Search Results (25 total)

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.

The β decay of the neutron-rich ₃₀⁸¹Zn₅₁ has been investigated at the PARRNe mass separator at the IPN Orsay. The sources of ⁸¹Zn were produced using the ISOL (Isotopic Separation On Line) technique by the fission of ^natU exposed to the neutron flux produced by the 26-MeV deuteron beam delivered by the MP-Tandem. With γ and γ-γ coincidence measurements, excited levels were attributed to ₃₁⁸¹Ga₅₀ for the first time. A partial decay scheme for ⁸¹Zn is proposed. The proposed level scheme is well reproduced by shell model calculations using the most recent effective empirical interaction. We show that the structure of this nucleus is consistent with that of the heavier odd-proton N=50 isotones within the assumption of strong proton Z=28 and neutron N=50 effective shell effects. The observed states can be associated to rather simple and clean configurations of three protons placed in the 1f_5/2 and 2p_3/2 orbits.

The energies of the excited states in very neutron-rich ⁴²Si and ^41,43P have been measured using in-beam γ-ray spectroscopy from the fragmentation of secondary beams of ^42,44S at 39A  MeV. The low 2⁺ energy of ⁴²Si, 770(19) keV, together with the level schemes of ^41,43P, provides evidence for the disappearance of the Z=14 and N=28 spherical shell closures, which is ascribed mainly to the action of proton-neutron tensor forces. New shell model calculations indicate that ⁴²Si is best described as a well-deformed oblate rotor.

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.

Nuclear structure of the neutron rich ^25-29Ne nuclei has been investigated through the in-beam γ-ray spectroscopy technique using fragmentation reactions of both stable and radioactive beams. Level schemes have been deduced for these Ne isotopes. In order to examine the importance of intruder fp configurations, they are compared to shell model calculations performed either in the restricted sd or in the larger sdpf valence space. The ^25,26Ne and ²⁷Ne nuclei were found to be in agreement with the sd shell model calculations, whereas ²⁸Ne exhibits signatures of the intruder fp shell contribution.

The excitation energy of the lowest-energy superdeformed band in ¹⁹⁶Pb is established using the techniques of time-correlated γ-ray spectroscopy. Together with previous measurements on ¹⁹²Pb and ¹⁹⁴Pb, this result allows superdeformed excitation energies, binding energies, and two-proton and two-neutron separation energies to be studied systematically, providing stringent tests for current nuclear models. The results are examined for evidence of a “superdeformed shell gap.”

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.

γ-ray production cross sections have been measured for γ-ray lines copiously emitted in the ³He bombardment of ¹⁶O nuclei: the 937-, 1042-, and 1081-keV lines of ¹⁸F and the 1887-keV line of ¹⁸Ne. Four Ge detectors with BGO shielding for Compton suppression were used to measure the angular distributions of the γ rays. The excitation functions have been obtained for ³He bombarding energies from 3.7 to 36 MeV. Total cross sections are tabulated for calculations relevant to γ-ray astronomy. The importance of these lines as diagnosis for the presence and properties of accelerated ³He in solar flares is discussed in light of the measured cross sections.

Lifetimes have been measured for states in four magnetic-rotational bands in ¹⁹⁶Pd using the Doppler-shift attenuation method. Line shapes of several transitions in each band have been analyzed in γ-ray coincidence spectra measured with the EUROBALL spectrometer array. The B(M1) values deduced from the lifetimes show a decrease with increasing spin within the bands as expected for the shears mechanism. The B(M1) values are compared to calculations within the framework of the tilted-axis cranking model.

The structure of neutron-rich ^40,42,44S nuclei has been investigated through in-beam γ-ray spectroscopy using the fragmentation reaction of a 60 MeV A ⁴⁸Ca beam on a thin Be target. E_γ, I_γ, γγ-coincidence, and γ-ray angular distributions were measured for each produced fragment. The level schemes previously containing only a single γ transition were extended, and spin values were proposed for the new states. The experimental results were interpreted by use of microscopic collective-model and large-scale shell-model calculations. The results of the model calculations are consistent with each other, and give a reasonable description of the experimental results. Both models predict an erosion of the N=28 shell closure at Z=16 and suggest a deformed ground state for ^40,42S and a spherical-deformed mixed configuration for ⁴⁴S.

Lifetimes have been measured for states in the yrast (band 1) and an excited superdeformed band (band 2) in ¹⁹⁶Pb. The excited band is thought to be built on an octupole vibration, rather than a quasiparticle excitation. Centroid shifts and line shapes of transitions in γ-ray coincidence spectra measured with the EUROBALL spectrometer have been analyzed. The transition quadrupole moments deduced from these data for the yrast and the excited superdeformed band are identical within the experimental uncertainties. This result shows that the excited band is not built on different deformation-driving orbitals compared to band 1. It is compatible with the previous suggestion of an octupole-vibrational excitation for the signature partner bands 2 and 3.

