Physics Division 1996 to 1998
Physics Division 1996 to 1998Historically, a cutting-edge experimental research program in nuclear structure, nuclear reactions, and nuclear data has given the Physics Division a strong background in traditional nuclear physics for many years. This program has focused on the spectroscopy of far-from-stability nuclei, giant resonance studies, and reaction dynamics. In the past few years, our research program has become stronger and more diverse with the establishment of a Nuclear Astrophysics Group, the advent of the Holifield Radioactive Ion Beam Facility (HRIBF), and the incorporation of the astrophysics research centered around the ORELA Facility. During this reporting period, all three groups have maintained a strong research program utilizing the capabilities of the HRIBF and other user facilities while constructing a wide range of state-of-the-art detection systems for the experimental programs at the HRIBF. Reports of these and other nuclear physics projects are presented in this section. As the varieties and intensities of the radioactive ion beams at the HRIBF increase, we expect that the focus of our experimental program will increasingly shift toward studies of physical phenomena that may be best addressed using these exotic beams.
During this reporting period, the Nuclear Data Evaluation effort has been both augmented and redirected to address more closely the needs of the research programs in this Section. In addition to the nuclear astrophysics data evaluation initiative, our nuclear structure work has focused on the compilation and evaluation of the data most relevant to the studies the far-from-stability nuclei.
The nuclear structure program has focused on studies of the far-from-stability nuclei, with emphasis on probing nuclear properties near the proton-drip line. Among the highlights of the recent results are: identification of five spherical and deformed proton emitters, establishment of level structures of nuclei located beyond the proton-drip line, first observation of superdeformation in an N=Z nucleus (60Zn) and systematic studies of superdeformation in the A~60 region, first observation of decay of a superdeformed state (in 58Cu) by emission of protons, discovery of collective bands in 56Ni which constitutes the first observation of superdeformation in a doubly-magic nucleus, extensive studies of structures of odd-odd and even-even N=Z nuclei from 52Fe to 68Se, investigations of possible existence of a proton-neutron superconductive phase in nuclear matter, isospin symmetry in isobaric analog nuclei, and level structures in and around 56Ni (N=Z=28) that are important for shell model calculations. In close collaboration with the ORNL-UT Theory Group, these results have been used to test, compare, and confront different theoretical approaches based on large-scale shell model calculations, Quantum Monte Carlo Diagonalization, and self-consistent Hartree-Fock calculations employing a variety of effective forces. These investigations will provide a better insight into the mechanisms responsible for producing large collectivity in the A~60 mass region, and will help determine a better parameter set for shell model calculations of medium-mass nuclei. Some of the N~Z nuclei studied under this program fall near the path of the rp-process and are of interest to the nuclear astrophysics program as well. The Nuclear Structure Group has taken the leadership in the installation and commissioning of a state-of-the-art Recoil Mass Spectrometer (RMS) and its associated detector systems. This system is operational with focal-plane detectors available for alpha, beta, gamma, and proton decay of reaction products. Commissionings of the CLARION (CLover Array for Radioactive IONs) Ge array and the HyBall charged particle detector array are scheduled for the spring of 1999. Along with the RMS and focal-plane detectors, these arrays provide the most sensitive tool available for nuclear structure studies with radioactive ion beams.
The Nuclear Astrophysics Group has completed their first experiment with a 17F radioactive beam from the HRIBF: The 17F(p,p) reaction was used to find a missing 18Ne resonance which plays an important role in the 17F(p,gamma)18Ne reaction which occurs in stellar explosions. An array of silicon detectors was developed and installed for this and similar measurements of astrophysically important reactions. The Daresbury Recoil Separator (DRS), which will be used to directly measure capture reactions that occur in astrophysical environments, has been installed and commissioning with stable beams is in progress. Another major thrust has been the measurement of neutron capture and transmission reactions on Ba, Nd, Pt, and Sr isotopes at the ORELA pulsed white neutron source. These measurements have helped determine the origin of the heavy elements synthesized in red giant stars, and help decipher the anomalous isotopic abundances found in meteorites.
The Nuclear Reactions Group has extended its program of research in the giant resonances to measurements of the giant dipole (GDR) strength function in the halo nuclei 11Be and 20O using the ORNL-TAMU-MSU BaF2 photon detector array, as well as measurements of the isovector giant quadrupole resonance in heavy, stable nuclei, all using the S800 spectrometer of the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University. The group has also begun a program of measurements with radioactive beams at the HRIBF, applying a thick-target, energy-loss resonance measurement technique to the determination of resonance parameters for proton scattering in inverse kinematics. Finally, they have developed detectors and techniques for gas filled operation of the Enge spectrometer in the measurement of heavy-ion fusion reactions using radioactive and normal beams. They will continue to develop these programs at the NSCL and at the HRIBF.