84 research outputs found
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Element No. 102
By the use of a radically new method they have succeeded in identifying unambiguously an isotope of element 102. In other careful experiments conducted over a period of many months they find that they are unable to confirm the element 102 discovery work of Fields et al. reported in 1957. The experiments at Berkeley were performed with the new heavy ion linear accelerator (HILAC) over a period of several weeks and culinated the chemical identification of an isotope of fermium (Fm{sup 250}) as the daughter of an alpha-particle-emitting isotope of element 102 (102{sup 254}). The method used to detect the isotope of element 102 was essentially a continuous milking experiment wherein the atoms of the daughter element 100 were separated from the parent element 102 by taking advantage of the recoil due to the element 102 alpha particle decay. The target consisted of a mixture of isotopes of curium (95% Cm{sup 244} and 4.5% Cm{sup 246}) mounted on a very thin nickel foil. The target was approximately 0.5 mg/cm{sup 2} thick and was covered with 75 {micro}gm/cm{sup 2} aluminum to prevent curium 'knockover'. The curium was bombarded with mono-energetic C{sup 12} ions at energies from 60 to 100 Mev. The transmuted atoms were knocked into helium gas to absorb the considerable recoil energy. It was found that with a sufficient electric field strength practically all of these positively charged atoms could be attracted to a moving negatively charged metallic belt placed directly beneath the target. These atoms would then be carried on this conveyer belt under a foil which was charged negatively relative to the belt. Approximately half of the atoms undergoing alpha decay would cause their daughter atoms to recoil from the surface of the belt to the catcher foil. The catcher foil was cut transversely to the direction of the belt motion into five equal length sections after a time of bombardement suited to the half-life of the daughter atom to be examined. The five foils were then alpha-pulse-analyzed simultaneously in a multiplex assembly consisting of five Frisch grid chambers, amplifiers, a single Wilkinson type 'kick-sorter', and a printer. With this equipment it was easily possible to make all the desired measurements for identifying the atoms caught on the catcher foils and thus to measure the half-life of the parent of the recoiling atoms. The method was first successfully used in bombardments of Pu{sup 240} with C{sup 12} ions to identify a new isotope of element 100, Fm{sup 248}. It was shown to have a half-life of 0.6 minutes by analysis of the amounts of the 20-minute Cf{sup 244} caught on the catcher foils
Attempts to Confirm the Existence of the 10-Minute Isotope of 102
In many score of experiments conducted in various ways over a period of many months they find that they are unable to confirm the element 102 discovery work of Fields et al. reported in 1957. These experimenters ascribed to an isotope of element 102 an alpha particle activity having an energy of 8.5 {+-} 0.1 Mev and a half-life of approximately 10 minutes. It was reported to be produced by bombardments of a 1 mg/cm{sup 2} curium target with 0.03-0.10 mter-microamperes of C{sup 13} ions of about 90 Mev energy in the internal beam of the Nobel Institute 225 cm cyclotron. Our attempts to reproduce this activity were made with the monoenergetic ion beam available from the Berkeley heavy ion linear accelerator (HILAC). Curium with a similar isotopic composition was used, except that instead of one target they used six separate electroplated targets, four with 0.4 mg/cm{sup 2} curium and two with 0.1 mg/cm{sup 2} curium. These were mounted in vacuum so that the heavy ion beam could pass through and knock the transmutation recoils into 0.9 mg/cm{sup 2} palladium foils. After a suitable bombardment the six catcher foils were dissolved in a few drops of concentrated aqua regia and an actinide element fraction quickly separated from palladium by elution with 2M HCl from a column packed with Dowex-1 anion exchange resin. It was possible to examine a trans-plutonium fraction within 8 minutes from the end of bombardment. A wide range of energies (60-100 Mev) of both C{sup 12} and C{sup 13} projectiles and (+6) ion currents up to 0.2 microamperes were used. In order to compare these bombardments with those which were reported to have produced the 8.5 Mev alpha activity one can compare the amounts of the other alpha particle activities that are produced in such bombardments. The nuclides Fm{sup 250}, Cf{sup 245}, Cr{sup 244}, and Cf{sup 246} were found in far greater amount in the experiments than in the aforementioned cyclotron runs. In the case of Cf{sup 246}, for example, which is produced with a relatively flat excitation function they found in a typical experiment about 40 alpha counts per minute. This should be compared with 0.1 alpha counts per minute of Cf{sup 246} found in the cyclotron experiment which was reported to have yielded four 8.5 Mev events
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DECAY PROPERTIES OF THE NUCLIDES FERMIUM-256, 257, and 258 AND MENDELEVIUM-255, 256, AND 257
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Fission of Gold by Carbon Ions
Angular distribution and kinetic-energy spectra of fragments, and cross sections for fission of gold with 68- to 124-Mev C{sup 12} ions have been obtained by observation of the fragments in two types of detectors, gas scintillation chambers and silicon p-n junctions. From the parameters used to fit the angular distributions to the theoretical curves of Halpern and Strutinski, we have obtained the average excitation energy of the fissioning nucleus at the time of fission. This quantity is approximately 25 Mev, which is nearly independent of bombarding energy, suggesting that fission is preceded by the emission of several particles from the compound nucleus. The fission cross section increases from a value of 100 mb at 68 Mev to 1.28 b. at 124 Mev. Over this range of bombarding energies, the total fragment kinetic-energy release rises from 142 {+-} 6 to 146 {+-} 6 Mev. At all bombarding energies, the variation of laboratory-system kinetic energy of the fragments with laboratory-system angle indicates full momentum transfer by the bombarding particle to the fissioning system
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