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Measured binary quasifission mass spectra in reactions with actinide nuclides show a large peak in yield near the doubly-magic $^{208}$Pb. This has generally been attributed to the enhanced binding energy of $^{208}$Pb causing a valley in the potential energy surface, attracting quasifission trajectories. To investigate this interpretation, binary quasifission mass spectra and cross-sections have been measured at near-barrier energies for reactions of $^{50}$Ti with actinide nuclides from $^{238}$U to $^{249}$Cf. Cross-sections have also been deduced for sequential fission (a projectile-like nucleus and two fragments from fission of the complementary target-like nucleus). Binary cross-sections fall from ∼70% of calculated capture cross-sections for $^{238}$U to only ∼40% for $^{249}$Cf, with a compensating increase in sequential fission cross-sections. The data are consistent with the $^{208}$Pb peak originating largely from sequential fission of heavier fragments produced in more mass-asymmetric primary quasifission events. These are increasingly suppressed as the heavy quasifission fragment mass increases above $^{208}$Pb. The important role of sequential fission calls for re-interpretation of quasifission characteristics and dynamics in superheavy element synthesis reactions.

International audience; The nature of quadrupole and octupole collectivity in $^{222}$Rn was investigated by determining the electric-quadrupole (E2) and octupole (E3) matrix elements using subbarrier, multistep Coulomb excitation. The radioactive $^{222}$Rn beam, accelerated to 4.23 MeV/u, was provided by the HIE-ISOLDE facility at CERN. Data were collected in the Miniball $\gamma$ -ray spectrometer following the bombardment of two targets, $^{120}$Sn and $^{60}$Ni. Transition E2 matrix elements within the ground-state and octupole bands were measured up to 10 ¯h and the results were consistent with a constant intrinsic electric-quadrupole moment, 518(11) $e$ fm$^2$ . The values of the intrinsic electric-octupole moment for the $0^+ \to 3^-$ and $2^+ \to 5^-$ transitions were found to be respectively $2360^{+300}_{-210}$ $e$ fm$_3$ and $2300^{+300}_{-500}$ $e$ fm$^3$ while a smaller value, $1200^{+500}_{-900}$ $e$ fm$^3$ , was found for the $2^+ \to 1^-$ transition. In addition, four excited non-yrast states were identified in this work via γ -γ coincidences.

International audience; Excited states in the extremely neutron-deficient nucleus Te107 have been identified from two separate experiments using the recoil-decay tagging technique. Two connected structures were observed on the basis of γγ-coincidence relations and tentatively assigned as built on the mixed-parentage νg7/2d5/2 and νh11/2 intruder configurations. The observed structures were compared with large-scale shell-model calculations and total Routhian surface calculations. Collective behavior was discovered to persist in the νh11/2 band of Te107 which highlights the shape-polarizing effect of a single valence neutron occupying the h11/2 intruder orbit as the N=50 shell closure is approached.

The competition between the dominant mass-asymmetric and rarer narrow mass-symmetric fission modes in actinide nuclei are controlled by deformed and spherical shell effects. The low energy fission of 80180Hg was recently observed to be strongly mass-asymmetric, indicating that despite spherical shell gaps in fragments around 4090Zr, the system does not fission mass-symmetrically. Several theoretical approaches have been used to explain this unexpected result.To investigate the underlying mechanism, systematic measurements of fission mass distributions for isotopes of Os, Pt, Hg and Pb, formed in fusion reactions with p, $^{12}$C, $^{32}$S, $^{40,48}$Ca projectiles, have been made for excitation energies above the fission saddle-point (Eeff⁎) between 2.8 and 28.2 MeV. Evidence for mass-asymmetric fission is widespread, manifested as flat topped mass distributions or significant deviations from a single Gaussian shape. The systematic trends seen cannot be attributed to quasifission. Comparing two-Gaussian fits at a wide range of E$^{⁎}$, it is concluded that the fit centroids reflect the low energy character of mass-asymmetric fission in the sub-lead region.Quantitative comparisons were made with microscopic calculations by Scamps and Simenel (2019) [33] of fission mass-asymmetries attributed to the influence of shell gaps in both neutrons (N=52, 56 for compact octuple deformations) and protons (Z=34 and Z=42, 44, 46 with large quadrupole deformations). For the predominant fission mode in the calculations, having one elongated and one compact fragment, the results are in extremely good agreement with all experimental values. This provides strong support for both the calculations, and the exploration of mass-asymmetric fission systematics through heavy ion fusion reactions. The total kinetic energy distributions for $^{176}$Pt and $^{180}$Pt do not show any evidence of a low TKE mass-symmetric fission mode, as had been reported for $^{178}$Pt by Tsekhanovich et al. (2019) [39].

