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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, 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.

The energies of the yrast levels extending up to I = 28 in even-even actinide nuclei were analyzed in terms of an angular velocity expansion E rot ( I ) = ∑ i =1 n α i ω I 2 i with n = 2−7 to test the application of this expansion to high spin. A strong ω 2 -dependence is observed for the Variable Moment of Inertia model (VMI) with two parameters α i ( ω 2 ), particularly at the higher spins. There is a marked difference in the ω 2 -dependence, of these parameters between the N = 142 and other nuclei studied in this region to indicate structure effects are present at the higher spin.

Rotational bands with K π = 1 + in the odd-odd nuclei 100,102 39 Y and 102,104 41 Nb are postulated from β − decay of t heir even-even parents. Each two-quasiparticle band has a moment of inertia very close to that of a rigid spheroidal nucleus with deformation β ∼ 0.3, hence these bands are nearly “pairing free”.

Rotational side-bands in 162Dy have been studied using the 160Gd(α, 2nγ)162Dy reaction. Seven side-bands are observed, with Kπ = 2+, 2−, (0)−, 0+, 5−, 4+ and (6−). Four of these bands have collective structure at low spin: the Kπ = 2+ γ-vibrational band, the Kπ = 0+ β-vibrational band, and the Kπ = 2− and (0)− octupole vibrational bands. Of the remaining bands, the 4+ band is deformation coupled while the 5− and (6−) bands are rotation-aligned. Several bandcrossings are observed in this nucleus. The β and γ-bands are crossed at I = 6 h and 12 h , respectively, by a highly aligned ( i 13 2 )2 S-band; extrapolation of this S-band to higher spin suggests that it crosses the g.s.b. between I = 18 h and 20 h . The 2− octupole band is crossed by the 5− band at I = 9 h and again by the (6−) band at I = 12 h . The latter bandcrossings are discussed in terms of two-quasiparticle plus rotor calculations.

The backbending or irregularities of band spacings of states near the yrast line were fit using simple rotational energy expressions which include band mixing of the yrast and yrare states. In our model the sharp backbending requires two large mixing terms, a spin-independent term arising from the pairing interaction and a Coriolis coupling term. Least-squares fitting of backbenders in the deformed even-even nuclei indicate a near cancellation of the mixing matrix elements near the virtual band crossing. Although the idealized and simplified expressions for the backbending spin in terms of the pairing gap Δ, high- j orbital spacing, high- j Coriolis matrix element J tT , and moment of inertia I may not be quantitatively correct, they do indicate the general conditions required for backbending.

Systematic examinations of the angular momentum generated by particle alignment were made for Dy and Er isotopes. Marked deviations are observed from the aligned spin values expected for full alignment of i 13 2 ( odd A) and ( i 13 2 ) 2 ( even A) particles. Suggested origins of these differences are presented.

In certain three-particle states of odd-A with A ∼ 100−110 and specific [p1p2n1] or [p1n1n2] configurations, the combination of a strong blocking effect and a particularly strong p-n coupling may cause the total collapse of pairing (2Δ → 0). The energy of such a level may sharply drop well below the pairing gap energy to occur near to the ground state. Evidence is presented that the well-known J π = 7 2 + anomalous isomeric states in odd-A 45Rh and 47Ag nuclei with Eexc {[ p 1 ( g 9 2 ) n 1 ( g 7 2 )] 1 + n 2 (r00 5/2 ) .