Deformed shell gap near $$N\sim 100$$ for Gd and Dy nuclei

The high spin excited states of $^{169}\mathrm{Tm}$ have been studied via the $^{169}\mathrm{Tm}$($^{32}\mathrm{S}$, $^{32}\mathrm{S}^{\ensuremath{'}}$)$^{169}{\mathrm{Tm}}^{*}$ reaction at a beam energy of 164 MeV. The $\ensuremath{\gamma}$ rays were detected using an array of 19 Compton-suppressed clover HPGe detectors. The band based on the $[411]1/{2}^{+}$ Nilsson orbital has been extended to observe the band crossing for the first time in this nucleus. A sharp backbending, similar to $^{165}\mathrm{Tm}$, is observed in this nucleus which is in complete contrast with its immediate neighbor $^{167}\mathrm{Tm}$. The data were interpreted in the cranked shell model approach by calculating the total Routhian surfaces, crossing frequencies, and the interaction strengths for these nuclei which are in excellent agreement with the experimental data. It is suggested that the $N=98$ deformed shell gap has larger effect in determining the nature of alignment than its shape driving effect.

We have measured in-beam \ensuremath{\gamma} rays in the neutron-rich ${}^{246}{\mathrm{Pu}}_{152}$ and ${}^{245}{\mathrm{Pu}}_{151}$ nuclei by means of $^{244}\mathrm{Pu}$($^{18}\mathrm{O}$, $^{16}\mathrm{O}$)$^{246}\mathrm{Pu}$ and $^{244}\mathrm{Pu}$($^{18}\mathrm{O}$, $^{17}\mathrm{O}$)$^{245}\mathrm{Pu}$ neutron transfer reactions, respectively. The \ensuremath{\gamma} rays emitted from $^{246}\mathrm{Pu}$ ($^{245}\mathrm{Pu}$) were identified by selecting the kinetic energy of scattered $^{16}\mathrm{O}$ ($^{17}\mathrm{O}$) detected by Si $\ensuremath{\Delta}E\ensuremath{-}E$ detectors. The ground-state band of $^{246}\mathrm{Pu}$ was established up to the ${12}^{+}$ state. We have found that the shell gap of $N=152$ is reduced in energy with decreasing atomic number by extending the systematics of the one-quasiparticle energies in $N=151$ nuclei into those in $^{245}\mathrm{Pu}$. This reduction of the shell gap clearly affects the ${2}^{+}$ energy of the ground-state band of $^{246}\mathrm{Pu}$.

The $\ensuremath{\gamma}$ decays of high spin states in ${}^{77}$Rb populated in the reaction ${}^{40}$Ca${(}^{40}$Ca,$3p)$${}^{77}$Rb were studied with the EUROGAM I $\ensuremath{\gamma}$-ray spectrometer at Daresbury Laboratory, in conjunction with the recoil mass separator. The data were used to extend or establish three positive parity and five negative parity bands in ${}^{77}$Rb. Level lifetimes and sidefeeding times were then measured with the NORDBALL $\ensuremath{\gamma}$-ray spectrometer, using the Doppler shift attenuation method. A very detailed level scheme including many weakly populated states was established and the bands were observed over large spin ranges. The deduced lifetimes demonstrate the high collectivity of the excited states and are consistent with their having large prolate deformations. The structures of the bands and their crossings are discussed in relation to the large shell gap at prolate deformation ${\ensuremath{\beta}}_{2}\ensuremath{\approx}0.4$.

The rotational band structure of the Z=104 nucleus (256)Rf has been observed up to a tentative spin of 20ℏ using state-of-the-art γ-ray spectroscopic techniques. This represents the first such measurement in a superheavy nucleus whose stability is entirely derived from the shell-correction energy. The observed rotational properties are compared to those of neighboring nuclei and it is shown that the kinematic and dynamic moments of inertia are sensitive to the underlying single-particle shell structure and the specific location of high-j orbitals. The moments of inertia therefore provide a sensitive test of shell structure and pairing in superheavy nuclei which is essential to ensure the validity of contemporary nuclear models in this mass region. The data obtained show that there is no deformed shell gap at Z=104, which is predicted in a number of current self-consistent mean-field models.

The ground-state bands of $^{248,250,252}\mathrm{Cf}$ have been established up to the ${10}^{+}$, ${12}^{+}$, and ${10}^{+}$ states, respectively, by in-beam $\ensuremath{\gamma}$-ray spectroscopy using neutron-transfer reactions with a $153$-MeV $^{18}\mathrm{O}$ beam and a highly radioactive Cf target. The deexcitation $\ensuremath{\gamma}$ rays in $^{248,250,252}\mathrm{Cf}$ were identified by taking coincidences with outgoing particles of $^{16\ensuremath{-}19}\mathrm{O}$ measured with Si $\ensuremath{\Delta}E\ensuremath{-}E$ detectors, and by selecting their kinetic energies. Moments of inertia of $^{248,250,252}\mathrm{Cf}$ were discussed in terms of the $N=152$ deformed shell gap.

