Search Results (69 total)

The E_γ-E_γ coincidence spectra from the electromagnetic decay of excited superdeformed states in ¹⁹⁴Hg reveal surprisingly narrow ridges, parallel to the diagonal. A total of 100–150 excited bands are found to contribute to these ridges, which account for nearly all the unresolved E2 decay strength. Comparison with theory suggests that these excited bands have many components in their wave functions, yet they display remarkable rotational coherence. This phenomenon can be explained in terms of the combination of shell effects and motional narrowing.

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.

The quasicontinuum of γ rays from the feeding and decay of superdeformed and normal bands in ¹⁵²Dy have been extracted in one- and two-dimensional spectra. The E_γ-E_γ correlations in the latter reveal strong ridges associated with superdeformed and normal states in this nucleus. The entry distributions for normal and superdeformed bands have been extracted from measured fold and sum-energy distributions. A Monte Carlo model was developed to simultaneously describe all the quasicontinuum and ridge spectra as well as the feeding intensity of the superdeformed bands. The rotational damping widths in the normal and superdeformed wells were derived based on a comparison of the data with model calculations of the continuum of γ rays at finite temperature.

The possibility that atomic nuclei possess stable, extremely elongated (hyperdeformed) shapes at very high angular momentum is investigated in the light of the most recent experimental results. The crucial role of the Jacobi shape transitions for the population of hyperdeformed states is discussed and emphasized. State-of-the-art mean-field calculations including the most recent parametrization of the liquid-drop energy together with thermal effects and minimization algorithms allowing the spanning of a large deformation space predict the existence of a region of hyperdeformed nuclei in the mass A∼120–130: Te, Cs, Xe, I, and Ba isotopes. In agreement with predictions presented in reviews by J. Dudek, K. Pomorski, N. Schunck, and N. Dubray [Eur. Phys. J. A 20, 15 (2003)] and J. Dudek, N. Schunck, and N. Dubray [Acta Phys Pol. B 36, 975 (2005)], our extended calculations predict that only very short hyperdeformed bands composed of a dozen discrete transitions at the most are to be expected–in contrast to the results known for the superdeformed bands. We stress the importance of the experimental research in terms of multiple-γ correlation analysis that proved to be very efficient for the superdeformation studies and seems very helpful in the even more difficult search for the discrete transitions in hyperdeformed nuclei.

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.

Excited states in ^98,100,102Mo have been studied via the ³⁰Si+¹⁶⁸Er-induced fission reaction at a beam energy of 142 MeV. Prompt γ rays were detected with the EUROBALL III multidetector array. The level schemes are extended with more than 20 new transitions and interpreted in the framework of a soft-octupole vibration model.

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.

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.

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.

The γ decay in the quasicontinuum is used to study the order-to-chaos transition in the thermally excited ¹⁶³Er nucleus. The experimental analysis is performed on high-statistics EUROBALL data, focusing on the spin region I≈(20–40)ℏ, and internal excitation energy up of ≈2.5  MeV. The results are compared to cranked shell model calculations for this nucleus, taking into account the dependence on the K quantum number. Two main topics are investigated. First, the validity of the selection rules associated with the K quantum number are studied as a function of the internal energy U above yrast. K-selection rules are found to be obeyed in the decay along discrete unresolved rotational bands up to U≈1.2  MeV, whereas in the interval U≈1.2–2.5  MeV, where the order-to-chaos transition is expected to take place, selection rules are found to be only partially valid. Second, the line-shape analysis of γ-γ coincidence spectra provides a direct experimental measurement of the rotational and compound damping widths (Γ_rot and Γ_μ), yielding values of 200 and 20 keV, respectively, in good agreement with theory.

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.

The gamma-ray decay of the giant dipole resonance (GDR) in the compound nucleus ²¹⁶Rn formed with the reaction ¹⁸O+¹⁹⁸Pt at the bombarding energy of 96 MeV was investigated. High-energy gamma-ray spectra in coincidence with both prompt and delayed low-energy transitions were measured. The obtained GDR width at the average temperature ⟨T⟩≈1  MeV was found to be larger than that at T=0  MeV and to be approximately constant as a function of spin. The measured width value of 7 MeV is found to be consistent with the predictions based on calculations of the nuclear shape distribution using the newest approach for the treatment of the fission barrier within the liquid drop model. The present study is the first investigation of the giant dipole resonance width from the fusion-evaporation decay channel in this nuclear mass range.

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).

High-spin states in ¹²³Cs were populated in the ⁶⁴Ni(⁶⁴Ni,p4n) reaction at a beam energy of 265  MeV. Gamma-ray coincidences were measured using the Gammasphere spectrometer. Two additional bands have been placed in the level scheme and the four previously known bands have been extended to higher spin. At the highest spins, two of the bands show irregular level sequences. These structures of noncollective excitations, which compete with collective rotation, are interpreted as band-terminating states. The results are compared to cranked Nilsson-Strutinsky calculations, and configuration assignments to the bands and to the terminating states are discussed.

