WHAT'S BEEN GOING ON STRUCTURE OF RARE EARTH ELEMENTS
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T THE Conference on Spectroscopyheld in July a t the Massachusetts Institute of Technology. Dr. W. F. Meggers, Chief of the Spectroscopic Section of the National Bureau of Standards, presented a summary of recent spectroscopic determinations of the electronic configurationsof the rare earth elements. Table 1 includes the results of workers a t Princeton and M.I.T., as well as a t the Bureau of Standards. Because of difficulties in obtaining pure samples, and due to the enormous number of lines in the spectra, no reliable catalog or interpretation of their emission lines bas been possible until recently, and all information concerning their structure has been purely theoretical. The premature addition of f-type electrons to fill or half fill the J shell explains in part th so-called anomalous valence of two. exhibited by samarium, europium, thulium, and ytterbium. It also explains the discrepancies between theoretical and experimental ground states, because the first theoretical predictions neglected to take account of this tendency of larger shells to hasten completion. Furthermore, the definition of a rare earth element as one which has three incomplete shells must be withdrawn, since it appears that the inner shell (4) is completed with ytterbium. --LAURENCE S. FOSTER
BOMBARDMENT OF URANIUM WITH FAST AND SLOW NEUTRONS
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HORTLY after the announcement of the discovery of the neutron (Chadwick, 1932) and of artificial radioactivity (M. and Mme. Joliot-Curie, 19341), Fermi and his co-workers (1934) announced that heavy elements, such as thorium and uranium, are capable of capturing neutrons and thereby becoming highly radioactive. The activity was found not to be associated with the transmutations of the target element into any of the elements between lead (atomic number 82) and uranium (atomic number 92) and they reached the startling conclusion that transuranic elements, (atomic numbers 93, 94--) had been synthesized. This interpretation was held to be correct quite generally until January, 1939, when Hahn and Strassmann demonstrated the presence of elements of about half the atomic weight of the uranium among the products of such bombardments. These observationsof the fission of the heaviest elements were confirmed almost immediately in laboratories scattered throughout the world, and the existence of Fermi's transuranic elements was open to very serious question. On June 7, 1940, however, E. McMillan, of the University of California, and P. H. Abelson, of the Carnegie Institute of Washington. D. C.. were able to announce that elements 93 and 94 actually are produced, but instead of having properties thought to characALE, herring, and sardine oils are to be processed for terize eka-rhenium and eka-osmium, their chemical properties rcraft lubricants by a new plant being built by the seem to correspond more closely to those of uranium itself, thus Japan Oil and Fat Company. We take it that fish oil is to go up. opening the possibility that as a consequence of inner-shell Judging from soap exports for September, 1939, which increased building a new series of elements analogous to, but chemically 44% to about $1,000,000, the rest of the world is going to cleanup. dissimilar to, the rare earths (atomic numbers 57-71). makes its The Bensel Brice Corporation of Hollywood announcesa proc- appearance, with element 93 as the first member. The series of ess, that is entirelv satisfactory for local distribution. for p a c k - transmutation reactions is evidently: ing fish, meat, fruits, and vegetables in Pliofilm bags and then cooking in the regular canning equipment. By this process about fifty per cent of the container cost is saved, which amounts to $8.00 per thousand. According to C. M. A. Stine, of du Pont de Nemours and Company, there are ten thousand different products being No alpha or beta ray activity, or any evidence of fission, has produced from cellulose, and the industry is still in its infancy. been detected which can be associated with the decay of [941rm. -ED. F. DEGERING It seems to be very long-lived. Except for amplification in
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details, this phase of the artificially induced radioactivity in rating isotopes have given too small a fractional difference to be dJ"'"by bombardment with both fast and slow neutrons seems practicable. to have been cleared up. The complex particle, formed by the capture of a thermal Even more astonishing is the explanation of the origin of the neutron by eU,U""",uursts in the inconceivably short time of elements of low atomic weight. It was noted very early that two seconds into a t least two atomic particles, the nature of which atomic products of fission were ejected in opposite directions a t remains uncertain because of their short half-life. They may be extremelv hieh velocities. thus carrying . enormous quantities of radioactive isotopes of Cs (40 seconds) and Rb (80 seconds) energy. Because of their low kinetic energies, particles of orU"' which have been shown to be the parent substances of Ba (90 and [9312" remain in the layer of target material where they are minutes or 15 hours), and Sr (7I/r hours), all of which have been produced, but the products of fission escape with such violence isolated from the products. The process of fission is highly that they may be caught on tissue paper a few centimeters from complex and in addition to the fast neutrons ejected, isotopes the target and examined chemically. The energies are of the of the following elements have been produced in the disintegraorder of magnitude of 200 Mev. or 4.6 X 10" calories per gram tions of atomic species resulting from fission: S ~ B&r, ~ , nRb, atomic weight of uranium. It is this tremendous quantity of &I, nY. d r , eCb, aMo, &la, and aSb, L z T ~831, , r X e , uCs, energy emitted that has given rise to the hope that atomic uBa, and a rare earth which may he .&a. The emission of the fission may be the source of "atomic power." fast neutrons has been shown to follow the process of fission and I n April, 1940. Nier of Minnesota and Booth, Dunning, and may arise as a result of nuclear excitation following the beta Grosse of Columbia announced that sub-microscopic quantities decay of the nuclear fragments. of the natural isotopes of uranium, nU'a4, dl2", and BU'", had The absurd extent to which "atomic power" has already been been separated by means of a mass spectrograph and subjected publicized is illustrated by a recent "headliner." On July 27, individually to neutron bombardment. It was found that only WBZ's new 50,000-watt transmitter (at Hull. Massachusetts) isotope dl"' undergoes fission with "thermal neutrons"; .Uaa' was turned on by a Geiger-Muller counter set off by the fission is also capable of absorbing "thermal neutrons," but is trans- of d J X a 6 . When the neutron source, a stainless steel capsule muted t o element [931z" just as with more energetic neutron containing $5000 worth of radium and beryllium, was brought projectiles. The exact sequence of events after the capture of close to the disc of uranium, three distinct clicks were broada slow neutron by dl's6 has not been worked out, but fission is cast to what the studio engineers undoubtedly hoped was an accompanied by the escape of many more fast neutrons than unseen audience of several million. were captured, which makes possible the initiating of a chain reaction. This can occur only in the presence of non-absorbing As an antidote to the following articles. LANGE, "Fast new objects of collision, such as water, which reduce the kinetic energy world," Colliers, July 6, 1940, and O'NEILL, "Enter atomic of the secondary neutrons down to thermal equilibrium values. power," Harpers, June, 1940, the student is referred to the excelThis possibility of a chain reaction has removed all but one lent summary. "Is atomic power a t hand?" by POTTERi n Scienbarrier to the development of "atomic power." The problem tific Monthly, 50, 570-1 (1940) and the more mature reader, to of separating the uranium isotopes on a commercial scale must "Nuclear fission," Rev. Mod. Physics, 12, 1-20 (1940). (For yet be solved. By mass spectrographic means, it has been original articles. consult the indexes of Physical Reviewand estimated that 75,000 years would be required to collect a single Chemical Abstracts.) -LAURENCES. FOSTER pound! The usual chemical and physical processes of sepa-
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