S. Messelt

Critical temperature for quenching of pair correlations

The level density at low spin in the 161,162-Dy and 171,172-Yb nuclei has been extracted from primary gamma rays. The nuclear heat capacity is deduced within the framework of the canonical ensemble. The heat capacity exhibits an S-formed shape as a function of temperature, which is interpreted as a fingerprint of the phase transition from a strongly correlated to an uncorrelated phase. The critical temperature for the quenching of pair correlations is found at Tc=0.50(4) MeV.

Puzzling γ‐ray strength functions in 44,45Sc and 50,51V

The nuclear physics group at the Oslo Cyclotron Laboratory (OCL) has developed a method to extract nuclear level density and γ‐ray strength function simultaneously from primary γ‐ray spectra. The resulting γ‐ray strength functions of 44,45Sc and 50,51V show an unexpected, large enhancement at low γ‐ray energies, indicating an increased probability of emitting low‐energy γ rays in the quasi‐continuum.

Experimental nuclear level densities and γ‐ray strength functions in Sc and V isotopes

The nuclear physics group at the Oslo Cyclotron Laboratory has developed a method to extract nuclear level density and γ‐ray strength function from first‐generation γ‐ray spectra. This method is applied on the nuclei 44,45Sc and 50,51V in this work. The experimental level densities of 44,45Sc are compared to calculated level densities using a microscopic model based on BCS quasiparticles within the Nilsson level scheme. The γ‐ray strength functions are also compared to theoretical expectations, showing an unexpected enhancement of the γ‐ray strength for low γ energies (Eγ⩽3 MeV) in all the isotopes studied here. The physical origin of this enhancement is not yet understood.

Experimental Level Densities and γ‐Strength Functions in rare earth nuclei

The level density and radiative strength function for 146,147Sm and 163,164Dy have been extracted from primary γ spectra using the Oslo method. As one approaches the closed N = 82 neutron shell, the structures in the level density become more pronounced due to shell effects. The experimental level densities can be used to explore thermodynamic properties of the nucleus within the microcanonical ensemble. Pygmy resonances, which are based on the scissors mode and seen in deformed rare‐earth nuclei, are not observed in near‐spherical 146,147Sm, as expected. Pygmy resonances in 163,164Dy were studied after 3He‐induced reactions and their width was found to be twice as wide as compared to results reported after neutron‐capture reactions.

Heating Nuclei in the Mass Region of A ∼ 40 – 50

The nuclear level densities of 44,45Sc and 50,51V have been measured using the Oslo method. From the level density thermal properties such as entropy and temperature are deduced. Also the radiative strength functions (RSF) of 50,51V have been extracted. The gross properties of the RSF are generally described by the low‐energetic tail of the giant electric dipole resonance (GEDR). At γ energies below ∼ 3 MeV, the RSFs show an unexpected enhancement.

Measurements of level densities and gamma ray strength functions

A method has been developed where primary γ-rays after nuclear reactions with one ejectile only, can be projected out from the total γ-ray spectrum. These primary γ-ray spectra are shown to be, under certain conditions, a new source for determination of nuclear level densities and γ-ray strength functions. Results for 162Dy, 166Er and 172Yb are presented. The level density is used for determination of nuclear temperature and heat capacity. Fine structures in the level density and the temperature suggest changes in the pair correlation.

New Measurements of Level Densities

An iterative procedure for simultaneous extraction of the level density and the γ-ray strength function from a set of primary γ-ray spectra is used. This procedure opens new perspectives in the search for thermodynamic phase transitions and the order to chaos transition, and has so far been used to study nine different nuclei in the rare earth region.

Identification of excited states in 226U: Evidence for octupole deformation

The level scheme of 226U has been deduced from the results of two experiments carried out at the University of Jyvaskyla, Finland. Both α- and γ-ray-spectroscopic techniques have been employed. The interleaved states of positive- and negative-parity indicate the octupole nature of this nucleus, and the behavior of the difference in aligned angular momentum between the positive- and negative-parity bands as a function of rotational frequency is consistent with that expected for a rotating reflection-asymmetric shape.

First observation of excited states in

The technique of recoil-decay tagging has been employed in order to observe excited yrast states in the neutron-deficient nucleus , for the first time. Interleaved bands of positive- and negative-parity states suggest the octupole nature of this nucleus, and the behaviour of the difference in aligned angular momentum between the negative- and positive-parity states with rotational frequency is consistent with that expected of a rotating reflection-asymmetric shape. This represents an extension of the known octupole-deformed nuclei to Z = 92. The interleaved bands of alternating parity are connected by strong electric dipole transitions, and values of , extracted from -ray branching ratios, suggest that possesses one of the largest E1 moments near to the ground state of the nuclei in this region.