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X2Q

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Originally appearing in Volume V22, Page 802 of the 1911 Encyclopedia Britannica.
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X2Q = )1P or Q/P =A1/A2 = T2/T1: where T2 and T1 are the half-periods of transformation of uranium and radium respectively. The work of Boltwood (34), Strutt (35) and McCoy (36) has conclusively shown that the ratio of radium to uranium in old minerals is a constant. Boltwood and Strutt determined the quantity of radium present in a mineral by the emanation method, and the amount of uranium by analysis. practically disappeared and the a rays arise entirely from radium C. Radium C has proved very valuable in radioactive measurements as providing an intense source of homogeneous a rays. Twenty-four hours after removal, the activity due to radium B and C has become exceedingly small. The wire, however, still shows a very small residual activity, first noted by Mme Curie. This residual activity measured by the a rays rapidly increases with the time and reaches a maximum in about three years. The active deposit of slow change has been examined in detail by Rutherford (23) and by Meyer and Schweidler (24). It has been shown to consist of three successive products called radium D, E and F. Radium D is a rayless substance of slow period of transformation. Its period has been calculated by Rutherford to be about 40 years, and by Meyer and Schweidler about 12 years. Antonoff (25) fixes the period of about 17 years. Radium D changes into E, a 13 ray product of period about 5 days, and E into F, an a ray product of period 140 days. It was at first thought that radium E was complex, but no evidence of this has been observed by Antonoff. The product radium F is of special interest, for it is identical with polonium—the first active body separated by Mme Curie. In a similar way it has been shown that radium D is the primary source of the activity observed in lead or " radiolead " separated by Hofmann. It is interesting to note what valuable results have been obtained from an examination of the minute residual activity observed on bodies exposed in the presence of the radium emanation. Radium Emanation.—The radium emanation is to be regarded as a typical radioactive product or transition element which exists in a gaseous form. It is produced from radium at a constant rate, and is transformed into radium A and helium. Its half-period of transformation is 3.86 days. The emanation from radium has been purified by condensing it in liquid air, and' pumping out the residual gases. The volume (26) of the emanation at normal pressure and temperature to be derived from one gram of radium in equilibrium is about o•6 cubic milli-metres. This small quantity of gas contains initially more than three-quarters of the total activity of the radium before its separation. In a pure state, the emanation is ioo,000 times as active weight for weight as pure radium. Pure emanation in a spectrum tube gives a characteristic spectrum of bright lines (27). The discharge in the gas is bluish in colour. With continued sparking, the emanation is driven into the walls of the tube and the electrodes. Notwithstanding the minute volume of emanation available, the boiling-point of the emanation has been determined at various pressures. At atmospheric pressure Rutherford (28) found the boiling-point to be -67° C., and Gray and Ramsay (29) 71° C. Liquid emanation appears colourless when first condensed; when the temperature is lowered, the liquid emanation freezes, and at the temperature of liquid air glows with a bright rose colour. The density of liquid emanation has been estimated at 5 or 6. Approximate estimates of the molecular weight of the radium emanation were early made by diffusion methods. The molecular weight in most cases came out about roo. In a comparison by Perkins of the rate of diffusion of the emanation with that of a monatomic vapour of high molecular weight, viz. mercury, the value deduced was 234. Since the radium atom in breaking up gives rise to one atom of the emanation and one atom of helium, its atomic weight should be 226—4=222. The emanation appears to have no definite chemical properties, and in this respect belongs to the group of inert monatomic gases of which helium and argon are the best known examples. It is partially soluble in water, and readily absorbed by charcoal. Thorium.—The first product observed in thorium was the emanation. This gives rise to the active deposit which has been analysed by Rutherford, Miss Brooks and by Hahn, and shown to consist of probably four products—thorium A, B, C and D. Thorium A is a rayless product of period 10.5 hours; thorium B an a ray product of period about one hour. The presence of thorium C has been inferred from the two types of a rays In order, however, to obtain a direct proof of the genetic relation between uranium and radium, it is necessary to show that radium appears after some time in a uranium compound from which all trace of radium has been initially removed. It can readily be calculated that the growth of radium should be easily observed by the emanation method in the course of one week, using a kilogram of uranium nitrate. Experiments of this kind were first made by Soddy (37), but initially no definite evidence was obtained that radium grew in the solution at all. The rate of production of radium, if it took place at all, was certainly less than 0,10 0th part of the amount to be expected if uranium were transformed directly into radium. It thus appeared probable that one or more products of slow period of trans-formation existed between uranium and radium. Since uranium must be transformed through these intermediate stages before radium appears, it is evident that. the initial rate of production of radium under these conditions might be extremely small. This conclusion has been confirmed by Soddy, who has shown that radium does appear in the solution which has been placed aside for several years. Since the direct parent of radium must be present in radio-active minerals, one of the constituents separated from the mineral must grow radium. This was shown to be the case by Boltwood (38), who found that actinium preparations produced radium at a fairly rapid rate. By the work of Rutherford and Boltwood, it was found that the growth of radium was not due to actinium itself, but to a new substance separated in some cases with the actinium. This new substance, which emits a rays, was separated by Boltwood (38), and called by him " lonium." It has chemical properties very similar to thorium. Soddy has shown that the period of ionium is probably not less than 20,000 years, indicating that ionium must exist in uranium minerals in not less than ten times the quantity of radium. It has not yet been directly shown that uranium produces ionium, but there can be no doubt that it does do so. Since ionium produces radium, Boltwood (38) has determined by direct experiment that radium is half transformed in 2000 years—a number in good agreement with other, data on that subject. The constant relation between uranium and radium will only hold for old minerals where there has been no opportunity for chemical alteration or removal of its constituents by the action of percolating water or other agencies. It is quite possible that altered minerals of no great age will not show this constant relation. • It seems probable that this is the explanation of some results of Mlle Gleditsch, where the relation between uranium and radium has been found not to be constant for some mineral specimens. Connexion of the Radioelements.