Isotope discovery

Our research at Berkeley has resulted in the discovery of element 94, demonstration of the slow neutron fissiona-bility of its isotope 94239, discovery and demonstration of the slow neutron fissionability of U23 3, spontaneous fission measurements on these isotopes, discovery of 93237, isolation of and nuclear measurements on U23, study of the chemical properties and methods of chemical separation of element 94, demonstration of the presence of small concentrations of 94 in nature and much related information. 

Ion chemistry is a product of the 20th century. J J Thomson discovered the electron in 1897 and identified it as a constituent of all matter. Free positive ions (as distinct from ions deduced to exist in solids or electrolytes) were first produced by Thomson just before the turn of the century. He produced beams of light ions, and measured their mass-to-charge ratios, in the early 1900s, culminating in the discovery of two isotopes of neon in 1912 [1]. This year also marked Thomson s discovery of which turns out to be the... 

The discoveries at Berkeley were made by bombarding a target of 249Cf with 12C nuclei of 71 MeV, and 13C nuclei of 69 MeV. The combination of 12C with 249Cf followed by instant emission of four neutrons produced Element 257-104. This isotope has a half-life of 4 to 5 s. 

In 1957 workers in the United States, Britain, and Sweden announced the discovery of an isotope of element 102 with a 10-minute half-life at 8.5 MeV, as a result of bombarding 244Gm with 13G nuclei. On the basis of this experiment, the name nobelium was assigned and accepted by the Gommission on Atomic Weights of the International Union of Pure and Applied Ghemistry. 

The acceptance of the name was premature because both Russian and American efforts now completely rule out the possibility of any isotope of Element 102 having a half-life of 10 min in the vicinity of 8.5 MeV. Early work in 1957 on the search for this element, in Russia at the Kurchatov Institute, was marred by the assignment of 8.9 +/- 0.4 MeV alpha radiation with a half-life of 2 to 40 sec, which was too indefinite to support discovery claims. 

The isotope produced was the 20-hour 255Fm. During 1953 and early 1954, while discovery of elements 99 and 100 was withheld from publication for security reasons, a group from the Nobel Institute of Physics in Stockholm bombarded 238U with 160 ions, and isolated a 30-min alpha-emitter, which they ascribed to 250-100, without claiming discovery of the element. This isotope has since been identified positively, and the 30-min half-life confirmed. 

The First Reactor. When word about the discovery of fission in Germany reached the United States, researchers thereafter found that (/) the principal uranium isotope involved was uranium-235 (2) slow neutrons were very effective in causing fission (J) several fast neutrons were released and (4) a large energy release occurred. The possibiUty of an atom bomb of enormous destmctive power was visualized. 

The historic discovery of radium in 1898 by Marie Curie initiated a remarkable use of this isotope as an early oceanic tracer. Less than 10 years after its discovery,... 

Discovery of a nitrogen isotope N by S. M. Naudd following the discovery of isotopes of O and C by others earlier in the same year. 

In the early years of this century the periodic table ended with element 92 but, with J. Chadwick s discovery of the neutron in 1932 and the realization that neutron-capture by a heavy atom is frequently followed by j6 emission yielding the next higher element, the synthesis of new elements became an exciting possibility. E. Fermi and others were quick to attempt the synthesis of element 93 by neutron bombardment of but it gradually became evident that the main result of the process was not the production of element 93 but nuclear fission, which produces lighter elements. However, in 1940, E. M. McMillan and P. H. Abelson in Berkeley, California, were able to identify, along with the fission products, a short-lived isotope of... 

F. W. Aston (Cambridge) discovery, by means of the mass spectrograph, of isotopes in a large number of non-radioactive elements and for enunciation of the whole-number rule. 

Fermi had been fascinated by the discovery of the neutron by James Chadwick in 1932. He gradually switched his research interests to the use of neutrons to produce new types of nuclear reactions, in the hope of discovering new chemical elements or new isotopes of known elements. He had seen at once that the uncharged neutron would not be repelled by the positively-charged atomic nucleus. For that reason the uncharged neutron could penetrate much closer to a nucleus without the need for high-energy particle accelerators. lie discovered that slow neutrons could... 

