High-spin structures of 51 121, 123, 125, 127Sb nuclei: Single proton and core-coupled states

The ^{121,123,125,127}Sb nuclei have been produced as fission fragments in three reactions induced by heavy ions: ¹²C + ²³⁸U at 90 MeV bombarding energy, ¹⁸O + ²⁰⁸Pb at 85 MeV, and ³¹P + ¹⁷⁶Yb at 152 MeV. Their level schemes have been built from gamma rays detected using the EUROBALL III and IV arrays. High-spin states of ^123,125,127Sb nuclei have been identified for the first time. Moreover isomeric states lying around 2.3 MeV have been established in ^123,125,127Sb from the delayed coincidences between the fission fragment detector SAPhIR and the gamma array. All the observed states can be described by coupling a d_5/2 or g_7/2 proton to an excited Sn core involving either vibrational states or broken neutron pairs.     

New high-spin states of 147Nd and 145Ce: Octupole correlation in the N = 87 isotones

High-spin states of the N = 87 nuclei, ¹⁴⁷Nd and ¹⁴⁵Ce, have been populated in the ¹²C + ²³⁸U and ¹⁸O + ²⁰⁸Pb fusion-fission reactions at 90 MeV and 85 MeV bombarding energy, respectively. The emitted γ-radiation was detected using the Euroball III and IV arrays. High-spin states of the ¹⁴⁷Nd isotope have been identified for the first time. The high-spin yrast and near-to-yrast structures of the ¹⁴⁵Ce nucleus have been considerably extended. The newly observed structures, discussed by analogy with the neighbouring isotones, show the coupling of an h_9/2 neutron to the quadrupole and octupole excitations of the core.     

Enhanced neutron pair transfer and collective excitations in the system 206Pb + 118Sn at barrier energies

At energies below the Coulomb barrier, neutron transfer and Coulomb excitation have been measured in a very heavy asymmetric nuclear system, in ²⁰⁶Pb + ¹¹⁸Sn. These are semi-magic nuclei showing super-fluid properties. Particle-γ coincidence techniques using 5 Euroball Cluster detectors (EB), combined in a set-up with the Heidelberg-Darmstadt NaI Crystal Ball (CB), have been used. Position-sensitive detectors allowed the observation of scattering processes covering angles from 110 up to 150 degrees. The fragments are identified via the known γ-decays of the lowest excited states using the high resolution of EB. Using the unique feature of the set-up with the CB, transfer to well-defined final channels with known quantum numbers is selected using the high-efficiency multiplicity filter of the CB with no second γ-ray, i.e. without feeding. The data are analysed using the semi-classical approach and transfer probabilities are obtained. Coulomb excitation has been analysed using known transition probabilities. The enhancement is deduced for the two-neutron transfer populating the low-lying super-fluid 2⁺ states in ¹²⁰Sn and ¹¹⁶Sn, while the 2n transition remains in the ground state for the ^20NPb nuclei. Large enhancements up to EF ≃ 10³ are observed. This is the first observation of neutron pair transfer enhancement for a heavy nuclear binary system with super-fluid properties with experimentally separated levels. The calculations with microscopic 2-neutron wave functions, with configuration mixing over six shell model configurations and using the coupled reaction channels approach, reproduce well the observed probabilities and the enhancement. Received: 27 August 2002 / Accepted: 9 December 2002 / Published online: 25 March 2003 RID="a" ID="a"e-mail: oertzen@hmi.de Communicated by D. Schwalm     

Gross structure in excitation functions for207Pb(p,p′)207Pb in the region of overlapping isobaric analogue resonances

Prominent resonances in the reaction²⁰⁷Pb(p, p′)²⁰⁷Pb leading to the low-lying states in²⁰⁷Pb(1/2^? g.s., 5/2^? 0.57 MeV, 3/2^? 0.89 MeV, 7/2^? 2.33 MeV) are observed betweenE _p =16 and 18 MeV. The structures in the excitation functions are interpreted as being due to analogues of states in²⁰⁸Pb which are mixtures of neutron particle-hole configurations.     

