Langevin description of the charge distribution of fragments originating from the fission of excited nuclei

The charge distribution of fragments originatingfrom the fission of the ²³⁶U compound nucleus is calculated within a stochastic approach based on Langevin equations. The elongation coordinate, the neck-thickness coordinate, and the charge-asymmetry coordinate are chosen as collective variables. The friction parameter of the charge mode is calculated on the basis of two nuclear-viscosity mechanisms, that of one-body and that of two-body dissipation. It is shown that the Langevin approach is applicable to studying isobaric distributions. In addition, the charge distribution in question is studied as a function of the excitation energy of the compound nucleus and as a function of the coefficient of two-body viscosity.     

Fission lifetimes of Th nuclei measured by crystal blocking

Crystal blocking lifetime measurements have been made for highly excited Th nuclei with neutron number well removed from the stability line. Thin W crystals were bombarded with ³²S ions in the energy range 170-180 MeV and the yield of fission fragments was measured for emission close to a 〈111〉 axis. The fission blocking dips are compared to the appropriately scaled ones for elastic scattering of the ³²S beam ions and no significant difference is seen between the dips. This implies that the fraction of nuclei fissioning with lifetimes longer than 10 as is less than 2%. Fission lifetimes are increased by viscosity in the nuclear mass flow and comparison with a statistical model calculation indicates that the viscosity parameter, η, must be lower than for Th and U nuclei near β-stability. The effect of the N = 126 magic number is discussed. Received: 2 October 2002 / Accepted: 16 January 2003 / Published online: 29 April 2003     

On the nature of the 17 μs isomer of the <Superscript>133</Superscript>Sb valence nucleus

The decay of the 17 μs isomer of ¹³³Sb was re-investigated experimentally. It was produced by thermal neutron induced fission of ²⁴¹Pu. Its detection is based on time correlation between fission fragments selected by the LOHENGRIN spectrometer at ILL (Grenoble), and the γ-rays, and conversion electrons from the isomer. The interpretation of the level scheme is based on shell model calculations, where empirical two-body matrix elements were employed. The good agreement between theory and experiment suggests that the isomer is the 21/2⁺ member of the 2p-1h πg _7/2ν(f _7/2 h _11/2 ⁻¹) configuration. Received: 20 January 2000 / Revised version: 28 February 2000     

Elastic, inelastic scattering and fusion of the 14N +59Co system at energies close to the coulomb barrier

Elastic and inelastic scattering differential cross sections were measured in the energy range 30 MeV ≤ E _lab ≤ 55 MeV, for the ¹⁴N +⁵⁹Co system. Ambiguities of the optical potential derived from the analysis of the elastic scattering data were removed by performing calculations at the radius of sensitivity and by comparison with the available fusion cross section data. A simultaneous analysis of the three mechanisms was performed by coupled channel calculations, and a unique energy independent nuclear potential was found to be able to fit the data. Discussions and comparisons concerning the optical model, the threshold anomaly, full and approximated coupled channel calculations are presented. Received: 6 February 1997 / Revised version: 1 August 1997     

One- and two-body dissipation in heavy-ion collisions

A microscopic theory has been formulated for one- and two-body dissipation in collisions between two heavy nuclei. With a nucleon-nucleon interaction as the basic perturbation in a density matrix approach with “linear response” approximations, the one- and two-body nuclear friction coefficients for the ⁴⁰Ca + ⁴⁰Ca system have been calculated and their dependence on relative kinetic energy and smearing of nuclear single-particle states was obtained. The results of our calculation show that: (a) the combined one- and two-body friction coefficients compare favourably with phenomenological values, (b) the one-body dissipation is more effective than two-body in kinetic energy damping, while both the mechanisms are comparable for the damping of relative angular momentum, (c) the importance of the two-body friction compared to one-body increases at higher relative kinetic energies and (d) the effect of introducing a smearing in nuclear levels appears as a lowering of nuclear friction.     

First energy and angle differential measurements of e+e−-pairs emitted by internal pair conversion of excited heavy nuclei

