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diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..d7b82bc --- /dev/null +++ b/.gitattributes @@ -0,0 +1,4 @@ +*.txt text eol=lf +*.htm text eol=lf +*.html text eol=lf +*.md text eol=lf diff --git a/LICENSE.txt b/LICENSE.txt new file mode 100644 index 0000000..6312041 --- /dev/null +++ b/LICENSE.txt @@ -0,0 +1,11 @@ +This eBook, including all associated images, markup, improvements, +metadata, and any other content or labor, has been confirmed to be +in the PUBLIC DOMAIN IN THE UNITED STATES. + +Procedures for determining public domain status are described in +the "Copyright How-To" at https://www.gutenberg.org. + +No investigation has been made concerning possible copyrights in +jurisdictions other than the United States. Anyone seeking to utilize +this eBook outside of the United States should confirm copyright +status under the laws that apply to them. diff --git a/README.md b/README.md new file mode 100644 index 0000000..1b51ccc --- /dev/null +++ b/README.md @@ -0,0 +1,2 @@ +Project Gutenberg (https://www.gutenberg.org) public repository for +eBook #69551 (https://www.gutenberg.org/ebooks/69551) diff --git a/old/69551-0.txt b/old/69551-0.txt deleted file mode 100644 index 08a0a34..0000000 --- a/old/69551-0.txt +++ /dev/null @@ -1,1203 +0,0 @@ -The Project Gutenberg eBook of The natural and artificial -disintegration of the elements, by Ernest Rutherford - -This eBook is for the use of anyone anywhere in the United States and -most other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms -of the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you -will have to check the laws of the country where you are located before -using this eBook. - -Title: The natural and artificial disintegration of the elements - An address by Professor Sir Ernest Rutherford - -Author: Ernest Rutherford - -Release Date: December 15, 2022 [eBook #69551] - -Language: English - -Produced by: Laura Natal Rodrigues (Images generously made available by - Hathi Trust Digital Library.) - -*** START OF THE PROJECT GUTENBERG EBOOK THE NATURAL AND ARTIFICIAL -DISINTEGRATION OF THE ELEMENTS *** - - -THE NATURAL -AND ARTIFICIAL DISINTEGRATION -OF THE ELEMENTS - - - - -AN ADDRESS BY - -Professor Sir ERNEST RUTHERFORD - -Kt., D. Sc., LL. D., Ph. D., D. Phys., F. R. S. - - - - -ON THE OCCASION OF THE CENTENARY CELEBRATION -OF THE FOUNDING OF - -THE FRANKLIN INSTITUTE - -AND THE INAUGURATION EXERCISES OF THE -BARTOL RESEARCH FOUNDATION -SEPTEMBER 17, 18, 19, 1924 - - - - -THE FRANKLIN INSTITUTE - -PHILADELPHIA - - - - -THE NATURAL -AND ARTIFICIAL DISINTEGRATION -OF THE ELEMENTS - - -_By_ Professor Sir ERNEST RUTHERFORD, Kt., D. Sc., - -LL. D., Ph. D., D. Phys., F. R. S. - - -IT is not my intention in this paper to give a detailed account of the -natural disintegration of the radio elements or of the methods employed -to effect the artificial disintegration of certain light elements. I -shall assume that you all have a general knowledge of the results of -these investigations, but I shall confine myself to a consideration of -the bearing of these results on our knowledge of the structure of the -nuclei of atoms. - -There is now a general agreement that the atoms of all elements have a -similar electrical structure, consisting of a central positively -charged nucleus surrounded at a distance by the appropriate number of -electrons. From a study of the scattering of _α_ particles by the atoms of -matter and from the classical researches of Moseley on X-ray spectra, we -know that the resultant positive charge on the nucleus of any atom, in -terms of the fundamental unit of electronic charge, is given numerically -by the atomic or ordinal number of the element, due allowance being made -for missing elements. We know that with few exceptions all nuclear -charges, from 1 for the lightest atom, hydrogen, to 92 for the heaviest -element, uranium, are represented by elements found in the earth. The -nuclear charge of an element controls the number and distribution of the -external electrons, so that the properties of an atom are defined by a -whole number, representing its nuclear charge, and are only to a minor -degree influenced by the mass or atomic weight of the atom. - -This minute but massive nucleus is, in a sense, a world of its own which -is little, if at all, influenced by the ordinary physical and chemical -forces at our command. In many respects, the problem of nuclear -structure is much more difficult than the corresponding problem of the -arrangement and motions of the planetary electrons, where we have a -wealth of available information, both physical and chemical, to test the -adequacy of our theories. The facts known about the nucleus are few in -number and the methods of attack to throw light on its structure are -limited in scope. - -It is convenient to distinguish between the properties assigned to the -nucleus and the planetary electrons. The movements of the outer -electrons are responsible for the X-ray and optical spectra of the -elements and their configuration for the ordinary physical and chemical -properties of the element. On the other hand, the phenomena of -radioactivity and all properties that depend on the mass of the atom are -to be definitely assigned to the nucleus. From a study of the -radioactive transformations, we know that the nucleus of a heavy atom -not only contains positively charged bodies but also negative electrons, -so that the nuclear charge is the excess of positive charge over -negative. In recent years, the general idea has arisen that there are -two definite fundamental units that have to do with the building up of -complex nuclei, viz., the light negative electron and the relatively -massive hydrogen nucleus which is believed to correspond to the positive -electron. - -This view has received very strong support from the experiments of Aston -on Isotopes in which he has shown that the masses of the various species -of atoms are represented nearly by whole numbers in terms of O = 16. -From the general electric theory, it is to be anticipated that the mass -of the hydrogen nucleus in the nucleus structure will be somewhat less -than its value 1.0077 in the free state on account of the very close -packing of the charged units in the concentrated nucleus. From Aston's -experiments, it appears that the average mass of the hydrogen nucleus, -or proton as it is now generally called, is very nearly 1.000 under -these conditions. We should anticipate that the whole number rule found -by Aston would hold only to a first approximation, since the mass of the -proton must be to some extent dependent on the detailed structure of the -nucleus. In the case of tin and xenon Aston has already signalized a -definite departure from the whole number rule, and no doubt a still more -accurate determination of the masses of the atoms will disclose other -differences of a similar kind. - -While our present evidence indicates that the proton and electron are -the fundamental constituents of the nucleus, it is very probable that -secondary combining units play a prominent part in nuclear constitution. -For example, the expulsion of helium nuclei from the radioactive bodies -indicates that the helium nucleus of mass 4 is probably a secondary unit -of great importance in atom building. On the views outlined, we should -expect the helium nucleus of charge to be built up of four protons and -two electrons. The loss of mass in forming this nucleus indicates that a -large amount of energy must be liberated during its formation. If this -be the case, the helium nucleus must be such a stable structure that the -combined energy of four or five of the swiftest _α_ particles would be -necessary to effect its disruption. Such a deduction is supported by our -failure to observe any evidence of disintegration of the swift particle -itself, whether it is used to bombard matter or whether the _α_ particle -is used to bombard other helium atoms. - -On these views, we should anticipate that the nucleus of radium of -atomic number 88 and atomic weight 22.6 contains in all 226 protons of -mass 1 and 138 electrons. While this gives us the numerical relation -between the two fundamental units, we have, at present, no definite -information of their arrangement in the minute nuclear volume, nor of -the nature and magnitude of the forces that hold them together. We -should anticipate that many of the protons and electrons unite to form -secondary units, _e. g._ helium nuclei, and that the detailed structure -of the nucleus may be very different from that to be expected if it -consists of a conglomeration of free protons and electrons. - -It is thus of great importance to obtain definite evidence of the nature -and arrangement of the components of the nucleus and of the forces that -hold them in equilibrium. We shall now consider some of the lines of -evidence which throw light on the actual dimensions of the nucleus and -the law of force operative in its neighborhood; the structure and modes -of vibration of the nucleus, together with the effects observed when -some light nuclei are disintegrated by bombardment with _α_ particles. - - - - -DIMENSIONS OF THE NUCLEI AND THE -LAW OF FORCE - - -The conception of the nucleus atom had its origin in 1911 in order to -explain the scattering of an _α_ particle through a large angle as the -result of a single collision. The observation that the _α_ particle is in -some cases deflected through more than a right angle as the result of an -encounter with a single atom first brought to light the intense forces -that exist close to the nucleus. Geiger and Marsden showed that the -number of particles scattered through different angles was in close -accord with the simple theory which supposed that, for the distance -involved, the _α_ particle and nucleus behaved like charged points, -repelling each other according to the law of the inverse square. The -accuracy of this law has been independently verified by Chadwick, so -that we are now certain that in a region close to the nucleus the -ordinary laws of force are valid. - -These scattering experiments also gave us the first idea as to the -probable dimensions of the nuclei of heavy atoms, for it is to be -anticipated that the law of the inverse square must break down if the _α_ -particle approaches closely to or actually enters the nuclear structure. -This variation in the law of force would show itself by a difference -between the observed and calculated numbers of _α_ particles scattered -through large angles. Geiger and Marsden, however, observed no certain -variation even when the _α_ particles of range about 4 cms. were -scattered through 100° by a gold nucleus. In such an encounter, the -closest distance of approach of the _α_ particle to the center of the -nucleus is about 5 x 10^-12 cm., so that it would appear that the radius -of the gold nucleus, assumed spherical, could not be much greater than -this value. - -There is another argument, based on radioactive data, which gives a -similar value for the dimensions of the radius of a heavy atom. The _α_ -particle escaping from the nucleus increases in energy as it passes -through the repulsive field of the nucleus. To fix a minimum limit, -suppose the _α_ particle from uranium, which is the slowest of all _α_ -particles expelled from a nucleus, gains all its energy from the -electrostatic field. It can be calculated on these data that the radius -of the uranium nucleus cannot be less than 6 x 10^-12 cm. This is based -on the assumption that the forces outside the nucleus are repulsive and -purely electrostatic. If, as seems not unlikely, there also exist close -to the nucleus strong attractive forces, varying more rapidly than an -inverse square law, the actual dimensions may be less than the value -calculated above. - -At this stage of our knowledge it is of great importance to test whether -the law of force breaks down for the distance of closest approach of an -_α_ particle to a nucleus. This can be done by comparing the observed -with the calculated number of _α_ particles scattered through angles of -nearly 180°. It seems almost certain that the inverse square law must -break down when swift _α_ particles are used. This can be seen from the -following argument. If an _α_ particle, of the same speed as that ejected -during the transformation of uranium, is fired directly at the uranium -nucleus, _it must penetrate into the nuclear structure_. If a still -swifter _α_ particle is used, _e. g._ that from radium C, which has about -twice the energy of the uranium _α_ particle, it is clear that it must -penetrate still more deeply into the nuclear structure. This is