The neutron-rich ^66,68Ni have been produced at GANIL via interactions of a 65.9A MeV ⁷⁰Zn beam with a ⁵⁸Ni target. Their reduced transition probability B(E2;0₁⁺→2⁺) has been measured for the first time by Coulomb excitation in a ²⁰⁸Pb target at intermediate energy. The B(E2) value for ⁶⁸Ni₄₀ is unexpectedly small. An analysis in terms of large scale shell model calculations stresses the importance of proton core excitations to reproduce the B(E2) values and indicates the erosion of the N  =  40 harmonic-oscillator subshell by neutron-pair scattering.

High spin states in ¹⁹³Tl have been populated by the ¹⁸¹Ta(¹⁶O, 6n) reaction at 110 MeV and γ-ray spectroscopy was performed using the EUROGAM II array. In addition to the two known superdeformed (SD) bands, three new superdeformed bands have been identified. The dynamic moments of inertia of two of these new bands display a sudden and opposite change of slope and two of them have identical transition energies within 2 keV. The behavior of their dynamic moments of inertia and their γ transitions allowed us to propose configurations for the proton orbitals involved around the SD shell closure Z=80. The intrinsic proton states identified are the two signature partners of the [411]1/2 and the negative signature of the [651]1/2 orbitals. Their properties are compared to the predictions of two theoretical calculations.

The neutron-rich nuclei ^109,111Pd have been produced as fission fragments following the fusion reactions ^28,30Si+ ¹⁷⁶Yb at 145 MeV and studied with the EUROGAM2 array. Two signature branches of a new band built on the νh_11/2 orbital have been identified in ¹⁰⁹Pd and the favored signature of the νh_11/2 band has been observed for the first time in ¹¹¹Pd. The spectroscopic properties as well as the rotational behavior of these bands are characteristic of bands expected for a prolate nuclear shape in this mass region. The effect of blocking of the first h_11/2 quasineutron in the odd-A Pd nuclei supports the interpretation of the first crossing in the yrast bands of ^110,112Pd as due to a ν(h_11/2)² pair.

The high-spin normally deformed and superdeformed states of ¹⁹⁴Hg have been populated in an exit channel of the ¹⁶O+¹⁸⁶W reaction at 110 MeV bombarding energy. The Doppler shift analysis of the energies of the γ transitions de-exciting both normally deformed and superdeformed states in ¹⁹⁴Hg indicates a lower recoil velocity of the ¹⁹⁴Hg nucleus compared to that of the ¹⁹⁶Pb nucleus obtained from the 6n exit channel of the complete-fusion compound nucleus. This suggests that incomplete fusion plays the main role in the population of normally deformed and superdeformed states of ¹⁹⁴Hg in this reaction.

Excited states in ¹⁸²Pt have been studied via the heavy-ion reactions ¹⁷⁰Yb(¹⁶O,4n), ¹⁶²Dy(²⁴Mg,4n), and ¹⁶³Dy(²⁴Mg,5n). γ-ray coincidence measurements were performed with arrays of HPGe detectors at the McMaster University Tandem Accelerator Laboratory (¹⁶O-induced reaction) and the Institut de Physique Nucléaire, Orsay (²⁴Mg-induced reactions). The ground-state rotational band has been extended to I=26ħ, and six new band structures have been identified and assigned quasiparticle configurations. The γ-vibrational band and the band built upon the first excited 0⁺ state have also been extended. Properties of the rotational bands are compared with cranked-shell-model and total-Routhian-surface calculations. Evidence concerning shape coexistence at low spin and band crossings at high spin is discussed.

The decay-out spectrum of the superdeformed yrast band in ¹⁹²Hg comprises a quasicontinuum, from which about 50 weak discrete transitions of energy between 1 and 3.2 MeV are resolved. The fluctuations of the one-dimensional quasicontinuum spectrum are studied with the fluctuation analysis method, which shows that of the order of a few thousand different transitions are available in the first step of the decay-out cascades. The experimental effective number of transitions is compared to schematic theoretical calculations.

The six superdeformed bands in ¹⁹³Pb have been studied with the EUROGAM 2 γ-ray spectrometer using the ¹⁶⁸Er(³⁰Si,5n)¹⁹³Pb reaction. The results are discussed in terms of cranked-Hartree-Fock-Bogolioubov-Lipkin-Nogami calculations. From the @FF ⁽²⁾ moment of inertia behavior as function of the rotational frequency and the M1 and E2 decay competition of the superdeformed states, the bands are interpreted as three pairs of signature partners based on quasineutron excitations. Dipole transitions linking two signature partner superdeformed bands have been observed and, for the first time in lead isotopes, the branching ratio B(M1)/B(E2)=0.15±0.04 μ_N²/e² b² has been extracted. © 1996 The American Physical Society.

