Search Results (67 total)

The reduced transition probabilities, B(E2;0_gs⁺→2₁⁺), have been measured in the radioactive isotopes ^108,106Sn using subbarrier Coulomb excitation at the REX-ISOLDE facility at CERN. Deexcitation γ rays were detected by the highly segmented MINIBALL Ge-detector array. The results, B(E2;0_gs⁺→2₁⁺)=0.222(19)e²b² for ¹⁰⁸Sn and B(E2;0_gs⁺→2₁⁺)=0.195(39)e²b² for ¹⁰⁶Sn were determined relative to a stable ⁵⁸Ni target. The resulting B(E2) values are ∼30% larger than shell-model predictions and deviate from the generalized seniority model. This experimental result may point towards a weakening of the N=Z=50 shell closure.

The structure of ^19-22N nuclei was investigated by means of in-beam γ-ray spectroscopic technique using fragmentation reactions of both stable and radioactive beams. Based on particle-γ and particle-γγ coincidence data, level schemes are constructed for the neutron-rich nitrogen nuclei. The experimental results are compared with shell model calculations. The strength of the N=14 and Z=8 shell closures and the weakening of the shell model interaction WBT are discussed.

Collective properties of the low-lying levels in the odd-A ^67–73Cu were investigated by Coulomb excitation with radioactive beams. The beams were produced at ISOLDE and postaccelerated by REX-ISOLDE up to 2.99  MeV/u. In ^67,69Cu, low-lying 1/2^-, 5/2^-, and 7/2^- states were populated. In ^71,73Cu, besides the known transitions deexciting the single-particle-like 5/2^- and core-coupled 7/2^- levels, γ rays of 454 and 135 keV, respectively, were observed for the first time. Based on a reanalysis of β-decay work and comparison with the systematics, a spin 1/2^- is suggested for these excited states. Three B(E2) values were determined in each of the four isotopes. The results indicate a significant change in the structure of the odd-A Cu isotopes beyond N=40 where single-particle-like and collective levels are suggested to coexist at very low excitation energies.

Neutron-rich, radioactive Zn isotopes were investigated at the Radioactive Ion Beam facility REX-ISOLDE (CERN) using low-energy Coulomb excitation. The energy of the 2₁⁺ state in ⁷⁸Zn could be firmly established and for the first time the 2⁺→0₁⁺ transition in ⁸⁰Zn was observed at 1492(1) keV. B(E2,2₁⁺→0₁⁺) values were extracted for ^74,76,78,80Zn and compared to large scale shell model calculations. With only two protons outside the Z=28 proton core, ⁸⁰Zn is the lightest N=50 isotone for which spectroscopic information has been obtained to date. Two sets of advanced shell model calculations reproduce the observed B(E2) systematics. The results for N=50 isotones indicate a good N=50 shell closure and a strong Z=28 proton core polarization. The new results serve as benchmarks to establish theoretical models, predicting the nuclear properties of the doubly magic nucleus ⁷⁸Ni.

High-spin states in ¹²⁶Xe have been populated in the ⁸²Se(⁴⁸Ca,4n)¹²⁶Xe reaction in two experiments, one at the VIVITRON accelerator in Strasbourg using the Euroball detector array, and a subsequent one with ATLAS at Argonne using the Gammasphere Ge-detector array. Levels and assignments made previously for ¹²⁶Xe up to I=20 have been confirmed and extended. Four regular bands extending to a spin of almost I=60, which are interpreted as two pairs of signature partners with opposite parity, are identified for the first time. The α = 0 partner of each pair is connected to the lower-lying levels, whereas the two α = 1 partners remain floating. A fractional Doppler shift analysis of transitions in the strongest populated (π,α)=(-,0) band provides a value of 5.2_0.5^0.4 b for the transition quadrupole moment, which can be related to a minimum in the potential-energy surface calculated by the ULTIMATE CRANKER cranked shell-model code at ε≈0.35 and γ≈5^°. The four lowest bands calculated for this minimum compare well with the two signature pairs experimentally observed over a wide spin range. A sharp upbend at ℏω~1170 keV is interpreted as a crossing with a band involving the j_15/2 neutron orbital, for which pairing correlations are expected to be totally quenched. The four long bands extend to within ∼5 spin units of a crossing with an yrast line defined by calculated hyperdeformed transitions and will serve as important stepping stones into the spin region beyond 60ħ for future experiments.

