M. Thoennessen

Reaching the limits of nuclear stability

The limits of nuclear stability have not been reached for most elements. Only for the lightest elements are the minimum and maximum number of neutrons necessary to form an isotope for a given element known. The current limits, novel features of nuclei at these limits as well as the future possibilities of pushing these limits even further will be discussed.

Discovery of 40Mg and 42Al suggests neutron drip-line slant towards heavier isotopes.

A fundamental question in nuclear physics is what combinations of neutrons and protons can make up a nucleus. Many hundreds of exotic neutron-rich isotopes have never been observed; the limit of how many neutrons a given number of protons can bind is unknown for all but the lightest elements, owing to the delicate interplay between single particle and collective quantum effects in the nucleus. This limit, known as the neutron drip line, provides a benchmark for models of the atomic nucleus. Here we report a significant advance in the determination of this limit: the discovery of two new neutron-rich isotopes—40Mg and 42Al—that are predicted to be drip-line nuclei. In the past, several attempts to observe 40Mg were unsuccessful; moreover, the observation of 42Al provides an experimental indication that the neutron drip line may be located further towards heavier isotopes in this mass region than is currently believed. In stable nuclei, attractive pairing forces enhance the stability of isotopes with even numbers of protons and neutrons. In contrast, the present work shows that nuclei at the drip line gain stability from an unpaired proton, which narrows the shell gaps and provides the opportunity to bind many more neutrons.

Computer-assisted assignments in a large physics class

Abstract The CAPA system, a software tool to implement a computer-assisted personalized approach for homework assignments and examinations, was used in a large introductory physics class for the first time. The students rated the system extremely favorably even though they spent significantly more time on the assignments compared to traditional classes. Fewer teaching assistants were needed and their time could be diverted from grading to more interactive contact with their students.

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.

Evidence for an l=0 ground state in 9He

Abstract The unbound nuclear systems 10 Li and 9 He were produced in direct reactions of 28 MeV/u 11 Be incident on a 9 Be target. The distributions of the observed velocity differences between the neutron and the charged fragment show a strong influence of final-state interactions. Since the neutron originates in a dominant l =0 initial state, a selection-rule argument allows a firm l =0 assignment for the lowest odd-neutron state in 10 Li. We report the results suggesting a very similar unbound state in 9 He, characterized by an s-wave scattering length more negative than −10 fm corresponding to an energy of the virtual state of less than 0.2 MeV. Shell-model calculations cast light on the reasons for the disappearance of the magic shell gap near the drip line.

Evidence for a Change in the Nuclear Mass Surface with the Discovery of the Most Neutron-Rich Nuclei with 17 ≤Z ≤ 25

The results of measurements of the production of neutron-rich nuclei by the fragmentation of a 76Ge beam are presented. The cross sections were measured for a large range of nuclei including 15 new isotopes that are the most neutron-rich nuclides of the elements chlorine to manganese (50Cl, 53Ar, ;{55,56}K, ;{57,58}Ca, ;{59,60,61}Sc, ;{62,63}Ti, ;{65,66}V, 68Cr, 70Mn). The enhanced cross sections of several new nuclei relative to a simple thermal evaporation framework, previously shown to describe similar production cross sections, indicates that nuclei in the region around 62Ti might be more stable than predicted by current mass models and could be an indication of a new island of inversion similar to that centered on 31Na.

Current Status and Future Potential of Nuclide Discoveries

Currently about 3000 different nuclei are known with about another 3000-4000 predicted to exist. A review of the discovery of the nuclei, the present status and the possibilities for future discoveries are presented.

Discovery of the cerium isotopes

The discovery of the 35 cerium isotopes presently known is discussed. Criteria for the discovery of isotopes are suggested and for each isotope a brief summary of the first refereed publication, including the production and identification method, is presented.

Evolution of the giant dipole resonance in excited 120Sn and 208Pb nuclei populated by inelastic alpha scattering

Abstract The evolution of the giant dipole resonance (GDR) in 120 Sn and 208 Pb nuclei at excitation energies in the range of 30–130 MeV and 40–110 MeV, respectively, were studied by measuring high energy γ rays from the decay of the resonance. The excited states were populated by inelastic scattering of α particles at beam energies of 40 and 50 MeV/nucleon for 120 Sn and 40 MeV/nucleon for 208 Pb. A systematic increase of the resonance width with increasing excitation energy was observed for both nuclei. The observed width evolution was compared to calculations employing a model that adiabatically couples the collective excitation to the nuclear shape, and to a model based on the collisional damping of nucleons. The adiabatic coupling model described the width evolution in both nuclei well, whereas the collisional damping calculation could describe the width evolution only in 208 Pb. Light-particle inelastic scattering populates low angular momentum states in the target nucleus. The observed width increase is therefore interpreted to be predominantly due to fluctuations in the nuclear shape induced by temperature. This interpretation is consistent with the adiabatic model calculations and with recent angular momentum-gated measurements of the GDR in excited Sn isotopes.

Study of Two-Neutron Radioactivity in the Decay of 26O

A new technique was developed to measure the lifetimes of neutron unbound nuclei in the picosecond range. The decay of 26O→24O+n+n was examined as it had been predicted to have an appreciable lifetime due to the unique structure of the neutron-rich oxygen isotopes. The half-life of 26O was extracted as 4.5(-1.5)(+1.1)(stat)±3(syst)  ps. This corresponds to 26O having a finite lifetime at an 82% confidence level and, thus, suggests the possibility of two-neutron radioactivity.