Radioactivity The ability possessed by some natural and synthetic isotopes to undergo nuclear transformation to other isotopes, 513 applications, 516-518 biological effects, 528-529 bombardment reactions, 514-516 diagnostic uses, 516t discovery of, 517 modes of decay, 513-514 nuclear stability and, 29-30 rate of decay, 518-520,531q Radium, 521-522 Radon, 528 Ramsay, William, 190 Random polymer 613-614 Randomness factor, 452-453 Raoult s law A relation between the vapor pressure (P) of a component of a solution and that of the pure component (P°) at the same temperature P — XP°, where X is the mole fraction, 268... 

Electrons. If the discovery of isotopes threatened ro undermine the periodic system, the discovery of the electron explained many of the periodic properties on which the table was based. J. J. Thomson attempted to explain the periodic system by postulating rings of electrons embedded in the positive charge that made up his phim pudding model of the atom. Thomson s model was quickly superseded by more sophisticated and elaborate mod-... 

However, an important development within atomic physics, namely the discovery of isotopy in the 1910s, led some philosophically minded chemists to reexamine Mendeleev s distinction and to rehabilitate it in a modified form. With the rapid discovery of isotopes it began to seem as though there were far more "elements" than the 90 or so which were displayed on periodic tables at the time. The work of Soddy [14], in particular, served to clarify the situation, and one that had been anticipated by Crookes,... 

But what would become of Mendeleev s periodic system which now seemed to consist of 300 or so "elements" To some chemists, the discovery of isotopes implied the end of the periodic system as it was known.3 These chemists suggested that it would be necessary to consider the individual new isotopes as the new "elements." But the chemist Paneth adopted a less reductionist approach, arguing that the periodic table of the familiar chemical elements should be retained because it dealt with the "elements" that were of interest to chemists. A justification for this view was provided by the fact that, with a few exceptions, the chemical properties of isotopes of the same element are indistinguishable.4 Moreover, Paneth appealed to Mendeleev s distinction between the two senses of the concept of an "element" in order to provide a philosophical rationale for the retention of the chemist s periodic table. Paneth argued that the discovery of isotopes of the elements represents the discovery of new elements as simple substances, whereas periodic... 

Isotopes. Toward the end of Mendeleev s life a growing body of evidence began to challenge his conception of the nature of tiie elements. Several revolutionary discoveries in physics showed that atoms were, in fact, reducible and that there was a sense in which all elements are composed of the same primary matter protons, neutrons, and electrons. Most alarmingly, there was even evidence to suggest that certain elements could be transformed into others through radioactivity. 

In 1913, J. J. Thomson4 demonstrated that neon consists of different atomic species (isotopes) having atomic weights of 20 and 22 g/mole. Thomson is considered to be the father of mass spectrometry. His work rests on Goldstein s (1886) discovery of positively charged entities and Wein s (1898) demonstration that positively charged ions can be deflected by electrical and magnetic fields. 

His researches and those of his pupils led to his formulation in the twenties of the concept of active catalytic centers and the heterogeneity of catalytic and adsorptive surfaces. His catalytic studies were supplemented by researches carried out simultaneously on kinetics of homogeneous gas reactions and photochemistry. The thirties saw Hugh Taylor utilizing more and more of the techniques developed by physicists. Thermal conductivity for ortho-para hydrogen analysis resulted in his use of these species for surface characterization. The discovery of deuterium prompted him to set up production of this isotope by electrolysis on a large scale of several cubic centimeters. This gave him and others a supply of this valuable tracer for catalytic studies. For analysis he invoked not only thermal conductivity, but infrared spectroscopy and mass spectrometry. To ex-... 

Historical That positive rays could be deflected in electric and magnetic fields was shown as early as 1898 by Wien, but it was not until 1912 that what was to become the forerunner of the modem mass spectrometers was built by JJ. Thompson, who became known as the father of mass spectrometry. The existence of two isotopes of neon (m/e 20 and 22) was demonstrated by Thompson with this instrument. The discovery of stable isotopes of elements has been generally considered the... 

The discoveries of Becquerel, Curie, and Rutherford and Rutherford s later development of the nuclear model of the atom (Section B) showed that radioactivity is produced by nuclear decay, the partial breakup of a nucleus. The change in the composition of a nucleus is called a nuclear reaction. Recall from Section B that nuclei are composed of protons and neutrons that are collectively called nucleons a specific nucleus with a given atomic number and mass number is called a nuclide. Thus, H, 2H, and lhO are three different nuclides the first two being isotopes of the same element. Nuclei that change their structure spontaneously and emit radiation are called radioactive. Often the result is a different nuclide. 

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