Low-spin structures in ^86,88Mo were populated using the ⁵⁸Ni(³⁶Ar, xαyp) heavy-ion fusion-evaporation reaction at a beam energy of 111 MeV. Charged particles and γ rays were emitted in the reactions and detected by the DIAMANT CsI ball and the EXOGAM Ge array, respectively. In addition to the previously reported low-to-medium spin states in these nuclei, new low-spin structures were observed. Angular correlation and linear polarization measurements were performed in order to unambiguously determine the spins and parities of intensely populated states in ⁸⁸Mo. Quasiparticle Random Phase Approximation (QRPA) calculations were performed for the first and second excited 2⁺ states in ⁸⁶Mo and ⁸⁸Mo. The results are in qualitative agreement with the experimental results, supporting a collective interpretation of the low-spin states for these transitional nuclei.

Shape coexistence in the light krypton isotopes was studied in two low-energy Coulomb excitation experiments using radioactive ⁷⁴Kr and ⁷⁶Kr beams from the SPIRAL facility at GANIL. The ground-state bands in both isotopes were populated up to the 8⁺ state via multi-step Coulomb excitation, and several non-yrast states were observed. Large sets of matrix elements were extracted for both nuclei from the observed γ-ray yields. Diagonal matrix elements were determined by utilizing the reorientation effect. In both isotopes the spectroscopic quadrupole moments for the ground-state bands and the bands based on excited 0₂⁺ states are found to have opposite signs. The experimental data are interpreted within a phenomenological two-band mixing model and model-independent quadrupole invariants are deduced for the relevant 0⁺ states using the complete sets of matrix elements and the formalism of quadrupole sum rules. Configuration mixing calculations based on triaxial Hartree-Fock-Bogolyubov calculations with the Gogny D1S effective interaction have been performed and are compared both with the experimental results and with recent calculations using the Skyrme SLy6 effective interaction and the full generator-coordinate method restricted to axial shapes.

Very neutron deficient uranium isotopes were produced in fusion evaporation reactions using ⁴⁰Ar ions on ¹⁸²W targets. The gas-filled recoil separator RITU was employed to collect the fusion products and to separate them from the scattered beam and other reaction products. The activities were implanted into a position sensitive silicon detector after passing through a gas-counter system. The isotopes were identified using spatial and time correlations between the implants and the decays. Two α-decaying states, with E_α=(8612±9) keV and T_1/2=(0.51_-0.10^+0.17) ms for the ground state and E_α=(10678±17) keV and T_1/2=(0.56_-0.14^+0.26) ms for an isomeric state, were identified in ²¹⁸U. In addition, the half-life and α-decay energy of ²¹⁹U were measured with improved precision. The measured decay properties deduced for ²¹⁸U suggest that there is no subshell closure at Z=92.

The first excited 2⁺ state of the unstable isotope ¹¹⁰Sn has been studied in safe Coulomb excitation at 2.82  MeV/u using the MINIBALL array at the REX-ISOLDE post accelerator at CERN. This is the first measurement of the reduced transition probability of this state using this method for a neutron deficient Sn isotope. The strength of the approach lies in the excellent peak-to-background ratio that is achieved. The extracted reduced transition probability, B(E2:0⁺→2⁺)=0.220±0.022e²b², strengthens the observation of the evolution of the B(E2) values of neutron deficient Sn isotopes that was observed recently in intermediate-energy Coulomb excitation of ¹⁰⁸Sn. It implies that the trend of these reduced transition probabilities in the even-even Sn isotopes is not symmetric with respect to the midshell mass number A=116 as ¹⁰⁰Sn is approached.

High-spin states in ¹⁶⁴Lu were populated in the ¹²¹Sb(⁴⁸Ca,5n) reaction at 215 MeV and γ-ray coincidences were measured with the Gammasphere spectrometer. Through this experiment the eight known triaxial superdeformed bands in ¹⁶⁴Lu could be confirmed. Some of these bands were extended to higher as well as to lower spins. Evidence is reported for the first time for weak ΔI=1,E1 transitions linking TSD3 and TSD1. This observation may imply coupling to octupole vibrational degrees of freedom. The decay mechanism is different from the one observed in the neighboring even-N isotopes, which exhibit wobbling excitations built on the πi_13/2 structure with E2(M1),ΔI=1 interband decay. An additional sequence decaying at high spin into TSD1 was observed up to I^π=(50^-). This band has a constant dynamic moment of inertia of ∼70ℏ²MeV^-1 and an alignment that is ∼2ℏ larger than that found for TSD1. A revision of the assumed spin-parity-assignment of TSD2 is based on the observed decay-out to normal-deformed structures. The parity and signature quantum numbers of TSD2 are now firmly assigned as (π,α)=(+,0), in disagreement with the former assignment of (π,α)=(-,1), which was based on the assumption that TSD2 is the signature partner of TSD1. TSD1 and TSD2 show an alignment gain at ℏω∼0.67 and 0.60 MeV, respectively. In TSD1 the involvement of the j_15/2 neutron orbital is suggested to be responsible for the high-frequency crossing.

