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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An in-beam study of 74Br has been carried out using y-ray spectroscopy in connection with the reactions (3He, p2n) and (d, 2n) on 74Se targets. Separate level schemes on top of the two/?-decaying isomers of 74Br are proposed. Two cascades connecting highspin states are assigned to populate the U =4 (+) isomer. In this level scheme, the excited states at 72.1,188.3 and 472.0 keV are found to have half-lives of 13.3, 0.7 and 0.7 ns, respectively. The low-spin isomer of 74Br is tentatively assigned as U = (0-). This state is populated by two other cascades. For the low-spin levels two-particle configurations of proton and neutron Nilsson orbitals are proposed. The high-spin states are tentatively interpreted as arising from the configurations (7~g9/2, vg9/2) and (=P3/z, Vgg/2). As a by-product, a cascade of four transitions has been observed during the 3He bombardment of V4Se and tentatively assigned to negative-parity states in 75Kr.

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