6 pags., 5 figs., 1 tab., There is sparse direct experimental evidence that atomic nuclei can exhibit stable "pear" shapes arising from strong octupole correlations. In order to investigate the nature of octupole collectivity in radium isotopes, electric octupole (E3) matrix elements have been determined for transitions in Ra222,228 nuclei using the method of sub-barrier, multistep Coulomb excitation. Beams of the radioactive radium isotopes were provided by the HIE-ISOLDE facility at CERN. The observed pattern of E3 matrix elements for different nuclear transitions is explained by describing Ra222 as pear shaped with stable octupole deformation, while Ra228 behaves like an octupole vibrator., The support of the ISOLDE Collaboration and technical teams is acknowledged. This work was supported by the following Research Councils and Grants: Science and Technology Facilities Council (UK) Grants No. ST/P004598/1, No. ST/L005808/1, No. ST/ R004056/1; Federal Ministry of Education and Research (Germany) Grants No. 05P18RDCIA, No. 05P15PKCIA, and No. 05P18PKCIA and the “Verbundprojekt 05P2018”; National Science Centre (Poland) Grant No. 2015/18/M/ ST2/00523; European Union’s Horizon 2020 Framework research and innovation programme 654002 (ENSAR2); Marie Skłodowska-Curie COFUND Grant (EU-CERN) 665779; Research Foundation Flanders and IAP Belgian Science Policy Office BriX network P7/12 (Belgium); GOA/2015/010 (BOF KU Leuven); RFBR (Russia) Grant No. 17-52-12015; and the Academy of Finland (Finland) Grant No. 307685.

The nuclei below lead but with more than 126 neutrons are crucial to an understanding of the astrophysical $r$-process in producing nuclei heavier than $A\sim190$. Despite their importance, the structure and properties of these nuclei remain experimentally untested as they are difficult to produce in nuclear reactions with stable beams. In a first exploration of the shell structure of this region, neutron excitations in $^{207}$Hg have been probed using the neutron-adding ($d$,$p$) reaction in inverse kinematics. The radioactive beam of $^{206}$Hg was delivered to the new ISOLDE Solenoidal Spectrometer at an energy above the Coulomb barrier. The spectroscopy of $^{207}$Hg marks a first step in improving our understanding of the relevant structural properties of nuclei involved in a key part of the path of the $r$-process., Comment: 6 pages, 5 figures

There is sparse direct experimental evidence that atomic nuclei can exhibit stable pear shapes arising from strong octupole correlations. In order to investigate the nature of octupole collectivity in radium isotopes, electric octupole ($E3$) matrix elements have been determined for transitions in $^{222,228}$Ra nuclei using the method of sub-barrier, multi-step Coulomb excitation. Beams of the radioactive radium isotopes were provided by the HIE-ISOLDE facility at CERN. The observed pattern of $E$3 matrix elements for different nuclear transitions is explained by describing $^{222}$Ra as pear-shaped with stable octupole deformation, while $^{228}$Ra behaves like an octupole vibrator. There is sparse direct experimental evidence that atomic nuclei can exhibit stable pear shapes arising from strong octupole correlations. In order to investigate the nature of octupole collectivity in radium isotopes, electric octupole ($E3$) matrix elements have been determined for transitions in $^{222,228}$Ra nuclei using the method of sub-barrier, multi-step Coulomb excitation. Beams of the radioactive radium isotopes were provided by the HIE-ISOLDE facility at CERN. The observed pattern of $E$3 matrix elements for different nuclear transitions is explained by describing $^{222}$Ra as pear-shaped with stable octupole deformation, while $^{228}$Ra behaves like an octupole vibrator.

International audience; Lifetimes of low- and high-spin states in the odd-A nucleus Hf177 were measured during the EXILL&FATIMA campaign using a spectrometer equipped with eight HPGe-Clover detectors and 16 fast-timing LaBr3(Ce) detectors. For the determination of lifetimes in the pico- to nanosecond regime, the well-established generalized centroid difference method was used. Lifetimes of the 9/2−, 9/2+, 11/2−, 11/2+ states were remeasured, the lifetimes of the 13/2−, 17/2−, 13/2+, 15/2+, and 17/2+ states were determined for the first time and an upper limit for the 19/2+ state has been established. From these lifetimes absolute reduced transition probabilities were extracted and compared to particle-rotor-model calculations and quasiparticle-phonon-model calculations showing the importance of including multipole-multipole interactions in the description of odd-A nuclei in the rare earth region.

