Search Results (16 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.

γ-ray production cross sections for proton and α-particle interactions with ¹²C, ¹⁶O, ²⁴Mg, and Fe have been measured in the energy range 5–25 MeV with proton beams and 5–40 MeV with α-particle beams. Isotopically pure foils of ²⁴Mg and foils of natural isotopical composition of C, MgO, and Fe have been used. γ-ray angular distributions were obtained with five high-purity Ge detectors with bismuth germanate Compton shields placed at angles of 45^° to 157.5^°. Cross sections for more than 50 different γ-ray transitions were extracted, and for many of them no data have been published before. Comparison of present data with data available in the literature shows mostly good to excellent agreement. In addition to the production cross sections, high-statistics, low-background line shapes of the 4.438 MeV ¹²C γ ray from inelastic scattering off ¹²C and spallation of ¹⁶O were obtained. Comparison with nuclear reaction calculations shows that these data place interesting constraints on nuclear reaction models.

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 ^92-96Zr nuclei were produced as fission fragments following the fusion reactions ²⁸Si+¹⁷⁶Yb and ³¹P+¹⁷⁶Yb at 145 and 152 MeV bombarding energy, respectively. Prompt γ rays emitted in the two reactions were detected with the EUROGAM II and EUROBALL IV arrays. Sequences of γ-ray transitions observed in coincidence were newly assigned to ^93-96Zr. The previously known level schemes have been extended to higher excitation energies and higher spins. The experimental results are discussed in the framework of shell-model calculations with ⁸⁸Sr assumed to be an inert core and the valence protons and neutrons filling the π(2p_1/2,1g_9/2) and ν(2d_5/2,3s_1/2) orbitals.

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.

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.

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.

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.

High angular momentum states in ^195-197Bi are populated in two reactions: ¹⁸³W(¹⁹F,xn)^202-xBi and ¹⁸¹Ta(²⁰Ne,xn)^201-xBi at beam energies of 108 and 123 MeV, respectively. Gamma rays were detected using the Gammasphere array. Three weakly populated rotational sequences have been found. They each have properties characteristic of other superdeformed bands in this mass region. On the basis of cross-bombardment information we believe that one band belongs to each of ¹⁹⁵Bi, ¹⁹⁶Bi, and ¹⁹⁷Bi. The properties of the bands in the odd-Bi nuclei are best reproduced if the odd proton occupies the favored signature of the [651]1/2 orbital, while the band in ¹⁹⁶Bi has this same proton configuration coupled to an additional N=7 (j_15/2) neutron. The relative behavior of the J⁽²⁾ moments of inertia can be qualitatively understood in terms of Pauli-blocking effects. © 1996 The American Physical Society.

High angular-momentum states in ^196,197Bi were populated in the reaction ¹⁸³W (¹⁹F,xn) at a beam energy of 108 MeV, and γ rays were detected with the Gammasphere array. Two weakly populated rotational bands, with energy spacings characteristic of superdeformation have been found. Both cascades can be assigned unambiguously to the Bi nuclei; however, their isotopic assignment to ¹⁹⁷Bi is tentative. The properties of the bands and their possible structures are discussed. Our results represent the first identification of superdeformed bands in a nucleus of the A∼190 mass region with Z≳82.

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.

Dipole transitions linking signature partner superdeformed bands in ¹⁹³Hg have been observed. Measurements of the photon decay branching ratios, taken together with the average superdeformed quadrupole moment measured in neighboring nuclei, enable the absolute M1 strengths to be determined. From these data, using the strong coupling model, we find (g_K-g_R)K/Q₀=-0.14±0.01 (e b)^-1, g_K=-0.65±0.14 with Ω=2.8±0.8. These data are consistent with the superdeformed neutron orbital being [512]5/2^- as predicted by cranked Woods-Saxon calculations.