.ACS Awwsard in M*urc Chentis&ry
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Nuclear Transformations in tlie New High Energy Ranges GLENN T. SEABORG, Department of Chemistry, University of California, Berkeley, Calif. UCLBAR science is about to enter, and i n fact has to some extent already entered, a n important new phase in its develop ment—namely, the area of ultra high ener gies. A limited entree into this region had already been permitted through observa tions with high energy cosmic rays, but these are of such low intensity as to limit seriously both the rate and the scope of progress which is possible. New ideas for design, together with new techniques for building, have led to prospects for the con struction in the rather immediate future of a number of machines capable of ac celerating electrons and light positive ions to an energy region comparable with that of the most interesting of the cosmic rays. This new phase is coming right on the heels of the amazing atomic energy de velopment, which began with the discovery of the nuclear fission reaction in uranium by O. Hahn and F. Strassmann in 1939 and culminated in the successful attainment of the self-sustaining nuclear chain-reaction and the manufacture in quantity of the synthetic element plutonium for applica tion to this purpose. The nuclear chainreaction is notable for the amount of energy which can be developed, since sub stantial quantities of fissionable material can be made to react and hence convert mass to energy. Since this is done through the medium of neutrons, another impor tant feature is the intensity and total quantity of neutrons which have become available, leading t o ohe transmutation of elements in macroscopic amounts and the production of radioactive isotopes on a heretofore undreamed of scale. I t is possible, and in fact quite probable, that the coming step into the very high energy region will lead to developments different in principle, in that i t will go beyond the rearrangement of nuclear particles and •enter the area of important insight into interactions between and the internal transformations of the fundamental nu clear particles themselves. Of course, even in the reactions which may be termed simple rearrangements of the fundamental constituents of nuclei, there will be im portant differences from previous observa tions which will be peculiar to the high energies involved. While we are standing on the threshold of this new adventure, it is interesting t o recall that the first induced nuclear trans formation was observed b y Rutherford less than 3 0 years ago. It was in 1919 a t t h e famous Cavendish Laboratory in England that the bombardment of nitro
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G. T. Seaborg receives award from F, D. Snell, representing Alpha Chi Sigma JL H E rules for the ACS Award in Pure Chemistry specify that a nominee "must have accomplished research of unusual merit for an individual on the threshold of his career.' ' I t is unnec essary to apply the final qualifying phrase to the 1947 recipient. E v e n before the war, he had won a place of distinction in the field of nu
gen with alpha particles led to t h e cloud chamber observation of its transmutation to oxygen with the emission of protons. The source of alpha particles in this case was one of the natural radioactive ele ments; and, in fact, it was not until more than a decade later, in 1932, that t h e first nuclear transmutation by totally artificial means was effected. In this now famous classic experiment the British scientists J. D . Cockcroft and E . T. S. Walton accelerated protons to an energy of about 0.5 m.e.v. b y a direct fall through a potential difference of this mag nitude. These protons were used t o bom bard the light element lithium and with the help of a cloud chamber the observa^ tion of the transmutation to t w o alpha particles was made. Although many of
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clear chemistry. E t was only natural that he be drawn into a position of responsibility in thte Manhattan Proj ect. At t h e Metallurgical Laboratory he directed researcrh oh t h e properties of the heavy isotopes and on the sepa ration of plutoniuna from, uranium and the fission producrts. T h e enormous plants at H a η ford areprcsent a stepping up of about one b>illionfo l d of tfae re sults of the researches directed fc>y our medalist. This work, and iihat o n the physical and chemical prop-ertics and oxiciation states of t h e heavy elements, necessi tated the development of new tech niques and new apparatus. T h e med alist is co-discoverer of "three elements —plutonium, amercium,.- and curium. He has worked oiat a n o w radio—decay series among the heavy elements in volving many isotopes which w e r e dis covered in his laboratory, While everyone irecogndzes t h a t many individuals contributed to the results achieved a t the MIetalluirgical Labora tory, it h a s been &aid t h a t "the success of this program w a s due· largely to the initiative and scientific judgment" o f our medalist. T h e work has b e e n char acterized a s "outstanding in the nistory of chemistry." I t is, therefore,, a great pleasure t o present t o the representative o f Alpha Chi Sigrua, \vhiclh finances t h e ACS Award in Pure Chemistry, Dr. Glenn Theodore Seaborg of th.e University o f California.
