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Isotopic yield measurements for thermal neutron induced fission of 245Cm at the Lohengrin fission-product mass separator are described. Using an ionisation chamber coupled to the Lohengrin mass separator and the stacked foil technique, we measured the isotopic yields from mass A=85 to mass A=115 (near symmetry region) corresponding to nuclear charges Z=33 (As) to Z=47 (Ag). This has considerably extended the data set previously known for the very light fission products providing isotopic yields from A=76 to 94. In the present work, the nuclear charge assignment was performed using the energy loss in a stack of foils of Parylene C, a technique developed and used previously. This allows 4 to 5 nuclear charges per mass to be measured. The global proton odd–even effect calculated from the present measurements is 9.3±0.5%, a value corresponding to the known systematics for the effect as a function of the Coulomb parameter. A study of the global proton odd–even effect as a function of the light fragment kinetic energy was additionally carried out and the intrinsic excitation energy at scission was deduced for different compound nuclei.

MINIBALL a highly efficient, position sensitive γ -spectrometer is under construction for upcoming radioactive-beam experiments. In contrast to high spin arrays the total full energy peak efficiency and not the resolving power is optimized with a factor of two larger efficiency for γ -ray energies up to 10 MeV and for low to medium γ -ray multiplicity events. Cluster detectors, incorporating three or four Ge crystals, will be employed to provide high flexibility and to accomodate the various demands of the experiments.

An experimental study of the 241Am incineration in a high-intensity thermal neutron flux was carried out at the high-flux reactor of the Institut Laue-Langevin in Grenoble. The combination of nuclear γ -ray spectroscopy and off-line mass spectrometry methods made possible the measurement of several parameters of the transmutation chain and the first experimental determination of the unknown 242gsAm thermal neutron capture cross section, which plays an essential role in the 241Am incineration process. During a 19 days irradiation in a thermal neutron flux of 5.6×10 14 n /( s cm 2 ) , (46±5) % of the initial 241Am was transmuted by neutron capture of which (22±8) % was incinerated by nuclear fission. A value of the thermal neutron cross section of 242gsAm( n ,γ ) of (330±50) barns was obtained. We show that this keeps the option open to incinerate 241Am by high-intensity moderated neutron fluxes.

The mass-separator Lohengrin was used to measure the yields of the light fission products with A=74−85 and their nuclear charge and kinetic energy distributions from the odd- Z compound nucleus 239 Np ∗ formed by double capture of thermal neutrons. The mass yield distribution reveals an influence of the fragment shell with N=50 affecting also the nuclear charge and kinetic energy distributions. An odd–even effect for protons is found in the very asymmetric mass division, increasing from 4% to 35% with increasing fission asymmetry. This is in contrast to findings in normal asymmetric fission (region of high fission yields) where no odd–even effect for protons was observed. An odd–even effect for neutrons is also found comparable in size with that for protons. The latter effect exists also in normal asymmetric fission and is at least partly attributed to prompt neutron emission from the fragments. Some information on the number of prompt neutrons emitted is also obtained. From this information and from the energy dependence of the odd–even effect for protons it is concluded that the very light fragments originate from a cold and nearly undeformed light sphere of a dumbbell scission configuration characterised by a double shell closure with 28 protons and about 50 neutrons. In contrast to the double shell closure at mass 132 ( Z=50 , N≈82 ) the two shell closures in the light sphere do not coincide at one mass but are realised at A=70 and 80, respectively. This leads to a layered structure of the light sphere of the dumbbell scission configuration.

Yields of light fission products (A = 68, 70-84, 87, 88, 94, 96, 98, 102 and 106-108), their kinetic energies and nuclear charge distributions (A = 71-84, 87 and 88) in the thermal neutron induced fission of the odd-Z nucleus Am-242m(Z = 95) were measured using the mass-separator Lohengrin at the Institute Laue-Langevin in Grenoble (France). The mass yield curve shows a fine structure at A = 70, probably due to shell and/or odd-even effects affecting also the nuclear charge distribution. The analysis of isotopic chain yields gives evidence for a very low excitation energy of the lightest fission fragments observed. A preferential formation of fragments with even Z is found for this odd-Z compound nucleus, Calculated values for the local odd-even effect are comparable with those for the neighbouring even-Z fissile nuclides and increase from 13% to 30% with increasing asymmetry of the mass split (A(L) = 84 to 68 and Z(L) = 35 to 28). The neutron odd-even effect shows a similar but less pronounced behaviour. (C) 1999 Elsevier Science B.V. All rights reserved.

An extensive level scheme for the transitional nucleus 191 Os has been constructed on the basis of (n, γ), (n, e − ) and (d, t) reaction data. γ-rays following thermal capture and average resonance capture (ARC) were detected with pair spectrometers. Secondary γ-ray spectra were obtained with the GAMS bent crystal spectrometers. The high precision of the resultant energies was the critical factor in developing a level scheme on the basis of Ritz combinations. γγ coincidences were obtained from both primary and secondary transitions. Conversion electrons, detected with the BILL spectrometer, gave multipolarity information. The (d, t) reaction measured with a Q3D magnetic spectrometer gave additional information on the hole states in 191 Os. A discussion of the resulting level scheme briefly reviews the fragmentation of negative-parity strength but focuses mostly on a rare example of an anti-aligned decoupled set of coexisting positive-parity states. For these, triaxial particle-plus-rotor calculations are carried out. Reasonable agreement is obtained and the calculations also explain the unusually strong population in (n, γ) of this set of mostly medium spin states.

