modified by the position of the kathode, to which they tend to travel. By placing the anode plate opposite a kathode having an aperture through the middle of it, the “ positive rays’ pass through, and the stream can be investigated on the other side of the kathode. Owing to the fact that these positive rays pass through a channel in the kathode plate they are known as the “ canal rays,” although “anode rays’? would, perhaps, be a better term.

As to the nature of these canal rays, there can be little doubt that in them we have a stream of positive ions. These ions are the atoms of the residual gas which have lost a negative corpuscle and, the original atom being electrically neutral, they have thus a positive charge equal and complementary to the charge of the corpuscles. As the mass of the positive ion of hydrogen is practically the same as that of the neutral atom, while the charge is no greater than that carried by the negative corpuscle, the velocity

—_

WALOGKE OF Fi RAYS 2t attained by the canal rays under a given electrical potential is not, comparatively speaking, great. We are thus acquainted with three types of radiation,

Fic. 3.—‘ Canal”? Ray Tube.

A=anode. K = perforated kathode. E = earthed screen.

two of them being particulate and the third apparently a form of etheric disturbance. These are—

(i) Canal or positive rays ; (ii) Kathode rays; Gili) X rays.

In investigating the nature of the rays spontaneously emitted by radium and the other radio-active elements, we meet with forms of radiation very closely analogous to the rays mentioned above.

The process of gaseous discharge may be briefly described as follows. In the residual gas contained in the exhausted tube, a very few ions are always to be found among the swiftly moving gas molecules. When the kathode plate becomes charged, any positive ions near are at once attracted and rush towards it. Just before they reach the kathode, their velocity may be great enough to ionise the gas molecules through which they pass, thus making more ions and, the negative ions being violently repelled, start away to ionise the gas further from the

22 THES GIENCE BOP GRABDIC AGIA [ii kathode. The function of the positive ions produced from either source when they reach the close proximity of the kathode, is to drag or pull the negative corpuscles out of it, thus tending to relieve it of its charge. When the corpuscles are free of the metal of the kathode, the violent repulsion which they experience causes them to fly away in straight lines, giving rise to the kathode stream. Their velocity is so great that the position of the anode has little or no effect on them unless the vacuum is very low, which means that they exhaust their velocity in collisions with the gas molecules, and the electrostatic pull of the positive electrode is thus felt.

Directly there is a tendency to lower the kathode potential by conducting away the corpuscles, more corpuscles are supplied along the conductor which connects the kathode with the source of current supply. The circuit is completed in low vacua by the direct rush of negative ions (7.¢. corpuscles) to the anode, and in high vacua there is an indirect flow.

We are led to consider negative corpuscles as constituents of all atoms for, in addition to the kathode stream of the vacuum tube, corpuscles are liberated from gases at a very high temperature, incandescent metals and carbon, metals under the radiation of ultra-violet light, and they can be generated in any gas by the passage of rays capable of causing ionisation. We see that a gas atom, although electrically neutral, is capable of losing a small fraction of its mass in the form of the negative corpuscle, which thus leaves the remainder of the atom with a positive charge or, in other words, a positive ion. On the withdrawal of the agency by which this forcible splitting up of the atom takes place, the ions will again recombine, but there is an appreciable interval before this

MATURE OF THE RAYS 23 recombination can be effected, and during this time the gas retains its conducting power. The negative corpuscle of the vacuum tube and the negative ion of ionised gases are identical in character, and to both the word “ electron” is now more usually applied, and this term will be used in the following chapters.

In all electrical phenomena, the electron appears to be the active agent, and the “electron theory ” is a new statement and interpretation of electrical science and the allied branches of physics.

The deflection of the kathode stream by an electrostatic field is demonstrated by using a tube as shown in the diagram. Prof. J. J. Thomson employed a similar apparatus in the determination of the properties of the electrons projected from the kathode plate, using, at the same time, a magnetic field acting at right angles to the electrostatic. The kathode stream tends to describe a circle in passing through a magnetic field of uniform intensity, the plane of the circle being at right angles to the lines of force. The electrostatic field can be made to act either with, or against, the magnetic by the polarity imparted to the plates and, with a proper adjustment of potential, the two opposed fields will neutralise each other, the deflection again being zero. The electrostatic action is one of attraction and repulsion, the kathode stream describing a curved path like that followed by a body projected at right angles to the earth’s surface. In another form of tube used in the determination of the velocity of the electrons, a zinc plate, negatively electrified, is exposed to a beam of ultra-violet light. Under these conditions a stream of corpuscles is projected from the plate and, after passing through an aperture in a screen connected to earth, falls on a plate to which an

24 THE SCIENCE VOMVRARIO- AC LIVIN electrometer is attached which shows a deflection by reason of the charge acquired. On the application of the magnetic field, the corpuscular stream is deflected and, by falling on a second plate in the side of the tube, produces a deflection in a second electrometer in connection with it. The radius of the curve followed by the corpuscles under the influence of the magnetic field is

Fic, 4.—Tube for showing the Electrostatic Deviation of the Kathode Rays.

K = kathode. A=anode. E=earthed screen. P,, P,' = charging plates.

determined by the relative positions of the two electrometer plates. By this method the velocity of the corpuscles can be calculated from a knowledge of the values of (i) the radius of the circle which they tend to describe, (ii) the intensity of the magnetic field necessary to produce that deflection, and (iii) the electric potential of the zinc plate from which they originate. This is thus expressed by

2V YH where V = potential, r = radius of circle, and H = magnetic field.

NATURE OF THE RAYS 25

As already mentioned, the velocity attained by cor- ‘puscles is variable within wide limits depending on conditions, the highest velocity which is produced in vacuum tubes being probably that found in the case of a “hard” X-ray tube.

In order to understand corpuscles it is necessary to

Fic. 5.—Tube for showing the Magnetic Deflection of the Electrons j projected from a negatively charged Zinc Plate under the influence of Ultra-violet Light.

N = negatively charged zinc plate. V = beam of ultra-violet light.

HK = earthed plate (perforated). P,, P, = plates connected with electrometers. § = screen.

know, not only the various velocities with which they can be endowed, but also their mass and the value of the negative charge they carry. The ratio which the charge bears to the mass, 2¢. the value of <, is of the highest importance. The various experimental methods by which this has been determined need not be given here; suffice to say that the value is always constant, no matter from

26 THE SCIENCE OF (RADIO-ACTIVI Ly.

what source the corpuscles may come. In the experiment quoted it would be expressed by

2V (rHy

The determination of the electrical charge carried by a corpuscle offers, both theoretically and experimentally, a very formidable problem. It has, however, been successfully attempted, the method employed being an application of a curious property of ions, both negative and positive, namely, that they become nuclei about which atoms and molecules can collect. Ions produced in air saturated with water vapour can cause a condensation in the form of a mist of droplets, each ion becoming a centre of condensation. It is not experimentally possible to directly count the drops, but the number may be arrived at by determining the volume of water deposited and dividing it by the volume of a drop. ‘This last factor can be found by observing the time taken by the drops to fall, as the time is dependent on their size. The total electrification divided by the number of droplets would give the charge carried by the corpuscles on the assumption that the number of droplets represented the same number of corpuscles. A complicating factor, however, is found in the fact that positive ions likewise provide centres of condensation, although not as readily as negative.

The negative corpuscle or electron is thus invariable, whatever its source. It has a mass about +) jth that of the lightest known atom, more exactly 71th of the hydrogen atom, and appears to be a fundamental unit common to all atoms.

Note.—The above formule are quoted from ‘The New Knowledge,” by Prof. R. K. Duncan, in which a popular account of the method will be found.