Rotational structures in the ¹⁶⁹Ta nucleus were studied via the ¹²⁴Sn(⁵¹V, 6n) reaction. These data were obtained as a side channel of an experiment focusing on ¹⁷¹Ta, but the sensitivity provided by the Gammasphere spectrometer proved sufficient for a significant extension of the level scheme of this rare-earth nucleus. Over 170 new transitions and four new band structures were placed in ¹⁶⁹Ta, including the intruder πi_13/2 structure. Linking transitions between all of the sequences were identified, and the relative excitation energies between the different configurations were determined for the first time. The rotational sequences were interpreted within the framework of the cranked shell model.

A medium-spin isomer in ¹⁹⁷Au is identified with t_1/2=150(5) ns following a multinucleon transfer reaction between an 850-MeV ¹³⁶Xe beam and a ¹⁹⁸Pt target. The transitions identified here are considered and possible configurations for the associated levels discussed. In addition, a newly observed out-of-beam transition in ¹⁹⁵Au is briefly reported.

High-spin states in ¹²⁴Ba were investigated in two experiments using the ⁶⁴Ni(⁶⁴Ni, 4n)¹²⁴Ba reaction at three different beam energies. In-beam γ-ray coincidences were measured with the Euroball and Gammasphere detector arrays. In the experiment with Euroball, the CsI detector array Diamant was employed to discriminate against charged-particle channels. Six new rotational bands were observed in ¹²⁴Ba, and previously known bands were extended to higher spins. One of the bands shows a transition from collective to noncollective behavior at high spins. Configuration assignments are suggested on the basis of comparison with cranked shell model and cranked Nilsson-Strutinsky calculations.

The reduced transition probabilities B(E2;0⁺→2₁⁺) of the neutron-rich ⁷⁴Zn and ⁷⁰Ni nuclei have been measured by Coulomb excitation in a ²⁰⁸Pb target at intermediate energy. These nuclei have been produced at Grand Accélérateur National d’Ions Lourds via interactions of a 60A  MeV ⁷⁶Ge beam with a Be target. The B(E2) value for ⁷⁰Ni₄₂ is unexpectedly large, which indicates that neutrons added above N=40 strongly polarize the Z=28 proton core. In the Zn isotopic chain, the steep rise of B(E2) values beyond N=40 continues up to ⁷⁴Zn₄₄. The enhanced proton core polarization in ⁷⁰Ni is attributed to the monopole interaction between the neutron in the g_9/2 and protons in the f_7/2 and f_5/2 spin-orbit partner orbitals. This interaction could result in a weakening of magicity in ⁷⁸Ni₅₀.

High-spin states in ¹⁶¹Lu were populated in the ¹³⁹La(²⁸Si, 6n) reaction and γ-ray coincidences were measured with the EUROBALL spectrometer. On the basis of these data, the previously known level scheme is extended with new band structures and is partly revised. Configuration assignments are made to all bands based on comparison of experimental properties with cranked shell model calculations. A strongly populated band with parity and signature (π,α)=(+,-1/2) is found to be yrast above spin I≃33. This band shows characteristics resembling those of two triaxial superdeformed bands in this nucleus based on the occupation of the shape-driving i_13/2 proton orbital. This structure, unique to ¹⁶¹Lu within the chain of even-N Lu isotopes, is discussed in terms of a quasiparticle configuration in a local triaxial minimum with a larger triaxiality and a smaller quadrupole deformation than calculated for the i_13/2 proton excitation.

High-spin states in ¹⁷¹Ta were populated in the ¹²⁴Sn(⁵¹V,4n) reaction at 228 MeV to search for evidence of stable triaxial deformation. Identification of a wobbling sequence based on the previously known πi_13/2 structure would provide a unique signature for this rarely observed shape. No such sequence was identified in these data, which suggests that the island of triaxial strongly deformed bands may be smaller than once thought. However, over 200 new transitions and two new bands were placed in the level scheme and the sequence based on the πi_13/2 orbital could be observed up to spin and parity I^π=(101/2⁺). The relative excitations of all the sequences were determined and the ground state of ¹⁷¹Ta was found to have 5/2⁺ quantum numbers, contrary to previous reports. All of the previously known structures were extended to much higher spin and their high-frequency band crossings are interpreted within the framework of the cranked shell model.

The level structure of ₆₀¹⁴⁰Nd₈₀ has been established up to spin 48 by in-beam γ-ray spectroscopy by use of the ⁹⁶Zr(⁴⁸Ca, 4n) reaction. High-fold γ-ray coincidences were measured with the EUROBALL spectrometer. Twelve new rotational bands have been discovered at high spins. They are interpreted as being formed in a deep triaxial minimum at ɛ₂≈0.25 and γ≈35^°. Possible configurations are assigned to the observed bands on the basis of configuration-dependent cranked Nilsson-Strutinsky calculations.