International audience; Lifetimes of excited states of the phosphorus isotopes 1533,34,35,36P have been measured by using the differential recoil-distance method. The isotopes of phosphorus were populated in binary grazing reactions initiated by a beam of S36 ions of energy 225 MeV incident on a thin Pb208 target mounted in the Cologne plunger apparatus. The combination of the PRISMA magnetic spectrometer and an early implementation of the AGATA γ-ray tracking array was used to detect γ rays in coincidence with projectile-like nuclear species. Lifetime measurements of populated states were made within the range from about 1 to 100 ps. The number of states for which lifetime measurements were possible was limited by statistics. For P33, lifetime limits were determined for the first 3/2+ and 5/2+ states at 1431 and 1848 keV, respectively; the results are compared with previous published lifetime values. The lifetime of the first 2+ state of P34 at 429 keV was determined and compared with earlier measurements. For P35, the states for which lifetimes, or lifetime limits, were determined were those at 2386, 3860, 4101, and 4493 keV, with Jπ values of 3/2+, 5/2+, 7/21−, and 7/22−, respectively. There have been no previous published lifetimes for states in this nucleus. A lifetime was measured for the stretched π(1f7/2)⊗ν(1f7/2)Jπ=(7+) state of P36 at 5212 keV and a lifetime limit was established for the stretched π(1d3/2)⊗ν(1f7/2)Jπ=(5−) state at 2030 keV. There are no previously published lifetimes for states of P36. Measured lifetime values were compared with the results of state-of-the-art shell-model calculations based on the PSDPF effective interaction. In addition, measured branching ratios, published mixing ratios, and electromagnetic transition rates, where available, have been compared with shell-model values. In general, there is good agreement between experiment and the shell model; however there is evidence that the shell-model values of the M1 transition rates for the 3/21+→1/2+ (ground state) and 5/21+→3/21+ transitions in P33 underestimate the experimental values by a factor between 5 and 10. In P35 there are some disagreements between experimental and shell-model values of branching ratios for the first and second excited 7/2− states. In particular, there is a serious disagreement for the decay characteristics of the second 7/2− state at 4493 keV, for which the shell-model counterpart lies at 4754 keV. In this case, the shell-model competing electromagnetic decay branches are dominated by E1 and M1 transitions.

International audience; The nature of low-lying 3− levels in Nd144 was investigated in the Nd143(n,γγ) cold neutron-capture reaction. The combination of the high neutron flux from the research reactor at the Institut Laue-Langevin and the high γ-ray detection efficiency of the EXILL setup allowed the recording of γγ coincidences. From the coincidence data precise branching ratios were extracted. Furthermore, the octagonal symmetry of the setup allowed angular-distribution measurements to determine multipole-mixing ratios. Additionally, in a second measurement the ultra-high resolution spectrometer GAMS6 was employed to conduct lifetime measurements using the gamma-ray induced Doppler-shift technique (GRID). The confirmed strong M1 component in the 33−→31− decay strongly supports the assignment of the 33− level at 2779keV as low-lying isovector octupole excitation. Microscopic calculations within the quasiparticle phonon model confirm an isovector component in the wave function of the 33− level, firmly establishing this fundamental mode of nuclear excitation in near-spherical nuclei.

International audience; The first excited level of 96Mo was investigated in a high-statistics experiment using the 95Mo(n, γγ) cold neutron capture reaction. The measurements used the high cold neutron flux from the research reactor at Institut Laue-Langevin and employed the highly-efficient EXILL array to detect γ-ray coincidences. The recorded statistics allow identification of decay branches with only a small relative intensity including the E3 decay. With the knowledge of the newly measured branching ratio and the known transition probability, the lifetime of the level was determined and, subsequently, the strength of the other decay branches of the octupole phonon were calculated. The extracted electromagnetic decay strengths are compared to the systematics of the stable even–even molybdenum isotopes and values calculated in a Skyrme-force based quasiparticle random phase approximation and in a cluster approach. Additionally, the decay branch to the low-lying quadrupole isovector level was observed.