Search Results (42 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 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 excited states of the neutron-rich nucleus ₁₅³⁷P₂₂ have been populated in grazing reactions, using a beam of ³⁶S ions (at 215 MeV) delivered onto a thin ²⁰⁸Pb target. Emitted γ rays from excited projectile-like nuclei were detected using the CLARA array of 25 escape-suppressed Ge clover detectors in coincidence with reaction products detected and identified with the PRISMA magnetic spectrometer. A level scheme is presented for ³⁷P together with proposed spin assignments. The level structure of ³⁷P is discussed within the context of shell-model calculations by using an improved sdpf effective interaction.

Excited states of N=22 ³⁶Si, populated in deep-inelastic processes produced by the interaction of a 215 MeV beam of ³⁶S ions with a ²⁰⁸Pb target, were studied in the present work. γ rays from the binary fragments detected using CLARA, an array of 25 Ge Clover detectors, were measured in coincidence with projectile-like fragments detected by PRISMA, a large solid angle magnetic spectrometer. Two new γ-ray photopeaks at energies of 1442 and 842 keV were observed and tentatively assigned to the 4⁺→2⁺ and 6⁺→4⁺ transitions, respectively. The systematics of the level structures of N=22 isotones are presented, and a comparison is made of the behavior of Si, Mg, and S isotopes. The level structure of ³⁶Si is also compared with the results of sdpf shell model 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.

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.

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.

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.

Excited states in ²¹⁶Th were investigated via prompt and delayed γ decays and the recoil-decay tagging method. The decay schemes of the I^π  =  (8⁺), t_1/2  =  128(8) μs, the I^π  =  (11^-), t_1/2  =  615(55) ns, and the I^π  =  (14⁺), t_1/2≥130 ns isomers were established. The configuration πh_9/2f_7/2 is assigned to the I^π  =  (8⁺) isomer, which implies that the h_9/2 and f_7/2 states are nearly degenerate. This is ascribed to increased binding of the f_7/2 orbital by its coupling to a low-lying I^π  =  (3^-) state at E_x  =  1687 keV. The role of octupole and pairing correlations for a Z  =  92 shell closure prediction is discussed on the basis of shell model calculations.

A new superdeformed rotational band has been observed in ¹⁵⁴Er using the Euroball Ge detector array. The new band and the one previously observed can be understood as based on coexisting superdeformed structures at prolate and triaxial shapes, respectively. The observation resolves long-standing difficulties in the theoretical interpretation of superdeformed states in ¹⁵⁴Er.

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.

Superdeformed (SD) states of ¹⁹⁰Hg have been studied with the Eurogam Phase 2 γ-ray spectrometer using the ¹⁶⁰Gd(³⁴S,4n) reaction. Two new excited SD bands have been found and identified as belonging to this nucleus, bringing the total number of SD bands in ¹⁹⁰Hg to 4. One of the new bands has a dynamic moment of inertia that is very similar to that of the yrast SD band of ¹⁹⁰Hg and most other SD bands in the A∼190 region. In contrast, the other band has a dynamic moment of inertia which is mainly constant as a function of rotational frequency and exhibits a dramatic increase at the lowest frequencies. The observed dynamic moments of inertia are compared with the results of random phase approximation calculations based on the cranked shell model. Finally, the known excited SD band has been extended towards lower frequencies and new transitions have been found linking this band to the yrast SD band. The extracted B(E1) values of the new linking transitions give further support for the possible octupole vibrational character of this band. © 1996 The American Physical Society.

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.

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.

An experiment using the Eurogam phase II γ-ray spectrometer confirms the existence of an excited superdeformed (SD) band in ¹⁹⁰Hg and its very unusual decay into the lowest SD band over 3–4 transitions. The energies of the transitions linking the two SD bands have been firmly established, and their angular distributions are consistent with a dipole character. Comparisons with calculations using random-phase approximation indicate that the excited SD band can be interpreted as an octupole-vibrational structure.

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.