The γ decay of the nuclei ^124,125Ba has been investigated by means of the EUROBALL spectrometer, coupled to the DIAMANT array of charged-particle detectors, using the reaction ⁶⁴Ni + ⁶⁴Ni at E_beam=255 and 261 MeV. In the nucleus ¹²⁵Ba six new E1 transitions have been found to link opposite-parity bands currently interpreted as νd_5/2(+g_7/2),νh_11/2 structures. The previously unknown J^π=3^- level in the nucleus ¹²⁴Ba has also been identified; its excitation energy is accurately reproduced by a microscopic calculation including octupole correlations. Both issues are bolstered by sizable B(E1)/B(E2) ratios.

An unexpected strong interaction between normal deformed and triaxial strongly deformed levels at high spin, ∼32ℏ, is observed in ¹⁶⁷Lu. This constitutes the first observation of accidental degeneracy, at such high spins, causing cross talk between levels in the normal deformed and in the second potential well in any known mass region. Furthermore, evidence of quasi-particle excitations highly competing with the wobbling excitation mode in a triaxial superdeformed well has been established in ¹⁶⁷Lu.

A new high-spin superdeformed band has been discovered in ₆₀¹⁴⁰Nd₈₀. It was populated in the ⁹⁶Zr(⁴⁸Ca,4n) reaction and investigated using the EUROBALL γ-ray spectrometer array. The band is observed in the approximate spin range of I=36 to 66. It is associated with shell gaps around Z=60 and at N=80 at large deformation. These gaps produce a pronounced minimum in the calculated total Routhian surfaces at a quadrupole deformation of ϵ₂=0.45. The new band which lies between the high-deformation bands in the A≈130 region and the superdeformed bands in A≈150 nuclei provides insight into the development of the deformation between these two regions. Two possible configurations are suggested involving four neutrons of i_13∕2 origin (ν6⁴) and either six protons of h_11∕2∕h_9∕2 origin (π5⁶) or five protons of h_11∕2∕h_9∕2 and one of i_13∕2 origin (π5⁵6¹).

Two high-spin regularly spaced rotational bands with large dynamical moments of inertia have been identified in ¹⁷⁵Hf with the Gammasphere spectrometer. These new bands are very similar to the previously identified triaxial superdeformed bands in the hafnium nuclei. However, the new bands in ¹⁷⁵Hf have been linked into the known level scheme and thereby provide the first firm spin assignments for these structures in this region. In order to understand the new bands, theoretical calculations have been performed based on the ULTIMATE CRANKER code. The new bands in ¹⁷⁵Hf are deduced to be built upon highly deformed structures. No experimental evidence for triaxiality was established and this work suggests that the structure of the so-called “triaxial” superdeformed bands in the Hf nuclei may be quite different from those identified in the lighter mass Lu nuclei. Since the two highly deformed bands in ¹⁷⁵Hf are associated with different deformations, this work also identifies the role of the intruder orbits in polarizing the nuclear shape.

The energy of the 2⁺→0⁺ transition in ²²⁶U has been measured as 81.3(6)  keV and the energy of the 4⁺→2⁺ transition in ²⁵⁴No has been measured as 101.1(6)  keV, both for the first time, by means of electron spectroscopy. The results are close to the estimates of the energies of these transitions from earlier γ-decay work. Absolute values of electromagnetic decay intensities have been measured for yrast transitions in both nuclei.

In-beam γ-ray spectroscopy using fragmentation reactions of both stable and radioactive beams has been performed in order to study the structure of excited states in neutron-rich oxygen isotopes with masses ranging from A=20 to 24. For the produced fragments, γ-ray energies, intensities, and γ-γ coincidences have been measured. Based on this information new level schemes are proposed for ^21,22O up to the neutron separation energy. The nonobservation of any γ-decay branch from ²³O and ²⁴O suggests that their excited states lie above the neutron decay thresholds. From this, as well as from the level schemes proposed for ²¹O and ²²O, the size of the N=14 and 16 shell gaps in oxygen isotopes is discussed in the light of shell-model calculations.

Lifetimes of states in the triaxial strongly deformed bands of ¹⁶³Lu have been measured with the Gammasphere spectrometer using the Doppler-shift attenuation method. The bands have been interpreted as wobbling-phonon excitations from the characteristic electromagnetic properties of the transitions connecting the bands. Quadrupole moments are extracted for the zero-phonon yrast band and, for the first time, for the one-phonon wobbling band. The very similar results found for the two bands suggest a similar intrinsic structure and support the wobbling interpretation. While the in-band quadrupole moments for the bands show a decreasing trend towards higher spin, the ratio of the interband to the in-band transition strengths remains constant. Both features can be understood by a small increase in triaxiality towards higher spin. Such a change in triaxiality is also found in cranking calculations, to which the experimental results are compared.