—We have already seen that a number of slowly transforming radioactive substances, viz. polonium (radium F), radiolead (radium D) and ionium are linked up to the uranium-radium series of transformations. Boltwood (39) has made a systematic examination of the relative activity in the form of very thin films due to each of the products present in the uranium-radium family. The results are shown in the following table, where the activity of pure uranium itself is taken as unity: Uranium r•oo Radium B . . 0.04(?) Ionium . . 0.34 Radium C . . 0.91 Radium . 0.45 Radium F . . o•46 Emanation 0.62 Actinium and its Radium A . 0.54 products . . 0.28 Total activity mineral, 4.64 times uranium. Taking into account the differences in the ionization due to an a particle from the various products, the results indicate that uranium expels two a particles for one from each of the other a ray products in the series of transformations. This indicates either that two particles are expelled during the transformation of the atom of uranium, or that another a ray product is present which has so far not been separated from the uranium. Although thorium is nearly always present in old uranium minerals and uranium in thorium minerals, there does notappear to be any radioactive connexion between enese two elements. Uranium and thorium are to be regarded as two distinct radioactive elements. With regard to actinium, there is still no definite information of its place in the scheme of transformations. Boltwood has shown that the amount of actinium in uranium minerals is proportional to the content of uranium. This indicates that actinium, like radium, is in genetic connexion with uranium. On the other hand, the activity of actinium with its series of a ray products is less than that of radium itself or uranium. In order to explain this anomaly, Rutherford has suggested that at a certain stage of disintegration of the uranium-radium series, the disintegration is complex, and two distinct kinds of matter appear, one in much larger quantity than the other. On this, view, the smaller fraction is actinium; so that the latter is a branch. descendant of the main uranium-radium series. End Products of Transformation.—It is now definitely established that the a particle expelled from any type of radioactive matter is an atom of helium, so that helium is a necessary accompaniment of radioactive changes involving the expulsion of a particles. After the radioactive transformations have come to an end, each of the elements uranium and thorium and actinium should give rise to an end or final product, which may be either a known element• or some unknown element of very slow period of transformation. Supposing, as seems probable, that the expulsion of an a particle lowers the atomic. weight of an element by four units—the atomic weight of helium—the atomic weights of each of the products in the uranium and radium series can be simply calculated. Since uranium expels two a particles, the atomic weight of the next ray product, ionium, is 238.5-8 or 230.5. The atomic weight of radium comes out to be 266.5, a number in good agreement with the experimental value. Similarly the atomic weight of polonium is 21o•5, and that of the final product after the trans-formation of polonium should be 206.5. This value is very close to the atomic weight of lead, and indicates that this sub-stance is the final product of the transformation of radium. This suggestion was first put forward by Boltwood (40), who has collected a large amount of evidence bearing on this subject. Since in old minerals the transformations have been in progress for periods of time, in some cases measured by hundreds of millions of years, it is obvious that the end product, if a stable element, should be an invariable companion of the radioelement and be present in considerable quantity. Boltwood has shown that lead always occurs in radioactive minerals, and in many cases in amount about that to be expected from their uranium, content and age. It is difficult to settle definitely this very important problem until it can be experimentally shown that radium is transformed into lead, or, what should prove simpler in practice, that polonium changes into helium and lead. Unfortunately for a solution of this problem within a reasonable time, a very large quantity of polonium would be necessary. Mme. Curie and Debierne have obtained a very active preparation of polonium containing about nth milligram of pure polonium. Rutherford and Boltwood and Curie and Debierne have both independently shown that polonium produces helium —a result to be expected, since it emits a particles. Production of Helium.—In 1902 Rutherford and Soddy suggested that the helium which is invariably found in radioactive minerals was derived from the disintegration of radioactive matter. In 1903 Ramsay and Soddy definitely showed that helium was produced by radium and also by its emanation. From the observed mass of the a particle, it seemed probable from the first that the a particle was an atom of helium. This conclusion was confirmed by the work of Rutherford and Geiger (41), who showed that the a particle was an atom of helium carrying two unit charges of electricity. In order to prove definitely this relation, it was necessary to show that the a particles, quite independently of the active matter from which they were expelled, gave rise to helium. This was done by Rutherford and Royds (42), who allowed the a particles from a large quantity of 'emanation to be fired through the very thin glass walls of the containing tube. The collected particle gave the spectrum of helium, showing, without doubt, that the a particle must be a helium atom. Since the a particle is an atom of helium, all radioactive matter which expels a 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 a 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 olidservations 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 a 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 a particle. It was found that the ionization per centimetre of path due to a narrow pencil of a 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 a 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 a particles have their velocity reduced by the same amount in their passage through the screen. The ranges in air of the a 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 ems. Bragg has shown that the range of an a 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 a particle of range R ems. in air is given by V2=K(R-l-I.25), where K is a constant. In his experiments he was unable to detect particles which had a velocity lower than 8.8XIo$ ems. 