[en] Mass measurements of ^96,97Kr using the ISOLTRAP Penning-trap spectrometer at CERN-ISOLDE are reported, extending the mass surface beyond N=60 for Z=36. These new results show behavior in sharp contrast to the heavier neighbors where a sudden and intense deformation is present. We interpret this as the establishment of a nuclear quantum phase transition critical-point boundary. The new masses confirm findings from nuclear mean-square charge-radius measurements up to N=60 but are at variance with conclusions from recent gamma-ray spectroscopy.

[en] Prompt γ-ray spectroscopy of the neutron-rich ⁹⁶Kr, produced in transfer-and fusion-induced fission reactions, has been performed using the combination of the Advanced γ Tracking Array and the VAMOS ++ spectrometer. A second excited state, assigned to J_π = 4⁺, is observed for the first time, and a previously reported level energy of the first 2⁺ excited state is confirmed. The measured energy ratio R_4/2 = E(4⁺)/E(2⁺) = 2.12(1) indicates that this nucleus does not show a well-developed collectivity contrary to that seen in heavier N = 60 isotones. This new measurement highlights an abrupt transition of the degree of collectivity as a function of the proton number at Z = 36, of similar amplitude to that observed at N = 60 at higher Z values. A possible reason for this abrupt transition could be related to the insufficient proton excitations in the g_9/2, d_5/2, and s_1/2 orbitals to generate strong quadrupole correlations or to the coexistence of competing different shapes. An unexpected continuous decrease of R_4/2 as a function of the neutron number up to N = 60 is also evidenced. This measurement establishes the Kr isotopic chain as the low-Z boundary of the island of deformation for N = 60 isotones. A comparison with available theoretical predictions using different beyond mean-field approaches shows that these models fail to reproduce the abrupt transitions at N = 60 and Z = 36. (authors)

No abstract available

[en] Low lying excited states of the nuclei ⁹⁴Kr and ⁹⁶Kr were studied after nucleon removal reactions at intermediate beam energies at the RIKEN Radioactive Isotope Beam Factory. Previously unknown γ transitions could be observed for both nuclei. The established level schemes confirm the already observed smooth onset of collectivity which is contrary to the findings in the zirconium and strontium isotopic chains. Additionally, the spectroscopic data show evidence for low-lying 0₂⁺ and 2₂⁺ states in ⁹⁶Kr. This indication of shape coexistence in the neutron rich krypton isotopes is in agreement with IBM-2 calculations based on the self-consistent beyond-mean-field approach using the Gogny D1S interaction.

No abstract available

[en] Nuclei in the neutron-rich A∝100 mass region are well suited for the understanding of the development of collectivity. By adding only a few neutrons to the N=50 shell closure, collective effects can quickly occur. For the Z=40 (Zr) isotopes, N=56 becomes an effective shell closure, so that ⁹⁶Zr is quoted as a doubly-magic nucleus. Adding only a few neutrons more, the Zr-isotopes get strongly deformed. This behavior indicates a shape phase transition at N=60 from spherical to deformed shapes. For the Z=38 (Sr) and Z=42 (Mo) isotopes the systematics show a similar behavior, whereas for the Z=44 (Ru) isotopes, this rapid change of shape seems to be attenuated. The aim of our work was to investigate the behavior of the Z=36 (Kr) isotopes in this phase transition region by determining the energies of the 2⁺₁ states and their E2 decay transition strengths to the ground state in ⁹⁴Kr (N=58) and ⁹⁶Kr (N=60). Therefore, we performed two experiments at REX-ISOLDE at CERN, utilizing the high-efficiency MINIBALL γ-ray spectrometer and analyzing the emitted γ-rays and scattered particles after the Coulomb-excitation reactions ¹⁹⁶Pt(⁹⁴Kr, ⁹⁴Kr^*)¹⁹⁶Pt^* and ¹⁹⁶Pt(⁹⁶Kr, ⁹⁶Kr^*)¹⁹⁶Pt^*. We show and discuss the preliminary results.

No abstract available

[en] We report on the study of excited states in "9"2,"9"4,"9"6Kr populated via projectile Coulomb excitation at safe energies. The radioactive ion beams at energies of 2.85 MeV/u were delivered by the REX-ISOLDE facility at CERN and impinged on self-supporting "1"9"4,"1"9"6Pt targets. The emitted γ-rays were detected by the Miniball detector-array. A detailed description of the experimental techniques used for extracting diagonal and transitional matrix elements and of the theoretical framework is given. The present experiment reveals the moderate evolution of the collective structure in the considered neutron-rich Kr isotopic chain, which is supported by the interacting boson model combined with the self-consistent mean-field method using a microscopic Gogny energy-density functional. The theory also suggests possible shape coexistence in the exotic nucleus "9"6Kr. (authors)

[en] Short communication. 2 refs

[en] Low-lying excited states of the neutron-rich ^94,95,96Kr were measured at the RIBF at the RIKEN Nishina Center for Accelerator-Based Science as part of the SEASTAR campaign. The nuclei of interest were populated in nucleon knockout reactions using the MINOS device surrounded by the DALI2 array for prompt γ-spectroscopy. Additionally, the EURICA array was run in a parasitic mode to measure the delayed isomeric transitions in ⁹⁵Kr. In all three nuclei, previously unknown γ-transitions were observed. Even-even Sr and Zr nuclei in the A=100 region show a sudden onset of deformation at N=60 while the lighter isotopes up to N=58 are rather spherical. Contrarily, the even krypton isotopes exhibit a smooth onset of collectivity up to N= 0. For ⁹⁶Kr, the measured new transitions imply the existence of low-lying low-spin non-yrast states, which we interpret as the shape coexisting states becoming yrast above N=60 in agreement with very recent IBM calculations. For ⁹⁵Kr, the analysis of prompt γ-radiation with and without coincidence of delayed radiation identified the prompt γ-rays as either feeding or bypassing the known isomeric state in ⁹⁵Kr.