based on -the assumption that the field due to a nucleus is approximately -symmetrical in all directions. If this is not true, it may happen that -only a fraction of the head-on collisions may be effective in -penetrating the nucleus. It is hoped soon to attack this difficult -problem experimentally. - -We have so far dealt with collisions of an _α_ particle with a heavy -atom. We know, however, from the results of Rutherford, Chadwick and -Bieler that in a collision of an _α_ particle with the lightest atom, -hydrogen, the law of the inverse square breaks down entirely when swift -particles are used. Not only are the numbers of H nuclei set in swift -motion much greater than is to be expected in the simple-point nucleus -theory, but the change of number with the velocity of the _α_ particle -varies in the opposite way from the simple theory. Such wide departures -between theory and experiment are only explicable if we assume either -that the nuclei have sensible dimensions or that the inverse square law -of repulsion entirely breaks down in such close collisions. If we -suppose the complexity in structure and in laws of force is to be -ascribed to the _α_ particle rather than to the hydrogen nucleus, -Chadwick and Bieler, as the result of a careful series of experiments, -concluded that the _α_ particle behaved as if it were a perfectly elastic -body, spheroidal in shape with its minor axis 4 x 10^-13 cm. in the -direction of motion and major axis 8 x 10^-13 cm. Outside this -spheroidal region the forces fell off according to the ordinary inverse -square law, but inside this region the forces increased so rapidly that -a particle was reflected from it as from a perfectly elastic body. No -doubt such a conception is somewhat artificial, but it does serve to -bring out the essential points involved in the collision, viz., that -when the nuclei approach within a certain critical distance of each -other, forces come into play which vary more rapidly than the inverse -square. It is difficult to ascribe this break-down of the law of force -merely to the finite size or complexity of the nuclear structure or to -its distortion, but the results rather point to the presence of new and -unexpected forces which come into play at such small distances. This -view has been confirmed by some recent experiments of Bieler in the -Cavendish Laboratory in which he has made, by scattering methods, a -detailed examination of the law of force in the neighborhood of a light -nucleus like that of aluminum. For this purpose he compared the relative -number of _α_ particles scattered within the same angular limit from -aluminum and from gold. For the range of angles employed, viz., up to -100°, it is assumed that the scattering of gold follows the inverse -square law. He found that the ratio of the scattering in aluminum -compared with that in gold depended on the velocity of the _α_ particle. -For example, for an _α_ particle of 3.4 cms. range, the theoretical ratio -was obtained for angles of deflection below 40° but was about 7 per -cent lower for an average angle of deflection of 80°. On the other -hand, for swifter particles of range 6.6 cms. a departure from the -theoretical ratio was much more marked and amounted to 29 per cent for -an angle of 80°. In order to account for these results he supposes that -close to the aluminum nucleus an attractive force is superimposed on the -ordinary repulsive forces. The results agreed best with the assumption -that the attractive force varies according to the inverse fourth power -of the distance and that the forces of attraction and repulsion balanced -at about 3.4 x 10^-13 cm. from the nuclear center. Inside this critical -radius the forces are entirely attractive; outside they are repulsive. - -While we need not lay too much stress on the accuracy of the actual -value obtained or of the law of attractive force, we shall probably not -be far in error in supposing the radius of the aluminum nucleus is not -greater than 4 x 10^-13 cm. It is of interest to note that the forces -between an _α_ particle and a hydrogen nucleus were found to vary rapidly -at about the same distance. - -It thus seems clear that the dimensions of the nuclei of light atoms are -small, and almost unexpectedly small in the case of aluminum when we -remember that 27 protons and 14 electrons are concentrated in such a -minute region. The view that the forces between nuclei change from -repulsion to attraction when they are very close together seems very -probable, for otherwise it is exceedingly difficult to understand why a -heavy nucleus with a large excess of positive charge can hold together -in such a confined region. We shall see that the evidence from various -other directions supports such a conception, but it is very unlikely -that the attractive forces close to a complex nucleus can be expressed -by any simple power law. - - - - -RADIOACTIVE EVIDENCE - - -A study of the long series of transformations which occur in uranium and -thorium provides us with a wealth of information on the modes of -disintegration of atoms, but unfortunately our theories of nuclear -structure are not sufficiently advanced to interpret these data with any -detail. The expulsion of high speed _α_ and _β_ particles from the -radioactive nucleus gives us some idea of the powerful forces resident -in the nucleus, for it can be estimated that the energy of emission of -the _α_ particle is in some cases greater than the energy that would be -acquired if the _α_ particle fell freely between two points differing in -potential by about 4 million volts. The energies of the _β_ and _γ_ rays -are on a similar scale of magnitude. - -Notwithstanding our detailed knowledge of the successive transformation -of the radio-elements, we have not so far been able to obtain any -definite idea of their nuclear structure, while the cause of the -disintegration is still a complete enigma. In comparing the uranium, -thorium, and actinium series of transformations, one cannot fail to be -struck by the many points of similarity in their modes of -disintegration. Not only are the radiations similar in type and in -energy, but, in all cases, the end product is believed to be an isotope -of lead. This remarkable similarity in the modes of transformation is -especially exemplified in the case of the "C" bodies, each of which is -known to break up in at least two distinct ways, giving rise to branch -products. For example, thorium C emits two types of _α_ rays, 65 percent -of range 8.6 cms. and 35 per cent of range 4.8 cms., and in addition -some _β_ rays. - -In order to explain these results, it has been suggested that a fraction -of the atoms of thorium C break up first with the expulsion of an _α_ -particle and the resulting product then emits a _β_ particle. The other -fraction breaks up in a reverse way, first expelling a _β_ particle, -while the subsequent product emits an _α_ particle. Similar dual changes -occur in radium C and actinium C, although the relative number of atoms -in each branch varies widely for the different elements. - -This remarkable similarity between the "C" bodies is still further -emphasized by the recent discovery of Bates and Rogers that both radium -C and thorium C give rise in small numbers to other groups of _α_ -particles, some of them moving at very high speeds. - -It has often been a matter of remark that the radioactive properties of -the "C" bodies seem to depend more on the atomic number, _i. e._, the -nuclear charge, than on the atomic weight. Confining our attention to -radium C and thorium C, which are best known, both have a nuclear charge -83, but the atomic mass of radium C is 214 and of thorium C 212. The -nucleus of radium C thus contains two protons and two electrons more -than that of thorium C. If it were supposed that the nuclei of these -elements consisted of a large number of charged units in ceaseless and -irregular motion, it is to be anticipated that the addition of the -protons and electrons to the complex structure would entirely alter the -nuclear arrangement and consequently its stability and mode of -transformation. On the other hand, we find that the modes of -transformation of these two nuclei have striking and unexpected points -of resemblance which are in entire disaccord with such a supposition. We -can, however, suggest a possible explanation of this anomaly by -supposing that the _α_ and _β_ particles which are liberated from these -elements are not built deep into the nuclear structure but exist as -_satellites_ of a central core which is common to both elements. These -satellites, if in motion, may be held in equilibrium by the attractive -forces arising from the core, and these forces would be the same for -both elements. On this view the manifestations of radioactivity are to -be ascribed not to the main core, but to the satellite distribution, -which must be somewhat different for the two elements although possibly -showing many points of similarity. It must be admitted that a theory of -this kind is highly speculative, but it does provide a useful working -hypothesis, not only to account for the similarity of the modes of -transformation of the two elements but also immediately suggests a -possible explanation of the liberation of a number of _α_ particles of -different ranges from the same element. There are two ways of regarding -this question. We may in the first place suppose that a certain amount -of surplus energy has to be liberated in the disintegration and that -this energy may be given to any one of a number of satellites. There -will be a certain probability that any particular particle will be given -this energy, and on this will depend the relative number of particles in -the different _α_ ray groups. The ultimate energy of ejection of an _α_ -particle will depend on its position in the field of force surrounding -the inner core at the moment of its liberation. On the other hand, we -may suppose that the same _α_ particle is always ejected but that the -particle may occupy in the atom one of a number of "stationary" -positions analogous to the "stationary states" of the electrons in -Bohr's theory of the outer atom. This rests on the assumption that all -the atoms will not be identical in satellite structure but there will be -a number of possible "excited" states of the atom as a consequence of -the previous disintegrations. This satellite theory is useful in another -connection. It has been suggested that possibly the high frequency _γ_ -rays from a radioactive atom may arise not from the movement of the -electrons as ordinarily supposed, but from the transfer of _α_ particles -from one level to another. In such a case, the difference in energies -between the various groups of _α_ particles from radium C and thorium C -should be connected by the quantum relation with the frequencies of -prominent _γ_ rays. The evidence at present available is not definite -enough to give a final decision on this problem, but points to the need -of very accurate measurements of the energies of the various groups of -_α_ particles. On account of the relatively small number of particles in -some of the groups, this is difficult of accomplishment. - -In considering the satellite theory in connection with the radioactive -bodies, it is at first sight natural to suppose, since the end product -of both the radium and thorium series is an isotope of lead, that one of -the isotopes of lead forms the central core. It may, however, well be -that the radioactive processes cease when there are still a number of -satellites remaining. If this be so, the core may be of smaller nuclear -charge and mass than that of lead. From some considerations, described -later, this core may correspond to an element near platinum of number 77 -and mass 192. - - - - -FREQUENCY OF VIBRATION OF THE NUCLEUS - - -One of the most interesting and important methods of throwing light on -nuclear structure is the study of the very penetrating _γ_ rays expelled -by some radioactive bodies. The _γ_ rays are identical in nature with -X-rays, but the most penetrating type of rays consists of waves of much -higher frequency than can be produced in an ordinary X-ray tube. The -work of the last few years has indicated very clearly that the major -part of the _γ_ radiation from bodies like radium B and C originates in -the nucleus. A determination of the frequencies of the _γ_ rays thus -gives us direct information on the modes of vibration of parts of the -nuclear structure. The frequency of some of the softer _γ_ rays excited -by radium B and radium C was measured by the crystal method by -Rutherford and Andrade, but it is difficult, if not impossible, by