The γ decay in the quasicontinuum from selected configurations of the rotational nucleus ¹⁶³Er has been measured with the EUROBALL array. A new analysis technique has allowed for the first time to directly measure the compound and rotational damping widths Γ_μ and Γ_rot. Values of Γ_μ≈20  keV and Γ_rot≈200  keV are obtained in the spin region I≈30–40ℏ︀, in good agreement with microscopic cranked shell model calculations. A dependence of Γ_μ and Γ_rot on the K-quantum number of the nuclear states is also presented.

Multiple band structures in ¹⁷⁹Au are established from γ-ray spectroscopic measurements with Gammasphere at the Argonne Fragment Mass Analyzer. The yrast band, based on the 13∕2⁺ proton, confirms the predicted drop in excitation energy of the prolate deformed band head as compared to the heavier isotopes. The implications for the prolate energy minimum in odd-mass Au nuclei beyond the neutron i_13∕2 midshell (N<102) are discussed.

Nine transitions of dipole character have been identified linking an excited superdeformed (SD) band in ¹⁵²Dy to the yrast SD band. As a result, the excitation energy of the lowest level in the excited SD band has been measured to be 14 238 keV. This corresponds to a 1.3 MeV excitation above the SD ground state. The levels in this band have tentatively been determined to be of negative parity and odd spin. The measured properties are consistent with an interpretation in terms of a rotational band built on a collective octupole vibration.

The nucleus ¹⁶³Lu has been populated through the reaction ¹³⁹La(²⁹Si,5n) with a beam energy of 157 MeV. Three triaxial, strongly deformed (TSD) bands have been observed with very similar rotational properties. The first excited TSD band has earlier been assigned as a one-phonon wobbling excitation built on the lowest-lying (yrast) TSD band. The large B(E2)_out/B(E2)_in value obtainable for one of four observed transitions between the second and first excited TSD bands is in good agreement with particle-rotor calculations for a two-phonon wobbling excitation.

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.

The excitation energy, spin, and parity of the yrast superdeformed band in ¹⁵²Dy have been firmly established. The evidence comes mainly from the measured properties of a 4011 keV single-step transition connecting the yrast superdeformed level fed by the 693 keV transition to the 27^- yrast state. Four additional, weaker, linking γ rays have been placed as well. The excitation energy of the lowest superdeformed band member is 10 644 keV and its spin and parity are determined to be 24⁺.

The nucleus ¹⁶³Lu has been populated through the fusion-evaporation reaction ¹³⁹La(²⁹Si,5n)¹⁶³Lu with a beam energy of 152 MeV. The electromagnetic properties of several connecting transitions between two presumably triaxial, strongly deformed (TSD) bands have been studied. Evidence is presented for the assignment of the excited TSD band as a wobbling mode built on the yrast TSD band, based on comparisons to new calculations in which an aligned particle is coupled to a strongly deformed triaxial rotor. The wobbling mode is uniquely related to triaxiality in nuclei.

Excited structures in the Z=80, ¹⁷⁸Hg (N=98), and ¹⁸⁰Hg (N=100) isotopes have been investigated with the Gammasphere spectrometer in conjunction with the recoil-decay tagging technique. The present data extend the previously known ground-state bands to higher spin and excitation energy. Negative parity bands with a complex decay towards the low spin states arising from both the prolate-deformed and the nearly spherical coexisting minima have been observed for the first time in both nuclei. It is shown that these sequences have characteristics in common with negative-parity bands in the heavier even-even Hg isotopes as well as in the Os and Pt isotones. These structures are interpreted as being associated at low spin with an octupole vibration which is crossed at moderate frequency by a shape driving, two-quasiproton excitation.

Unresolved γ rays of ¹¹⁴Te sorted into one- and two-dimensional spectra were studied. The reaction ⁶⁴Ni(E_beam  =  230–270 MeV)+⁵⁴Cr was used and the γ rays were detected with the EUROBALL array. The effective moment of inertia for the quasicontinuum was found to be almost constant for I  =  (20–40)ħ, in contrast with the decreasing behavior of the terminating yrast band. A comparative analysis with the nucleus ¹⁶⁴Yb is presented and the results from a fluctuation analysis were compared with cranked shell model calculations. The comparison shows consistency in the scaling of the rotational damping width with mass number.

High-spin states in ¹²⁴Xe were populated in the fusion-evaporation reaction ¹¹⁰Pd(¹⁸O,4n)¹²⁴Xe at a beam energy of 86 MeV. The experiment was performed at the NORDBALL spectrometer in Risø, Denmark. The decay out of a magnetic dipole band was established for the first time in this nucleus. By means of angular correlations from oriented states the multipole mixing ratios for 9 of the 12 gamma transitions of the magnetic dipole band were deduced. The experimental B(M1;I→I-1)/B(E2;I→I-2) values were compared to estimates of the Dönau-Frauendorf semiclassical model. The dipole band is inferred to be based on a prolate four-quasiparticle configuration of the type [ν(h_11/2⊗g_7/2)]⊗[π(h_11/2⊗d_5/2/g_7/2)].