D. Schwalm

Correlated e+ e− peaks observed in heavy-ion collisions

Abstract Three very narrow e + e − sum-energy peaks around 610, 750, and 810 keV have been observed in U+Th as well as in U+Ta collisions at beam energies around the Coulomb barrier. As no processes involving conventional atomic and nuclear physics were found to describe their origin, the data were in particular confronted with the hypothesis that the lines are due to the two-body decay of neutral objects in an e + e + pair. Although the 810 keV sum-energy line observed in U+Th is consistent with the prompt two-body e + e − decay of a neutral object if created nearly at rest in the heavy-ion center-of-mass system, the other lines require at least a considerably more complicated scenario if they are to be explained in the context of a two-body decay.

Collective Rotation of 238U at High Spins

An experimental study of the actinide nucleus 238U was performed by Coulomb excitation with 208Pb projectiles. The E2-transition matrix elements in the ground state band were measured up to spin 28 by comparing experimental Coulomb excitation yields to semiclassical calculations and by analysing Doppler broadened line shapes measured in coincidence with a gamma-ray sum spectrometer. The E2-metrix elements agree within their errors with the rigid rotor predictions. Neither an increase of the collective transition strength at high spin due to centrifugal stretching as predicted by the Rotation Vibration Model nor a cutoff in collectively as postulated by the Interacting Boson Model is observed. The excitation energies in the ground state band show the presence of an aligned angular momentum of 5 at spin 30 which can be explained by a crossing of an aligned band at ω ≈ 250 keV. The octupole band shows a constant angular momentum alignment of 3 (above spin 7) and no indication of a bandcrossing.

The Proton Microprobe: A Powerful Tool for Nondestructive Trace Element Analysis

A proton microprobe capable of focusing proton beams with energies up to 6 million electron volts to a spot size of 2 x 2 square micrometers has been used for chemical analysis of small grains of minerals in lunar samples by proton-induced x-ray emission. The proton microprobe is preferable to the electron microprobe for analyzing trace elements whose concentrations are below the detection limit of the latter and for analyzing objects with numerous major and trace elements with a wide range of atomic numbers. Application of the proton microprobe to biological samples is feasible.

Development of segmented Ge detectors for future γ-ray arrays

The EUROBALL Cluster detector is composed of seven encapsulated Ge detectors in a common cryostat with a total volume of 2000 ccm of HP Gemanium. The development and the performance of the Cluster detector is summarized. Up to six Cluster detectors were used in pre-EUROBALL experiments at the S-DALINAC Darmstadt, at the tandem-postaccelerator facility of the MPI-K Heidelberg and at the UNILAC at GSI. Examples of these experiments - the excitation of dipole modes with (γ, γ′)-reactions and the first coincidence spectroscopy of the N=Z-nucleus 68Se with a CLUSTER Cube - are discussed. The development of segmented encapsulated Ge detectors for a MINIBALL at the radioactive beam facility REX-ISOLDE has been launched. The status of the project is presented.

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.

Triaxiality and its dynamics in 104Ru investigated by multiple Coulomb excitation

Abstract The electromagnetic properties of 104 Ru have been investigated by multiple Coulomb excitation. Thin and thick targets of 104 Ru have been bombarded with 4.6 MeV/u 208 Pb. From the measurement of particle-particle-γ coincidences and the γ-multiplicity (thin target) and γγ coincidences (thick target) the level scheme of 104 Ru could be extended appreciably. Numerous B (E2) values for the ground-state band and excited bands as well as static quadrupole moments for the ground-state band up to I π = 8 + could be determined by comparing experimental Coulomb excitation yields (thin target) to semiclassical calculations. The results are discussed in the framework of different collective models as the generalized collective model, the interacting boson model and the asymmetric rotor model considering especially the aspects of triaxial deformation and γ-softness. The electromagnetic moments show the first excited 0 + state not to be the predicted collective excitation but to belong to an intruder configuration.

Magnetic moments in the backbending region of 158Dy

Abstract Yrast levels in the backbending region of 158 Dy were Coulomb excited with a 4.7 MeV/u 208 Pb beam. Employing the transient field technique with a thin magnetized iron foil, the precessions of the angular correlations of decay γ-rays from levels up to spin I π = 16 + were measured. The results show a clear reduction of the g -factor for states in the backbending region relative to that of the low-spin levels, thus demonstrating that the backbending in 158 Dy is mainly caused by the alignment of i 13 2 neutrons. In a similar experiment, precession measurements on Coulomb excited low-spin levels of 164 Dy served to determine the static hyperfine field of Dy in Fe and the g -factor of the 6 + state in 164 Dy.

Reduced collectivity at high spins in 158Dy

Abstract Lifetimes of the yrast states in 158 Dy between I π = 4 + and I π = 26 + were measured applying the recoil distance technique to the inverse reaction 26 Mg( 136 Xe, 4n). A reduction in the E2 transition probabilities for all yrast states above the upbending region is found. From the time structure of the sidefeeding an average B (E2) value for the pre-yrast transitions could be deduced, which shows a reduction similar to that observed for the yrast states.

252Cf fission revisited — new insights into the fission process

Abstract γ-ray energies and multiplicities and neutron multiplicities from Cf spontaneous fission were measured with Nal detectors in 4π at the Heidelberg-Darmstadt Crystal Ball together with the mass and kinetic energy of the fission fragments. The correlation of neutron multiplicity with fragment mass and kinetic energy is presented and discussed in the context of nascent fragment deformation. The γ-multiplicity unfolded for individual fragments is found to be rather independent of mass. A high-energy component in the γ-spectra is found in the vicinity of symmetric fission. The results are discussed in the framework of current fission models.

Feeding times of high-spin states in 152, 154Dy: Probes of nuclear structure above the yrast line

Abstract Measurements of feeding times of high-spin yrast states up to spin 30 ℏ in 154 Dy and 36 ℏ in 152 Dy were utilized to obtain information about possible spin-dependent shape changes. The reactions 25 Mg( 134 Xe, 5n), 124 Sn( 34 S, 4n) and 25 Mg( 132 Xe, 5n), 122 Sn( 34 S, 4n) were used to populate the high-spin states in 154 Dy and 152 Dy, respectively. Feeding times as well as lifetimes were determined with the recoil-distance technique. In 152 Dy only long feeding times (⩾ 10 ps) could be identified, indicating that the aligned-particle yrast states are fed through configurations of similar character, with little direct population from collective cascades in the continuum region. In 154 Dy discrete states with I ⩽ 30 ℏ have lifetimes which are characteristically collective, whereas the preyrast cascades exhibit both fast (≲1 ps) and slow (∼10 ps) feeding components. The latter imply a change with increasing spin from collective to aligned-particle character, probably associated with a prolate to oblate shape transition.