this -method to determine the frequencies of the very penetrating rays. -Fortunately, due largely to the work of Ellis and Fräulein Meitner, a -new and powerful method has been devised for this purpose. It is well -known that the _β_ rays from radium B and radium C give a veritable -spectrum in a magnetic field, showing the presence of a number of groups -of _β_ rays each expelled with a definite speed. It is clear that each of -the groups of _β_ rays arises from conversion of the energy of a _γ_ ray -of definite frequency into a _β_ ray in one or other of the electronic -levels in the outer atom. The energy ω required to move an electron -from one of these levels to the outside of the atom is known from a -study of X-ray absorption spectra. The frequency ν of the _γ_ ray is -thus given by the quantum relation hν = E + ω, where E is the measured -energy of the _β_ particle. - -Since each _γ_ ray may be converted in any one of the known electronic -levels in the outer atom, a single _γ_ ray is responsible for the -appearance of a number of groups of _β_ rays, corresponding to conversion -in the K, L, M, etc., levels. In this way, an analysis of the _β_ ray -spectrum allows us to fix the frequency of the more intense _γ_ rays -which are emitted from the nucleus. The energy of the shortest wave -measured in this way by Ellis corresponds to more than two million -volts, while other evidence shows that probably still shorter waves are -emitted in small quantity from radium C. - -Ellis and Skinner have shown that the energies of these rays show -certain combination differences, such as are so characteristic of the -energies of the X-rays arising from the outer electrons. A series of -energy levels may thus be postulated in the nucleus similar in character -to the electron levels of the outer atom, and the _γ_ rays have their -origin in the fall either of an electron or of an _α_ particle between -these levels. This is a significant and important result, indicating -that the quantum dynamics can be applied to the nucleus as well as to -the outer electronic structure. - -The probability of levels in the nuclear structure is most clearly seen -on the satellite hypothesis, but in our ignorance of the laws of force -near the core we are at the moment unable to apply the quantum dynamics -directly to the problem. The outlook for further advances in this -direction is hopeful, but is intimately connected with a further -development of our knowledge of the laws of force that come into play -close to the nucleus in the region occupied by the satellites. - - - - -ARTIFICIAL DISINTEGRATION OF ELEMENTS - - -We have seen that it is believed that the nuclei of all atoms are -composed of protons and electrons and that the number of each of these -units in any nucleus can be deduced from its mass and nuclear charge. It -is, however, at first sight rather surprising that no evidence of the -individual existence of protons in a nucleus is obtained from a study of -the transformations of the radioactive elements, where the processes -occurring must be supposed to be of a very fundamental character. As far -as our observations have gone, electrons and helium nuclei, but no -protons, are ejected during the long series of transformations of -uranium, thorium and actinium. One of the most obvious methods for -determining the structure of a nucleus is to find a method of -disintegrating it into its component parts. This is done spontaneously -for us by nature to a limited extent in the case of the heavy -radioactive elements, but evidence of this character is not available in -the case of the ordinary elements. - -As the swift _α_ particle from the radioactive bodies is, by far, the -most energetic projectile known to us, it seemed from the first possible -that occasionally the nucleus of a light atom might be disintegrated as -the result of a close collision with an _α_ particle. On account of the -minute size of the nucleus, it is to be anticipated that the chance of -a direct hit would be very small and that consequently the -disintegration effects, if any, would be observed only on a very minute -scale. During the last few years Dr. Chadwick and I have obtained -definite evidence that hydrogen nuclei or protons can be removed by -bombardment of _α_ particles from the elements boron, nitrogen, fluorine, -sodium, aluminum and phosphorus. In these experiments the presence of H -nuclei is detected by the scintillation method, and their maximum -velocity of ejection can be estimated from the thickness of matter which -can be penetrated by these particles. The number of H nuclei ejected -even in the most favorable case is relatively very small compared with -the number of bombarding _α_ particles, viz., about one in a million. - -In these experiments the material subject to bombardment was placed -immediately in front of the source of _α_ particles and observations on -the ejected particles were made on a zinc sulphide screen placed in a -direct line a few centimetres away. Using radium C as a source of _α_ -rays, the ranges of penetration, expressed in terms of centimetres of -air, were all in these cases greater than the range of free nuclei (30 -cms. in air) set in motion in hydrogen by the _α_ particles. By inserting -absorbing screens of 30 cms. air equivalent in front of the zinc -sulphide screen the results were quite independent of the presence of -either free or combined hydrogen as an impurity in the bombarded -materials. Some of the lighter elements were examined for absorptions -less than this, but, in general, the number of H particles due to -hydrogen contamination of the source and the materials was so large that -no confidence could be placed in the results. - -In such experiments many scintillations can be observed, but it is very -difficult to decide whether these can be ascribed in part to an actual -disintegration of the material under examination. The presence of -long-range particles of the _α_ ray type from the source of radium C -still further complicates the question, since in general the number of -such particles is large compared with the disintegration effect we -usually observe. - -To overcome these difficulties, inherent in the direct method of -observation, Dr. Chadwick and I have devised a simple method by which we -can observe with certainty the disintegration of an element when the -ejected particles have a range of only 7 cms. in air. This method is -based on the assumption, verified in our previous experiments, that the -disintegration particles are emitted in all directions relative to the -incident rays. A powerful beam of _α_ rays falls on the material to be -examined and the liberated particles are observed at an average angle of -90° to the direction of the incident _α_ particles. By means of screens -it is arranged that no _α_ particles can fall directly on the zinc -sulphide screen. - -This method has many advantages. We can now detect particles of range -more than 7 cms. with the same certainty as particles of range above 30 -cms. in our previous experiments, for the presence of hydrogen in the -bombarded material has no effect. This can be shown at once by -bombarding a screen of paraffin wax, when no particles are observed on -the zinc sulphide screen. On account of the very great reduction in -number of H nuclei or _α_ particles by scattering through 90°, the -results are quite independent of H nuclei from the source or of the -long-range _α_ particles. The latter are just detectable under our -experimental conditions when a heavy element like gold is used as -scattering material, but are inappreciable for the lighter elements. - -A slight modification of the arrangement enables us to examine gases as -well as solids. - -Working in this way we have found that in addition to the elements -boron, nitrogen, fluorine, sodium, aluminum, and phosphorus, which give -H particles of maximum range in the forward direction between 40 and 90 -cms., the following give particles of range above 7 cms.: neon, -magnesium, silicon, sulphur, chlorine, argon, and potassium. The numbers -of the particles emitted from these elements are small compared with the -number from aluminum under the same conditions, varying between ⅓ and -¹⁄₂₀. The ranges of the particles have not been determined with -accuracy. Neon appears to give the shortest range, about 16 cms., under -our conditions, the ranges of the others lying between 18 cms. and 30 -cms. By the kindness of Dr. Rosenhain we were able to make experiments -with a sheet of metallic beryllium. This gave a small effect, about -¹⁄₃₀ of that of aluminum, but we are not yet certain that it may -not be due to the presence of a small quantity of fluorine as an -impurity. The other light elements, hydrogen, helium, lithium, carbon, -and oxygen, give no detectable effect beyond 7 cms. It is of interest to -note that while carbon and oxygen give no effect, sulphur, also probably -a "pure" element of mass 4n, gives an effect of nearly one-third that of -aluminum. This shows clearly that the sulphur nucleus is not built up -solely of helium nuclei, a conclusion also suggested by its atomic -weight of 32.07. - -We have made a preliminary examination of the elements from calcium to -iron, but with no definite results, owing to the difficulty of obtaining -these elements free from any of the "active" elements, in particular, -nitrogen. For example, while a piece of electrolytic iron gave no -particles beyond 7 cms., a piece of Swedish iron gave a large effect, -which was undoubtedly due to the presence of nitrogen, for after -prolonged heating _in vacuo_ the greater part disappeared. Similar -results were experienced with the other elements in this region. - -We have observed no effects from the following elements: nickel, copper, -zinc, selenium, krypton, molybdenum, palladium, silver, tin, xenon, gold -and uranium. The krypton and xenon were kindly lent by Dr. Aston. - - - - -EXAMINATION OF LIGHT ELEMENTS FOR PARTICLES -OF RANGE LESS THAN 3 CMS. OF AIR - - -When _α_ particles are scattered from light elements, the simple theory -shows that the velocity of the scattered particles depends on the angle -of scattering. For example, using bombarding _α_ particles of range 7 -cms., the range of the _α_ particles scattered through more than 90° -cannot be greater than 1.0 cm. for lithium (7), 2.0 cms. for beryllium -(10), 2.5 cms. for carbon, 3.2 cms. for oxygen, 4.3 cms. for aluminum, -and 6.8 cms. for gold. - -Provided we introduce sufficient thickness of absorber to stop the _α_ -particles scattered through 90°, we can examine for disintegrated -particles from carbon, for example, whose range exceeds 2.5 cms. Certain -difficulties arise in this type of experiment which are absent when the -thickness of absorber is greater than 7 cms.; any heavy element present -as an impurity will give scattered _α_ particles of range greater than -those from carbon and thus complicate the observations. In addition, -serious troubles may arise due to the volatilization or escape of active -matter from the source. This is especially marked if the vessel -containing the radioactive source is exhausted. To overcome this -difficulty, we have found it desirable to cover the source with a thin -layer of celluloid of 2 or 3 mm. stopping power for _α_ rays. By this -procedure we have been able to avoid serious contamination and to -examine the lighter elements by this method. We have been unable to -detect any appreciable number of particles from lithium or carbon for -ranges greater than 3 cms. If carbon shows any effect at all, it is -certainly less than one tenth of the number from aluminum under the same -conditions. This is in entire disagreement with the work of Kirsch and -Patterson (Nature, April 26, 1924), who found evidence of a large number -of particles from carbon of range 6 cms. A slight effect was observed in -beryllium in accordance with our other experiments. No effect was noted -in oxygen gas. Apart from beryllium, no certain effect has been noted -for elements lighter than boron. - -Under the conditions of our experiment, it seems clear that neither H -nuclei nor other particles of range greater than 3 cms. can be liberated -in appreciable numbers from these elements in a direction at right -angles to the bombarding _α_ rays. This is, in a sense, a disappointing -result, for, unless these elements are very firmly bound structures, it -was to be anticipated that an _α_ particle bombardment would resolve them -into their constituent particles. - -We hope to examine this whole question still more thoroughly, as it is a -matter of great importance to the theory of nuclear constitution to be -certain whether or