S. J. Zhu

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.

Cold binary and ternary fragmentations in spontaneous fission of252Cf

SummaryThe phenomenon of cold (neutronless) binary and ternary fission in spontaneous fission of252 Cf was experimentally observed by triple gamma coincidence technique with Gammasphere with 72 gamma-ray detectors. Many correlated pairs for both binary and ternary (4He,10Be) fission were observed in the spontaneous fission of252 Cf. Yields of cold ternary and cold binary fission were extracted. These results are compared with the theoretical calculations using M3Y nucleon-nucleon potential and WKB approximation for barrier penetration.

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.

Collective Bands in 104,106,108Mo

We have used our analysis of γ-γ-γ data (5.7 × 1011 triples and higher folds) taken with Gammasphere from prompt γ rays emitted in the spontaneous fission of 252Cf to study the collective bands in 104,106,108Mo. The one-phonon and two-phonon γ-vibrational bands and known two-quasiparticle bands in neutron-rich 104,106Mo were extended to higher spins. The one-and two-phonon γ-vibrational bands have remarkably close energies for transitions from the same spin states and identical moments of inertia. Several new bands are observed and are proposed as quasiparticle bands in 104,106Mo, along with the first β-type vibrational band in 106Mo. The quasiparticle bands have essentially constant moments of inertia near the rigid-body value that indicate blocking of the pairing interaction. Candidates for chiral doublet bands in 106Mo are strong. These are the first reported chiral vibrational bands in an even-even nucleus.

Identical and shifted identical bands

SummarySpontaneous fission of252Cm was studied with 72 large Compton suppressed Ge detectors in Gammasphere. New isotopes160Sm and162Gd were identified. Through X-ray-γ and γ-γ-γ coincidence measurements, level energies were established to spins 14+ to 20+ in152, 154, 15660Nd92, 94, 96156, 158, 16062Sm94, 96, 98 and160, 16264Gd96, 98. These nuclei exhibit a remarkable variety of identical bands and bands where the energies and moments of inertia are shifted by the same constant amounts for every spin state from 2+ to 12+ for various combinations of nuclei differing by 2n, 4n, 2p, 4p and α.

Identification of new collective bands in neutron-rich {sup 102}Zr

The level scheme of {sup 102}Zr has been expanded by measuring high-fold prompt {gamma}-ray coincidence events in the spontaneous fission of {sup 252}Cf with the Gammasphere array. Previous identifications have been confirmed. A two-quasi-particle band with a band head energy of 2926.4 keV has been identified for the first time, as well as two rotational collective bands, with band head energies of 1386.3 and 1652.7 keV, respectively. Spins and parties are assigned based on level systematics and angular correlation measurements.

Identification of {sup 162}Gd and a new type of identical bands

From γ-γ-γ coincidence measurements in spontaneous fission of 252Cf, level energies were established to spins 14+ to 20+ in many neutron-rich nuclei. New isotope 162Gd was identified. Yrast bands of 152,154,156Nd, 156,158,160Sm, and 160,162Gd exhibit near-identical transition energies and moments of inertia (MOI) shifted by the same constant amounts for every spin state from 2+ to 12+ or 14+ for nuclei differing by 2n, 4n, 2p, 4p, α, α+2p, and α+2n. These shifted identical bands (SIB) are a new phenomenon. Analysis of all known even-even proton- to neutron-rich nuclei from Ba(Z=56) to Os(Z=76) reveal no SIB for proton-rich nuclei and few cases of SIB for the most neutron-rich pairs around N=98–102 separated by 2n and 2p.

A new phenomenon—shifted identical bands and new region of stable octupole deformation

SummaryFrom γ-γ-γ coincidence measurements in the spontaneous fission of252Cf, level energies were established for spins 14+ to 20+ in many neutron-rich nuclei. New isotopes160Sm and162Gd were identified. The yrast bands of152,154,156Nd,156,158,160Sm, and160,162Gd exhibit remarkably near-identical transition energies and moments of inertia which are shifted by the same constant amounts for every spin state from 2+ to 12+ or 14+ for nuclei differing by 2n, 4n, 2p, 4p, α, α+2p, and α+2n. These shifted identical bands (SIB) are a new phenomenon. Analysis of all known even-even proton- to neutron-rich nuclei from Ba (Z = 56) to Os (Z = 76) reveal no SIB for proton-rich nuclei and only a few additional cases of SIB for the most neutron-rich pairs aroundN = 98–102 separated by 2n and 2p. The levels in8107Mo were extended and levels in109Mo identified for the first time.Conversion coefficient measurements and level patterns help establish a new region of stable octupole deformation for107,109Mo withN = 65, 67, associated with theν(d5/2-h11/2) orbitals. This is the first region based on only one pair of orbitals.

Collective Motions in A = 100-112 Nuclei

With a very high statistics data set with 5.7×1011 triple and higher fold γ coincidences from the spontaneous fission of 252Cf, a wealth of fascinating new collective motions in nuclei with A = 100–112 have been observed. Nothing was known previously about one‐ and two‐phonon γ‐bands in odd‐A nuclei. We discovered the first one‐and two‐ phonon γ vibrational bands in an odd N‐even Z nucleus, 105Mo. The energies of the one‐ and two‐ phonon bands in 104Mo, 105Mo, 106Mo and 108Mo were recently studied and are remarkably similar in energies. The first chiral doublet bands in e‐e nuclei were observed in triaxial 106,108Mo and 110,112Ru while such chiral band in 108Ru are perturbed by its γ‐soft shape. The experimental evidences, including our recent angular correlation studies are presented along with what these new bands are telling us about the evolving collective structures and deformation in this region.

Stable and vibrational octupole modes in neutron rich nuclei

Evidence for stable octupole deformation in neutron-rich nuclei bounded by Z=54–58 and N=85–92 is described. More vibrational type octupole modes are observed to either side of this region in 140Ba and 152,154Nd. The largest stable octupole deformation (β3∼0.1) is reported in 144Bagg. The theoretically predicted quenching (β3→0) of stable octupole deformation with increased rotation is observed in 146Ba. There is good agreement between theory and experiment for the strongly varying electric dipole moments as a function of mass for 142–148Ba. In odd-A 143Ba and odd-Z, 145La, two pairs of positive and negative parity bands with opposite spins, parity doublets are observed. In 145La the ground band with symmetric shape coexists with the asymmetric octupole shape which stabilizes above about spin 19/2+. The isotope 109Mo was identified and a new region of stable octupole deformation is identified in 107,109Mo centered around N=64–66 as earlier predicted. This is the first case of stable octupole deformation i...