Detailed spectroscopy of superdeformed actinide nuclei

Progress in the experimental techniques used to investigate superdeformed fission isomers in the actinides allowed for detailed spectroscopic results of collective properties as well as for the identification of the rotational structure of multiphonon vibrational excitations. A novel approach could be established to determine the depth of the second potential well.     

Conversion-electron spectroscopy in the decay of superdeformed 135Nd

We have investigated the structure of the superdeformed band in ¹³⁵Nd using conversion-electron spectroscopy. The transitions within the superdeformed band show conversion coefficients which are consistent with an E2 multipolarity. From the conversion coefficients of transitions linking this band with known low-lying states its positive parity was established. Hence, a direct determination of the parity of a high-spin superdeformed band could be performed for the first time.     

Fragments' mass and energy characteristics in the spontaneous fission of 236Pu, 238Pu, 240Pu, 242Pu,and 244Pu

The results of a systematic study of the mass and energy of fission fragments emitted in the spontaneous fission of ^{236,238,240,242,244}Pu are reported. A comparison of the fragments' mass and energy distributions demonstrates the occurrence of different fission modes with varying relative probability. These results are discussed in terms of the random neck rupture model as well as in terms of the scission point model, showing the influence of the neutron number of the fissioning system. Finally an improved method of analysis allows the investigation of the cold fission region.     

Vibrational resonances below 5 MeV in the fission of 240Pu

Using the ²³⁹Pu(d, pf) reaction, the next lower K^π = 0⁺ vibrational resonance below 5 MeV in the deformed potential of ²⁴⁰Pu was located at 4.55 MeV. The resultant spacing of fission vibrations near 5 MeV in the second minimum is 500 keV. A resonance at 4.70 MeV is interpreted as the K^π = 0^? vibration coupled to the 4.55 MeV K^π = 0⁺ state, which yields E(K^π = 0^?) = 150 keV relative to the ground state of the second minimum.     

Resonances in the isomeric and prompt fission probabilities of240Pu

The prompt and isomeric fission probabilities of²⁴⁰Pu have been studied using the²³⁹Pu(d,p) reaction. A number of resonances are observed in the subbarrier population of the 4 ns fission isomer for excitation energies between 4.0 and 6.2 MeV. Apart from a structure at 4.3 MeV, they do not coincide with transmission resonances found in prompt fission. Calculations with an extended doorway state model which simultaneously reproduce the measured delayed and prompt fission probabilities yield revised fission barrier parameters as well as spectroscopic information on fission and gamma widths of highly excited states in the second minimum.     

Conversion--electron gamma--ray coincidence spectroscopy of superdeformed 135Nd

Three mini-orange conversion–electron spectrometers and four Euroball Ge Cluster detectors have been used for γ–e^- coincidence spectroscopy of superdeformed ¹³⁵Nd. Transitions within the superdeformed band are shown to have the expected E2 multipolarity. The 766.5–keV transition which links the band to a positive-parity state has a conversion coefficient consistent with M1 multipolarity. Consequently, positive parity is deduced for the superdeformed band. No evidence for E0 transitions was found. Received: 22 December 1997 / Revised version: 19 January 1998     

An interpretation of the structure of subthreshold fission cross section of240Pu

The structure recently observed in the subthreshold fission cross section of²⁴⁰Pu is interpreted as being due to intermediate states of the fissioning nucleus associated with the second minimum of the potential energy as a function of deformation postulated byStrutinsky.     

Fission Barrier for ^240Pu in the Quadrupole Constrained Relativistic Mean Field Approach

The fission barrier for ^240pu is investigated beyond the second saddle point in the potentiM energy surface by the constrained relativistic mean field method with the newly proposed parameter set PK1. The microscopic correction for the centre-of-mass motion is essentiM to provide the correct potential energy surface. The shell effects that stabilize the nuclei against the fission is also investigated by the Strutinsky method. The shapes for the ground state, fission isomer and saddle-points, etc, are studied in detail.     

Microscopic structure of superdeformed states in Th, U, Pu and Cm isotopes with Gogny force

The structure properties of the even-even nuclei ^{226, 228, 230, 232, 234}Th, ^{230, 232, 234, 236, 238, 240}U, ^{240, 242, 244, 246}Pu, and ^{242, 244, 246, 248}Cm have been investigated at normal and superdeformed shapes in microscopic mean-field calculations based on Gogny force. Collective levels are predicted from constrained Hartree-Fock-Bogoliubov and configuration mixing calculations. Two quasiparticle states are also predicted from blocking calculations for neutron and proton configurations. Predictions are shown and compared with experimental data at superdeformed shapes. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: michel-g.girod@cla.tr     

Angular momentum dependence of moments of inertia and fission barriers of 240Pu

Minimization with respect to a constraint on angular momentum leads to a strong angular momentum dependence of nuclear properties. The second minimum of the potential energy surface of ²⁴⁰Pu becomes lower than the first one for J >24.     

Direct decays from superdeformed states in 192Pb observed using time-correlated gamma-ray spectroscopy

Correlations of decays above and below isomeric states in the normally deformed minimum of 192Pb have been used to identify discrete transitions in the decay of the superdeformed (SD) band. The data establish the absolute excitation energy of the lowest observed SD level as 4.425 MeV. Extrapolation to the bandhead indicates that the excitation energy of the superdeformed well in 192Pb is 0.5 MeV lower than in the heavier isotope 194Pb. The results confirm the trend to decreasing excitation energy with decreasing neutron number predicted by both a macroscopic Strutinsky method approach and microscopic mean field calculations.     

Excitation energies of superdeformed States in 196Pb: towards a systematic study of the second well in Pb isotopes

The excitation energy of the lowest-energy superdeformed band in 196Pb is established using the techniques of time-correlated gamma-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."     

Confirmation of the superdeformed band in192Pb

High-spin states in¹⁹²Pb were populated via the reaction¹⁷³Yb(²⁴Mg,5n) at a beam energy of 140 MeV, and the resulting-rays were deteced using GAMMASPHERE. A previously observed superdeformed band in¹⁹²Pb has been confirmed.     

Characterization of the superdeformed band in189Hg

New measurements of the pionic double charge exchange on⁵⁶Fe exhibit a pronounced energy dependence of this reaction at low energies in accordance with similar observations on other nuclei. We show that this peculiar energy dependence finds its natural explanation by a resonance in theNN subsystem withJ ^p =0^–,T=0 and a mass of 2.065 GeV.This work has been funded by the German Federal Minister for Research and Technology (BMFT) under the contract numbers 06 TÜ 656 and 06 KA 266 and by the DFG (Mu 704/3, Graduiertenkolleg)     

Dipole strength distribution and specific properties of excitations in the second minimum in actinide nuclei

Vibrational states in the first minimum in ²³⁸U and ²⁴⁰Pu and in the second minimum in ²⁴⁰Pu have been calculated within the quasiparticle-phonon nuclear model. The results of calculation of vibrational states in both minima are compared. Specific properties of the vibrational states in the second minimum in actinide nuclei are discussed.     

Single-step link of the superdeformed band in 143Eu

A discrete γ-ray transition with an energy of 3360.6 keV deexciting the second lowest SD state in ¹⁴³ Eu has been discovered. It carries 3.2 % of the full intensity of the band and feeds into a nearly spherical state which is above the I = 35/2⁽⁺⁾, E_x=4947 keV level. The exact placement of the single-step link is, however, not established due to the specially complicated level scheme in the region of interest. The energy of the single-step link agrees well with the previously determined two-step links.