Recent developments for high-precision mass measurements of the heaviest elements at SHIPTRAP

A <sup>160</sup>Gd beam was accelerated to an energy of 1000 MeV and, separately, bombarded thick targets of <sup>154</sup>Sm and <sup>164</sup>Dy in order to observe neutron-rich, rare-earth nuclei via deep-inelastic collision processes. Gammasphere was utilized to observe g-ray emissions. Here, many new states and transitions were observed in <sup>160</sup>Gd as a result of so-called "unsafe" Coulomb excitation. The ground-state band in <sup>160</sup>Gd has been extended to I<sup>π</sup> = 20<sup>+</sup> and a rotational band based on the K<sup>π</sup> = 4<sup>+</sup> state, previously associated with a hexadecapole vibration, was observed up to 18<sup>+</sup>. The quasiparticle configuration of the K<sup>π</sup> = 4<sup>+</sup> band has been determined, and its unusual alignment behavior may result from a possible quenching of static neutron pairing. In addition, the band based on the [523]5/2 quasineutron orbital in <sup>161</sup>Gd was extended from 11/2<sup>–</sup> to 33/2<sup>–</sup>, and also displays the same unusual alignment behavior.

Structural properties and α-decay chains of transfermium nuclei (101≤Z≤110)

The excited states of 169Tm have been studied via the 169Tm(32S,32S′)169Tm* reaction at the beam energy of 164MeV. The $\gamma$-rays were detected using the Indian National Gamma Array (INGA) setup, composed of 19 Compton-suppressed clover HPGe detectors. A new level scheme of 169Tm with 11 newly placed $ \gamma$-rays has been proposed. A band crossing in the $ \pi [541]1/2^{-}$ band and several interband E1 transitions between this and the $ \pi [411]1/2^{+}$ ground-state band have been observed for the first time in this nucleus. The role of the $N=98$ deformed shell gap has been discussed by comparing the band crossing parameters of the negative parity bands in Tm and other neighboring nuclei. The origin of the interband E1 transitions has been investigated in terms of coupling to octupole degrees of freedom. The shape evolution of the Tm isotopes around $ N=98$ have been studied in the projected and cranked shell model approaches, both of which predict a change in shape from an axial prolate to a triaxial one after band crossing in these nuclei. The new data and the calculations help to understand the unusual structural phenomena reported for the nuclei with $ N=98$.

Here, the decay of a K<sup>π</sup> = 8<sup>–</sup> isomer in <sup>244</sup>Pu and the collective band structures populating the isomer were studied using deep inelastic excitations with <sup>47</sup>Ti and <sup>208</sup>Pb beams, respectively. Precise measurements of M1/E2 branching ratios in the band confirm a 9/2<sup>–</sup>[734]<sub>ν</sub>Ⓧ7/2<sup>+</sup>[624]<sub>ν</sub> configuration assignment for the isomer, validating the systematics of K<sup>π</sup> = 8<sup>–</sup>, two-quasineutron isomers observed in even-Z, N = 150 isotones. These isomers around the deformed shell gap at N = 152 provide critical benchmarks for theoretical predictions of single-particle energies in this gateway region to superheavy nuclei.

The exotic nuclei are a fertile source of new features of nuclear structure. The evolution of new shell gaps accompanied by quenching of classical magic numbers is one of the marked features in these nuclei. These studies aimed to search and explore such behavior and find major significance on both the experimental and theoretical fronts. Here, we present an inclusive study and significant evidence of the existence of deformed shell closure at the neutron number N = 100 in rare earth Nd, Sm, Gd and Dy nuclei, obtained from the persistence of a peak in the analysis of symmetry energy and its bulk and surface components, over the isotopic chains of these nuclei, within the coherent density fluctuation model (CDFM). The relativistic mean field densities, employing the NL3 and recently developed IOPB-I parameter sets, have been used as an input within CDFM. This result is in excellent agreement with our earlier prediction of deformed magic shell closure at N = 100 in rare earth nuclei [Satpathy et al (2004 J. Phys. G 30 771); Ghouri et al (2012 Phys. Rev. C 85 064327)] and further reinforced by the experimental confirmation of deformed magicity at N = 100 in 162Sm and 164Gd nuclei [Patel et al (2014 Phys. Rev. Lett. 113 262502)]. An important consequence of the work is that N = 100 isotopes of these nuclei can serve as a waiting point in r-process nucleosynthesis and influence the heavy nuclei formation in astrophysical entities.