not the light elements can be disintegrated by swift -_α_ particles. - -In considering the results of our new and old observations, some points -of striking interest emerge. In the first place, all the elements from -fluorine to potassium inclusive suffer disintegration under _α_ ray -bombardment. As far as our observations have gone, there seems little -doubt that the particles ejected from all these elements are H nuclei. -The odd elements, B, N, F, Na, Al, P, all give long-range particles -varying in range from 40 cms. to 90 cms. in the forward direction, the -even elements, C, O, Ne, Mg, Si, S, either give few particles or none at -all as in the case of C and O, or give particles of much less range than -the adjacent odd numbered elements. The differences between the ranges -of even-odd elements become much less marked for elements heavier than -phosphorus. - -This obvious difference in velocity of expulsion of the H nuclei from -even and odd elements is a matter of great interest. Such a distinction -can be paralleled by other observations of an entirely different -character. Harkins has shown that elements of even atomic number are -much more abundant in the earth's crust than elements of odd atomic -number. In his study of Isotopes, Aston has shown that in general odd -numbered elements have only two isotopes differing in mass by two units, -while even numbered elements in some cases contain a large number of -isotopes. This remarkable distinction between even and odd elements -cannot but excite a lively curiosity, but we can at present only -speculate on its underlying cause. - - - - -VELOCITY OF ESCAPE OF HYDROGEN NUCLEI - - -We have seen that the experiments of Bieler on the scattering of _α_ rays -by aluminum and magnesium indicate that a powerful attractive force -comes into play very close to the nuclei of these atoms. If this be the -case, the forces of attraction and repulsion must balance at a certain -distance from the nucleus. Outside this critical point the forces on a -positively charged body are entirely repulsive. Certain important -consequences follow from this general view of nuclear forces. Suppose, -for example, that, due to a collision with a swift _α_ particle, a -hydrogen nucleus is liberated from the nuclear structure. After passing -across the critical surface, it will acquire energy in passing through -the repulsive field. It is clear, on this view, that the energy of a -charged particle after escape from the atom cannot be less than the -energy acquired in the repulsive field; consequently we should expect to -find evidence that there is a minimum velocity of escape of a -disintegration particle. We have obtained definite evidence of such an -effect both in aluminum and sulphur by examining the absorption of H -nuclei from these elements. The number of scintillations for a thin film -was found to be nearly constant for absorption between 7 and 12 cms., -but falls off rapidly for greater thicknesses. This is exactly what is -to be expected on the views outlined. No doubt the limiting velocity -varies somewhat for the different elements, but a large amount of -experiment will be required to fix this limit with accuracy. From these -results it is possible to form a rough estimate of the potential of the -field at the critical surface, and this comes out to be about 3 million -volts for aluminum. The value for sulphur is somewhat greater. This -brings out in a striking way the extraordinary smallness of the nuclei -of these elements, for it can be calculated that the critical surface -cannot be distant more than 6 x 10^-13 cm. from the centre of the -nucleus. These deductions of the critical distance are in excellent -accord with those made by Bieler from observations of the scattering of -_α_ particles. - -Another important consequence follows. It is clear that an _α_ particle -fired at the nucleus will not be able to cross this critical surface and -thus be in a position to produce disintegration, unless its velocity -exceeds that corresponding to the critical potential. In an experiment -made a few years ago, we found that the number of H nuclei liberated -from aluminum fell off rapidly with diminution of the velocity of the _α_ -particle and was too small in number to detect when the range of the _α_ -particle was less than 4.9 cms. This corresponds to the energy of an _α_ -particle falling between about 3 million volts--a value in good accord -with that calculated from the escape of H nuclei. - -Further experiments are required with other elements to test if this -relation between the minimum velocity of H nuclei and the minimum -velocity of the _α_ particle to produce disintegration holds generally; -but the results as far as they go are certainly very suggestive. - -It is of interest to note that these results afford a definite proof of -the nuclear conception of the atom and give us some hope that we may -determine the magnitude of the critical potential for a number of the -light elements. - - - - -EVOLUTION OF NUCLEI - - -In concluding, I would like to make a few remarks of a more speculative -character dealing with the fundamental problem of the origin and -evolution of the elements from the two fundamental building units, the -positive and negative electrons. It must be confessed that there is -little information to guide us with the exception of our knowledge of -the nuclear charges and masses of the various species of elements which -survive to-day. It has always been a matter of great difficulty to -imagine how the more complex nuclei can be built up by the successive -additions of protons and electrons, since the proton must be endowed -with a very high speed to approach closely to the charged nucleus. I -have already discussed in this paper the evidence that powerful -attractive forces varying very rapidly with the distance are present -close to the nuclear structure and it seems probable that these forces -must ultimately be ascribed to the constituent proton. In such a case it -may be possible for an electron and proton to form a very close -combination, or neutron, as I have termed it. The probable distance -between the centre of this doublet is of the order of 3 x 10^-13 cm. The -forces between two neutrons would be very small except for distance of -approach of this order of magnitude, and it is probable that the -neutrons would collect together in much the same fashion as a number of -small movable magnets would tend to form a coherent group held together -by their mutual forces. - -In considering the origin of the elements, we may for simplicity suppose -a large diffused mass of hydrogen which is gradually heated by its -gravitational condensation. At high temperatures the gas would consist -mainly of free hydrogen nuclei and electrons, and some of these would in -course of time combine to form neutrons, emitting energy in the process. -These neutrons would collect together in nuclear masses of all kinds of -complexity. Now the tendency of the groups of neutrons would be to form -more stable nuclear combinations, such as helium nuclei of mass four, -and possibly intermediate stages of masses two and three. Energy would -be emitted in these processes probably in the form of swift surplus -electrons which were not necessary for the stability of the system. In a -sense, all these nuclear masses would be radioactive, but some of them -in their transformation may reach a stable configuration which would -represent the nucleus of one of our surviving elements. If we suppose -that nuclear masses over a wide range of mass can be formed before -serious transformation occurs, it is easy to see how every possible type -of stable element will gradually emerge. If we take the helium nucleus -as a combining unit which emits in its formation the greatest amount of -energy, we should ultimately expect many of the neutrons in a heavy -nucleus to form helium nuclei. These helium nuclei would tend to collect -together and form definite systems and it seems not unlikely that they -will group themselves into orderly structures, analogous in some -respects to the regular arrangement of atoms to form crystals, but with -much smaller distances between the structural units. In such a case, -some of the elements may consist of a central crystal type of structure -of helium nuclei surrounded by positive and negatively charged -satellites in motion round this central core. Assuming that such orderly -arrangements of helium nuclei are possible, it is of interest to note -that the observed relations between atomic charge and atomic mass for -the elements can be approximately obtained on a very simple assumption. -Suppose that helium nuclei form a point centred cubic lattice with an -electron at the centre of a crystal unit of eight helium nuclei. A few -of the possible types of grouping are given in the following table, with -corresponding masses and nuclear charges. The structure 4. 3. 2. means -a rectangular arrangement with sides containing 4. 3. 2. nuclei -respectively. It will thus contain 24 helium nuclei, have a mass 96, and -will contain 6 intranuclear electrons. Its nuclear charge will therefore -be 48 - 6 = 41. - - -Structural arrangement of Calculated Calculated Known element of - helium nuclei nuclear charge Mass equal charge - -3. 2. 2. 22 48 Ti 48 - -3. 3. 2. 32 72 Ge 74, 72, 70 - -3. 3. 3. 46 108 Pd 106.7 - -4. 2. 2. 29 64 Cu 63.35 - -4. 3. 2. 42 96 Mo 96 - -4. 3. 3. 60 144 Nd 144 - -4. 4. 3. 78 192 Pt 195 - - -While the agreement is far from perfect for all these structures, there -is a general accord with observation. If we take the view that some of -these structures can grow by the addition of satellites, there is room -for adjustment of masses and to include the intervening elements. This -point of view is admittedly very speculative and there may well be other -types of structure involved. At the same time, the general evidence -suggests that there are some basal structures on which the heavier atoms -are progressively built up. The failure of the whole number rule for the -mass of isotopes, observed in some cases by Aston, _e.g._, between tin -and xenon, certainly supports such a conception. From a study of the -artificial disintegration of the elements we have seen that carbon and -oxygen represent very stable structures probably composed of helium -nuclei. It is possible that oxygen nuclei, for example, may be the -structural basis of some of the elements following oxygen, but our -information is at present too meagre to be at all certain on this point. - -I think, however, it will be clear from this lecture what a difficult -but fascinating problem is involved in the structure of nuclei. Before -we can hope to make much advance, it is essential to know more of the -nature of the forces operative close to protons and electrons, and we -may hope to acquire much information by a detailed study of the -scattering of swift _α_ rays and _β_ rays by nuclei. Fortunately, there is -now a number of distinct lines of attack on this problem, and from a -combination of the results obtained we may hope to make steady, if not -rapid, progress in the solution of this, the greatest problem in -Physics. - -*** END OF THE PROJECT GUTENBERG EBOOK THE NATURAL AND ARTIFICIAL -DISINTEGRATION OF THE ELEMENTS *** - -Updated editions will replace the previous one--the old editions will -be renamed. - -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the -United States without permission and without paying copyright -royalties. 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You may copy it, give it away or re-use it under the terms -of the Project Gutenberg License included with this eBook or online -at <a href="https://www.gutenberg.org">www.gutenberg.org</a>. If you -are not located in the United States, you will have to check the laws of the -country where you are located before using this eBook. -</div> - -<p style='display:block; margin-top:1em; margin-bottom:0; margin-left:2em; text-indent:-2em'>Title: The natural and artificial disintegration of the elements</p> -<p style='display:block; margin-left:2em; text-indent:0; margin-top:0; margin-bottom:1em;'>An address by Professor Sir Ernest Rutherford</p> -<p style='display:block; margin-top:1em; margin-bottom:0; margin-left:2em; text-indent:-2em'>Author: Ernest Rutherford</p> -<p style='display:block; text-indent:0; margin:1em 0'>Release Date: December 15, 2022 [eBook #69551]</p> -<p style='display:block; text-indent:0; margin:1em 0'>Language: English</p> - <p style='display:block; margin-top:1em; margin-bottom:0; margin-left:2em; text-indent:-2em; text-align:left'>Produced by: Laura Natal Rodrigues (Images generously made available by Hathi Trust Digital Library.)