The neutron-rich rare isotope 190W is discussed as a candidate for a prolate-oblate transitional nucleus with maximum γ-softness. The collectivity of this isotope is assessed for the first time by the measurement of the reduced E2 transition probability of its first 2+ state to the ground state. The experiment employed the FAst TIming Array (FATIMA), comprised of 36 LaBr3(Ce) scintillators, which was part of the DESPEC setup at GSI, Darmstadt. The 41+ and 21+ states of 190W were populated subsequently to the decay of its 127(12) μs isomeric Jπ=10− state. The mean lifetime of the 21+ state was determined to be τ=274(28) ps, which corresponds to a B(E2;21+→01+) value of 95(10) W.u. The results motivated a revision of previous calculations within an energy-density functional-based interacting boson model-2 approach, yielding E2 transition properties and spectroscopic quadrupole moments for tungsten isotopes. From comparison to theory, the new data suggest that 190W is at the transition from prolate to oblate structure along the W isotopic chain, which had previously been discussed as a nuclear shape-phase transition.

Pulse shape discrimination of a liquid scintillator-based detector (BC501A) has been studied using a digital approach. The objective of this work is to develop a new neutron-gamma discrimination algorithm to improve the discrimination efficiency. The new approach is a combination of two parts. The first part uses an adaptive Sallen-Key filter to reduce noise in the pulse followed by traditional approaches of either charge comparison or time over threshold. The approach of adaptive Sallen-Key combined with time over threshold was found to have a much better capability to discriminate neutron events from that of gamma. The pulse shape discrimination performance obtained for a 5 inches by 5 inches (length by diameter) liquid scintillator detector using the combination of the adaptive Sallen-Key filter with time over threshold is found to be significantly higher compared to that obtained using traditional methods. Moreover, this combination has the potential for real-time hardware implementation.

Fission dynamics of ^188 Pt, a neutron deficient nucleus in newly discovered mass asymmetric sub-Pb region, have been explored via fission fragments mass-angle and mass-total kinetic energy distributions, at energies around and above the barrier. The observed correlations of fragment mass and emission angle indicate the presence of fission events originating from a non-equilibrated source. Mass-total kinetic energy spectra are relatively broader and the dependence of measured mean total-kinetic-energy on fragment mass is also broader than the expected parabolic dependence for liquid drop fission behaviour, at all studied energies. The measured mean total kinetic energy values are higher than the prediction of Viola systematics. The widths of measured total kinetic energy distributions are also inconsistent with the observed systematic behaviour of compound nucleus (CN) in this mass region. These observation of mass-angle and mass-total kinetic energy distributions revealed the clear signatures of the presence of slow quasifission in the fission of ^188 Pt compound nucleus. These findings indicate the role of entrance channel parameters such as mass asymmetry and charge product Z_P Z_T in Fusion-Fission and quasi-fission dynamics. Dynamical model calculations do not predict the presence of quasi-fission for the present reaction system.

Reduced transition probabilities have been extracted between excited, yrast states in the N=Z+2 nucleus 94Pd. The transitions of interest were observed following decays of the Iπ=14+, Ex=2129-keV isomeric state, which was populated following the projectile fragmentation of a 124Xe primary beam at the GSI Helmholtzzentrum für Schwerionenforschung accelerator facility as part of FAIR Phase-0. Experimental information regarding the reduced E2 transition strengths for the decays of the yrast 8+ and 6+ states was determined following isomer-delayed Eγ1−Eγ2−△T2,1 coincidence method, using the LaBr3(Ce)-based FATIMA fast-timing coincidence gamma-ray array, which allowed direct determination of lifetimes of states in 94Pd using the Generalized Centroid Difference (GCD) method. The experimental value for the half-life of the yrast 8+ state of 755(106) ps results in a reduced transition probability of B(E2:8→+6+) = 205−25+34 e2 fm4, which enables a precise verification of shell-model calculations for this unique system, lying directly between the N=Z line and the N=50 neutron shell closure. The determined B(E2) value provides an insight into the purity of (g9/2)n configurations in competition with admixtures from excitations between the (lower) N=3pf and (higher) N=4gds orbitals for the first time. The results indicate weak collectivity expected for near-zero quadrupole deformation and an increasing importance of the T=0 proton-neutron interaction at N=48.

We have measured the fission fragment mass-angle and mass-total kinetic energy (TKE) distributions for the neutron-deficient Pt-190 compound nucleus (CN) populated via C-12 + Hf-178 reaction, at around and above barrier energies. No mass-angle correlation was observed in the fission of Pt-190 signifying the absence of quasi-fission events in the studied reaction. The observed mass-TKE distributions have expected triangular shape and TKE distributions are well described with the single Gaussian fits, and mean TKE shows parabolic dependence on fragment mass as predicted based on liquid drop fission behaviour. The widths of measured TKE distributions agree well with the observed systematics for CN fission in this mass region. Though the CN is relatively neutron deficient, these observations suggest a clear picture of true CN fission behaviour for the chosen reaction in the studied energy domain.