Background: Quasifission is the main reaction channel hindering the formation of superheavy nuclei (SHN). Its understanding will help to optimize entrance channels for SHN studies. Quasifission also provides a probe to understand the influence of shell effects in the formation of the fragments.Purpose: Investigate the role of shell effects in quasifission and their interplay with the orientation of the deformed target in the entrance channel.Methods: $^{48}\mathrm{Ca}+^{249}\mathrm{Bk}$ collisions are studied with the time-dependent Hartree-Fock approach for a range of angular momenta and orientations.Results: Unlike similar reactions with a $^{238}\mathrm{U}$ target, no significant shell effects which could be attributed to the $^{208}\mathrm{Pb}$ ``doubly magic'' nucleus are found. However, the octupole deformed shell gap at $N=56$ seems to strongly influence quasifission in the most-central collisions.Conclusions: Shell effects similar to those observed in fission affect the formation of quasifission fragments. Mass-angle correlations could be used to experimentally isolate the fragments influenced by $N=56$ octupole shell gaps.

Using state-of-the-art γ-ray spectroscopic techniques, the first rotational band of a superheavy element, extending up to a spin of 20 ℏ, was discovered in the nucleus 256Rf. To perform such an experiment at the limits of the present instrumentation, several developments were needed. The most important of these developments was of an intense isotopically enriched 50Ti beam using the MIVOC method. The experimental set-up and subsequent analysis allowed the 256Rf ground-state band to be revealed. The rotational properties of the band are discussed and compared with neighboring transfermium nuclei through the study of their moments of inertia. These data suggest that there is no evidence of a significant deformed shell gap at Z = 104.

We have measured de-excitation $\ensuremath{\gamma}$ rays in $^{249}\mathrm{Cm}$ populated by one-neutron stripping reactions with a $^{248}\mathrm{Cm}$ target and 162-MeV $^{16}\mathrm{O}$, 162-MeV $^{18}\mathrm{O}$, and 120-MeV $^{13}\mathrm{C}$ beams. $\ensuremath{\gamma}$ rays in $^{249}\mathrm{Cm}$ were identified by measuring kinetic energies of outgoing particles using Si $\ensuremath{\Delta}E\text{\ensuremath{-}}E$ detectors. It was demonstrated that high-$j$ orbitals were selectively populated in the ($^{16}\mathrm{O}$, $^{15}\mathrm{O}$) reaction having a large negative $Q$ value. We have observed eight quasiparticle states above the deformed shell gap of $N=152$. The $1/{2}^{+}[620]$, $1/{2}^{\ensuremath{-}}[750]$, and $7/{2}^{+}[613]$ bands were extended up to $19/{2}^{+}$, $19/{2}^{\ensuremath{-}}$, and $13/{2}^{+}$ states, respectively. We have established the $9/2 9/{2}^{+}[615]$ state at 526 keV, the $9/2 9/{2}^{+}[604]$ state with a short life of ${T}_{1/2}\ensuremath{\ll}2$ ps at 1030 keV, and the $11/2 11/{2}^{\ensuremath{-}}[725]$ state with ${T}_{1/2}=19(1)$ ns at 375 keV. Furthermore, the $17/2 {1/2}^{+}[880]$ state, having a large component of the ${k}_{17/2}$ spherical single-particle state, has been identified at 1505 keV. We discuss the properties of those quasiparticle states in the framework of a deformed shell model.

The excited states of $^{187}\mathrm{Os}$ have been studied via $^{186}\mathrm{W}(^{4}\mathrm{He},3n)^{187}\mathrm{Os}$ reaction at a beam energy of 36 MeV. The $\ensuremath{\gamma}$ rays were detected using the Indian National Gamma Array at the Variable Energy Cyclotron Centre having seven Compton-suppressed clover high-purity germanium (HPGe) detectors and one low-energy photon spectrometer (LEPS) detector with a digital data acquisition system. The level scheme of $^{187}\mathrm{Os}$ has been extended substantially up to $\ensuremath{\approx}$3.86 MeV of excitation energy and $37/2\phantom{\rule{0.16em}{0ex}}\ensuremath{\hbar}$ of spin with the placement of more than 90 new $\ensuremath{\gamma}$ rays. All known bands have been extended and new band structures have been identified. The results show evidence of triaxial shapes for different configurations of $^{187}\mathrm{Os}$ and different manifestations of nonaxial shape have been observed in the same nucleus. A comparison of the observed band crossing frequency in $^{187}\mathrm{Os}$ with neighboring nuclei gives evidence of a deformed shell gap at $N=110$. The experimental results are well explained using total Routhian surface calculations.