D. Shi

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

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.

Octupole deformation in 142,143Ba and 144Ce: new band structures in neutron-rich Ba-isotopes

Abstract New, high spin band structures are established for the neutron-rich nuclei 142,143,145,147Ba, and 144Ce, and levels of 144,146Ba extended to higher spins from the study of γ-γ and γ-γ-γ coincidence studies in spontaneous fission. Alternating parity sequences connected by strong electric dipole transitions are identified in 142,143Ba and 144Ce but not in 145,147Ba to confirm theoretical predictions of stable octupole deformation for N = 86.

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

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.}

Identification of 152Ce and unexpected variations in moments of inertia with neutron number and spin in 142-148Ba, 144-152Ce and 146-156Nd

Levels in 152Ce are identified for the first time along with new high spin states in 142-148Ba, 144-150Ce and 150-156Nd from X-X, gamma -X and gamma - gamma - gamma -ray coincidence measurement in spontaneous fission of 252Cf and 242Pu. The N=88, 90 Ba nuclei have very nearly identical J1 for the yrast bands (2+ to 10+) and for the octupole bands (3- to 15-). The N=92, 94 Nd nuclei have essentially identical yrast J1 (0+ to 16+).

Structure of neutron-rich nuclei and neutron multiplicities in spontaneous fission

Abstract New insights into the fission process and the structure of neutron-rich nuclei have been gained by measuring γ-γ and γ-γ-γ coincidences of the prompt γ-rays following the spontaneous fission of 252Cf. The data were taken with the Compton Suppression Spectrometer System at HHIRF. New high spin levels were observed in 143,145,147Ba Only in 143Ba are intertwined bands observed with the characteristics of the octupole structure found only in 144Ba. New high spin states observed in the N = 84 136 Te, 138 Xe, 140 Ba nuclei exhibit a symmetry pattern suggestive of a pseudospin f - shell . For the first time in fission, the relative yields and neutron multiplicities were measured directly. Yields and multiplicities were measured directly for the ZrCe and MoBa correlated fragment pairs by observing the coincidences between the γ-rays in each fragment. Zero-neutron to ten-neutron emission yields were determined; the latter for the first time. The zero-neutron emission can be understood as a new example of cluster radioactivity. These results demonstrate that the use of the high resolution γ-γ-γ coincidence technique employed here for the first time opens up a new approach to the study of the fission process that will provide previously inaccessible details of the process.

New Spontaneous Fission Mode for {sup {bold 252}}Cf: Indication of Hyperdeformed {sup {bold 144,145,146}}Ba at Scission

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.}

New Spontaneous Fission Mode for252Cf: Indication of Hyperdeformed144,145,146Baat Scission

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.}

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

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.}

Neutron Multiplicities in Spontaneous Fission and Nuclear Structure Studies

New insights into the fission process can be gained by better quantitative knowledge of how the energy released in fission is distributed between the kinetic energy of the two fragments, the excitation energy of the two fragments and the number of neutrons emitted. Studies of prompt gamma-rays emitted in spontaneous fission (SF) with large arrays of Compton suppressed Ge detector arrays are providing new quantitative answers to longstanding questions concerning fission as well as new insights into the structure of neutron-rich nuclei. For the first time the triple gamma coincidence technique was employed in spontaneous fission studies. Studies of SF of 252Cf and 242Pu have been carried out. These γ-γ-γ data provide powerful ways to identify uniquely gamma rays from a particular nucleus in the very complex gamma-ray spectra given off by the over 100 different nuclei produced. The emphasis of this paper is on the first quantitative measurements of the multiplicities of the neutrons emitted in SF and the energy levels populated in the fragments. Indeed, in the break up into Mo-Ba pairs, we have identified for the first time fragments associated with from zero up to ten neutrons emitted and observed the excited energy states populated in these nuclei. The zero neutron emission pairs like 104Mo - 148Ba, 106Mo - 146Ba and 104Zr - 148Ce observed in this work are particularly interesting because they represent a type of cold fission or a new mode of cluster radioactivity as proposed by Greiner, Sandulescu and co-workers1,2. These data provide new insights into the processes of cluster radioactivity and cold fission.