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Rapid shape changes are observed for neutron-rich nuclei with A around 100. In particular, a sudden onset of ground-state deformation is observed in the Zr and Sr isotopic chains at N=60: low-lying states in N≤58 nuclei are nearly spherical, while those with N≥60 have a rotational character. Nuclear lifetimes as short as a few ps can be measured using fast-timing techniques with LaBr 3 (Ce)-scintillators, yielding a key ingredient in the systematic study of the shape evolution in this region. We used neutron-induced fission of 241 Pu and 235 U to study lifetimes of excited states in fission fragments in the A∼100 region with the EXILL-FATIMA array located at the PF1B cold neutron beam line at the Institut Laue-Langevin. In particular, we applied the generalized centroid difference method to deduce lifetimes of low-lying states for the nuclei 98 Zr (N=58), 100 Zr and 102 Zr (N≥60). The results are discussed in the context of the presumed phase transition in the Zr chain by comparing the experimental transition strengths with the theoretical calculations using the Interacting Boson Model and the Monte Carlo Shell Model.

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A record number of 100 Sn nuclei was detected and new isotopic species toward the proton dripline were discovered at the RIKEN Nishina Center. Decay spectroscopy was performed with the high-efficiency detector arrays WAS3ABi and EURICA. Both the half-life and the β-decay end point energy of 100 Sn were measured more precisely than the literature values. The value and the uncertainty of the resulting strength for the pure 0 þ → 1 þ Gamow-Teller decay was improved to B GT ¼ 4.4 þ0.9 −0.7 . A discrimination between different model calculations was possible for the first time, and the level scheme of 100 In is investigated further.Sn and its neighboring nuclei comprise a unique testing ground for modern large scale shell model (LSSM) calculations with realistic nuclear interactions. 100 Sn is the heaviest doubly magic N ¼ Z nucleus that is particle stable and decays via a pure and very fast Gamow-Teller (GT) β decay. The 100 Sn region is located in the nuclear chart close to the end of the astrophysical rapid proton capture process path. Thus, it is of particular interest concerning fundamental challenges in both nuclear physics and astrophysics [1].According to the extreme single particle model (ESPM) [2], 100 Sn decays via a pure GT transition of a proton (π) from the completely filled π0g 9=2 orbital into a neutron (ν) in the empty spin-orbit partner, the ν0g 7=2 orbital of 100 In. The ESPM GT strength is predicted to be B GT ¼ 17.78 [1]. However, the experimental values obtained up to now are smaller: 9.1 þ3.0 −2.6 [3] and 5.8 þ5.5 −3.2 [4,5]. These experiments [3,5,6] revealed the smallest log(ft) value-even smaller than the values of nuclei which decay by a Superallowed Fermi decay-throughout the nuclear chart. However, the PHYSICAL REVIEW LETTERS 122, 222502 (2019) 0031-9007=19=122 (22)=222502(6) 222502-1

Quantum electrodynamics in very strong Coulomb fields is one scope which has not yet been tested experimentally with sufficient accuracy to really determine whether the perturbative approach is valid. One sensitive test is the determination of the 1s Lamb shift in highly-charged very heavy ions. The 1s Lamb shift of hydrogen-like lead (Pb 81+ ) and gold (Au 78+ ) has been determined using the novel detector concept of silicon microcalorimeters for the detection of hard x-rays. The results of  260 53 ( ) eV for lead and  211 42 () eV for gold are within the error bars in good agreement with theoretical predictions. To our knowledge, for hydrogen-like lead, this represents the most accurate determination of the 1s Lamb shift.

Half-lives and energies of γ rays emitted in the decay of isomeric states of nuclei in the vicinity of the doubly magic 100 Sn were measured in a decay spectroscopy experiment at Rikagaku Kenkyusho (The Institute of Physical and Chemical Research) of Japan Nishina Center. The measured half-lives, some with improved precision, are consistent with literature values. Three new results include a 55-keV E2 γ ray from a new (4 + ) isomer with T 1/2 = 0.23(6) μs in 92 Rh, a 44-keV E2 γ ray from the (15 + ) isomer in 96 Ag, and T 1/2 (6 + ) = 13(2) ns in 98 Cd. Shell-model calculations of electromagnetic transition strengths in the (p 1/2 ,g 9/2 ) model space agree with the experimental results. In addition, experimental isomeric ratios were compared to the theoretical predictions derived with an abrasion-ablation model and the sharp cutoff model. The results agreed within a factor of 2 for most isomers. From the nonobservation of time-delayed γ rays in 100 Sn, new constraints on the T 1/2 , γ -ray energy, and internal conversion coefficients are proposed for the hypothetical isomer in 100 Sn.

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.

As a first step of the EXL project scheduled for the New Experimental Storage Ring at FAIR a precursor experiment (E105) was performed at the ESR at GSI. For this experiment, an innovative differential pumping concept, originally proposed for the EXL recoil detector ESPA, was successfully applied. The implementation and essential features of this novel technical concept will be discussed, as well as details on the detectors and the infrastructure around the internal gas-jet target. With 56 Ni(p, p) 56 Ni elastic scattering at 400 MeV u −1 , a nuclear reaction experiment with stored radioactive beams was realized for the first time. Finally, perspectives for a next-generation EXL-type setup are briefly discussed.