D. Habs

Spin dependence and angular correlation of giant resonances studied in (HI, xn) reactions

Abstract The high energy component above 10 MeV in the γ decay following the 128 Te + 34 S compound nucleus reaction is studied by exploiting the new experimental possibilities provided by the NaI crystal ball spectrometer.

Quasirotational groundstate bands of even-even nuclei with 50≦N, Z≦82

The quasirotational groundstate bands of140Sm,142Sm,144Gd and136Nd have been determined by studying the prompt γ-decay after (α,xn+yp+zα.) reactions using α-particles of up to 104 MeV and targets of144Sm,142Nd,141Pr,140Ce,136Ce and139La. The new levels continue the trends of the neighbouring even-even nuclides smoothly. These systematics of the groundstate bands in the region 50<N,Z<82 are not disturbed by expected subshell closure effects.

Evidence for molecular-cluster states in222Th

The groundstate and a negative parity band of222Th have been observed in the208Pb (180, 4n)222Th reaction by conversion electron and γ-ray spectroscopy identifying the evaporation residues in a recoil separator. The de-excitation pattern is characterized by strong El transitions interconnecting the levels of alternating parity. The yrast levels may be interpreted as a molecularcluster band.

The spherical to asymmetric shape transition in the mass region with 50<(N, Z)<82

Collective potential energy surfaces were determined in the mass region with 50<(N, Z)<82 by fitting experimental level spectra andB(E 2)-values on the basis of the generalized collective model of Gneuss and Greiner. While the nuclides withN=80 and withZ=52 are rather soft vibrators we find an abrupt transition to asymmetric rotators forN=78 and forZ=54 tending to more symmetric shapes further away from the closed shells. The results predict the position of levels not yet observed andB(E 2)-values.

Electron impact ionization and dielectronic recombination of sodium-like iron ions

Abstract Absolute rates and cross sections for dielectronic recombination and ionization of Na-like Fe15+ (1s22s22p63s) ions were measured at electron impact energies between 0 and 1030 eV using the Heidelberg heavy ion storage ring TSR with the cooling device as an electron target. In the region where excitation-autoionization and related resonant processes are relevant, variations of the ionization cross sections below 0.5% were observed with an energy resolution of 4 to 5 eV FWHM. The theoretical calculations are in good overall agreement with the measured cross sections but do not reproduce all the details of the rich resonance structures revealed by our experiment. In the lower energy range we made use of the magnetically expanded electron beam with a transverse temperature corresponding to only kT⊥ = (15 ± 3) meV. This resulted in an excellent energy resolution in the measurements of dielectronic recombination associated with 3s → 3p, 3d, 4l core excitations. Here, theory and experiment are in very good agreement.

Laser-induced recombination studies with the adiabatically expanded electron beam of the Heidelberg TSR

The enhancement of spontaneous radiative recombination of C6+ ions with free electrons in a resonant laser field has been investigated at the Heidelberg cooler ring TSR for different transverse electron temperatures realized by adiabatically expanding the magnetically guided electron beam. The recombination spectra close to the ionization threshold strongly deviate from calculations for free independent electrons, showing important contributions at energies below this threshold. Shape and relative size of these contributions change significantly as the transverse temperature is varied. These changes are not consistent with the expected behaviour of sub-threshold contributions due to electric stray fields.

Determination of the fission barrier of232Pu from β-delayed fission and the problem of the first barrier

A delayed fission activity with a half-life of (55±7) s and a production cross section of (5±1)nb has been observed in the irradiation of237Np with 104 MeVα particles. It is tentatively assigned to the electron capture decay of232Am followed by fission of232Pu With a probability of (1.3−0.8+4)· 10−2 A barrier height of (5.3±0.4) MeV is extracted for232Pu exceeding theoretical barrier heights by about 2MeV. Systematic deviations between experimental and theoretical barrier heights previously observed in particular for U and Th isotopes are now also established for neutron deficient Pu isotopes indicating severe defects in the procedure of calculating static fission barriers.

Fast scintillators as radiation resistant heavy-ion detectors

Abstract Three promising scintillation materials, namely GSO(Ce), YAP(Ce), and CsI (pure), were investigated under 50MeV 12 C and 100MeV 32 S heavy-ion irradiation. The radiation resistance, light output and time structure of the scintillation were measured. The measurements were complemented by studies of the transmittance and thermoluminescence of the irradiated crystals. GSO(Ce) and CsI(pure) show an excellent radiation resistance together with a high light output. Maintaining a time resolution in the nanosecond regime, their applicability for heavy-ion detection with accumulated doses up to 5 × 10 13 ions/cm 2 was demonstrated. This enables their continuous operation in counting applications with high average ion rates occurring, e.g., at heavy-ion storage rings. A CsI(pure) scintillation detector realized on the basis of these measurements for electron-ion recombination experiments at the Heidelberg heavy-ion storage ring TSR is described.

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.

Physics with 4π−γ-detectors

Abstract The broad range of applications of the Darmstadt-Heidelberg crystal ball is illustrated. First results are discussed which have been obtained in investigations of the nuclear structure of rapidly rotating nuclei, in studies of nuclear reaction mechanisms, and in experiments with radioactive sources.