Lifetime measurements of low-lying excited states in the semimagic (N=50) nucleus Rh95 have been performed by means of the fast-timing technique. The experiment was carried out using γ-ray detector arrays consisting of LaBr3(Ce) scintillators and germanium detectors integrated into the DESPEC experimental setup commissioned for the Facility for Antiproton and Ion Research () Phase-0, Darmstadt, Germany. The excited states in Rh95 were populated primarily via the β decays of Pd95 nuclei, produced in the projectile fragmentation of a 850 MeV/nucleon Xe124 beam impinging on a 4g/cm2Be9 target. The deduced electromagnetic E2 transition strengths for the γ-ray cascade within the multiplet structure depopulating from the isomeric Iπ=21/2+ state are found to exhibit strong deviations from predictions of standard shell model calculations which feature approximately conserved seniority symmetry. In particular, the observation of a strongly suppressed E2 strength for the 13/2+→9/2+ ground state transition cannot be explained by calculations employing standard interactions. This remarkable result may require revision of the nucleon-nucleon interactions employed in state-of-the-art theoretical model calculations, and might also point to the need for including three-body forces in the Hamiltonian. Published by the American Physical Society 2024

The nuclear two-photon or double-gamma (2γ) decay is a second-order electromagnetic process whereby a nucleus in an excited state emits two gamma rays simultaneously. To be able to directly measure the 2γ decay rate in the low-energy regime below the electron-positron pair-creation threshold, we combined the isochronous mode of a storage ring with Schottky resonant cavities. The newly developed technique can be applied to isomers with excitation energies down to ∼100  keV and half-lives as short as ∼10  ms. The half-life for the 2γ decay of the first-excited 0^{+} state in bare ^{72}Ge ions was determined to be 23.9(6) ms, which strongly deviates from expectations.

Ever-lasting interest in the structure of 100 Sn and neighbouring nuclei is still well justified by the fact that it is the heaviest doubly-magic nucleus with N=Z. State-of-the-art experimental techniques involving stable and radioactive beam facilities have enabled access to these exotic nuclei. In particular, the analysis of experimental data obtained in two DESPEC experiments at GSI Darmstadt extends the information on the shell structure and its evolution towards N = Z = 50, and allows the study of seniority conservation and proton-neutron interaction in the g9/2 orbit. Several theoretical approaches for shell-model investigations are discussed and their predictive power assessed. The calculated systematics of the reduced transition probabilities for high- to medium-spin states in N∼Z isotopes with active g9/2 orbit is presented for the first time.

An experiment was performed at GSI with the objective of measuring theβ-intensity distribution in the decay of Hg, Au and Pt isotopes around N=126 using the total absorption gamma-ray spectroscopy technique. The aim is to benchmark theoretical models used to make predictions of half-life and neutron emission probabilities of exotic nuclei involved in the rapid neutron capture process, leading to the synthesis of very heavy elements. This paper presents some experimental details and the current status of the analysis.

The Monte-Carlo simulated response for gamma-ray detection of the FAst TIMing Array (FATIMA) for exploitation within the DEcay SPECtroscopy (DESPEC) experimental system at the FAIR Phase-0 facility at Darmstadt, Germany is presented. In this configuration, FATIMA consisted of 36 LaBr3(Ce) detectors surrounding the AIDA, position-sensitive charged-particle active stopper. The decay of the I-pi=8(+) isomer-fed decay cascade in Pd-96, measured in the first DESPEC experiment at the FAIR-0 facility was used to validate the simulations. The experimental data yielded in-situ full-energy peak efficiency values for FATIMA of 11.2(11)%, 6.8(7)%, 3.8(4)% and 2.1(4)% at 106, 325, 684 and 1415 keV respectively, consistent with the values derived from the simulated response.

Direct lifetime measurements via gamma-gamma coincidences using the FATIMA fast-timing LaBr3(Ce) array were performed for the excited states below previously reported isomers. In the N = 50 semi-magic 96Pd nucleus, lifetimes below the I pi = 8+ seniority isomer were addressed as a benchmark for further analysis. The results for the I pi = 2+ and 4+ states confirm the published values. Increased accuracy for the lifetime value was achieved for the 4+ state.

Direct lifetime measurements via gamma -gamma coincidences using a fast timing detector array consisting of LaBr3(Ce) scintillators has been applied to determine the lifetime of low-lying states in the semimagic (N = 50) nucleus Ru-94. The experiment was carried out as the first in a series of "FAIR-0" experiments with the DESPEC experimental setup at the Facility for Antiproton and Ion Research (FAIR). Excited states in Ru-94 were populated primarily via the beta-delayed proton emission of Pd-95 nuclei, produced in the projectile fragmentation of an 850 MeV/nucleon Xe-124 beam impinging on a 4 g/cm(2) Be-9 target. While the deduced E2 strength for the 2(+) -> 0(+) transition in the yrast cascade follows the expected behavior for conserved seniority symmetry, the intermediate 4(+) -> 2(+) transition exhibits a drastic enhancement of transition strength in comparison with pure-seniority model predictions as well as standard shell model predictions in the f pg proton hole space with respect to doubly magic Sn-100. The anomalous behavior is ascribed to a subtle interference between the wave function of the lowest seniority v = 2, I-pi = 4(+) state and that of a close-lying v = 4 state that exhibits partial dynamic symmetry. In addition, the observed strongly prohibitive 6(+) -> 4(+) transition can be attributed to the same mechanism but with a destructive interference. It is noted that such effects may provide stringent tests of the nucleon-nucleon interactions employed in state-of-the-art theoretical model calculations.

