and 8.007 MeV. A more detailed account of the energy levels in 25X Fm populated by the a-particle decay of 255 No is the subject of a future publication. 9 Based on our experience with the identification of nobelium, we feel that a conclusive atomicnumber identification can be obtained with the observation of far fewer coincident events than were recorded in this experiment. We are currently applying this technique to the identification and study of a -active isotopes of transnobelium elements.We wish to express our thanks to Dr. M. L. Mallory and E. D. Hudson for pioneering ion source and cyclotron development, to A. W. Riikola and the operating crew of the Oak Ridge isochronous cyclotron for providing copious quantities of 12 C +4 ions for this experiment, and to Dr. R. D. Baybarz for preparing the 249 Cf target. * Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corporation.Extensive discussions on the physical 1 and chemical properties 2 of superheavy elements began after Myers and Swiatecki 3 pointed out that shell effects may lead to an island of relative stability beyond the present periodic table and after Meldner and Roper calculated that the next shell closure after 82 should probably occur at proton number 114. Detailed lifetime calculations were carried out for superheavy nuclei with even proton numbers by Nilsson 5i6 and Greiner 7 and their collaborators, and it was emphasized that additional stability is expected for odd-Z and/or odd-i\T nuclei. 7 * 8 These studies indicate for certain superheavy nuclei half-lives sufficiently long 5 for survival throughout geologic time, i.e., T x / 2 g 10 8 yr, and for occurrence in the cosmic radiation, T 1/2^1 0 5 yr, provided that such nuclei are produced in nucleosynthesis.
ZusammenfassungMit schnellen radiochemischen Trennverfahren wurden die Hauptkomponenten unter den Vorläufern verzögerter Neutronen bei der Spaltung von 235 U durch thermische Neutronen ermittelt. Bestimmt wurden die Beiträge der Brom-und Jodisotope, der gesamten Nichthalogene und einiger Nichthalogenelemente, darunter Rubidium, Cäsium und Yttrium. Ein neuer Neutronenvorläufer, 0,8-sec 140 J, wurde identifiziert und ein weiterer, l,l-eec ,8 > M Y, bestätigt. Aus den relativen Häufig-keiten der Vorläufer wurden durch Normierung über die Ausbeute der 22-sec Neutronengruppe die Neutronenausbeuten der Vorläufer beim untersuchten Spaltprozeß und, durch Vergleich mit den Spaltausbeuten, ihre Neutronenemissionswahrscheinlichkeiten abgeleitet. Die Addition der Neutronenausbeuten aller bekannten Vorläufer liefert innerhalb der einzelnen Neutronengruppen Gesamtausbeuten, die mit den Gruppenausbeuten übereinstimmen.
SummaryRapid radiochemical separation methods have been used to determine the main components of the delayed-neutron precursors in fission of 2S5 U by thermal neutrons. The contributions of bromine and iodine isotopes and the total contribution of the nonhalogen elements have been determined as well as the contributions of individual nonhalogen elements like rubidium, cesium, and yttrium. A new delayed-neutron precursor, 0.8 sec 110 I, has been identified and another one, 1.1 sec 98 >"Y, has been confirmed. By normalizing relative abundances of delayed-neutron precursors via the yield of the 22-sec neutron group, the neutron yields of the precursors in the fission process studied were obtained, and their neutron emission probabilities were derived by comparing neutron yields with fission yields. Summing-up the neutron yields of all known precursors within each group results within the error limits in total yields equal to the group yields.
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