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Register a new userPairing-quadrupole interplay in the neutron-deficient tin nuclei: first lifetime measurements of low-lying states in $^{106,108}$Sn
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The lifetimes of the low-lying excited states $2^+$ and $4^+$ have been directly measured in the neutron-deficient $^{106,108}$Sn isotopes. The nuclei were populated via a deep-inelastic reaction and the lifetime measurement was performed employing a differential plunger device. The emitted $gamma$ rays were detected by the AGATA array, while the reaction products were uniquely identified by the VAMOS++ magnetic spectrometer. Large-Scale Shell-Model calculations with realistic forces indicate that, independently of the pairing content of the interaction, the quadrupole force is dominant in the $B(E2; 2_1^+ to 0_{g.s.}^+)$ values and it describes well the experimental pattern for $^{104-114}$Sn; the $B(E2; 4_1^+ to 2_1^+)$ values, measured here for the first time, depend critically on a delicate pairing-quadrupole balance, disclosed by the very precise results in $^{108}$Sn. This result provides insight in the hitherto unexplained $B(E2; 4_1^+ to 2_1^+)/B(E2; 2_1^+ to 0_{g.s.}^+) < 1$ anomaly.
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We report on the experimental study of quadrupole collectivity in the neutron-deficient nucleus uc{104}{Sn} using intermediate-energy Coulomb excitation. The $B(E2; 0^+_1 rightarrow 2^+_1)$ value for the excitation of the first $2^+$ state in uc{104}{Sn} has been measured to be $0.180(37)~e^2$b$^2$ relative to the well-known $B(E2)$ value of uc{102}{Cd}. This result disagrees by more than one sigma with a recently published measurement cite{Gua13}. Our result indicates that the most modern many-body calculations remain unable to describe the enhanced collectivity below mid-shell in Sn approaching $N=Z=50$. We attribute the enhanced collectivity to proton particle-hole configurations beyond the necessarily limited shell-model spaces and suggest the asymmetry of the $B(E2)$-value trend around mid-shell to originate from enhanced proton excitations across $Z=50$ as $N=Z$ is approached.
The properties of the low-lying 2^+ states in the even-even nuclei around 132Sn are studied within the quasiparticle random phase approximation. Starting from a Skyrme interaction in the particle-hole channel and a density-dependent zero-range interaction in the particle-particle channel, we use the finite rank separable approach in our investigation. It is found that the fourth 2^+ state in 132Te could be a good candidate for a mixed-symmetry state.
Shape coexistence in the $Z approx 82$ region has been established in mercury, lead and polonium isotopes. Even-even mercury isotopes with $100 leq N leq 106$ present multiple fingerprints of this phenomenon, which seems to be no longer present for $N geq 110$. According to a number of theoretical calculations, shape coexistence is predicted in the $^{188}$Hg isotope. The $^{188}$Hg nucleus was populated using two different fusion-evaporation reactions with two targets, $^{158}$Gd and $^{160}$Gd, and a beam of $^{34}$S, provided by the Tandem-ALPI accelerators complex at the Laboratori Nazionali di Legnaro. The channels of interest were selected using the information from the Neutron Wall array, while the $gamma$ rays were detected using the GALILEO $gamma$-ray array. The lifetimes of the excited states were determined using the Recoil Distance Doppler-Shift method, employing the dedicated GALILEO plunger device. Using the two-bands mixing and rotational models, the deformation of the pure configurations was obtained from the experimental results. The extracted transition strengths were compared with those calculated with the state-of-the-art symmetry-conserving configuration-mixing (SCCM) and five-dimentional collective Hamiltonian (5DCH) approaches in order to shed light on the nature of the observed structures in the $^{188}$Hg nucleus. An oblate, a normal- and a super-deformed prolate bands were predicted and their underlying shell structure was also discussed.
The electric quadrupole transition from the first 2+ state to the ground 0+ state in 18C was studied through lifetime measurement by an upgraded recoil shadow method applied to inelastically scattered radioactive 18C nuclei. The measured mean lifetime is 18.9 +/- 0.9 (stat) +/- 4.4 (syst) ps, corresponding to a B(E2;2+ -> 0+) value of 4.3 +/- 0.2 +/- 1.0 e2fm4, or about 1.5 Weisskopf units. The mean lifetime of the first 2+ state in 16C was remeasured to be 18.0 +/- 1.6 +/- 4.7 ps, about four times shorter than the value reported previously. The discrepancy between the two results was resolved by incorporating the gamma-ray angular distribution measured in this work into the previous measurement. These transition strengths are hindered compared to the empirical transition strengths, indicating that the anomalous hindrance observed in 16C persists in 18C.
Excitation functions of elastic and inelastic 7Be+p scattering were measured in the energy range between 1.6 and 2.8 MeV in the c.m. An R-matrix analysis of the excitation functions provides strong evidence for new positive parity states in 8B. A new 2+ state at an excitation energy of 2.55 MeV was observed and a new 0+ state at 1.9 MeV is tentatively suggested. The R-matrix and Time Dependent Continuum Shell Model were used in the analysis of the excitation functions. The new results are compared to the calculations of contemporary theoretical models.
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