γ
-ray spectroscopy of fission fragments is a powerful method for studies of nuclear structure properties. Recent results on the spectroscopy of fission fragments, using the combination of the AGATA
...γ
-ray tracking array and the VAMOS++ large acceptance magnetic spectrometer at GANIL, are reported. A comparison of the performance of the large germanium detector arrays EXOGAM and AGATA illustrates the advances in
γ
-ray spectroscopy of fission fragments. Selected results are highlighted for prompt
γ
-ray spectroscopy studies, measurements of short lifetimes of excited states with the Recoil Distance Doppler-Shift method, using both AGATA and VAMOS++ and prompt-delayed
γ
-ray spectroscopy studies using AGATA, VAMOS++ and EXOGAM.
The collective structure of 106Cd is elucidated by multi-step Coulomb excitation of a 3.849 MeV/A beam of 106Cd on a 1.1 mg/cm2208Pb target using GRETINA-CHICO2 at ATLAS. Fourteen E2 matrix elements ...were obtained. The nucleus 106Cd is a prime example of emergent collectivity that possesses a simple structure: it is free of complexity caused by shape coexistence and has a small, but collectively active number of valence nucleons. This work follows in a long and currently active quest to answer the fundamental question of the origin of nuclear collectivity and deformation, notably in the cadmium isotopes. The results are discussed in terms of phenomenological models, the shell model, and Kumar-Cline sums of E2 matrix elements. The 〈02+||E2||21+〉 matrix element is determined for the first time, providing a total, converged measure of the electric quadrupole strength, 〈Q2〉, of the first-excited 21+ level relative to the 01+ ground state, which does not show an increase as expected of harmonic and anharmonic vibrations. Strong evidence for triaxial shapes in weakly collective nuclei is indicated; collective vibrations are excluded. This is contrary to the only other cadmium result of this kind in 114Cd by C. Fahlander et al. (1988) 38, which is complicated by low-lying shape coexistence near midshell.
Lifetimes of states in the ground-state bands of (70)Se and (72)Se were measured using the recoil-distance Doppler shift method. The results deviate significantly from earlier measurements, requiring ...a revision of the conclusions drawn from a recent Coulomb excitation experiment concerning the shape of (70)Se. The new results lead to a coherent picture of shape coexistence in the neutron-deficient selenium and krypton isotopes. The coexistence and evolution of oblate and prolate shapes in this mass region is for the first time consistently described by new Hartree-Fock-Bogolyubov-based configuration-mixing calculations which were performed using the Gogny D1S interaction.
Lifetimes of excited states in ^{99}$Y , ^{101}$Y, ^{101}$Nb, ^{103}$Nb, and ^{105}$Nb were measured in an experiment using the recoil distance Doppler shift method at GANIL (Grand Accélérateur ...National d’Ions Lourds). The neutron-richnuclei were produced in fission reactions between a ^{238}$U beam and a ^9$Be target. Prompt γ rays were measuredwith the EXOGAM array and correlated with fission fragments that were identified in mass and atomic numberwith the VAMOS$\ {++}$ spectrometer. The measured lifetimes, together with branching ratios, provide $\it B(M1)$ and$\it B(E2)$ values for the strongly coupled rotational bands built on the 4225/2$^+$ ground state in the Y and Nb nucleiwith neutron number N $\geq$ 60. The comparison of the experimental results with triaxial particle-rotor calculationsprovides information about the evolution of the nuclear shape in this mass region.
The lifetimes of the first excited 2{sup +} states in {sup 62}Fe and {sup 64}Fe have been measured for the first time using the recoil-distance Doppler shift method after multinucleon transfer ...reactions in inverse kinematics. A sudden increase of collectivity from {sup 62}Fe to {sup 64}Fe is observed. The experimental results are compared with new large-scale shell-model calculations and Hartree-Fock-Bogolyubov-based configuration-mixing calculations using the Gogny D1S interaction. The results give a deeper understanding of the mechanism leading to an onset of collectivity near {sup 68}Ni, which is compared with the situation in the so-called island of inversion around {sup 32}Mg.
A long-standing prediction of nuclear models is the emergence of a region of long-lived, or even stable, superheavy elements beyond the actinides. These nuclei owe their enhanced stability to closed ...shells in the structure of both protons and neutrons. However, theoretical approaches to date do not yield consistent predictions of the precise limits of the 'island of stability'; experimental studies are therefore crucial. The bulk of experimental effort so far has been focused on the direct creation of superheavy elements in heavy ion fusion reactions, leading to the production of elements up to proton number Z = 118 (refs 4, 5). Recently, it has become possible to make detailed spectroscopic studies of nuclei beyond fermium (Z = 100), with the aim of understanding the underlying single-particle structure of superheavy elements. Here we report such a study of the nobelium isotope 254No, with 102 protons and 152 neutrons-the heaviest nucleus studied in this manner to date. We find three excited structures, two of which are isomeric (metastable). One of these structures is firmly assigned to a two-proton excitation. These states are highly significant as their location is sensitive to single-particle levels above the gap in shell energies predicted at Z = 114, and thus provide a microscopic benchmark for nuclear models of the superheavy elements.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The proton-rich isotope 68Br was discovered in secondary fragmentation reactions of fast radioactive beams. Proton-rich secondary beams of 70,71,72Kr and 70Br, produced at the RIKEN Nishina Center ...and identified by the BigRIPS fragment separator, impinged on a secondary 9Be target. Unambiguous particle identification behind the secondary target was achieved with the ZeroDegree spectrometer. Based on the expected direct production cross sections from neighboring isotopes, the lifetime of the ground or long-lived isomeric state of 68Br was estimated. The results suggest that secondary fragmentation reactions, where relatively few nucleons are removed from the projectile, offer an alternative way to search for new isotopes, as these reactions populate preferentially low-lying states.
The low energy excited 02,3+ states in 96Sr are amongst the most prominent examples of shape coexistence across the nuclear landscape. In this work, the neutron 2s1/22 content of the 01,2,3+ states ...in 96Sr was determined by means of the d(95Sr, p) transfer reaction at the TRIUMF-ISAC2 facility using the SHARC and TIGRESS arrays. Spectroscopic factors of 0.19(3) and 0.22(3) were extracted for the 96Sr ground and 1229 keV 0+ states, respectively, by fitting the experimental angular distributions to DWBA reaction model calculations. A detailed analysis of the γ-decay of the isomeric 03+ state was used to determine a spectroscopic factor of 0.33(13). The experimental results are compared to shell model calculations, which predict negligible spectroscopic strength for the excited 0+ states in 96Sr. The strengths of the excited 02,3+ states were also analyzed within a two-level mixing model and are consistent with a mixing strength of a2=0.40(14) and a difference in intrinsic deformations of |Δβ|=0.31(3). These results suggest coexistence of three different configurations in 96Sr and strong shape mixing of the two excited 0+ states.
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