</p> -<div style='margin-top:2em; margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK THE NATURAL AND ARTIFICIAL DISINTEGRATION OF THE ELEMENTS ***</div> - -<div class="figcenter" style="width: 500px;"> -<img src="images/cover.jpg" width="500" alt="500"> -</div> - - -<h1>THE NATURAL<br> -AND ARTIFICIAL DISINTEGRATION<br> -OF THE ELEMENTS</h1> - -<p><br><br></p> - -<h4>AN ADDRESS BY</h4> - -<h2>Professor Sir ERNEST RUTHERFORD</h2> - -<h5>Kt., D. Sc., LL. D., Ph. D., D. Phys., F. R. S.</h5> - -<p><br><br></p> - -<h4>ON THE OCCASION OF THE CENTENARY CELEBRATION<br> -OF THE FOUNDING OF</h4> - -<h3>THE FRANKLIN INSTITUTE</h3> - -<h4>AND THE INAUGURATION EXERCISES OF THE<br> -BARTOL RESEARCH FOUNDATION<br> -SEPTEMBER 17, 18, 19, 1924</h4> - -<p><br><br></p> - -<h5>THE FRANKLIN INSTITUTE</h5> - -<h5>PHILADELPHIA</h5> - -<p><br><br><br></p> - -<h4>THE NATURAL -AND ARTIFICIAL DISINTEGRATION -OF THE ELEMENTS</h4> - - -<h5><i>By</i> Professor Sir ERNEST RUTHERFORD, Kt., D. Sc., -<br> -LL. D., Ph. D., D. Phys., F. R. S.</h5> - -<p> -IT is not my intention in this paper to give a detailed account of the -natural disintegration of the radio elements or of the methods employed -to effect the artificial disintegration of certain light elements. I -shall assume that you all have a general knowledge of the results of -these investigations, but I shall confine myself to a consideration of -the bearing of these results on our knowledge of the structure of the -nuclei of atoms. -</p> -<p> -There is now a general agreement that the atoms of all elements have a -similar electrical structure, consisting of a central positively -charged nucleus surrounded at a distance by the appropriate number of -electrons. From a study of the scattering of <i>α</i> particles by the -atoms of matter and from the classical researches of Moseley on X-ray -spectra, we know that the resultant positive charge on the nucleus of -any atom, in terms of the fundamental unit of electronic charge, is -given numerically by the atomic or ordinal number of the element, due -allowance being made for missing elements. We know that with few -exceptions all nuclear charges, from 1 for the lightest atom, hydrogen, -to 92 for the heaviest element, uranium, are represented by elements -found in the earth. The nuclear charge of an element controls the number -and distribution of the external electrons, so that the properties of an -atom are defined by a whole number, representing its nuclear charge, and -are only to a minor degree influenced by the mass or atomic weight of -the atom. -</p> -<p> -This minute but massive nucleus is, in a sense, a world of its own which -is little, if at all, influenced by the ordinary physical and chemical -forces at our command. In many respects, the problem of nuclear -structure is much more difficult than the corresponding problem of the -arrangement and motions of the planetary electrons, where we have a -wealth of available information, both physical and chemical, to test the -adequacy of our theories. The facts known about the nucleus are few in -number and the methods of attack to throw light on its structure are -limited in scope. -</p> -<p> -It is convenient to distinguish between the properties assigned to the -nucleus and the planetary electrons. The movements of the outer -electrons are responsible for the X-ray and optical spectra of the -elements and their configuration for the ordinary physical and chemical -properties of the element. On the other hand, the phenomena of -radioactivity and all properties that depend on the mass of the atom are -to be definitely assigned to the nucleus. From a study of the -radioactive transformations, we know that the nucleus of a heavy atom -not only contains positively charged bodies but also negative electrons, -so that the nuclear charge is the excess of positive charge over -negative. In recent years, the general idea has arisen that there are -two definite fundamental units that have to do with the building up of -complex nuclei, viz., the light negative electron and the relatively -massive hydrogen nucleus which is believed to correspond to the positive -electron. -</p> -<p> -This view has received very strong support from the experiments of Aston -on Isotopes in which he has shown that the masses of the various species -of atoms are represented nearly by whole numbers in terms of O = 16. -From the general electric theory, it is to be anticipated that the mass -of the hydrogen nucleus in the nucleus structure will be somewhat less -than its value 1.0077 in the free state on account of the very close -packing of the charged units in the concentrated nucleus. From Aston's -experiments, it appears that the average mass of the hydrogen nucleus, -or proton as it is now generally called, is very nearly 1.000 under -these conditions. We should anticipate that the whole number rule found -by Aston would hold only to a first approximation, since the mass of the -proton must be to some extent dependent on the detailed structure of the -nucleus. In the case of tin and xenon Aston has already signalized a -definite departure from the whole number rule, and no doubt a still more -accurate determination of the masses of the atoms will disclose other -differences of a similar kind. -</p> -<p> -While our present evidence indicates that the proton and electron are -the fundamental constituents of the nucleus, it is very probable that -secondary combining units play a prominent part in nuclear constitution. -For example, the expulsion of helium nuclei from the radioactive bodies -indicates that the helium nucleus of mass 4 is probably a secondary unit -of great importance in atom building. On the views outlined, we should -expect the helium nucleus of charge to be built up of four protons and -two electrons. The loss of mass in forming this nucleus indicates that a -large amount of energy must be liberated during its formation. If this -be the case, the helium nucleus must be such a stable structure that the -combined energy of four or five of the swiftest <i>α</i> particles would be -necessary to effect its disruption. Such a deduction is supported by our -failure to observe any evidence of disintegration of the swift particle -itself, whether it is used to bombard matter or whether the <i>α</i> -particle is used to bombard other helium atoms. -</p> -<p> -On these views, we should anticipate that the nucleus of radium of -atomic number 88 and atomic weight 22.6 contains in all 226 protons of -mass 1 and 138 electrons. While this gives us the numerical relation -between the two fundamental units, we have, at present, no definite -information of their arrangement in the minute nuclear volume, nor of -the nature and magnitude of the forces that hold them together. We -should anticipate that many of the protons and electrons unite to form -secondary units, <i>e. g.</i> helium nuclei, and that the detailed -structure of the nucleus may be very different from that to be expected -if it consists of a conglomeration of free protons and electrons. -</p> -<p> -It is thus of great importance to obtain definite evidence of the nature -and arrangement of the components of the nucleus and of the forces that -hold them in equilibrium. We shall now consider some of the lines of -evidence which throw light on the actual dimensions of the nucleus and -the law of force operative in its neighborhood; the structure and modes -of vibration of the nucleus, together with the effects observed when -some light nuclei are disintegrated by bombardment with <i>α</i> particles. -</p> - -<p><br><br><br></p> - -<h4>DIMENSIONS OF THE NUCLEI AND THE -LAW OF FORCE</h4> - -<p> -The conception of the nucleus atom had its origin in 1911 in order to -explain the scattering of an <i>α</i> particle through a large angle as the -result of a single collision. The observation that the <i>α</i> particle is -in some cases deflected through more than a right angle as the result of an -encounter with a single atom first brought to light the intense forces -that exist close to the nucleus. Geiger and Marsden showed that the -number of particles scattered through different angles was in close -accord with the simple theory which supposed that, for the distance -involved, the <i>α</i> particle and nucleus behaved like charged points, -repelling each other according to the law of the inverse square. The -accuracy of this law has been independently verified by Chadwick, so -that we are now certain that in a region close to the nucleus the -ordinary laws of force are valid. -</p> -<p> -These scattering experiments also gave us the first idea as to the -probable dimensions of the nuclei of heavy atoms, for it is to be -anticipated that the law of the inverse square must break down if the -<i>α</i> particle approaches closely to or actually enters the nuclear -structure. This variation in the law of force would show itself by a -difference between the observed and calculated numbers of <i>α</i> -particles scattered through large angles. Geiger and Marsden, however, -observed no certain variation even when the <i>α</i> particles of range -about 4 cms. were scattered through 100° by a gold nucleus. In such an -encounter, the closest distance of approach of the <i>α</i> particle to -the center of the nucleus is about 5 x 10<sup>-12</sup> cm., so that it -would appear that the radius of the gold nucleus, assumed spherical, -could not be much greater than this value. -</p> -<p> -There is another argument, based on radioactive data, which gives a -similar value for the dimensions of the radius of a heavy atom. The -<i>α</i> particle escaping from the nucleus increases in energy as it -passes through the repulsive field of the nucleus. To fix a minimum -limit, suppose the <i>α</i> particle from uranium, which is the slowest -of all <i>α</i> particles expelled from a nucleus, gains all its energy -from the electrostatic field. It can be calculated on these data that -the radius of the uranium nucleus cannot be less than 6 x -10<sup>-12</sup> cm. This is based on the assumption that the forces -outside the nucleus are repulsive and purely electrostatic. If, as seems -not unlikely, there also exist close to the nucleus strong attractive -forces, varying more rapidly than an inverse square law, the actual -dimensions may be less than the value calculated above. -</p> -<p> -At this stage of our knowledge it is of great importance to test whether -the law of force breaks down for the distance of closest approach of an -<i>α</i> particle to a nucleus. This can be done by comparing the -observed with the calculated number of <i>α</i> particles scattered -through angles of nearly 180°. It seems almost certain that the inverse -square law must break down when swift <i>α</i> particles are used. This -can be seen from the following argument. If an <i>α</i> particle, of -the same speed as that ejected during the transformation of uranium, is -fired directly at the uranium nucleus, <i>it must penetrate into the -nuclear structure</i>. If a still swifter <i>α</i> particle is used, -<i>e. g.