The neutron-rich nuclei 94;96 Kr were studied via projectile Coulomb excitation at the REX-ISOLDE facility at CERN. Level energies of the first excited 2 þ states and their absolute E2 transition strengths to the ground state are determined and discussed in the context of the Eð2 þ 1 Þ and BðE2; 2 þ 1 ! 0 þ 1 Þ systematics of the krypton chain. Contrary to previously published results no sudden onset of deformation is observed. This experimental result is supported by a new proton-neutron interacting boson model calculation based on the constrained Hartree-Fock-Bogoliubov approach using the microscopic Gogny-D1M energy density functional. DOI: 10.1103/PhysRevLett.108.062701 PACS numbers: 25.70.De, 27.60.+j, 29.30.Kv, 29.38.Gj Since the availability of high-intensity radioactive ion beams, the extension of the concept of quantum phase transitions to exotic nuclei is of great interest in nuclear physics [1]. Quantum phase transitions occur in atomic nuclei as a function of the number of protons or neutrons and describe changes of the ground-state shapes [2]. The so-called A % 100 mass region of the nuclear chart around 100 Zr is one of the most popular regions for the study of this phenomenon since the zirconium (Z ¼ 40) and strontium (Z ¼ 38) isotopes undergo a shape transition from almost spherical to strongly deformed shapes when going from neutron number N ¼ 58 to N ¼ 60 [3][4][5][6][7]. This

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In the EXILL campaign a highly efficient array of high purity germanium (HPGe) detectors was operated at the cold neutron beam facility PF1B of the Institut Laue-Langevin (ILL) to carry out nuclear structure studies, via measurements of γ-rays following neutron-induced capture and fission reactions. The setup consisted of a collimation system producing a pencil beam with a thermal capture equivalent flux of about 108 n s−1cm−2 at the target position and negligible neutron halo. The target was surrounded by an array of eight to ten anti-Compton shielded EXOGAM Clover detectors, four to six anti-Compton shielded large coaxial GASP detectors and two standard Clover detectors. For a part of the campaign the array was combined with 16 LaBr3:(Ce) detectors from the FATIMA collaboration. The detectors were arranged in an array of rhombicuboctahedron geometry, providing the possibility to carry out very precise angular correlation and directional-polarization correlation measurements. The triggerless acquisition system allowed a signal collection rate of up to 6 × 105 Hz. The data allowed to set multi-fold coincidences to obtain decay schemes and in combination with the FATIMA array of LaBr3:(Ce) detectors to analyze half-lives of excited levels in the pico- to microsecond range. Precise energy and efficiency calibrations of EXILL were performed using standard calibration sources of 133Ba, 60Co and 152Eu as well as data from the reactions 27Al(n,γ)28Al and 35Cl(n,γ)36Cl in the energy range from 30 keV up to 10 MeV.

An array of eight Ge detectors for coincidence measurements of γ rays from neutroncapture reactions has been constructed at the PF1B cold-neutron facility of the Institut Laue-Langevin. The detectors arranged in one plane every 45 • can be used for angular correlation measurements. The neutron collimation line of the setup provides a neutron beam of 12 mm in diameter and the capture flux of about 10 8 /(s×cm 2 ) at the target position, with a negligible neutron halo. With the setup up to 10 9 γγ and up to 10 8 triple-γ coincidence events have been collected in a day measurement. Precise energy and efficiency calibrations up to 10 MeV are easily performed with 27 Al(n, γ) 28 Al and 35 Cl(n, γ) 36 Cl reactions. Test measurements have shown that neutron binding energies can be determined with an accuracy down to a few eV and angular correlation coefficients measured with a precision down to a percent level. The triggerless data collected with a digital electronics and acquisition allows to determine half-lives of excited levels in the nano-to microsecond range. The high resolving power of double-and triple-γ time coincidences allows significant improvements of excitation schemes reported in previous (n, γ) works and complements high-resolution γ-energy measurements at the double-crystal Bragg spectrometer GAMS of ILL.

Lifetimes of intermediate-spin states in two rotational bands of 99 Zr have been measured. These states were populated following the neutron-induced fission of 235 U at the PF1B beamline of the Institut Laue-Langevin, Grenoble, during the EXILL-FATIMA campaign. The nucleus 99 Zr 59 exhibits shape coexistence and lies precisely on the border of an abrupt change in ground-state deformation when going from N = 58 to N = 60, making its study interesting for understanding the mechanisms involved in the rapid onset of deformation here. The B(E 2) values extracted for decays in the ν3/2[541] band allow quadrupole deformations of β 2 = 0.34(1) and 0.26(3) to be determined for the 821.6-and 1236.6-keV members, whereas β 2 = 0.32(3) was found for the 850.5-keV member of the ν3/2 [411] band. Some of the excited states known in 99 Zr have been reasonably described with interacting boson-fermion model (IBFM) calculations. Type-II shell evolution is proposed to play a major role in modifying single-particle energies in 99 Zr.