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Lifetimes of excited states in the very exotic nuclides 108Zr68 and 110Zr70 were measured for the first time applying high-resolution in-beam γ-ray spectroscopy at relativistic energies. The experiment was carried out at the Radioactive Isotope Beam Factory in RIKEN, using nucleon removal reactions of radioactive Nb ions to produce 108Zr and 110Zr. Emitted γ rays from the excited states were measured by the High-resolution Cluster Array at RIBF (HiCARI) and lifetimes were extracted from the observed line shapes caused by relativistic Doppler-shift effects. In addition to the new lifetime information, the precision of the E(21+) and R4/2=E(41+)/E(21+) values of 110Zr could be significantly improved. The low-lying 22+ states in both nuclei are also newly observed, which are indicative of the triaxiality. The various physical quantities were compared with theoretical models to investigate the collectivity in Zr isotopes towards N=70. Together the experimental and theoretical results point at a reduction of the axial deformation while triaxial collectivity increases.

Detailed γ-ray spectroscopy of the exotic neon isotope 28Ne has been performed for the first time using the one-neutron removal reaction from 29Ne on a liquid hydrogen target at 240 MeV/nucleon. Based on an analysis of parallel momentum distributions, a level scheme with spin-parity assignments has been constructed for 28Ne and the negative-parity states are identified for the first time. The measured partial cross sections and momentum distributions reveal a significant intruder p-wave strength providing evidence of the breakdown of the N=20 and N=28 shell gaps. Only a weak, possible f-wave strength was observed to bound final states. Large-scale shell-model calculations with different effective interactions do not reproduce the large p-wave and small f-wave strength observed experimentally, indicating an ongoing challenge for a complete theoretical description of the transition into the island of inversion along the Ne isotopic chain.

The nucleus is a quantum many-body complex system governed by the nuclear force, and it is prone to global changes such as deformation, rotation, vibration, fission, and clustering. In the past >30 a, we have witnessed the rapid expansion of the experimentally attainable nuclear chart and new discoveries and breakthroughs in studies on unstable nuclei. Examples include the halo nuclei and the associated exotic structural phenomena, the shell evolution observed using in-beam γ spectroscopy through the application of the achromatic magnetic spectrometer, the measurement of the basic properties of unstable nuclei, and the discovery of new magic numbers and rich phenomena in multi-nucleon correlations along with the formation of clusters and molecules. In the coming years, the expanded area of the nuclear chart—particularly the medium-heavy-mass neutron-rich region—will be the host of extreme exotic structures, the astrophysical r-process, and the reaction pathways to reach the superheavy island. Therefore, many new-generation radioactive ion-beam facilities are under development worldwide, and essential breakthroughs are foreseen.

Situated in the so-called “island of inversion,” the nucleus 32Mg is considered as an archetypal example of the disappearance of magicity at N=20. We report on high statistics in-beam spectroscopy of 32Mg with a unique approach, in that two direct reaction probes with different sensitivities to the underlying nuclear structure are employed at the same time. More specifically, states in 32Mg were populated by knockout reactions starting from 33Mg and 34Si, lying inside and outside the island of inversion, respectively. The momentum distributions of the reaction residues and the cross sections leading to the individual final states were confronted with eikonal-based reaction calculations, yielding a significantly updated level scheme for 32Mg and spin-parity assignments. By fully exploiting observables obtained in this measurement, a variety of structures coexisting in 32Mg was unraveled. Comparisons with theoretical predictions based on shell-model overlaps allowed for clear discrimination between different structural models, revealing that the complete theoretical description of this key nucleus is yet to be achieved.