the first acceleration machines were of the type i n vhicih the total accelerating voltage w a s simply a h i g h voltage applied once, such, as t h e a.c. rectification and voltage multiplier units «and tfcie electro static generator, the principle o f resonance acceleration in which a, particle is sub jected to a small accelerating field applied repeatedly over a long£ path, was being developed simultskneousXy. T h i s found its most practical application in t h e magnetic resonance accelerator or cyclotron de veloped b y E. O. Lawremce at t l i e Univer sity of California.. Thus instrument was widely used in t3ie 1930's m d even the early models comtinue t o yield important results. The n e w madiines a l l are char acterized by the application o f this reso nance principle aaid in ^fact a r e also char-
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acterized b y the application of less high voltage and more brain power than has been the case for the machines of the past. The acceleration machines in the past, and also the chain-reacting piles» have given rise to bombarding particles with energies in the range of millions of electron volts, extending in a few cases into the region of tens of. millions of electron volts. This energy region is, roughly speaking, just that of the nuclear binding energies— that is, the energy of binding of the ele mentary and small particles such as tluneutrons, protons, and alpha particles. The transformations have therefore been characterized by the addition or subtrac tion o f one» or two such particles leading to changes of something like one or two units i n the atomic number and mass number o f the I*»iiil»ard«»d material. Entering Λ>ιτ Energy Range Although it is of course an arbitrary matter as t o when we may consider our selves to have entered a new energy range, i t is g»»nt»mlly considered that this is the case a s wt» &> into the region of hundreds of millions of electron volts. Although we have as one reason for this simply the historical fact that because of the course which the development of accelerators has taken, this energy- region is being entered by making a large jump, of the order of a factor of 10 or more, essentially all in one step, we have in addition a more scientific reason for singling out t his area. It seems reasonable to expect that it will be in this region that a first step in the interconversion of the elementary' nu clear particles might take place. It may be possible with such energies to create meso trons by artificial means for the first time. The mesotron, which has been observed and studied for about 10 years as an im portant constituent of the cosmic rays, seems to have a rest mass of some 200 times that of the electron and hence the energy equivalent of its mass is about 100 m.e.v. Thus, it might IKÎ expected that with the acceleration of particles to a kinetic energyabove 10O m.e.v., a transformation of kinetic energy- might occur with its a p pearance a s mass in the form of the mesotron. Actually there are some experimental, together with a number of theoretical, considerations which lead to the view that this transformation, in order to occur with any significant probability, must involve the creation of a pair of mesotrons, one negatively and the other positively charged» and thus a minimum of 200 m.e.v. of kinetic energy will be needed. This amount must be further increased in order t o allow a sufficient amount of kinetic energy to g o into the products and other participants of the transformation so that the laws of conservation of momentum may b e obeyed, and therefore, depending upon the particular reactions involved, the actual threshold may be somewhere above 200 m.e.v. (Known examples of transformations of the type in which ele5820
mentary particles change their character or are created or destroyed are the processes o f "pair production" in which positron-electron pairs are created from gamma-rays and "annihilation" in which gamma-rays arc produced from positronelectron pairs.) The mesotron plays an important role in the current theories of nuclear structure in that it is supposed to be intimately involved in the very fundamental interaction between neutrons and piotons through which they are held together to create a nucleus. It i s therefore hoped that their study as a result of their production in quantity by artificial means might give just the key which i s needed to the fundamental understanding of t h e nucleus of the atom. Although the accelerators for the region of hundreds of millions of electron volts a l l use t h e resonance principle, there are a number of different methods by which this is done, and thus we have a number of devices to considt r such as the synchro-cyclotron, the betatron, the synchrotron, and the linear accelerator. Although it will be beyond the scope of this discussion to describe these in detail, or to mention all the places which have the various machines under construction, some of these aspects will be discussed. Acceleration Devices In t h e synehro-cyclotron, as in the ordinary cyclotron, particles such