</i> that from radium C, which has about twice the energy of the -uranium <i>α</i> particle, it is clear that it must penetrate still -more deeply into the nuclear structure. This is based on the assumption -that the field due to a nucleus is approximately symmetrical in all -directions. If this is not true, it may happen that only a fraction of -the head-on collisions may be effective in penetrating the nucleus. It -is hoped soon to attack this difficult problem experimentally. -</p> -<p> -We have so far dealt with collisions of an <i>α</i> particle with a -heavy atom. We know, however, from the results of Rutherford, Chadwick -and Bieler that in a collision of an <i>α</i> particle with the -lightest atom, hydrogen, the law of the inverse square breaks down -entirely when swift particles are used. Not only are the numbers of H -nuclei set in swift motion much greater than is to be expected in the -simple-point nucleus theory, but the change of number with the velocity -of the <i>α</i> particle varies in the opposite way from the simple -theory. Such wide departures between theory and experiment are only -explicable if we assume either that the nuclei have sensible dimensions -or that the inverse square law of repulsion entirely breaks down in such -close collisions. If we suppose the complexity in structure and in laws -of force is to be ascribed to the <i>α</i> particle rather than to the -hydrogen nucleus, Chadwick and Bieler, as the result of a careful series -of experiments, concluded that the <i>α</i> particle behaved as if it -were a perfectly elastic body, spheroidal in shape with its minor axis 4 -x 10<sup>-13</sup> cm. in the direction of motion and major axis 8 x -10<sup>-13</sup> cm. Outside this spheroidal region the forces fell off -according to the ordinary inverse square law, but inside this region the -forces increased so rapidly that a particle was reflected from it as -from a perfectly elastic body. No doubt such a conception is somewhat -artificial, but it does serve to bring out the essential points involved -in the collision, viz., that when the nuclei approach within a certain -critical distance of each other, forces come into play which vary more -rapidly than the inverse square. It is difficult to ascribe this -break-down of the law of force merely to the finite size or complexity -of the nuclear structure or to its distortion, but the results rather -point to the presence of new and unexpected forces which come into play -at such small distances. This view has been confirmed by some recent -experiments of Bieler in the Cavendish Laboratory in which he has made, -by scattering methods, a detailed examination of the law of force in the -neighborhood of a light nucleus like that of aluminum. For this purpose -he compared the relative number of <i>α</i> particles scattered within -the same angular limit from aluminum and from gold. For the range of -angles employed, viz., up to 100°, it is assumed that the scattering of -gold follows the inverse square law. He found that the ratio of the -scattering in aluminum compared with that in gold depended on the -velocity of the <i>α</i> particle. For example, for an <i>α</i> -particle of 3.4 cms. range, the theoretical ratio was obtained for -angles of deflection below 40° but was about 7 per cent lower for an -average angle of deflection of 80°. On the other hand, for swifter -particles of range 6.6 cms. a departure from the theoretical ratio was -much more marked and amounted to 29 per cent for an angle of 80°. In -order to account for these results he supposes that close to the -aluminum nucleus an attractive force is superimposed on the ordinary -repulsive forces. The results agreed best with the assumption that the -attractive force varies according to the inverse fourth power of the -distance and that the forces of attraction and repulsion balanced at -about 3.4 x 10<sup>-13</sup> cm. from the nuclear center. Inside this -critical radius the forces are entirely attractive; outside they are -repulsive. -</p> -<p> -While we need not lay too much stress on the accuracy of the actual -value obtained or of the law of attractive force, we shall probably not -be far in error in supposing the radius of the aluminum nucleus is not -greater than 4 x 10<sup>-13</sup> cm. It is of interest to note that the -forces between an <i>α</i> particle and a hydrogen nucleus were found -to vary rapidly at about the same distance. -</p> -<p> -It thus seems clear that the dimensions of the nuclei of light atoms are -small, and almost unexpectedly small in the case of aluminum when we -remember that 27 protons and 14 electrons are concentrated in such a -minute region. The view that the forces between nuclei change from -repulsion to attraction when they are very close together seems very -probable, for otherwise it is exceedingly difficult to understand why a -heavy nucleus with a large excess of positive charge can hold together -in such a confined region. We shall see that the evidence from various -other directions supports such a conception, but it is very unlikely -that the attractive forces close to a complex nucleus can be expressed -by any simple power law. -</p> - -<p><br><br><br></p> - -<h4>RADIOACTIVE EVIDENCE</h4> - -<p> -A study of the long series of transformations which occur in uranium and -thorium provides us with a wealth of information on the modes of -disintegration of atoms, but unfortunately our theories of nuclear -structure are not sufficiently advanced to interpret these data with any -detail. The expulsion of high speed <i>α</i> and <i>β</i> particles -from the radioactive nucleus gives us some idea of the powerful forces -resident in the nucleus, for it can be estimated that the energy of -emission of the <i>α</i> particle is in some cases greater than the -energy that would be acquired if the <i>α</i> particle fell freely -between two points differing in potential by about 4 million volts. The -energies of the <i>β</i> and <i>γ</i> rays are on a similar scale of -magnitude. -</p> -<p> -Notwithstanding our detailed knowledge of the successive transformation -of the radio-elements, we have not so far been able to obtain any -definite idea of their nuclear structure, while the cause of the -disintegration is still a complete enigma. In comparing the uranium, -thorium, and actinium series of transformations, one cannot fail to be -struck by the many points of similarity in their modes of -disintegration. Not only are the radiations similar in type and in -energy, but, in all cases, the end product is believed to be an isotope -of lead. This remarkable similarity in the modes of transformation is -especially exemplified in the case of the "C" bodies, each of which is -known to break up in at least two distinct ways, giving rise to branch -products. For example, thorium C emits two types of <i>α</i> rays, 65 -percent of range 8.6 cms. and 35 per cent of range 4.8 cms., and in -addition some <i>β</i> rays. -</p> -<p> -In order to explain these results, it has been suggested that a fraction -of the atoms of thorium C break up first with the expulsion of an -<i>α</i> particle and the resulting product then emits a <i>β</i> -particle. The other fraction breaks up in a reverse way, first expelling -a <i>β</i> particle, while the subsequent product emits an <i>α</i> -particle. Similar dual changes occur in radium C and actinium C, -although the relative number of atoms in each branch varies widely for -the different elements. -</p> -<p> -This remarkable similarity between the "C" bodies is still further -emphasized by the recent discovery of Bates and Rogers that both radium -C and thorium C give rise in small numbers to other groups of <i>α</i> -particles, some of them moving at very high speeds. -</p> -<p> -It has often been a matter of remark that the radioactive properties of -the "C" bodies seem to depend more on the atomic number, <i>i. e.</i>, -the nuclear charge, than on the atomic weight. Confining our attention -to radium C and thorium C, which are best known, both have a nuclear -charge 83, but the atomic mass of radium C is 214 and of thorium C 212. -The nucleus of radium C thus contains two protons and two electrons more -than that of thorium C. If it were supposed that the nuclei of these -elements consisted of a large number of charged units in ceaseless and -irregular motion, it is to be anticipated that the addition of the -protons and electrons to the complex structure would entirely alter the -nuclear arrangement and consequently its stability and mode of -transformation. On the other hand, we find that the modes of -transformation of these two nuclei have striking and unexpected points -of resemblance which are in entire disaccord with such a supposition. We -can, however, suggest a possible explanation of this anomaly by -supposing that the <i>α</i> and <i>β</i> particles which are liberated -from these elements are not built deep into the nuclear structure but -exist as <i>satellites</i> of a central core which is common to both -elements. These satellites, if in motion, may be held in equilibrium by -the attractive forces arising from the core, and these forces would be -the same for both elements. On this view the manifestations of -radioactivity are to be ascribed not to the main core, but to the -satellite distribution, which must be somewhat different for the two -elements although possibly showing many points of similarity. It must be -admitted that a theory of this kind is highly speculative, but it does -provide a useful working hypothesis, not only to account for the -similarity of the modes of transformation of the two elements but also -immediately suggests a possible explanation of the liberation of a -number of <i>α</i> particles of different ranges from the same element. -There are two ways of regarding this question. We may in the first place -suppose that a certain amount of surplus energy has to be liberated in -the disintegration and that this energy may be given to any one of a -number of satellites. There will be a certain probability that any -particular particle will be given this energy, and on this will depend -the relative number of particles in the different <i>α</i> ray groups. -The ultimate energy of ejection of an <i>α</i> particle will depend on -its position in the field of force surrounding the inner core at the -moment of its liberation. On the other hand, we may suppose that the -same <i>α</i> particle is always ejected but that the particle may -occupy in the atom one of a number of "stationary" positions analogous -to the "stationary states" of the electrons in Bohr's theory of the -outer atom. This rests on the assumption that all the atoms will not be -identical in satellite structure but there will be a number of possible -"excited" states of the atom as a consequence of the previous -disintegrations. This satellite theory is useful in another connection. -It has been suggested that possibly the high frequency <i>γ</i> rays -from a radioactive atom may arise not from the movement of the electrons -as ordinarily supposed, but from the transfer of <i>α</i> particles -from one level to another. In such a case, the difference in energies -between the various groups of <i>α</i> particles from radium C and -thorium C should be connected by the quantum relation with the -frequencies of prominent <i>γ</i> rays. The evidence at present -available is not definite enough to give a final decision on this -problem, but points to the need of very accurate measurements of the -energies of the various groups of <i>α</i> particles. On account of the -relatively small number of particles in some of the groups, this is -difficult of accomplishment. -</p> -<p> -In considering the satellite theory in connection with the radioactive -bodies, it is at first sight natural to suppose, since the end product -of both the radium and thorium series is an isotope of lead, that one of -the isotopes of lead forms the central core. It may, however, well be -that the radioactive processes cease when there are still a number of -satellites remaining. If this be so, the core may be of smaller nuclear -charge and mass than that of lead. From some considerations, described -later, this core may correspond to an element near platinum of number 77 -and mass 192. -</p> - -<p><br><br><br></p> - -<h4>FREQUENCY OF VIBRATION OF THE NUCLEUS</h4> - -<p> -One of the most interesting and important methods of throwing light on -nuclear structure is the study of the very penetrating <i>γ</i> rays -expelled by some radioactive bodies. The <i>γ</i> rays are identical in -nature with X-rays, but the most penetrating type of rays consists of -waves of much higher frequency than can be produced in an ordinary X-ray -tube. The work of the last few years has indicated very clearly that the -major part of the <i>γ</i> radiation from bodies like radium B and C -originates in the nucleus. A determination of the frequencies of the -<i>γ</i> rays thus gives us direct information on the modes of -vibration of parts of the nuclear structure. The frequency of some of -the softer <i>γ</i> rays excited by radium B and radium C was measured -by the crystal method by Rutherford and Andrade, but it is difficult, if -not impossible, by this method to determine the frequencies of the very -penetrating rays. Fortunately, due largely to the work of Ellis and -Fräulein Meitner, a new and powerful method has been devised for this -purpose. It is well known that the <i>β</i> rays from radium B and -radium C give a veritable spectrum in a magnetic field, showing the -presence of a number of groups of <i>β</i> rays each expelled with a -definite speed. It is clear that each of the groups of <i>β</i> rays -arises from conversion of the energy of a <i>γ</i> ray of definite -frequency into a <i>β</i> ray in one or other of the electronic levels -in the outer atom. The energy ω required to move an electron from one -of these levels to the outside of the atom is known from a study of -X-ray absorption spectra. The frequency ν of the <i>γ</i> ray is thus -given by the quantum relation hν = E + ω, where E is the measured -energy of the <i>β</i> particle. -</p> -<p> -Since each <i>γ</i> ray may be converted in any one of the known -electronic levels in the outer