Nuclear Structure Group

Research Activities

A fundamental goal of low energy nuclear physics is to understand the nature of nucleonic matter by studying the collective properties and microscopic structure of nuclei. The nuclear structure group is following three interconnected research themes which push towards this goal by using a broad range of experimental facilities and state-of-the-art detector systems. These themes have substantial overlap with the stated goals of the nuclear structure community and will remain at the focus of research for the foreseeable future. The three themes may be summarized as follows:

New Phenomena in Neutron-Rich Nuclei

Scientific goals: Large neutron excess may amplify aspects of nuclear behavior or give rise to new phenomena. Predictions include modifications to the effective nucleon-nucleon interaction, effects on pairing due to continuum coupling, and radically different proton and neutron density distributions, leading to modified shell structure and new collective modes.

Current and future activities: Neutron-rich nuclei have been investigated using deep-inelastic transfer reactions at Gammasphere and through incomplete-fusion and fusion-evaporation reactions at LIBERACE+STARS. The level structure of F and Na nuclei close to the neutron drip-line has been investigated using fragmentation beams at MSU. Studies close to the drip-line, which are most likely to reveal unexpected new phenomena, will require radioactive beams combined with a powerful gamma-ray spectrometer such as GRETINA or GRETA.

Gammasphere is currently the world’s most powerful Ge detector array. Originally conceived by Berkeley Lab scientists, it was constructed by an LBNL consortium of National Laboratories and Universities. It is currently sited at the ATLAS accelerator of Argonne National Laboratory.

Nucleon Pairing

Scientific goals: Nucleon pairing is a topic of considerable interest in nuclear structure research and outstanding issues include: pairing at high excitation energy, neutron-proton pairing in the T=0 channel, pairing fluctuations near closed-shell nuclei, the development of a static pair gap with nucleon number, and pairing in weakly bound nuclei. These topics are of general interest for understanding finite Fermi systems.

Current and future activities: Experimental activities are focused on using two-nucleon transfer reactions to populate pairing states in nuclei of interest. For instance, (3He, p) reactions on N=Z nuclei have been performed using the FMA at ATLAS and with the LIBERACE+STARS detector system at the 88-Inch Cyclotron. These studies will be extended along the N=Z line up to 100Sn in order to search for the development of T=0 n-p pairing. Two-neutron transfer reactions from weakly bound nuclei such as 6He and 11Li are being used to search for new pairing effects. The possibility of using break-up reactions of light-ions has been investigated at the 88-Inch Cyclotron and experiments using beams of 6He and 11Li are planned at radioactive-beam facilities such as SPIRAL and TRIUMF.

The left panel is a top view of the LIBERACE Ge array currently comprising six Compton-suppressed clover detectors in a horizontal plane around the target chamber. The right panel shows an annular Si detector of the type used in STARS.

Nuclear Behavior at High Excitation Energy

Scientific goals: The behavior of nuclei far above the yrast line, where level densities become high, remains largely unexplored. Outstanding issues include the melting of shell structure, the conservation of quantum numbers, the conditions leading to the onset of quantum chaos, and fundamental collective excitations at high excitation energy. Again, these are topics of general interest for the physics of mesoscopic systems.

Current and future activities: Gammasphere has been used to investigate rotational damping and the onset of chaos at high temperature in rotational nuclei such as 168Yb. LIBERACE+STARS provide a unique combination of high-resolution gamma-ray and particle detection enabling new studies of collective resonances, resonant states near particle thresholds, and single-particle structure at high excitation energy. Future investigations of resonance structures will take advantage of the increased efficiency and good energy resolution at high gamma-ray energy of the GRETINA array. Studies in the region of large level density will take advantage of the ability of GRETA to provide high-multiplicity gamma-ray coincidence data. These studies will require stable and radioactive beams.

A schematic of how GRETINA/GRETA may be coupled with the BGS at the 88-Inch Cyclotron. This would open a new era in the prompt- g spectroscopy of heavy elements.

In addition to these main research activities we have recently started to collaborate with the heavy-element research group at LBNL in an effort to use the Berkeley Gas-filled Separator (BGS) at the 88-Inch Cyclotron to investigate the structure of the heaviest elements.

A list of publications and some of the recent talks given by group members can be found by following the links below:

Publications

Talks and Presentations