as protons, deuterons, or helium ions are kept in expanding circular orbits by a strong magnetic field and are repeatedly acted upon by a small accelerating electric field. It differs from t h e ordinary cyclotron in that it is capable of accelerating the particle into a n energy region where its relativistic mass i s appreciably larger than its rest mass. In the ordinary cyclotron the accelerating voltage is applied to the particle i n its orbits a t equally spaced intervals of time and thus the particle gets out of phase when the mass increases because the time of revolution then increases. The sv-nchro-cyclotron overcomes this by the frequency modulation principle in which the accelerating voltage is applied at time intervals which are lengthened in synchronization with the traveling of the relativisticaliy heavier particle. As an example, with a 200 m.e.v. deuteron, because the energy equivalent of its rest mass i s about 2,000 m.e.v., its relativistic mass i s 10% greater than its res*, mass; therefore the frequency with which the accelerating voltage is applied to it in its widening orbits is decreased until ' it is 10% l e s s in the outermost orbit than it was at the beginning of its path in the small orbits at the center of the machine. A huge synchro-cyclotron is already in operation a t the "University of California and others arc planned for a number of places including the Universities of Chicago and Rochester. The betatron, invented by D . Kerst of the University of Illinois, accelerates electrons. T h i s uses an induction prin-
CHEMICAL
ciple by which a time-varying central magnetic field provides an induced electric field to accelerate a stream of electrons which move in an encompassing circular path, the distribution of the magnetic flux at and within the orbit meeting certain requirements in order to ensure that the electrons remain in the fixed stable orbit. The General Electric Co. has one in operation which generates electrons with 100 m.e.v. of energy and Kerst is in the process of constructing one at the University of Illinois to give 300 m.e.v. electrons. The electrons of this energy and also the high energy x-rays which are generated when these impinge on matter give rise to reactions which are somewhat different from those produced with the positively charged particles such as protons and deuterons. Another ingenious device, proposed independently by Ε. Μ. McMillan and by V. Veksler, is the synchrotron. In this machine the particle is accelerated in a fixed circular orbit by the repeated appli cation of an accelerating voltage and is held in the orbit by increasing the magnetic field strength there a t the proper rate so as to keep it in the orbit until it gains its full energy, which depends on the radius of the orbit and the final strength of the field. For acceleration to a corresponding energy the synchrotron is a much smaller and therefore much cheaper machine than the betatron because the strong timevarying central field of the betatron is dispensed with, thus greatly reducing the size of the magnet. Also, such an instru ment will probably be capable of ulti mately reaching much higher energies than the betatron because electrons accelerated in the latter lose energy by radiation with out continual replenishment, as is the case for the synchrotron where the electron is fed energy as it receives an acceleration pulse on each revolution. A synchrotron designed to give electrons with 300 m.e.v. of energy is nearing completion under McMillan's direction at the University of California, and one which gives 80 m.e.v.. electrons is already in operation in the re search laboratory of the General Electric Co. in Schenectady, Ν . Υ. There is good reason to expect that the former might be the first machine to effect the artificial creation of mesotrons because it seems likely that there might be less energy needed above the 200 m.e.v. threshold for mesotron pair production when elec trons are used as compared with heavier particles such as protons because less energy is "lost" in the momentum con servation process. Another device, applicable to the ac celeration of either electrons or protons, depending upon the design, is the linear accelerator. In this there is no magnetic field. The particles move down a straight tube in which they are exposed to acceler ating electric fields which are applied a t various locations along the path just in time to act on t h e particles as they arrive
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at the location. The advantage of this device may ultimately lie in its reaching the very highest energy regions at a lower cost because here there is a linear relation ship between cost and energy while most of the other machines g o up in cost as some higher power of the energy. Such a de vice for the acceleration of protons is being constructed under the direction of L·. W. Alvarez at the University of California, while electron accelerators to work on this principle are planned for t h e Massachu setts Institute of Technology under J. C. Slater and also at a number of other places.
