J. H. Hamilton

New insights from studies of spontaneous fission with large detector arrays

Abstract The new insights into a) the rapidly varying structures of neutron-rich nuclei up to spins as high as 20+ and 19−, b) the spontaneous fission process itself, and c) modes of cluster radioactivity that have come out of studies of spontaneous fission of 242Pu, 248Cm and 252Cf with large detector arrays are presented. The studies include γ-γ-γ coincidences with the second generation (14–20 Compton-suppressed Ge arrays) and new, third generation (35–45 Compton-suppressed Ge arrays, Gammasphere and Eurogam) gamma-ray detector arrays. The neutron-rich nuclei observed span the full range from superdeformed (β2 ≥ 0.4) double-magic ground states to spherical double-magic nuclei and competing shapes in between. New structure insights include the following: New regions of identical ground state bands to spins of 10+ to 16+ were discovered; one at the sudden onset of large deformation at N=60,62 in both even- and odd-A 98–101Sr, one at midshell N = 64,66 in 108,110Ru, one for N = 88–90 144,146Ba, one for N = 92–94 152,154Nd, and at high spin in 156Nd and also in excited bands in these regions. New shape coexisting structures in both even- and odd-A for A = 96–102 Sr and Zr nuclei are found. Evidence is found in both even-even and odd-A nuclei for a new region of octupole deformation around Z = 56 and N = 86–88. Rapid changes in moments of inertia are observed with changes in Z and N of two units with sudden changes in the moments of inertia associated with band crossings. The levels in many neutron-rich nuclei are observed for the first time. Earlier, plunger studies yielded lifetimes of low spin states and recently the Doppler Shift Line Shape Analysis yielded lifetimes of high spin states. For the first time, direct measurements of yields and neutron multiplicities have been made for five correlated pairs of fission fragments of SrNd, ZrCe, MoBa, RuXe and PdTe nuclei. Neutron multiplicities from 0–10 v emission (10v for the first time) were observed in MoBa correlated pairs with the 0 and 7–10 neutron-emission yields enhanced compared to gross yields for all fragments. The MoBa data provide evidence for two different fission modes. By unfolding the observed MoBa yields, the masses and excitation energies and mass distributions at scission were extracted. These data revealed a new mode involving the high neutron multiplicities that occurs essentially through one pair, 108Mo144Ba, 107Mo145Ba, or 106Mo146Ba or some combination where the 144Ba, 145Ba and/or 146Ba at scission are hyperdeformed with a long-to-short axis ratio of 3:1. The zero-neutron emission channels provide new examples of the cold rearrangements of nucleons in a new type of cluster radioactivity. The cluster radioactivity model predicted the observed enhancements of the zero-neutron channels for odd-A-odd-A nuclei as well as the observed fine structure, that is, the spin distributions of the two fragments. The measurements of intensities and γ-γ-γ coincidences in SF with large detector arrays open a new era in the determination of previously inaccessible properties of neutron-rich nuclei and the fission process.

New cold and ultra hot binary and cold ternary spontaneous fission modes for 252Cf and new band structures with gammasphere

Abstract Prompt γ-γ-γ and x-γ-γ coincidence studies following the spontaneous fission of 252Cf were carried out first at Oak Ridge then at Gammasphere first with 36 and later with 72 Ge and two x-ray detectors and a long x-ray-γ coincidence experiment at Idaho. Many new cases of correlated pairs in cold (zero neutron emission) binary fission are observed and for the first time the heavier correlated pairs are identified in ternary fission where the third partners are α, 6He (or α2n), 10Be and, tentatively, 14C. Theoretical calculations were carried out of the relative yields of cold binary and ternary fission. There is reasonable agreement between the relative theoretical and experimental yields. New level structures and isotopes include new octupole deformations, identical bands and other structures. Stable octupole deformation is now observed in N=86, 140Xe, 142Ba and 144Ba and 143Ba, 148Ba and to higher spin in 144Ba. The 142–146Ba data provide the first evidence for the predicted disappearance of stable octupole deformation at high spins from band crossings. Identical yrast bands are observed with widely different neutron and proton numbers in 98,100Sr, 108,110Ru, 112,116Pd, 144,146Ba, 152,154,156Nd, 156Sm, 160Gd and a new type of shifted identical bands in 156,158,160Sm as well as the first identical octupole bands in 142,144Ba. Other new level structures are found. Yields and neutron multiplicities were measured directly for SrNd, ZrCe, MoBa, RuXe, and PdTe correlated pairs. A new ultra hot fission mode was discovered going via only 108Mo144Ba, 107Mo145Ba, and/or 106Mo146Ba pairs where one or more of 44,145,146Ba are hyperdeformed at scission with 3:1 axis ratio.

Review of even element super-heavy nuclei and search for element 120

Abstract.The reaction 54Cr

Zero neutron emission in spontaneous fission of 252Cf: a form of cluster radioactivity

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Internal conversion studies in the decay of Ag110m

248Cm was investigated at the velocity filter SHIP at GSI, Darmstadt, with the intention to study production and decay properties of isotopes of element 120. Three correlated signals were measured, which occurred within a period of 279ms. The heights of the signals correspond with the expectations for a decay sequence starting with an isotope of element 120. However, a complete decay chain cannot be established, since a signal from the implantation of the evaporation residue cannot be identified unambiguously. Measured properties of the event chain are discussed in detail. The result is compared with theoretical predictions. Previously measured decay properties of even element super-heavy nuclei were compiled in order to find arguments for an assignment from the systematics of experimental data. In the course of this review, a few tentatively assigned data could be corrected. New interpretations are given for results which could not be assigned definitely in previous studies. The discussion revealed that the cross-section for production of element 120 could be high enough so that a successful experiment seems possible with presently available techniques. However, a continuation of the experiment at SHIP for a necessary confirmation of the results obtained in a relatively short irradiation of five weeks is not possible at GSI presently. Therefore, we decided to publish the results of the measurement and of the review as they exist now. In the summary and outlook section we also present concepts for the continuation of research in the field of super-heavy nuclei.

