Shao-Jiang Zhu

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 Srue5f8Nd, Zrue5f8Ce, Moue5f8Ba, Ruue5f8Xe and Pdue5f8Te nuclei. Neutron multiplicities from 0–10 v emission (10v for the first time) were observed in Moue5f8Ba correlated pairs with the 0 and 7–10 neutron-emission yields enhanced compared to gross yields for all fragments. The Moue5f8Ba data provide evidence for two different fission modes. By unfolding the observed Moue5f8Ba 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, 108Moue5f8144Ba, 107Moue5f8145Ba, or 106Moue5f8146Ba 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.

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.

Shape trends and triaxiality in neutron-rich odd-mass Y and Nb isotopes

New level schems of Y and Nb isotopes are proposed based on measurements of prompt gamma rays from 252Cf fission at Gammasphere. Shape trends regarding triaxiality and quadrupole deformations are studied.

Signature of the shape coexistence in 72Kr: discontinuities of the moment of inertia at low spin

Abstract The low-lying level structure of the N=Z nucleus 72Kr was studied by 40 Ca ( 35 Cl, p , 2 n γ) 72 Kr at 95 MeV. Gamma ray coincidences are used to construct for the first time a level scheme for this nucleus based on coincidence gating with the previously identified 2+−0+ transition. The new level scheme indicates strong prolate-oblate mixing of two shape coexisting bands. The results support theoretical predictions of a stable oblate minimum in this region.

IBM-1 description of the fission products 108,110,112Ru

Abstract IBM-1 calculations for the fission products 108,110,112 Ru have been carried out. The even–even isotopes of Ru can be described as transitional nuclei situated between the U(5) (spherical vibrator) and SO(6) ( γ -unstable rotor) symmetries of the interacting Boson Model. At first, a Hamiltonian with only one- and two-body terms has been used. Excitation energies and B (E2) ratios of gamma transitions have been calculated. A satisfactory agreement has been obtained, with the exception of the odd–even staggering in the quasi- γ bands of 110,112 Ru. The observed pattern is rather similar to the one for a rigid triaxial rotor. A calculation based on a Hamiltonian with three-body terms was able to remove this discrepancy. The relation between the IBM and the triaxial rotor model was also examined.

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+).

High-spin states in neutron-rich even-even Pd isotopes

Transitions among high-spin states in 112,114,116Pd are observed through prompt -ray spectroscopy following the spontaneous fisson of 252Cf. Spins and parities are assigned to many newly observed levels based on angular correlation analysis and comparison with known spins and parities of less neutron-rich Pd nuclides. The -band structures are significantly extended. New rotational band structures are observed for the first time, and tentatively assigned negative parity. IBA-1 model calculations are performed for the ground state and -bands.

Identification of 181Hg and shape coexistence in odd-A Hg isotopes

Abstract In-beam γ-ray transitions in 181 Hg, the lightest odd-A Hg isotope known thus far, have been identified from fragment mass-γ and γ-γ coincidence measurements. Five prolate deformed rotational bands were placed in the level scheme. A decoupled band built on the strongly prolate deformed 1 2 − [521] ground state was observed up to 29 2 − . A 5 2 − [512] configuration is suggested for a pair of strongly coupled bands displaying no signature splitting. The other two bands are also signature partner bands. They are populated with the largest intensity and exhibit splitting. They have been associated with the mixed neutron i 13 2 orbitals and are proposed to decay to an i 13 2 isomeric state associated with an oblate state.

Search for the chiral doublet bands in 122Cs

High spin states in the odd–odd nucleus 122Cs have been studied by in-beam γ-ray spectroscopy with a 107Ag(19F,1p3n) fusion-evaporation reaction at a beam energy of 85 MeV. A previously known πh11/2 ⊗ νh11/2 yrast band was confirmed and its spin I was re-examined. A ΔI = 2 band was identified, which together with another known ΔI = 2 band forms a side band of the πh11/2 ⊗ νh11/2 yrast band. The two observed πh11/2 ⊗ νh11/2 bands are proposed as a pair of chiral doublet bands in 122Cs based on a systematic comparison with those in the neighbouring odd–odd Cs isotopes, as well as the relativistic mean field and the triaxial particle-rotor-model calculations.