atom, a single <i>γ</i> ray is -responsible for the appearance of a number of groups of <i>β</i> rays, -corresponding to conversion in the K, L, M, etc., levels. In this way, -an analysis of the <i>β</i> ray spectrum allows us to fix the frequency -of the more intense <i>γ</i> rays which are emitted from the nucleus. -The energy of the shortest wave measured in this way by Ellis -corresponds to more than two million volts, while other evidence shows -that probably still shorter waves are emitted in small quantity from -radium C. -</p> -<p> -Ellis and Skinner have shown that the energies of these rays show -certain combination differences, such as are so characteristic of the -energies of the X-rays arising from the outer electrons. A series of -energy levels may thus be postulated in the nucleus similar in character -to the electron levels of the outer atom, and the <i>γ</i> rays have their -origin in the fall either of an electron or of an <i>α</i> particle between -these levels. This is a significant and important result, indicating -that the quantum dynamics can be applied to the nucleus as well as to -the outer electronic structure. -</p> -<p> -The probability of levels in the nuclear structure is most clearly seen -on the satellite hypothesis, but in our ignorance of the laws of force -near the core we are at the moment unable to apply the quantum dynamics -directly to the problem. The outlook for further advances in this -direction is hopeful, but is intimately connected with a further -development of our knowledge of the laws of force that come into play -close to the nucleus in the region occupied by the satellites. -</p> - -<p><br><br><br></p> - -<h4>ARTIFICIAL DISINTEGRATION OF ELEMENTS</h4> - -<p> -We have seen that it is believed that the nuclei of all atoms are -composed of protons and electrons and that the number of each of these -units in any nucleus can be deduced from its mass and nuclear charge. It -is, however, at first sight rather surprising that no evidence of the -individual existence of protons in a nucleus is obtained from a study of -the transformations of the radioactive elements, where the processes -occurring must be supposed to be of a very fundamental character. As far -as our observations have gone, electrons and helium nuclei, but no -protons, are ejected during the long series of transformations of -uranium, thorium and actinium. One of the most obvious methods for -determining the structure of a nucleus is to find a method of -disintegrating it into its component parts. This is done spontaneously -for us by nature to a limited extent in the case of the heavy -radioactive elements, but evidence of this character is not available in -the case of the ordinary elements. -</p> -<p> -As the swift <i>α</i> particle from the radioactive bodies is, by far, -the most energetic projectile known to us, it seemed from the first -possible that occasionally the nucleus of a light atom might be -disintegrated as the result of a close collision with an <i>α</i> -particle. On account of the minute size of the nucleus, it is to be -anticipated that the chance of a direct hit would be very small and that -consequently the disintegration effects, if any, would be observed only -on a very minute scale. During the last few years Dr. Chadwick and I -have obtained definite evidence that hydrogen nuclei or protons can be -removed by bombardment of <i>α</i> particles from the elements boron, -nitrogen, fluorine, sodium, aluminum and phosphorus. In these -experiments the presence of H nuclei is detected by the scintillation -method, and their maximum velocity of ejection can be estimated from the -thickness of matter which can be penetrated by these particles. The -number of H nuclei ejected even in the most favorable case is relatively -very small compared with the number of bombarding <i>α</i> particles, -viz., about one in a million. -</p> -<p> -In these experiments the material subject to bombardment was placed -immediately in front of the source of <i>α</i> particles and -observations on the ejected particles were made on a zinc sulphide -screen placed in a direct line a few centimetres away. Using radium C as -a source of <i>α</i> rays, the ranges of penetration, expressed in -terms of centimetres of air, were all in these cases greater than the -range of free nuclei (30 cms. in air) set in motion in hydrogen by the -<i>α</i> particles. By inserting absorbing screens of 30 cms. air -equivalent in front of the zinc sulphide screen the results were quite -independent of the presence of either free or combined hydrogen as an -impurity in the bombarded materials. Some of the lighter elements were -examined for absorptions less than this, but, in general, the number of -H particles due to hydrogen contamination of the source and the -materials was so large that no confidence could be placed in the -results. -</p> -<p> -In such experiments many scintillations can be observed, but it is very -difficult to decide whether these can be ascribed in part to an actual -disintegration of the material under examination. The presence of -long-range particles of the <i>α</i> ray type from the source of radium C -still further complicates the question, since in general the number of -such particles is large compared with the disintegration effect we -usually observe. -</p> -<p> -To overcome these difficulties, inherent in the direct method of -observation, Dr. Chadwick and I have devised a simple method by which we -can observe with certainty the disintegration of an element when the -ejected particles have a range of only 7 cms. in air. This method is -based on the assumption, verified in our previous experiments, that the -disintegration particles are emitted in all directions relative to the -incident rays. A powerful beam of <i>α</i> rays falls on the material -to be examined and the liberated particles are observed at an average -angle of 90° to the direction of the incident <i>α</i> particles. By -means of screens it is arranged that no <i>α</i> particles can fall -directly on the zinc sulphide screen. -</p> -<p> -This method has many advantages. We can now detect particles of range -more than 7 cms. with the same certainty as particles of range above 30 -cms. in our previous experiments, for the presence of hydrogen in the -bombarded material has no effect. This can be shown at once by -bombarding a screen of paraffin wax, when no particles are observed on -the zinc sulphide screen. On account of the very great reduction in -number of H nuclei or <i>α</i> particles by scattering through 90°, the -results are quite independent of H nuclei from the source or of the -long-range <i>α</i> particles. The latter are just detectable under our -experimental conditions when a heavy element like gold is used as -scattering material, but are inappreciable for the lighter elements. -</p> -<p> -A slight modification of the arrangement enables us to examine gases as -well as solids. -</p> -<p> -Working in this way we have found that in addition to the elements -boron, nitrogen, fluorine, sodium, aluminum, and phosphorus, which give -H particles of maximum range in the forward direction between 40 and 90 -cms., the following give particles of range above 7 cms.: neon, -magnesium, silicon, sulphur, chlorine, argon, and potassium. The numbers -of the particles emitted from these elements are small compared with the -number from aluminum under the same conditions, varying between ⅓ and -¹⁄₂₀. The ranges of the particles have not been determined with -accuracy. Neon appears to give the shortest range, about 16 cms., under -our conditions, the ranges of the others lying between 18 cms. and 30 -cms. By the kindness of Dr. Rosenhain we were able to make experiments -with a sheet of metallic beryllium. This gave a small effect, about -¹⁄₃₀ of that of aluminum, but we are not yet certain that it may -not be due to the presence of a small quantity of fluorine as an -impurity. The other light elements, hydrogen, helium, lithium, carbon, -and oxygen, give no detectable effect beyond 7 cms. It is of interest to -note that while carbon and oxygen give no effect, sulphur, also probably -a "pure" element of mass 4n, gives an effect of nearly one-third that of -aluminum. This shows clearly that the sulphur nucleus is not built up -solely of helium nuclei, a conclusion also suggested by its atomic -weight of 32.07. -</p> -<p> -We have made a preliminary examination of the elements from calcium to -iron, but with no definite results, owing to the difficulty of obtaining -these elements free from any of the "active" elements, in particular, -nitrogen. For example, while a piece of electrolytic iron gave no -particles beyond 7 cms., a piece of Swedish iron gave a large effect, -which was undoubtedly due to the presence of nitrogen, for after -prolonged heating <i>in vacuo</i> the greater part disappeared. Similar -results were experienced with the other elements in this region. -</p> -<p> -We have observed no effects from the following elements: nickel, copper, -zinc, selenium, krypton, molybdenum, palladium, silver, tin, xenon, gold -and uranium. The krypton and xenon were kindly lent by Dr. Aston. -</p> - -<p><br><br><br></p> - -<h4>EXAMINATION OF LIGHT ELEMENTS FOR PARTICLES -OF RANGE LESS THAN 3 CMS. OF AIR</h4> - -<p> -When <i>α</i> particles are scattered from light elements, the simple -theory shows that the velocity of the scattered particles depends on the -angle of scattering. For example, using bombarding <i>α</i> particles -of range 7 cms., the range of the <i>α</i> particles scattered through -more than 90° cannot be greater than 1.0 cm. for lithium (7), 2.0 cms. -for beryllium (10), 2.5 cms. for carbon, 3.2 cms. for oxygen, 4.3 cms. -for aluminum, and 6.8 cms. for gold. -</p> -<p> -Provided we introduce sufficient thickness of absorber to stop the <i>α</i> -particles scattered through 90°, we can examine for disintegrated -particles from carbon, for example, whose range exceeds 2.5 cms. Certain -difficulties arise in this type of experiment which are absent when the -thickness of absorber is greater than 7 cms.; any heavy element present -as an impurity will give scattered <i>α</i> particles of range greater than -those from carbon and thus complicate the observations. In addition, -serious troubles may arise due to the volatilization or escape of active -matter from the source. This is especially marked if the vessel -containing the radioactive source is exhausted. To overcome this -difficulty, we have found it desirable to cover the source with a thin -layer of celluloid of 2 or 3 mm. stopping power for <i>α</i> rays. By this -procedure we have been able to avoid serious contamination and to -examine the lighter elements by this method. We have been unable to -detect any appreciable number of particles from lithium or carbon for -ranges greater than 3 cms. If carbon shows any effect at all, it is -certainly less than one tenth of the number from aluminum under the same -conditions. This is in entire disagreement with the work of Kirsch and -Patterson (Nature, April 26, 1924), who found evidence of a large number -of particles from carbon of range 6 cms. A slight effect was observed in -beryllium in accordance with our other experiments. No effect was noted -in oxygen gas. Apart from beryllium, no certain effect has been noted -for elements lighter than boron. -</p> -<p> -Under the conditions of our experiment, it seems clear that neither H -nuclei nor other particles of range greater than 3 cms. can be liberated -in appreciable numbers from these elements in a direction at right -angles to the bombarding <i>α</i> rays. This is, in a sense, a -disappointing result, for, unless these elements are very firmly bound -structures, it was to be anticipated that an <i>α</i> particle -bombardment would resolve them into their constituent particles. -</p> -<p> -We hope to examine this whole question still more thoroughly, as it is a -matter of great importance to the theory of nuclear constitution to be -certain whether or not the light elements can be disintegrated by swift -<i>α</i> particles. -</p> -<p> -In considering the results of our new and old observations, some points -of striking interest emerge. In the first place, all the elements from -fluorine to potassium inclusive suffer disintegration under <i>α</i> ray -bombardment. As far as our observations have gone, there seems little -doubt that the particles ejected from all these elements are H nuclei. -The odd elements, B, N, F, Na, Al, P, all give long-range particles -varying in range from 40 cms. to 90 cms. in the forward direction, the -even elements, C, O, Ne, Mg, Si, S, either give few particles