Excited states in the doubly-odd¹⁸⁴Au nucleus have been studied by in-beam γ-ray spectroscopy. This nucleus was produced through the fusion-evaporation reactions ¹⁶⁵Ho(²⁴Mg, 5n), ¹⁷⁰Yb(¹⁹F, 5n), and ¹⁶¹Dy(²⁷Al, 4n). Different rotational band structures have been observed and interpreted as specific couplings of proton and neutron single-particle excitations present in neighboring odd Au and Pt nuclei. © 1996 The American Physical Society.

From the observed competition between M1 and E2 γ transitions deexciting the states of the two signature partner yrast superdeformed bands in ¹⁹³Tl, the i_{13 / 2} (Ω=5 / 2) intruder proton orbital is found to have (g_K-g_R)K / Q₀=0.138±0.008. A model dependent value of g_s^eff=(0.7±0.2)g_s^free is deduced. A saturation of the T² moments of inertia, at rotational frequency ℏω>0.32 MeV, is observed for the two bands. This feature is discussed in terms of exhausted quasineutron alignment in the presence of substantially reduced quasiproton alignment due to Pauli blocking.

Constrained Hartree-Fock-Bogoliubov calculations based on Gogny’s force have been performed to determine the potential energy surfaces, collective masses, and moments of inertia used to build a five-dimension collective Hamiltonian treating quadrupole motion in the ^{184,186,188,190}Hg isotopes. Many collective states have been predicted at low excitation energy, some of them forming γ-vibrational bands. To expand experimental information on γ bands, high statistics measurements on the β⁺ and electron capture decay of ¹⁹⁰Tl^g,m and ¹⁸⁶Tl^g,m have been performed at the ISOCELE facility. For the first time, γ bands have been identified in ¹⁹⁰Hg and ¹⁸⁶Hg. These new results together with previous experimental information available on the ground state and K=0 excited bands in ^{184,186,188,190}Hg form a database which has been analyzed and discussed in the present theoretical framework. It is argued that the first K=0 excited band in ¹⁹⁰Hg is a β-vibrational band, and that ¹⁸⁶Hg is a nucleus in which shape coexistence occurs not only for the K=0 bands but also for the γ bands.

The complete spectrum of γ rays following the decay of superdeformed (SD) states in ¹⁹²Hg has been extracted. The spectrum, which comprises a quasicontinuous component and sharp lines, reveals the decay mechanism and defines the excitation energies of the SD band. The decay of SD states results from the coupling between a cold, ordered SD system with a hot, chaotic one with normal deformation.

High-spin states in ¹⁸⁹Tl have been populated through the ¹⁶⁵Ho(²⁸Si,4n) reaction and studied with in-beam γ-ray spectroscopy techniques. Both oblate and prolate structures associated with the i13/2 proton orbit are confirmed and extended to higher spins (19/2 and 41/2, respectively). However, only the oblate structure related to the πh9/2 has been observed meaning that the associated prolate structure is nonyrast contrary to expectation. This experimental result points clearly to a large amplitude of (πh9/2)² in the wave function of the prolate minimum of ¹⁸⁸Hg.

The nature of the first band crossing in the heavy rare-earth region is an important open spectroscopic problem. Both h(9/2 protons and i(13/2 neutrons are likely to be responsible for this phenomenon. The results of a double blocking experiment in ¹⁷⁸Re strongly suggest that in fact a pair of h(9/2 protons and of i(13/2 neutrons are simultaneously playing a role.

Levels in ¹⁰⁴Ag and ¹⁰²Ag were studied using mainly the ⁹⁴Mo(¹²C, pn) and the ⁸⁹Y(¹⁶O, 3n) reactions, respectively. A set of standard in-beam measurements involving relative excitation functions of γ rays, γ-γ-t coincidences, conversion electrons, angular distributions, and linear polarization of emitted γ rays have been performed. Excited states with spin values up to 17^- and 14^- are populated in ¹⁰⁴Ag and ¹⁰²Ag, respectively. The experimental data are discussed in the framework of the axial plus two quasiparticle model and some simple rules have been deduced to interpret the behavior of odd-odd transitional nuclei at high angular momentum.

NUCLEAR STRUCTURE ⁹⁴Mo(¹²C, pn), E=41-57 MeV, ⁹²Mo(¹²C, pn), E=50 MeV, ⁸⁹Y(¹⁶O, 3n), E=60-80 MeV, ⁹²Mo(¹⁴N, 2p2n), E=72 MeV; measured I_γ[E(¹²C)], I_γ[E(¹⁶O)], I_γ(θ), P_γ, γ-γ coin, γ(t), E_CE, I_CE. ¹⁰⁴Ag and ¹⁰²Ag deduced levels, J, π, ICC, γ multipolarity, t_{1 / 2}. Enriched ^92,94Mo targets, Ge(Li) detectors, intrinsic Ge Compton polarimeter, electron orange spectrometer. Calculated levels from axial rotor plus two quasiparticle model.