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 a particle, it was at first thought that it pursued a rectilinear path in the gas without appreciable deflection due to its encounters with the molecules. Geiger (48) has, however, shown by the scintillation method that the a 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 a particle. A small fraction of the a particles falling on a thick screen are deflected through more than a right angle, and emerge again on the side of incidence. Rutherford and Geiger (49) have devised an electrical method of counting the a particles expelled from radioactive matter. The a 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 a 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 a particles are ejected per second from one gram of radium itself and from each of its a ray products in equilibrium with it. By measuring the charge on a counted number of a particles, it was found that the a particle carries a positive charge of 9.3 X ro`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 ro 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 milli- metres per gram of radium . .585 •6 Volume of helium in cubic millimetres pro- 158 169 duced per year per gram of radium. Heating effect of radium per gram per hour 113 118 in gram calories Half-period of transformation of radium 176o 2000 in year 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 (5o) that the a 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 a particles incident on the screen, determined by the electrical method of counting. This shows that each a particle produces a visible flash of light when it falls on a suitable zinc sulphide screen. The scintillations produced by a 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 a particles. Kinoshita has shown that the number of a particles can also be counted by the photographic method, and that each particle must produce a detectable effect. Absorption of 13 Rays.—We have seen that the 13 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 3 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 /3 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 /3 rays are all expelled at the same speed. In addition, it has been supposed that the /3 particles do not decrease much in velocity in passing through matter. Wilson has recently made experiments upon homogeneous /3 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 $ particles decrease in velocity in traversing matter. Experiments upon the absorption of /3 rays are greatly complicated by the scattering of the 3 rays in their encounters with the molecules. For example, if a pencil of 13 rays falls on a metal, a large fraction of the rays are scattered II sufficiently to emerge on the side of incidence. This scattering of the 0 rays has been investigated by Eve, McLennan, Schmidt, Crowther and others. It has been found that the scattering for different chemical elements is connected with their atomic weight and their position in the periodic table. McCelland and Schmidt have given theories to account for the absorption of rays by matter. The whole problem of absorption and scattering of particles by substances is very complicated, and the question is still under active examination and discussion. The negative charge carried by the /3 rays has been measured by a number of observers. It has been shown by Rutherford and Makower that the number of /3 particles expelled. per second from one gram of radium in equilibrium is about that to be expected if each atom of the (3 ray products in breaking up emits one /3 particle. Heat Emission of Radioactive Matter.—In 1903 it was shown by Curie and Laborde (52) that a radium compound was always hotter than the surrounding medium, and radiated heat at a constant rate of about ioo gram calories per hour per gram of radium. The rate of evolution of heat by radium has been measured subsequently by a number of observers. The latest and most accurate determination by Schweidler and Hess, using about half a gram of radium, gave 118 gram calories per gram per hour (53). There is now no doubt that the evolution of heat by radium and other radioactive matter is mainly a secondary phenomenon, resulting mainly from the expulsion of a particles. Since the latter have a large kinetic energy and are easily absorbed by matter, all of these particles are stopped in the radium itself or in the envelope surrounding it, and their energy of motion is transformed into heat. On this view, the evolution of heat from any type of radioactive matter is proportional to the kinetic energy of the expelled a particles. The view that the heating effect of radium was a measure of the kinetic energy of the a particles was strongly confirmed by the experiments of Rutherford and Barnes (54). They showed that the emanation and its products when removed from radium were responsible for about three-quarters of the heating effect of radium in equilibrium. The heating effect of the radium emanation decayed at the same rate as its activity. In addition, it was found that the ray products, viz. the emanation radium A and radium C, each gave a heating effect approximately proportional to their activity. Measurements have been made on the heating effect of uranium and thorium and of pitchblende and polonium. In each case, the evolution of heat has been shown to be approximately a measure of the kinetic energy General treatises are: P. Curie, fEuvres, 1908; E. Rutherford, Radioactive Transformations, 1906; F. Soddy, Interpretation of Radium, 1909; R. J. Strutt, Becquerel Rays and Radium, 1904; W. Makower, Radioactive Substances, 1908; J. Joly, Radioactivity and Geology, 1909. See also Annual Reports of the Chemical Society. (E. Ru.)
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