We present the first energy and angle resolved measurements of e⁺e⁻-pairs emitted from heavy nuclei (Z≥ 40) at rest by internal pair conversion (IPC) of transitions with energies of less than 2 MeV as well as recent theoretical results using the DWBA method, which takes full account of relativistic effects, magnetic substates and finite size of the nucleus. The 1.76 MeV E0 transition in ⁹⁰Zr (⁹⁰Sr source) and the 1.77 MeV M1 transition in ²⁰⁷Pb (²⁰⁷Bi source) have been investigated experimentally using the essentially improved setup at the double-ORANGE β-spectrometer of GSI. The measurements prove the capability of the setup to cleanly identify the IPC pairs in the presence of five orders of magnitude higher β⁻ and γ background from the same source and to yield essentially background-free sum spectra despite the large background. Using the ability of the ORANGE setup to directly determine the opening angle of the e⁺e⁻–pairs (Θ_e+e−), the angular correlation of the emitted pairs was measured within the range covered experimentally (40°≤Θ_e+e−≤ 180°). In the ⁹⁰Zr case the correlation could be deduced for a wide range of energy differences E _Δ of the pairs (−530 keV ≤E _Δ≤ 530 keV). The ⁹⁰Zr results are in good agreement with recent theory. The angular correlation deduced for the M1 transition in ²⁰⁷Pb is in strong disagreement with theoretical predictions derived within the Born approximation and shows almost isotropic character. This is again in agreement with the new theoretical results. Received: 22 April 1997 / Revised version: 16 December 1997     

Polarization observables in (γ,NN) reactions

The formalism of ( γ, NN) reactions is given where the incident photon is polarized and the outgoing-nucleon polarization is detected. Sixteen structure functions and fifteen polarization observables are found in the general case, while only eight structure functions and seven polarization observables survive in coplanar kinematics. Numerical examples are presented for the ¹⁶O(γ, np) and ¹⁶O(γ, pp) reactions. The transitions to the ground state of ¹⁴C and ¹⁴N are calculated in a model where realistic short-range and tensor correlations are taken into account for the np pair, while short-range and long-range correlations are included in a consistent way for pp pairs. The effects of the one-body and two-body components of the nuclear current and the role of correlations in cross-sections and polarizations are studied and discussed. Received: 7 June 2001 / Accepted: 26 September 2001     

Fragment dependence of high energy γ-ray emission in the spontaneous fission of 252Cf

The high energy γ-ray emission accompanying the spontaneous fission of ²⁵²Cf has been measured in coincidence with individual fission fragments selected by discrete γ-ray transitions. The enhancement of the γ-ray emission probability in the energy range E_γ= 3–8 MeV has been observed for the fission fragments in the region of nearly symmetric mass splitting, confirming results reported in previous investigations. The γ-γ coincidence technique employed in the present work clearly demonstrate that the major contribution to this enhancement is caused by the fission channels where one fragment is near to the N= 82 or Z= 50 shell closures. The high energy γ-ray emission probability does not show any significant dependence on the number of neutrons emitted in the fission process, supporting the hypothesis that high energy γ-rays are mainly emitted from the fragments after the neutron evaporation. Received: 22 December 1998     

Production of medium-weight isotopes by fragmentation in 750 A MeV 238U on 208Pb collisions

Projectile fragmentation of ²³⁸U in a lead target was investigated at a bombarding energy of 750 A MeV. Isotopic production cross sections of about 250 different projectile fragments in the element range Z= 30–53 were measured with the FRagment Separator (FRS). The magnetic selection and the kinematical analysis of the measured isotopes allowed to disentangle fission and fragmentation residues. The mass loss of these residues indicates a violent collision where a large amount of energy is dissipated. The position of the fragmentation corridor defined by the measured residues was used to determine an effective proton-evaporation barrier. Received: 3 October 1997 / Revised version: 27 February 1998     

Production of neutron-rich isotopes in fission of uranium induced by neutrons of 20 MeV average energy

In the context of a parameter study conducted by several laboratories for future European radioactive beam facilities based on fast-neutron induced fission, in particular for the SPIRAL-II project at GANIL, we have measured the yields of neutron-rich isotopes in the mass range of 88 to 144. These nuclei were obtained as fission products of natural uranium bombarded by neutrons of 20 MeV average energy emitted by a thick carbon target irradiated by 50 MeV deuterons. Yields have been measured using on-line mass separation with the ion-guide method. Compared with proton-induced fission at 25 MeV the magnitude of cross-sections, except for the symmetric region, is similar. Z-distributions of isobars have the same width, 0.7 charge units, but their maxima are shifted by about 0.8 charge units, favouring production of the neutron-richer isobars. Our data allow calculations of absolute cross-sections for fission of natural uranium induced by neutrons of about 20 MeV. Received: 10 July 2000 / Accepted: 27 October 2000     

Further measurement of the 7Be(p,γ)8B cross section at low energies with the coulomb dissociation of 8B

We have measured the dissociation of ⁸B in the Coulomb field of ²⁰⁸Pb at E_in=51.9 MeV/nucleon and extracted the cross section of the ⁷Be(p,γ)⁸B reaction at 0.4 ≤ E_rel≤ 3 MeV, which is of importance for the ⁸B solar-neutrino production rate. The extracted astrophysical S₁₇ factors are consistent with our earlier Coulomb dissociation measurement, and agree with the values deduced from the direct capture measurements by Filippone et al., Vaughn et al. and Hammache et al. The S factor at zero energy was extracted to be 18.9±1.8 eV-b with the help of theoretical energy-dependence. Received: 15 August 1998     

