A. Schiller

Evidence for the ground-state resonance of 26O.

Evidence for the ground state of the neutron-unbound nucleus (26)O was observed for the first time in the single proton-knockout reaction from a 82 MeV/u (27)F beam. Neutrons were measured in coincidence with (24)O fragments. (26)O was determined to be unbound by 150(-150)(+50) keV from the observation of low-energy neutrons. This result agrees with recent shell-model calculations based on microscopic two- and three-nucleon forces.

Observation Of Ground-State Two-Neutron Decay

Neutron decay spectroscopy has become a successful tool to explore nuclear properties of nuclei with the largest neutron-to-proton ratios. Resonances in nuclei located beyond the neutron dripline are accessible by kinematic reconstruction of the decay products. The development of two-neutron detection capabilities of the Modular Neutron Array (MoNA) at NSCL has opened up the possibility to search for unbound nuclei which decay by the emission of two neutrons. Specifically this exotic decay mode was observed in 16Be and 26O.

Exploring the neutron dripline two neutrons at a time: The first observations of the 26O and 16Be ground state resonances

The two-neutron unbound ground state resonances of 26O and 16Be were populated using one-proton knockout reactions from 27F and 17B beams. A coincidence measurement of 3-body system (fragment + n + n) allowed for the decay energy of the unbound nuclei to be reconstructed. A low energy resonance, < 200 keV, was observed for the first time in the 240 + n + n system and assigned to the ground state of 26O. The 16Be ground state resonance was observed at 1.35 MeV. The 3-body correlations of the 14Be + n + n system were compared to simulations of a phase-space, sequential, and dineutron decay. The strong correlations in the n-n system from the experimental data could only be reproduced by the dineutron decay simulation providing the first evidence for a dineutron-like decay.

Gamma-ray strength functions and their relation to astrophysics

The nuclear γ‐ray strength function is one of the indispensable inputs needed for reaction‐rate calculations, and is particularly important for the neutron‐capture cross section. The nuclear physics group at the Oslo Cyclotron Laboratory has developed a method to extract simultaneously nuclear level density and γ‐ray strength function from particle‐γ coincidence measurements. Data on the strength functions of Sn nuclei as well as for lighter elements are presented. The Sn isotopes all display a resonance‐like structure close to the neutron threshold, that could possibly be due to the neutron‐skin oscillation mode. This so‐called pygmy dipole resonance greatly influences the neutron‐capture rates. In the lighter nuclei, an enhancement of the strength function at low γ energies is observed. The possible impact of this increase on Maxwellian‐averaged reaction rates has been investigated.

Efficiency of the Modular Neutron Array (MoNA)

The efficiency of the Modular Neutron Array (MoNA), located at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University, was measured and compared to simulations. The Coulomb dissociation of a 90 MeV/u beam of 11Be in a gold target was used to produce neutrons. The expected neutron production rate was calculated using the virtual photon method. The measured efficiency agrees with the efficiency calculated with GEANT simulations. The current configuration of MoNA has a 73% intrinsic detection efficiency for 90 MeV neutrons.

Test of level density models from reactions of {sup 6}Li on {sup 58}Fe and {sup 7}Li on {sup 57}Fe

The reactions of {sup 6}Li on {sup 58}Fe and {sup 7}Li on {sup 57}Fe have been studied at 15 MeV beam energy. These two reactions produce the same compound nucleus, {sup 64}Cu. The charged particle spectra were measured at backward angles. The data obtained have been compared with Hauser-Feshbach model calculations. The level density parameters of {sup 63}Ni and {sup 60}Co have been obtained from the particle evaporation spectra. We also find contributions from the break up of the lithium projectiles to the low energy region of the {alpha} spectra.

Level densities and radiative strength functions

This paper presents a summary of the latest results from experiments performed at the Oslo Cyclotron. The Oslo group has developed a technique to extract simultaneously the level density and radiative strength function from primary γ‐ray spectra. A small (pygmy) resonance in the radiative strength function has been observed at around 3 MeV in several deformed rare earth nuclei. This resonance is shown to be of M1 character and the origin is thought to be the scissors mode. This resonance vanishes for the spherical Sm nuclei, as expected, since the scissors mode is dependent on deformation. Experiments performed in Oslo found the resonance in Dy isotopes to be twice as wide as the width extracted in an indirect way by comparing simulations with two‐step cascade spectra from neutron capture experiments. In 116,117Sn an increase in the slope of the radiative strength function around Eγ = 4.5 MeV indicates the onset of resonance‐like structures in both nuclei, resulting in a significant enhancement of the rad...

Thermodynamic properties of atomic nuclei with T<1 MeV

The thermal motion of single quasi‐particles represents the largest contribution to nuclear entropy and is essential for the description of thermodynamic properties. The entropy is an approximately extensive (additive) quantity and the concept of thermal quasi‐particle entropy is presented. Various applications of thermodynamics in heated nuclei are discussed.

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.

Level densities and γ‐strength functions in Sm isotopes

The level density and radiative strength functions (RSF) for 146,147Sm have been extracted. As one approaches the closed N = 82 neutron shell we see that the structures in the level density become more pronounced due to shell effects. The experimental level densities can be used to explore thermodynamical properties of the nucleus within the microcanonical ensemble.