The level scheme of 137 Nd has been investigated by methods of in-beam γ-ray and conversion electron spectroscopy as well as by observing the γ-transitions following the β-decay of 137 Pm . We measured γ-ray and conversion electron singles spectra, γγ coincidences and angular distributions of prompt γ-rays and conversion electrons. The experimental data result in a detailed level scheme of 137 Nd and allow the assignment of spin and parity to the β-decaying level in 137 Pm , i.e. I π = 11 2 − . Members of a disturbed rotational band based on the 11 2 − isomeric state of 137 Nd and populated by prompt γ-transitions (high spin levels) as well as by the β-transitions of 137 Pm (low spin levels) are identified. The negative parity levels are discussed in a particle-plus-rotator model with Coriolis coupling. Though the analysis results in evidence for a weak prolate deformation of 137m Nd , a quantitative description of the observed level scheme is not achieved. There are features which suggest that a γ-deformation of the core has to be taken into account.

Gamma rays and conversion electrons have been measured following thermal neutron capture in 113 Cd using the crystal spectrometers GAMS and the β-spectrometer BILL at the High Flux Reactor of the ILL at Grenoble. Primary γ-rays following thermal and average resonance neutron capture at E n = 2 keV and 24 keV were recorded at the High Flux Beam Reactor at the Brookhaven National Laboratory. The 113 Cd(d, p) 114 Cd reaction was studied with the Q3D spectrograph at the Munich tandem accelerator. Combining all these experimental results an almost complete level scheme of 114 Cd was constructed up to 3.3 MeV including 48 excited levels with spin and parity information. The level scheme is discussed in terms of particle-hole excitations across the Z = 50 closed shell coupled to collective states, as well as in an interacting boson configuration mixing scheme.

A high-resolution study of the 76Se(n, γ) reaction was carried out with curved-crystal and pair spectrometers and conversion electrons were measured following slow-neutron capture. The resulting data yield very precise level energies and spin and parity assignments for most of the levels. The neutron separation energy of 77Se was measured as 7418.85 ± 0.07 keV. The experimental data were compared with theoretical results for the level energies in 77Se and the E2, M1 and E1 branching ratios obtained from the SU(6) particle-vibration model (PTQM). We used 76Se as a slightly perturbed SU(5) vibrational core and the particle-vibration interaction strengths from the PTQM calculation for 75Se. In this frame there is evidence for a Δn = 2 term in the E2 operator. This term has not been included so far in TQM and IBM calculations.

The γ and e − spectra following thermal neutron capture in 74 Se were studied with curved-crystal, β, and pair spectrometers. Precise energies have been obtained for the transitions and levels at low energies. Two primary E2 transitions were found. The neutron separation energy for 75 Se was determined as 8027.6 keV. Precise γ-energies following the electron capture decay of 75 Se were also measured, resulting in precise level energies in 75 As. The calculation of the energy levels in 75 Se has been performed in the SU(6) particle-vibrational model (PTQM) and 27 theoretical states have been tentatively assigned to the experimental levels. The spectrum of the core nucleus 74 Se has been calculated in the SU(6) quadrupolephonon model (TQM). The structure of theoretical states, the relation to SU(3) and SU(5) limits, and potential energy surface are discussed. The E2, M1 and E1 transitions have been calculated in PTQM and compared to the experiment. Also, an overview is presented of theoretical explanations of the I = j , j −1, j −2 anomalous triplet emphasizing the rule with shell-model classification corrected for quadrupole phonons.

The level scheme of 135 Pr has been investigated by means of off-beam γ-ray and conversion electron measurements as well as by methods of in-beam γ-spectroscopy. We measured γ-ray and conversion electron singles spectra, off-beam and in-beam γγ coincidences and γ-angular distributions. From the experimental data a detailed scheme of 22 excited levels for 135 Pr has been constructed. In addition to members of the decoupled band based on the 11 2 − state six further negative parity states were identified. These states are strongly populated in the β-decay of a 9 2 isomer in the parent nucleus 135 Nd. A comparison of all the negative parity states with predictions of the alignment coupling model showed a nearly quantitative agreement, if softness of the core is taken into account. In this case, a prolate deformation with a deformation parameter β ≈ 0.13 was obtained for the 11 2 − isomer in 135 Pr.

PACS conv.GSI; The isotopic distributions and recoil velocities of the fission fragments produced in the spallation reaction 208Pb + p at 500 A MeV have been measured using the inverse-kinematics technique, a lead beam onto a liquid-hydrogen target, and the high-resolution spectrometer FRS at GSI. The shapes of the different distributions are found in good agreement with previously published data while the deduced total fission cross-section is higher than expected from existing systematics and some previous measurements. From the experimental data, the characteristics of the average fissioning system can be reconstructed in charge, mass and excitation energy, and the average number of post-fission neutrons can be inferred. The results are also compared to different models describing the spallation reaction. The intranuclear cascade code INCL4 followed by the de-excitation code ABLA is shown to describe reasonably well the evolution of the isotopic distribution shapes between 500 and 1000 A MeV.