Measurement of the 24Mg(p, t)22Mg reaction for the states near the 21Na + p threshold

Differential cross-sections of the ²⁴Mg(p, t)²²Mg reaction were measured at 34.68 MeV for the states near the proton threshold at 5.502 MeV in ²²Mg. Among them, the new states at 5.962, 6.046, 6.246 and 6.323 MeV, which were reported previously, have been confirmed. Angular distributions for these states were analyzed by distorted-wave Born-approximation calculations to deduce the spins and parities. The angular distribution for the 5.714 MeV state, which is considered to be most crucial for the stellar reaction ²¹Na(p, γ)²²Mg, has been found to be consistent with J ^π = 2⁺ assignment. The 6.046 MeV state is newly assigned to have J ^π = 0⁺, and the 5.962 MeV state is tentatively assigned to have J ^π = (1^-). These two states will also play an important role for ²²Mg production in novae. Received: 7 March 2002 / Accepted: 7 May 2002     

Prompt dipole γ-ray emission in fusionlike heavy-ion reactions

The ³²S + ¹⁰⁰Mo and ³⁶S + ⁹⁶Mo fusionlike reactions were studied at incident energy of E _lab = 298 MeV and 320 MeV, respectively, with the aim of probing the influence of the entrance channel charge asymmetry on the dipole γ-ray emission. The excitation energy and spin distribution of the compound nucleus created in these reactions were identical, the only difference being associated with the unequal charge asymmetry of the two entrance channels. High-energy γ-rays were detected in an array of 9 seven-pack BaF₂ clusters. Coincidence with fusionlike residues detected in four PPAC ensured the selection of central reaction events. By studying the differential γ-ray multiplicity associated with the two reactions it was shown that the dipole strength excited in the compound nucleus increases with the entrance channel charge asymmetry. From the linearized spectra, the increase of the GDR γ-ray intensity was found to be ∼ 25% for the more charge asymmetric system. The results are discussed and compared with those of previous data obtained at different incident energies. Received: 21 October 2002 / Accepted: 23 December 2002 / Published online: 1 April 2003     

Nuclear levels in 228Th populated in the decay of 228Pa (II)

The electron-capture decay of ²²⁸Pa to levels in ²²⁸Th was studied using mass-separated sources and a γ-ray detection system consisting of five Compton-suppressed Ge detectors. A total of 87 levels were observed up to an excitation energy of 2 MeV which are connected by approximately 500 γ-ray transitions. The complete octupole quadruplet, three excited K ^π=0⁺ bands and two K ^π=2⁺ bands were identified below 1.4 MeV. The observed level structure is compared to calculations within the quasiparticle-phonon nuclear model. The surprisingly good agreement indicates that ²²⁸Th has less transitional character than hitherto assumed. Received: 30 March 1998     

Narrow α + <Superscript>28</Superscript>Si elastic-scattering states at high excitation in <Superscript>32</Superscript>S

The excitation function and angular distributions of elastic α-particle scattering on ²⁸Si have been measured in the laboratory energy range 6-28 MeV using a backscattering technique on a thick target, yielding a continuous energy distribution. More than 200 narrow states are observed, with widths in the range ∼ 30-100 keV at excitation energies E ^* = 13-32 MeV. Angular distributions at backward angles were measured, and angular momentum values of more than 83 states have been deduced. The analysis gives spin-parities J ^π, α-partial widths Γ_α and reduced widths of the narrow high-lying resonant states in ³²S. The experimentally observed states display both the negative- and the positive-parity rotational-like sequences with seemingly no parity splitting, a finding which is at variance with most potential-model predictions. The deduced effective moment of inertia indicates a more extended structure than the ground-state configuration. The observed strength of each ℓ-value is analyzed in terms of an underlying split doorway state of Lorentzian form, which yields an interpretation as fragmented rotational α + ²⁸Si states. Received: 26 June 2000 / Accepted: 16 September 2002 / Published online: 4 February 2003 RID="a" ID="a"e-mail: kkallman@abo.fi RID="b" ID="b"Present address: Swedish Polytechnic, FIN-65200 Vasa, Finland. Communicated by D. Guerreau     

