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The level structure of 7 Se was investigated via the decay of Br produced in the Ni ("O,pn) reaction. 32 transitions were assigned to Se on the basis of half-life, energy, and/ or relative y-ray yields for different beam energies with 18 transitions confirmed by coincidence data. 32 transitions are placed into the decay scheme which consists of the following levels: 862.0, 3225.9; and 3239.5 keV. The half-life of~~B r was measured to be 1.31+ 0.04 min. RADIOACTIVITY +Br; measured Tii2, E&, I&, pp coin. Se deduced levels, Jp.

The levels in 74 » 76 Kr were studied with in-beam y-spectroscopy techniques and the j3 + decay of 7G Rb. The energies of the 2 t + states in 74,76 Kr deviate from smooth behavior compared with the higher spin levels. The yrast cascade B (E2)'s are highly collective. The 74 » 76 Kr ground states have unusually large deformation. The origin of this deformation and of shape coexistence in this region is described in terms of the protons driving the deformationo PACS numbers: 23.20. Ck, 23.20.Lv, 27.50. + e The 0 2 + energies have a deep minimum in 7O,72Q G an( j 72,74g e anc j are near or below the 2 X + energies. 1 These and other data have led to various suggestions of shape coexistence in these nuclei, where the low-lying 0 2 + states are more deformed than the ground states. 1 " 4 However, questions have been raised about shape coexistence and deformation in these nuclei, in part because well-developed, deformed bands built on the 0 2 + states in 70,72 Ge are not seen. 1 * 3 In this paper we suggest that the origin of shape coexistence for iV«38 nuclei is related to the number of protons which delicately controls whether a deformed shape or near-spherical shape is lowest in nuclei in this region. Our 74 » 76 Kr data give evidence that their ground states have remarkably large deformation.The origin of strong deformation and shape coexistence in this region can be attributed to the gaps in the single-particle spectrum seen in Fig. 1514

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The Coulomb excitation of states in 229Th has been observed using 17.0-MeV 4He2+ ions. For the 5/2+[633] ground-state rotational band, we deduced an intrinsic quadrupole moment, Q20, of 8.816 ± 0.090 eb and an intrinsic hexadecapole moment, Q40, of 3.69 ± 0.72 eb2. This Q40 value for 229Th is 43% larger than that for the even-even neighbor, 230Th, and is the largest thus far observed in the actinide region. Large E3 matrix elements are extracted for states at 512, 562, and 611 keV, which supports the assignment of these states as members of a Kπ = 0− octupole-vibrational band built on the 5/2+[633] ground state. This 0− octupole excitation occurs at much higher energy than the one inferred from earlier decay studies that is built on the 3/2+[631] state. This suggests state-dependent octupole correlations consistent with octupole shape transitional models of 229Th.

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Precision Coulomb-excitation experiments using He ions have been performed in the actinide region {230~A -248) by the observation of elastic and inelastically scattered projectiles using a split-pole magnetic spectrometer equipped with a position-sensitive proportional detector. 12 even-A targets from 30Th to Cm have been investigated and the reduced quadrupole matrix element, (2lK(E2) l0), and the reduced hexadecapole matrix element, (4)(K (E4) ()0), have been determined from the experimental excitation probabilities of the 0+, 2+, and 4+ states in the ground-state rotational bands. The values of B(E4, 0 4) range from 167 single-particle units for 4U to essentially zero single-particle units for 2 4'2 8'24 Cm. Model-dependent deformation parameters, p20 and p40, are extracted from the measured E2 and E4 transition moments for distributions of nuclear charge represented by deformed Fermi distributions and by a deformed homogeneous distribution. P NUCI EAR REACTIONS 230 232Th (Qt & ) 238U (0 &I) E ] 6 and ] 7 MeV 234U 19 MeV 238U (& Otl) E 16 18 MeV 238, 240, 242, 244Pu (af Cm {e, n'), E =17 MeV; measured 0(E~i; 8=150) deduced B(E2), B(E4). Enriched targets. Extracted. model-dependent deformations, p20 and p40.