The PbPb collision

The reaction ²⁰⁸Pb on ²⁰⁸Pb was studied at bombarding energies of 7.0 and 7.57 MeV/u. One-particle inclusive measurements using a large-area position-sensitive ionisation chamber delivered the kinetic energy, charge and scattering angle of the reaction products. A precise calibration of the stopping power for very heavy ions in the detector gas was performed. The measured Wilczynski diagrams show, for increasing loss of kinetic energy, an increase of the mean scattering angle. It is attributed to the dominance of the repulsive Coulomb forces with respect to the attractive nuclear forces. The element distribution for the ²⁰⁸Pb on ²³⁸U reaction at 7.5 MeV/u was also measured and compared to the PbPb and UU reactions. Fission probabilities are derived as a function of charge and total kinetic energy loss. The most striking result is seen in the σ_z² versus TKEL correlation: the average rate of energy loss per nucleon exchange is abnormally large. It is shown that this behaviour is associated with the double magic closed shell character of the colliding nuclei. Nuclear structure information is extracted through a simple parametrisation.     

The neutron and proton two-particle nucleus 134Sb: New low-spin states observed in the decay of 134Sn and an estimate of the energy of the 7- isomer

Excited states in the ¹³⁴Sb nucleus, populated in the β^--decay of ¹³⁴Sn, have been studied at the mass separator OSIRIS. The ¹³⁴Sn activity was produced via fast neutron-induced fission of ²³⁸U target. A main result was the discovery of a very low-lying first-excited state of ¹³⁴Sb, at 13keV, which has led to a strong revision of the level scheme. The new results are compared with different theoretical calculations and with the known data for the analogous neutron and proton two-particle nucleus in the ²⁰⁸Pb region. On the basis of this comparison, the energy of the ( πg _7/2νf _7/2)_7- isomer is estimated to be about 250keV, some 100keV lower than previously reported. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: anovak@fuw.edu.pl     

Angular correlations from 6Li break-up near the Coulomb barrier of 118Sn and 208Pb

Absolute coincidence cross sections for three-particle final states have been measured inplane in the ⁶Li + ¹¹⁸Sn and ⁶Li +²⁰⁸Pb reactions at incident energies of 22.2 and 23.0 MeV, respectively. Most strongly populated is the α + d branch, proceeding sequentially through the first excited state (E_x = 2.18 MeV, J^π = 3⁺) of ⁶Li. The angular distributions are fitted by DWBA calculations including Coulomb interaction in the excitation of the projectile. The main competing reaction channels above and at the Coulomb barrier (¹¹⁸Sn: E/E_c ≈ 1.3; ²⁰⁸Pb: E/E_c ≈ 0.9) are: neutron transfer (⁶Li, αp) and the non-sequential α + d break-up of ⁶Li. The latter spectra are consistent with a quasi-free break-up mechanism. No ³He or tritons have been found in the coincidence spectra, as well as no evidence for a three-particle dissociation of ⁶Li into α + p + n.     

Effect of neutron transfer channels in fusion reactions with weakly bound nuclei at subbarrier energies

The role of neutron transfer in fusion reactions of weakly-bound nuclei at subbarrier energies is studied within the empirical model of channel coupling. The results from calculating the fusion cross sections for the ⁷Li + ²⁰⁹Bi, ^{9, 11}Li + ^{208, 206}Pb, ^{6, 7, 9, 11}Li + ¹⁵²Sm reactions are presented. Good agreement with the available experimental data is shown. Several combinations of colliding nuclei for which the strong enhancement of subbarrier fusion due to the effect of neutron transfer processes are predicted.     

Strong enhancement of two-neutron transfer in the system 206Pb+118Sn

One and two neutron transfer has been measured in the heaviest asymmetric nuclear system with semi magic nuclei showing superfluid properties, in ²⁰⁶Pb+¹¹⁸Sn collisions at an energy well below the Coulomb barrier with scattering orbits covering the largest angles. Particle-γ coincidence techniques using 5 Euroball-Cluster detectors (EB) combined in a set-up with the Heidelberg-Darmstadt NaI-Crystal Ball (CB) have been used. Transfer channels are identified with EB via their known γ-decays of the lowest excited states. Using the unique feature of the set-up with the CB, transfer to well defined final states with known quantum numbers (without feeding) are selected using the high efficiency multiplicity filter of the CB (no second γ-ray). The data are analysed using the semiclassical approach and transfer probabilities are obtained. The enhancement for the two-neutron transfer populating the low lying superfluid 2⁺ state in ¹²⁰Sn (and ¹¹⁶Sn), while the Pb-branch is in the groundstate is deduced by comparison with the strongest single neutron transfer transition. Large enhancements (EF ≃ 10³) are observed. This is the first direct measurement of enhancement for a heavy nuclear binary system with experimentally separated levels suggesting a strong contribution from superfluid pair transfer. Received: 18 December 1998 / Revised version: 23 February 1999     