Microscopic three-body force for asymmetric nuclear matter

Brueckner calculations including a microscopic three-body force have been extended to isospin-asymmetric nuclear matter. The effects of the three-body force on the equation of state and on the single-particle properties of nuclear matter are discussed with a view to possible applications in nuclear physics and astrophysics. It is shown that, even in the presence of the three-body force, the empirical parabolic law of the energy per nucleon vs. isospin asymmetry β = (N - Z)/A is fulfilled in the whole asymmetry range 0≤β≤1 up to high densities. The three-body force provides a strong enhancement of the symmetry energy which increases with density in good agreement with the predictions of relativistic approaches. The Lane's assumption that proton and neutron mean fields linearly vary vs. the isospin parameter is violated at high density due to the three-body force, while the momentum dependence of the mean fields turns out to be only weakly affected. Consequently, a linear isospin split of the neutron and proton effective masses is found for both cases with and without the three-body force. The isospin effects on multifragmentation events and collective flows in heavy-ion collisions are briefly discussed along with the conditions for direct URCA processes to occur in the neutron star cooling. Received: 18 February 2002 / Accepted: 16 May 2002     

Nuclear matter properties in relativistic mean-field model with σ- ω coupling

The σ-ω coupling is introduced phenomenologically in the linear σ-ω model to study the nuclear matter properties. It is shown that not only the effective nucleon mass M^* but also the effective σ meson mass m _σ ^* and the effective ω meson mass m _ω ^* are nucleon-density-dependent. When the model parameters are fitted to the nuclear saturation point, with the nuclear radius constant r ₀ = 1.14 fm and volume energy a ₁ = 16.0 MeV, as well as to the effective nucleon mass M ^* = 0.85M, the model yields m _σ ^* = 1.09m _σ and m _ω ^* = 0.90m _ω at the saturation point, and the nuclear incompressibility K ₀ = 501 MeV. The lowest value of K₀ given by this model by adjusting the model parameters is around 227 MeV. Received: 23 March 2001 / Accepted: 8 June 2001     

Determination of nonlinear σ-ω-ρ model parameters in the relativistic mean-field theory by nuclear-matter properties

The parameters of the σ-ω-ρ model in the relativistic mean-field theory with nonlinear σ-meson self-interaction are determined by nuclear-matter properties, which are taken as those extracted by fits to data based on nonrelativistic nuclear models. The values of the relevant parameters are C _σ ²∼ 94, C _ω ²∼ 32, C _ρ ²∼ 26, b∼ - 0.09, c∼ 1, and the σ-meson mass m _σ∼ 370 MeV, while the value of the calculated nuclear- surface thickness is t∼ 1.4 fm. The field system is shown to be stable, since the σ-meson self-interaction energy is a lower bound in this whole parameter region with positive c. On the other hand, the effective nucleon mass M^* is larger than 0.73M, if the symmetry incompressibility K_s is assumed to be negative and the nuclear-matter incompressibility K₀ is kept less than 300 MeV. Received: 27 June 2001 / Accepted: 5 October 2001     

High-spin study of odd- A 49In isotopes beyond the neutron mid-shell

The ^{115, 117, 119, 121}In nuclei have been produced as fission fragments in three reactions induced by heavy ions: ¹²C + ²³⁸U at 90MeV bombarding energy, ¹⁸O + ²⁰⁸Pb reaction at 85MeV, and ³¹P + ¹⁷⁶Yb at 152MeV. Their level schemes have been built from gamma-rays detected using the EUROBALL III and IV arrays. High-spin states of ^{117, 119, 121}In nuclei have been identified for the first time. Moreover, isomeric states lying around 2.5MeV have been established in ^{119, 121}In from the delayed coincidences between the fission fragment detector SAPhIR and the gamma array. Most of the observed states can be described by coupling a proton g _9/2 hole to a Sn core, while the intruder band based on an orbital from the π[g _7/2/d _5/2] sub-shells behaves as the ground-state band of neighbouring Cd isotopes. Received: 24 May 2002 / Accepted: 11 June 2002 / Published online: 19 November 2002 RID="a" ID="a"e-mail: porquet@csnsm.in2p3.fr RID="b" ID="b"Present address: CEA-DIF, DAM/DCRE/SDE/LDN, 91680 Bruyères-le-Chatel, France. RID="c" ID="c"Present address: CSNSM IN2P3-CNRS and Université Paris-Sud 91405 Orsay, France. RID="d" ID="d"Present address: CEA/Saclay, DSM/DAPNIA/SPhN, 91191 Gif-sur-Yvette Cedex, France. Communicated by D. Schwalm     

Search for β-delayed fission of the heavy neutron-rich isotope 230Ac

The ²³⁰Ra has been produced via multinucleon transfer reaction and dissipative fragmentation by 60 MeV/u ¹⁸O ion irradiation of ²³²Th targets. The radium was radiochemically separated from the mixture of thorium and reaction products. Thin Ra sources were prepared and exposed to the mica fission track detectors, and measured by a HPGe gamma detector. It is likely that the β-delayed fission of ²³⁰Ac was observed for the first time and the β-delayed fission probability of ²³⁰Ac was tentatively found to be (1.19±0.40)×10^-8. Received: 2 November 2000 / Accepted: 30 January 2001