or none at -all as in the case of C and O, or give particles of much less range than -the adjacent odd numbered elements. The differences between the ranges -of even-odd elements become much less marked for elements heavier than -phosphorus. -</p> -<p> -This obvious difference in velocity of expulsion of the H nuclei from -even and odd elements is a matter of great interest. Such a distinction -can be paralleled by other observations of an entirely different -character. Harkins has shown that elements of even atomic number are -much more abundant in the earth's crust than elements of odd atomic -number. In his study of Isotopes, Aston has shown that in general odd -numbered elements have only two isotopes differing in mass by two units, -while even numbered elements in some cases contain a large number of -isotopes. This remarkable distinction between even and odd elements -cannot but excite a lively curiosity, but we can at present only -speculate on its underlying cause. -</p> - -<p><br><br><br></p> - -<h4>VELOCITY OF ESCAPE OF HYDROGEN NUCLEI</h4> - -<p> -We have seen that the experiments of Bieler on the scattering of <i>α</i> -rays by aluminum and magnesium indicate that a powerful attractive force -comes into play very close to the nuclei of these atoms. If this be the -case, the forces of attraction and repulsion must balance at a certain -distance from the nucleus. Outside this critical point the forces on a -positively charged body are entirely repulsive. Certain important -consequences follow from this general view of nuclear forces. Suppose, -for example, that, due to a collision with a swift <i>α</i> particle, a -hydrogen nucleus is liberated from the nuclear structure. After passing -across the critical surface, it will acquire energy in passing through -the repulsive field. It is clear, on this view, that the energy of a -charged particle after escape from the atom cannot be less than the -energy acquired in the repulsive field; consequently we should expect to -find evidence that there is a minimum velocity of escape of a -disintegration particle. We have obtained definite evidence of such an -effect both in aluminum and sulphur by examining the absorption of H -nuclei from these elements. The number of scintillations for a thin film -was found to be nearly constant for absorption between 7 and 12 cms., -but falls off rapidly for greater thicknesses. This is exactly what is -to be expected on the views outlined. No doubt the limiting velocity -varies somewhat for the different elements, but a large amount of -experiment will be required to fix this limit with accuracy. From these -results it is possible to form a rough estimate of the potential of the -field at the critical surface, and this comes out to be about 3 million -volts for aluminum. The value for sulphur is somewhat greater. This -brings out in a striking way the extraordinary smallness of the nuclei -of these elements, for it can be calculated that the critical surface -cannot be distant more than 6 x 10<sup>-13</sup> cm. from the centre of the -nucleus. These deductions of the critical distance are in excellent -accord with those made by Bieler from observations of the scattering of -<i>α</i> particles. -</p> -<p> -Another important consequence follows. It is clear that an <i>α</i> -particle fired at the nucleus will not be able to cross this critical -surface and thus be in a position to produce disintegration, unless its -velocity exceeds that corresponding to the critical potential. In an -experiment made a few years ago, we found that the number of H nuclei -liberated from aluminum fell off rapidly with diminution of the velocity -of the <i>α</i> particle and was too small in number to detect when the -range of the <i>α</i> particle was less than 4.9 cms. This corresponds -to the energy of an <i>α</i> particle falling between about 3 million -volts—a value in good accord with that calculated from the escape -of H nuclei. -</p> -<p> -Further experiments are required with other elements to test if this -relation between the minimum velocity of H nuclei and the minimum -velocity of the <i>α</i> particle to produce disintegration holds -generally; but the results as far as they go are certainly very -suggestive. -</p> -<p> -It is of interest to note that these results afford a definite proof of -the nuclear conception of the atom and give us some hope that we may -determine the magnitude of the critical potential for a number of the -light elements. -</p> - -<p><br><br><br></p> - -<h4>EVOLUTION OF NUCLEI</h4> - -<p> -In concluding, I would like to make a few remarks of a more speculative -character dealing with the fundamental problem of the origin and -evolution of the elements from the two fundamental building units, the -positive and negative electrons. It must be confessed that there is -little information to guide us with the exception of our knowledge of -the nuclear charges and masses of the various species of elements which -survive to-day. It has always been a matter of great difficulty to -imagine how the more complex nuclei can be built up by the successive -additions of protons and electrons, since the proton must be endowed -with a very high speed to approach closely to the charged nucleus. I -have already discussed in this paper the evidence that powerful -attractive forces varying very rapidly with the distance are present -close to the nuclear structure and it seems probable that these forces -must ultimately be ascribed to the constituent proton. In such a case it -may be possible for an electron and proton to form a very close -combination, or neutron, as I have termed it. The probable distance -between the centre of this doublet is of the order of 3 x -10<sup>-13</sup> cm. The forces between two neutrons would be very small -except for distance of approach of this order of magnitude, and it is -probable that the neutrons would collect together in much the same -fashion as a number of small movable magnets would tend to form a -coherent group held together by their mutual forces. -</p> -<p> -In considering the origin of the elements, we may for simplicity suppose -a large diffused mass of hydrogen which is gradually heated by its -gravitational condensation. At high temperatures the gas would consist -mainly of free hydrogen nuclei and electrons, and some of these would in -course of time combine to form neutrons, emitting energy in the process. -These neutrons would collect together in nuclear masses of all kinds of -complexity. Now the tendency of the groups of neutrons would be to form -more stable nuclear combinations, such as helium nuclei of mass four, -and possibly intermediate stages of masses two and three. Energy would -be emitted in these processes probably in the form of swift surplus -electrons which were not necessary for the stability of the system. In a -sense, all these nuclear masses would be radioactive, but some of them -in their transformation may reach a stable configuration which would -represent the nucleus of one of our surviving elements. If we suppose -that nuclear masses over a wide range of mass can be formed before -serious transformation occurs, it is easy to see how every possible type -of stable element will gradually emerge. If we take the helium nucleus -as a combining unit which emits in its formation the greatest amount of -energy, we should ultimately expect many of the neutrons in a heavy -nucleus to form helium nuclei. These helium nuclei would tend to collect -together and form definite systems and it seems not unlikely that they -will group themselves into orderly structures, analogous in some -respects to the regular arrangement of atoms to form crystals, but with -much smaller distances between the structural units. In such a case, -some of the elements may consist of a central crystal type of structure -of helium nuclei surrounded by positive and negatively charged -satellites in motion round this central core. Assuming that such orderly -arrangements of helium nuclei are possible, it is of interest to note -that the observed relations between atomic charge and atomic mass for -the elements can be approximately obtained on a very simple assumption. -Suppose that helium nuclei form a point centred cubic lattice with an -electron at the centre of a crystal unit of eight helium nuclei. A few -of the possible types of grouping are given in the following table, with -corresponding masses and nuclear charges. The structure 4. 3. 2. means -a rectangular arrangement with sides containing 4. 3. 2. nuclei -respectively. It will thus contain 24 helium nuclei, have a mass 96, and -will contain 6 intranuclear electrons. Its nuclear charge will therefore -be 48 - 6 = 41. -</p> - - -<table style="border-spacing: 0px;padding: 1px;border-width: 0px;"> -<colgroup><col span="4"> -</colgroup> -<thead> -<tr> -<th style="width:200px">Structural arrangement of<br> -helium nuclei</th> -<th style="width:150px">Calculated<br> -nuclear charge</th> -<th style="width:150px">Calculated<br> -Mass</th> -<th style="width:150px">Known element of<br> -equal charge</th> -</tr> -</thead> -<tbody> -<tr> -<td style="text-align: center;">3. 2. 2.</td> -<td style="text-align: center;">22</td> -<td style="text-align: center;">48</td> -<td style="text-align: center;">Ti 48</td> -</tr> -<tr> -<td style="text-align: center;">3. 3. 2.</td> -<td style="text-align: center;">32</td> -<td style="text-align: center;">72</td> -<td style="text-align: center;">Ge 74, 72, 70</td> -</tr> -<tr> -<td style="text-align: center;">3. 3. 3.</td> -<td style="text-align: center;">46</td> -<td style="text-align: center;">108</td> -<td style="text-align: center;">Pd 106.7</td> -</tr> -<tr> -<td style="text-align: center;">4. 2. 2.</td> -<td style="text-align: center;">29</td> -<td style="text-align: center;">64</td> -<td style="text-align: center;">Cu 63.35</td> -</tr> -<tr> -<td style="text-align: center;">4. 3. 2.</td> -<td style="text-align: center;">42</td> -<td style="text-align: center;">96</td> -<td style="text-align: center;">Mo 96</td> -</tr> -<tr> -<td style="text-align: center;">4. 3. 3.</td> -<td style="text-align: center;">60</td> -<td style="text-align: center;">144</td> -<td style="text-align: center;">Nd 144</td> -</tr> -<tr> -<td style="text-align: center;">4. 4. 3. </td> -<td style="text-align: center;">78</td> -<td style="text-align: center;">192</td> -<td style="text-align: center;">Pt 195</td> -</tr> -</tbody></table> - - -<p> -While the agreement is far from perfect for all these structures, there -is a general accord with observation. If we take the view that some of -these structures can grow by the addition of satellites, there is room -for adjustment of masses and to include the intervening elements. This -point of view is admittedly very speculative and there may well be other -types of structure involved. At the same time, the general evidence -suggests that there are some basal structures on which the heavier atoms -are progressively built up. The failure of the whole number rule for the -mass of isotopes, observed in some cases by Aston, <i>e.g.</i>, between tin -and xenon, certainly supports such a conception. From a study of the -artificial disintegration of the elements we have seen that carbon and -oxygen represent very stable structures probably composed of helium -nuclei. It is possible that oxygen nuclei, for example, may be the -structural basis of some of the elements following oxygen, but our -information is at present too meagre to be at all certain on this point. -</p> -<p> -I think, however, it will be clear from this lecture what a difficult -but fascinating problem is involved in the structure of nuclei. Before -we can hope to make much advance, it is essential to know more of the -nature of the forces operative close to protons and electrons, and we -may hope to acquire much information by a detailed study of the -scattering of swift <i>α</i> rays and <i>β</i> rays by nuclei. Fortunately, -there is now a number of distinct lines of attack on this problem, and -from a combination of the results obtained we may hope to make steady, if -not rapid, progress in the solution of this, the greatest problem in -Physics. -</p> - -<p><br><br><br></p> -<div style='display:block; margin-top:4em'>*** END OF THE PROJECT GUTENBERG EBOOK THE NATURAL AND ARTIFICIAL DISINTEGRATION OF THE ELEMENTS ***</div> -<div style='text-align:left'> - -<div style='display:block; margin:1em 0'> -Updated editions will replace the previous one—the old editions will -be renamed. -</div> - -<div style='display:block; margin:1em 0'> -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. 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