Abstract Excited states in 63,65,67Mn were studied via in-beam γ -ray spectroscopy following knockout reactions from 68Fe. Similar level schemes, consisting of the 11/2−, 9/2−, 7/2− and 5/2 g. s. − level sequence, connected by I → I − 1 transitions, were established, the first time for 65,67Mn. Their level structures show features consistent with strongly-coupled rotational bands with K = 5 / 2. State-of-the-art shell-model calculations with the modified LNPS effective interaction reproduce the observed levels remarkably well and suggest the dominance of 4-particle-4-hole neutron configurations for all the states. The data on the low-lying excited states of odd-mass 53−67Mn provide a textbook example of nuclear structure evolution from weak coupling through decoupling to strong coupling along a single isotopic chain on the n-rich side of the β stability line. These results help to deepen our understanding of the N = 40 “island of inversion”. [ABSTRACT FROM AUTHOR]

In beam geometries where a directed γ -ray beam hits the surface of a coaxial high purity germanium detector (HPGe), the detector efficiency is sensitive to the position where γ -rays initially hit the detector surface because the structure of the detector is nonuniform. This may cause inaccuracy of the detector efficiency when measured using standard sources that are point-like sources emitting γ -rays isotropically. Obtaining a precise estimation of the full energy peak efficiency of the coaxial HPGe detector in the beam geometry for on-axis and off-axis measurements requires a Monte Carlo simulation. We performed Monte Carlo simulations that calculate the detector efficiency in the beam geometry. The effects of the off-axis distance and γ -ray beam size on the efficiency are quantitatively analyzed. We found that the intrinsic efficiency in the beam geometry is maximized when the beam hits the detector at specific off-axis distances. Our Monte Carlo calculations have been supported by nuclear resonance fluorescence experiments using laser Compton scattering γ -ray beams. • Calculating efficiency of coaxial HPGe detector for directed γ-ray beams by Geant4. • Revealing the sensitivity of efficiency to γ-ray beam size and off-axis distance. • Off-axis efficiency is higher than on-axis efficiency, depending on γ-ray energy. • Maximizing efficiency by selecting the optimum off-axis distance. • LCS γ-ray beams up to 3 MeV validate calculations experimentally. [ABSTRACT FROM AUTHOR]

The TRIUMF-ISAC Gamma-Ray Escape Suppressed Spectrometer is a high energy-resolution, high efficiency γ-ray detector array for radioactive beam experiments. It consists of high-purity germanium detectors with scintillator suppressors and waveform digitization. TIGRESS can operate in modes optimized for either maximum efficiency or for high peak-to-total, and can switch between these modes within a day. The digitized waveforms provide 6 mm FWHM position resolution for incident 137Cs γ rays. The array hosts a suite of auxilliary detectors for charged particles and neutrons. [ABSTRACT FROM AUTHOR]

Abstract: A detector has been designed and built to assist in-beam γ-ray spectroscopy with fusion–evaporation reactions. It measures with high efficiency the evaporation residues that recoil out of a thin target into the angular interval from 1.8° to 9.0° at an adjustable distance of 1000–1350mm from a target, in coincidence with γ-rays detected in a Ge-detector array. This permits filtering of such γ-rays out of a much stronger background of other reaction products and scattered beam. Evaporation residues are identified by their time-of-flight and the pulse height using a pulsed beam. The velocity vector of the γ-emitting recoil is also measured in the event-by-event mode, facilitating to correct the registered γ-ray energy for the Doppler shift, with the resulting significant improvement of the energy resolution. The heavy-ion detection scheme uses emission of secondary electrons caused by the recoiling ions when hitting a thin foil. These electrons are then electrostatically accelerated and focused onto a small scintillator that measures the summed electron energy, which is proportional to the number of electrons. The detector is able to operate at high frequency of the order of 1MHz and detect very heavy nuclei with as low kinetic energy as 5MeV. The paper describes the properties of the detector and gives examples of measurements with the OSIRIS, GAREL+ and EUROBALL IV γ-ray spectrometers. The usefulness of the technique for spectroscopic investigations of nuclei with a continuous beam is also discussed. [Copyright &y& Elsevier]

The high-spin states of157Yb have been studied via the reaction of144Sm(16O, 3n) at16O energy of 90 MeV using techniques of in-beam γ-ray spectroscopy. Measurement of γ-γ- t coincidences was performed with 11 BGO(AC)HPGe detectors. Based on the γ-γ coincidence relationships and the measured results of γ-ray anisotropies and DCO ratios, the level scheme for157Yb was established. The shape co-existence and structural evolution of the vi13/2 band with increasing angular momentum in157Yb have been discussed. The systematics of the vi13/2 bands in the N=87 odd-A isotones have been compared. [ABSTRACT FROM AUTHOR]