Neutron activation cross sections at 14.4 MeV For some naturally occurring heavy elements in the region 76 ≦ Z ≦ 82

By using the triple gamma coincidence technique with 20 Ge detectors at the Holifield Laboratory in the spontaneous fission of 252Cf, neutronless fragmentations, like 104Mo-148Ba, 106Mo-146Ba, 108Mo-144Ba and 104Zr-148Ce, are experimentally observed directly for the first time. When zero neutron emission spontaneous fission occurs, essentially all the available energy goes into the total kinetic energy of the fragments (cold fission). This process is seen theoretically to be an extension of cluster radioactivity, which involves the emission of one light fragment like 14C, 20O, (24)Ne or 28Mg to nearly equal fragments. In the neutronless spontaneous fission reported here, double fine structures (i.e. decays to the excited states of both fragments) are observed experimentally in contrast to fine structure in only the heavy partner populated by the light partner in earlier cluster radioactivity work. Neutronless spontaneous fission is discussed in the framework of cluster radioactivity.

The energy levels of U233

The internal conversion spectrum from the decay Ag110m has been studied in an iron-free, double-focusing spectrometer. The K and in some cases L+M conversion electron intensities were measured for the following transitions in Cd110 : 446.5, 620.2, 657.6, 677.3, 686.5, 706.4, 743.9, 763.6, 818.0, 884.5, 937.4 and 1384.0 keV and the 433.8 and 613.7 keV transitions in Pb108. The energies were determined with an accuracy of a few parts in 104. The K conversion coefficients of the 2+ → 0+ 657.6, 4+ → 2+ 884.5 and 6+ → 4+ 937.4 keV transitions were measured by the internal-external conversion method to be (2.64±0.10)×10−3, (1.26±0.06)×10−3 and (1.12±0.08)×10−3, respectively. These αK results are in good agreement with the theoretical values for E2 transitions.

New Spontaneous Fission Mode for 252Cf: Indication of Hyperdeformed 144,145,146Ba at Scission.

Abstract Neutron activation cross sections at 14.4 MeV were determined for isotopes of the following naturally occurring heavy elements: Os, Pt, Au, Hg, Tl and Pb. The mixed-powder method with Ge(Li) γ-detection was used, and the cross sections in mb are as follows. For (n, 2n) reactions ( m = metastable ; g = ground state ); 192 Os, 1993±200 ( m+g ); 192 Pt , 2035±150; 198 Pt , 1009±100 ( m ); 198 Pt , 707±85 ( g ); 197 Au , 151 ±18 ( m ); 197 Au , 1986±150( m+g ); 196 Hg , 1617±160( m ); 196 Hg , 363±54( g ); 198 Hg , 885±80( m ); 198 Hg , 1125±100( g ); 200Hg, 789±120( m ); 204 Hg , 2077±166; 203 TI , 1950±200; 204 Pb , 1737±140 . For (n, n′) reactions: 199Hg, 128±20( m ); 204 Pb , 51±10( m ) . For (n, 3n) reactions: 197Au, 61±20. For (n, p) reactions: 194Pt, 4.3±0.6; 196 Pt , 1.1±0.2; 198 Hg , 4.5±0.5; 199 Hg , 2.3±0.3; 208 Pb , 0.46±0.06 . For the (n, α) reaction; 203Tl, 2.2±0.4. For the [(n, np)+(n, pn)+(n, d)] reaction: 199Hg, 0.40±0.06. Cross sections calculated with the statistical-model equation of Pearlstein and an optimized level-density parameter equation are compared with a consistent set of measured (n, 2n) values over the region 30 ≦ Z ≦ 82; it is shown that deviations of the calculated cross sections from the respective measured values become larger as the energy needed to emit the second neutron increases, probably because fewer exit channels are available and the continuum assumption in the statistical model of the compound nucleus becomes invalid.

The shape of the intense Au198 beta spectrum and the K conversion coefficient of the 412-keV transition

Abstract The beta decay of Pa233 was studied by means of high resolution permanent-magnet electron spectrographs, an iron free double-focussing beta spectrometer and a curved-crystal gamma ray spectrometer. Measured values of relative conversion electron and photon intensities are used to determine conversion coefficients and to assign multipolarities and multipole mixing ratios for the transitions in U233. The quantum assignments of the levels in U233 are considered, by comparisons of observed beta and photon branching ratios to those predicted by theory. Several new transitions were observed and some of these are used to postulate new levels in U233.

The structure of high spin states in 184Hg and 186Hg

From {gamma}-ray coincidence studies following spontaneous fission of {sup 252}Cf, direct measurements of yields and neutron multiplicities were made for Sr-Nd, Zr-Ce, Mo-Ba, Ru-Xe, and Pd-Te correlated pairs. A strong enhancement of the 7-10 neutron channels for Mo-Ba pairs is observed. A new fission mode associated with the enhanced neutron yields is identified. These data can be interpreted in terms of one or more of {sup 144,145,146}Ba being hyperdeformed at scission. Mean field calculations predict a hyperdeformed third minimum in {sup 252}Cf and an extremely deformed {sup 146}Ba fragment at scission. {copyright} {ital 1996 The American Physical Society.}