Research

During the past several years our group has concentrated on studies of the interplay between the collective and single particle degrees of freedom in nuclei, that are responsible for producing large deformations in nuclei. This leads to experimental studies of highly deformed structures characterized by rotational bands nuclei, termed "superdeformed (SD)", with major to minor axis ratios between (1.5-2.0):1.0. These nuclei are produced in fusion reactions of colliding heavy nuclei, in which the maximum possible angular momentum is brought into the fused system.

The properties of nuclei investigated include: the energy spectra, measurements of the static and dynamic moments of inertia, as well as the transition quadrupole moments [derived from lifetime measurements in the range (1-50)x10^-15s]. These results are compared with theoretical models to learn about the strength of the polarizing influence of the deformation driving high L orbitals.

We have concentrated our efforts on the medium (A ~ 80-90) and light mass systems (A ~ 40-50). Since the population of SD structures in these systems by fusion reactions occurs only 1 to 10 times per 10,000 reactions, powerful gamma-ray spectroscopic techniques are needed. To this end, the Gammasphere Array (consisting of ~ 100 large Compton suppressed Ge spectrometers) is used. The power of Gammasphere is further enhanced by the "Microball", our 4 charged-particle multi-detector array, that detects efficiently protons and alpha particles and provides their energies. This allows us to select exit channels and reduce the unwanted background. From the particle momenta we can determine the recoiling nucleus direction and thus do precise Doppler corrections on an event-by-event basis. As a result the energy resolution for gamma rays improves by as much as a factor of 3 depending on exit channel.

However, in order to reach the most neutron deficient nuclei along the N=Z line and even more neutron deficient systems approaching the proton drip line (proton binding energy vanishes) one needs to combine the above techniques with the additional selection provided by the rarely emitted neutrons. A high efficiency Shell of 30 Neutron Detectors was designed and constructed for this purpose. It aids tremendously the study of neutron deficient nuclei, which we found to exhibit shape coexistence between spherical and well-deformed structures. We found that some of these nuclei such as ^58,59Cu, and ⁵⁶Ni are proton of alpha particle emitters from the lowest states of their deformed structures. These investigations are continuing.

Independent studies in the light mass systems (A ~ 40-50) will aim to obtain definitive evidence for superdeformation, alpha clustering in nuclei, and the role played by the proton-neutron pairing interaction in determining the high-spin structure of nuclei with N = Z neutron and proton numbers. We will be looking for proton-neutron pairing effects in ⁹²Pd and for charge independence of the nn, np, nn interaction in ³⁸Ca.

As a new direction our group, with the initiative of Dr. W. Reviol, is moving in the direction of the high spin spectroscopy of heavy and trans lead nuclei. There, we plan to look for effects on structure of the ultra high L orbitals like the j_15/2 and k_17/2 ones. These are orbitals that drive the nucleus to stable large deformation configurations. Shape with 3:1 axes ratio are predicted to occur in selected nuclei in the heavy transuranium elements. In addition, exotic shapes of large oblate deformations are predicted in the light Pb isotopes (^184,186Pb ).

These species can only be formed in fusion reactions but fission presents a serious problem. To eliminate this enormous background of unwanted gamma rays, we have built a new detector system that is capable of identifying the fusion products by virtue of their slow velocity (large time of flight). The device bares the acronym HERCULES (High Efficiency Evaporation Residue Counter Under a Lot of Elastic Scattering). It consists of 4 rings of 64 very thin scitillators (< 1 mg/cm²) viewed by fast photomultiplier tubes. The detectors are not protected from elastic scattering and they are designed with very fast electronics to be able to operate at counting rates of 1-2 MHz per detector. For a full list of publications go here.