This paper reports results of the first experiment of the DESPEC Phase-0 campaign at GSI, which focused on the study of neutron-deficient nuclei approaching 100Sn. These data provide the first extended commissioning experiment for the DESPEC collaboration within NuSTAR. We present results on electromagnetic transition rates associated with the decays from excited states populated following the formation of I pi = 8+ proton 'seniority -isomer' states in the N = 50 isotones 94Ru and 96Pd. Direct half-life measurements via gamma-gamma coincidences using the FATIMA detector array consisting of 36 LaBr3(Ce) scintillators have determined the reduced matrix elements associated with decays between low-lying states in these semi-magic nuclei. The extracted half-lives for yrast I pi = 6+ and 4+ states in 96Pd and the 6+ state in 94Ru are consistent with the published, highest-precision values for these nuclei.

The HISPEC-DESPEC collaboration aims at investigating the struc-ture of exotic nuclei formed in fragmentation reactions with decay spectroscopymeasurements, as part of the FAIR Phase-0 campaign at GSI. This paper reportson first results of an experiment performed in spring 2021, with a focus on beta-decaystudies in the Po-Fr nuclei in the 220 < A <230 island of octupole deformationexploiting the DESPEC setup. Ion-beta correlations and fast-timing techniques arebeing employed, giving an insight into this difficult-to-reach region.

A new pulse-processing electronics architecture for photomultipliers coupled to fast scintillators used in nuclear physics experiments is described. The system comprises a field programmable gate array (FPGA) based time-to-digital converter (TDC), as well as custom analog electronics for amplification and efficient analog signal processing. These electronics include separate fast and slow discrete amplifiers to extract the timing and energy information of the incident radiation on cerium doped lanthanum bromide scintillators. With the slow amplifier, a novel circuit that is able to convert energy information directly into pulse width with a linear dependence is introduced. Advanced and highly demanded features such as low-processing time, high data rate handling capabilities and long collection windows have been tested with radioactive ion-beams. These characteristics of pulse-processing modules will further push the boundaries of experimental freedom.

The DEcay SPECtroscopy (DESPEC) setup for nuclear structure investigations was developed and commissioned at GSI, Germany in preparation for a full campaign of experiments at the FRS and Super-FRS. In this paper, we report on the first employment of the setup in the hybrid configuration with the AIDA implanter coupled to the FATIMA LaBr3(Ce) fast-timing array, and high-purity germanium detectors. Initial results are shown from the first experiments carried out with the setup. An overview of the setup and function is discussed, including technical advancements along the path.

The pre- and post-scission neutron multiplicities are measured for the fission of 208Rn, populated with the reactions 30Si+178Hf and 48Ti+160Gd in the excitation energy range of 54.5 - 80.8 MeV. We found significant differences in the pre-scission neutron multiplicities of these two systems. Also, both the reactions exhibit an enhancement in the pre-scission multiplicity with respect to the existing data for the 16O+194Pt reaction populating 210Rn. The measured multiplicities are compared with the theoretical predictions obtained from the Langevin dynamical model for fission. Dynamical simulations are also performed to estimate the capture cross-sections. Contribution of the neutrons, emitted during the fusion process, is shown to be crucial in determining the neutron multiplicities for the present systems. Moreover, in case of the 48Ti+160Gd reaction, rapid quasi-fission dynamics prevents neutron emission from the thermalized target-projectile composite. The present measurement helps us to understand the relative importance of fusion and quasi-fission processes and their timescales depending on the entrance channel mass asymmetry.

Summary. — This paper reports preliminary results of the DESPEC campaign at GSI, focused on the study of neutron-deficient nuclei approaching 100 Sn. The results presented show the isomeric decays of excited states with I π = 14 + and 8 + in 96 Pd and 94 Pd, respectively. The detailed characterisation of the DESPEC set-up and analysis methodologies, proven in this experimental run, are crucial for the future campaigns.

G R Umapathy1,* Sunil Ojha1, Rudra N Sahoo2, A. Banerjee3, Pankaj K Giri4, Nabendu K Deb5, M M Hosamani6, S Chopra1 and D Kanjilal1 1 Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India 2Department of Physics, Indian Institute of Technology Ropar, Rupnagar 140 001, India 3GSI Helmholtzzentrum f ur Schwerionenforschung, Planckstrasse 1, 64291 Darmstadt, Germany 4Department of Physics, Central University of Jharkhand, Ranchi, 835205, India 5Department of Physics, Gauhati University, Guwahati, 781014, Assam, India 6Department of Studies in Physics, Karnatak University, Dharwad 580003, India . * Email: umagrphysics@gmail.com