Role of the
Chemist
Before proceeding t o a consideration of the actual results which h a v e been at tained through the use of the machines al ready in operation, it m a y be well to say a few words concerning the role which the chemist plays in this work. One of the most powerful means of determining a nuclear reaction is to identify chemically the radioactive products of the reaction. After the machines h a v e effected the trans mutations, the chemist goes t o work, and he finds himself faced with some extremely interesting and challenging problems. It is his task to identify the atomic number of the transmutation products and his services here are practically indispensable. Usually there are a great number of radio active isotopes of m a n y elements produced during the irradiation of t h e target ma terial. After the nuclear transformations have been effected, the target material is usually dissolved and the products are separated from each other b y methods which are, for the most part, characteristic of ordi nary inorganic and analytical chemistry. It is usually necessary, especially in the case of precipitation reactions, t o add a small amount of those inactive elements which arc isotopic with t h e expected or suspected transmutation products. This added material is usually designated by the term "carrier." T h e chemical problem thereafter is one of separating the carrier elements and the target element from each other, but often with a degree of complete ness beyond that encountered in ordinary chemical separations. The radiation prop erties of the chemical fraction are then de termined to complete the identification. It can probably be said that most such chemical separation procedures rely on the formation of insoluble substances and precipitation reactions characteristic of analytical chemistry, or the closely similar coprecipitation process in which the radio active material is carried b y an insoluble compound of another element which incor porates it by one of several mechanisms. However, there are other methods of ex treme power and usefulness of which men tion should be made. Among these, parti tion between solvents, in which the desired radioactive substance is usually extracted from water into an organic solvent and purified by several passes back and forth
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between the same or different solvents, is a very useful method, particularly because it is often possible to isolate the substance in the absence» of carrier material. Other methods such as distillation and electroly sis are useful in a number of cases. The use of adsorption techniques, analogous to "chromatography," have proved very ef fective. By the use of adsorbants of the ion exchange type in columns, and some organic types have proved especially effecti c, it has been possible to make difficult separations quite efficiently. T h e ex treme power of this method is illustrated by the fact that it is capable of making clean separations of neighboring rare earth elements. Although a number of high energy ac celerators are under construction, and it is beyond the scope of this discussion to try to survey this tremendous program from the standpoint of geographical distribu tion of the machines, at the present time only a couple of them arc in actual opera tion. A number of results of interest have already been obtained through the use of the General Electric Co.'s betatron, which accelerates electrons to, and produces electromagnetic radiation of energy up to, 100 m.e.v., and the Berkeley 184-inch synchro-cyclotron or f.m. (frequency modulated) cyclotron, which produces douterons and helium ions of energy about 200 and 400 m.e.v. respectively. Using the high energy x-rays from the General Electric's betatron, G. C. Baldwin and G. S. Klaiber have found reactions in which several neutrons or protons or com binations of neutrons and protons are ejected during the irradiation of a number of elements. As an example, we may cite the formation of the isotope Na 2 4 from the irradiation of silicon (Si 28 ), a reaction in which three protons plus a neutron seem to be ejected in arriving a t the product. T h e observations so far seem to indicate that this electromagnetic radiation is in sufficient in energy to effect the artificial creation of mesotrons in observable yield. The results with the 184-inch cyclotron have been spectacular. The bombard ment of elements anywhere in the periodic table with 200 m.e.v. deuterons or 400 m.e.v. helium ions gives rise in each case to a tremendous number of radioactive product isotopes often extending over an atomic number range of 10 or 15 or 20. For each of these elements in turn there is produced a number of radioactive isotopes so that in each bombardment the number of transmutation products is truly large. This leads to great complexity and a great strain is placed upon the chemical and physical measurements needed in order to unravel the results from each bombard ment.
Results and Task of Interpretation Already a number of results have been obtained of which it will be possible here to cite only a few. H. H . Hopkins and Β . Β. Cunningham have found among the
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many products from the bombardment of arsenic with 400 m.e.v. helium ions the 37-minute CI 38 isotope. This corresponds to a reduction of 16 atomic number units, as the atomic number of chlorine is 17 while that, of arsenic is 3 3 ; the correspond ing decrease in mass units is 37. In another example, D . R. Miller and R. C. Thompson have found among the prod ucts of the irradiation of copper with 200 m.e.v. deuterons and 400 m.e.v. helium ions, in th' manganese fraction alone, a range of radioactive isotopes from mass number 51 up to 56. M. Lindner and I. Perl man find that the bombardment of antimony (atomic number 51) with 200 m.e.v. deuterons gives rise to radioactive isotope -s extending from molybdenum (atomic number 42) to tellurium (atomic number 52) and the observations are still far from complete. These results, chosen quite a t random, are meant to be only illustrative of the scope of the reactions whir : . occur. The occurrence of nuclear reactions in volving the ejection of so many particles has raised a question as to the method for writing thp reactions. The ordinary no menclature is inadequate because in many cases the path by which the final product is obtained is indeterminate. For example, in the production of 46-minute M n " from helium ions on copper, it is not known whether the reaction is due primarily to the direct ejection of (assuming the isotope Cu 65 is involved) 6 protons and 12 neutrons or 3 alpha particles and 6 neutrons or 5 protons and 13 neutrons to form an Fe 5 1 which goes to Mn 6 1 by positron emission, etc.; probably a combination of several of these paths is involved. Consequently we have adopted the convention of writing the reaction CueB (a, 6zl8a)Mn 5 1 , in which the ζ and a correspond to the loss in charge and mass units, respectively. Accord ing to this convention, for example, the above-mentioned production of CI38 would be written As78(