Study of giant pairing vibrations with neutron-rich nuclei

We investigate the possible signature of the presence of giant pairing states at an excitation energy of about 10 MeV via two-particle transfer reactions induced by neutron-rich weakly bound projectiles. Performingparticle-particle RPA calculations on ²⁰⁸Pb and BCS + RPA calculations on ¹¹⁶Sn, we obtain the pairing strength distribution for two-particle addition and removal modes. Estimates of two-particle transfer cross sections can be obtained in the framework of the macroscopic model. The weak-binding nature of the projectile kinematically favors transitions to high-lying states. In the case of the (⁶He, ⁴He) reaction, we predict a population of the Giant Pairing Vibration with cross sections of the order of a millibarn, dominating over the mismatched transition to the ground state.     

<Emphasis Type="Italic">E</Emphasis>2 transitions between excited single-phonon states: Role of ground-state correlations

The probabilities for E2 transitions between low-lying excited 3^? and 5^? single-phonon states in the ²⁰⁸Pb and ¹³²Sn magic nuclei are estimated on the basis of the theory of finite Fermi systems. The approach used involves a new type of ground-state correlations, that which originates from integration of three (rather than two, as in the random-phase approximation) single-particle Green’s functions. These correlations are shown to make a significant contribution to the probabilities for the aforementioned transitions.     

Dynamical model of fission fragment angular distributions

A dynamical model of fission fragment angular distributions is developed. The experimental data on the angular anisotropy of fission fragments is analyzed for the ¹⁶O + ²⁰⁸Pb, ²³²Th, ²³⁸U, and ²⁴⁸Cm reactions at energies of the incident ¹⁶O ions ranging from 90 to 160 MeV. This analysis allows us to extract the relaxation time for the tilting mode. It was also demonstrated that the angular distributions are sensitive to the deformation dependence of the nuclear friction.

     

RPA-calculations in 208Pb with a density dependent interaction

Excitation energies and excitation probabilities of the low lying states of ²⁰⁸Pb are calculated within the theory of Finite Fermi System (FFS), using a density dependent residual interaction. A strong M2-transition at 7.51 MeV is predicted.     

A STUDY OF EXITATION ENERGIES SHARING BETWEEN FRAGMENTS IN HEAVY-ION REACTIONS

In this paper the excitation energies sharing between fragments in heavy-ion reaction were analysed based on Coherent Surface Excitation Model (CSEM) For ⁵⁶Fe+²³⁸U at E_Lab=476MeV and ⁸⁶Kr+²⁰⁸Pb at E_Lab=1565MeV reactions,a comparison between experimental data and calculated values has been made.     

201 MeV proton excitation of giant resonances in 208Pb: Macroscopic and microscopic analysis

The region of the giant resonances in ²⁰⁸Pb has been investigated by inelastic scattering of 201 MeV protons. To test the analysis, angular distributions were measured for the low-lying 3^?, 5^?, 2⁺ and 4⁺ collective states. The giant isoscalar quadrupole resonance (ISGQR) is split into two structures, one at 9.0 MeV with a full width at half-maximum Γ = 1.0 MeV, the other one at 10.6 MeV (Γ = 2.0 MeV), with fine structures at 8.9, 9.3, 10.1, 10.6 and 11 MeV. A macroscopic analysis using the distorted-wave Born approximation (DWBA) leads for the low-lying collective levels, as well as for the ISGQR, to transition probabilities too small by a factor of two, compared with those obtained in other reactions. Microscopic analysis using the distorted-wave impulse approximation (DWIA), with three different sets of random phase approximation (RPA) transition densities, is in very good agreement with the data. At forward angles, in the 12 to 16 MeV excitation energy region, a strong resonance at 13.5 MeV (Γ = 3.6 MeV) is accounted for by the Coulomb excitation of the isovector giant dipole resonance (IVGDR); at larger angles the results are compatible with the excitation of the isoscalar monopole resonance (ISGMR) located at 13.9 MeV (Γ = 2.6 MeV).A resonance located at 21.5 MeV (Γ = 5.7 MeV) appears as the superposition of an isovector quadrupole resonance (IVGQR) excited by Coulomb interaction and a resonance of multipolarity L = 1 ΔT = 0 (ISGDR “squeezing mode”).     

Evidence for multiphonon giant resonances in electromagnetic fission of 238U

Differential cross sections for electromagnetic fission of 238U projectiles (500 MeV/u) in C, Sn, and Pb targets are measured and analyzed in terms of single- and multiphonon giant resonance excitations as doorway states to fission. A novel experimental method exploits the linear relationship between neutron multiplicity and the primary 238U excitation energy. Multiphonon states contribute up to 20% of the cross section; a component at high excitation energies is indicated that may arise from three-phonon dipole and two-phonon GDR x GQRiv giant resonance excitations.     

Evidence of projectile polarisation effects on the reaction 208Pb(17O, 16O)209Pb

Coupled reaction channels calculations (CRC) of the reactions ²⁰⁸Pb(¹⁷O, ¹⁶O)²⁰⁹Pb leading to different states of ²⁰⁹Pb are compared with DWBA predictions at projectile energies of 78, 86 and 102 MeV. The calculations exhibit strong effects of multistep processes on Q-value and angular-momentum-mismatched transfer reactions. It is shown that the contribution to the transfer through the inelastic excitation of ¹⁷O contains a major part of the multistep effect. A simple three-channel model comprising the elastic, inelastic and transfer channels is constructed which simulates the CRC effects on the transfer cross sections. The polarization effects of the eliminated channels give rise to effective potentials which are mainly imaginary.     

Inelastic scattering and nucleon transfer in the system232Th+206Pb near the Coulomb barrier

Nucleon transfer accompanied by Coulomb excitation was studied in the system²⁰⁶Pb+²³²Th atE _Lab=6.4 MeV/u. Particle-particle-gamma coincidence techniques were used to identify excited states of reaction products populated through inelastic scattering and nucleon transfer reactions. The mean excitation energy was measured by means of aγ-ray energy and multiplicity filter consisting of 6 NaI detectors. Large cross sections for one-neutron and two-neutron pick-up from²³²Th are observed. The impact-parameter dependence of the neutron transfer is analyzed in terms of semiclassical barrier penetration models. Using realistic neutron potentials with a diffuse surface, the experimental data are in accordance with the assumption of a “cold” transfer to states near the yrast line.     

Self-consistent description of <Emphasis Type="Italic">EL</Emphasis> transitions between one-phonon states in magic nuclei

Transition probabilities between low-lying one-phonon states of magic nuclei are for the first time computed self-consistently within an approach to anharmonic effects based on the quantum theory of many-body systems. In the adopted approach, three-quasiparticle correlations in the ground state are taken into account, and the nuclear mean field is interrelated with the effective nucleon–nucleon interaction. These quantities are derived using the energy density functional method with known parameters of the Fayans functional. The E1 and E2 transitions in the ¹³²Sn and ²⁰⁸Pb nuclei are considered as an example, and a reasonably good agreement with the data on these nuclei is reached. Three-quasiparticle correlations in the ground state are shown to make a significant contribution to the probabilities of the discussed transitions.     

Angular anisotropy of the fusion-fission and quasifission fragments

The anisotropy in the angular distribution of the fusion-fission and quasifission fragments for the ¹⁶O + ²³⁸U , ¹⁹F + ²⁰⁸Pb and ³²S + ²⁰⁸Pb reactions is studied by analyzing the angular-momentum distributions of the dinuclear system and compound nucleus which are formed after capture and complete fusion, respectively. The orientation angles of the axial symmetry axes of the colliding nuclei relative to the beam direction are taken into account for the calculation of the variance of the projection of the total spin onto the fission axis. It is shown that there is a large contribution of the quasifission fragments in the ³²S + ²⁰⁸Pb reaction to the deviation of the experimental angular anisotropy from the statistical model results. Enhancement of anisotropy at low energies in the ¹⁶O + ²³⁸U reaction is connected with the quasifission of the dinuclear system having low temperature and relatively small effective moment of inertia.