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Insights into Nuclear Structure Featured at 1995 DNP Fall Meeting

New insights into nuclear structure, superheavy elements, and future applications of basic nuclear research were among the topics featured at the annual fall meeting of the APS Division of Nuclear Physics (DNP), held 25-28 October 1995 at the Indiana University Memorial Union in Bloomington, Indiana. The meeting consisted of six invited sessions, including the plenary session on basic research in nuclear physics, and 22 contributed sessions. A town meeting was also held on Friday afternoon to provide an opportunity for a large segment of the nuclear science community to be exposed to and contribute to the ongoing discussion regarding future challenges and priorities for the field.

Future of Nuclear Physics Research

Nuclear physics played a major role in the development of nuclear reactors and nuclear weapons 50 years ago. Today, according to John C. Browne of Los Alamos National Laboratory, neutrons have the potential of playing an even more important role, thanks to advances in high power accelerator technology initially developed for nuclear physics research. Browne opened the Thursday morning plenary session on the future of nuclear physics with a discussion of possible applications, focusing on concepts for burning defense and civilian nuclear waste using intense sources of spallation neutrons in accelerator-driven subcritical systems.

Electrostatic accelerators are finding important applications in materials analysis and materials modification. Particle-induced x-ray emission is used in fields from art history through environmental sciences, and in the area of security, x-ray imaging using electron beams and fast-pulsed neutron analysis is proving useful for plastic explosive and drug detection. Accelerator-based mass spectrometry is used in a number of fields which rely on counting extremely rare isotopes in small samples, and materials modification is having a significant impact on the semiconductor industry. According to G.A. Norton of the National Electrostatics Corporation, virtually all semiconductor devices now rely on ion implantation with ion beam energies ranging from a few kilovolts to several MeV.

Recent developments in magnetic resonance imaging (MRI) have significantly improved anatomical imaging, and have also added novel dimensions, such as the ability to measure and image functional, physiological and metabolic parameters in the human brain. Furthermore, the recent introduction of high magnetic fields for the use of human studies aids these applications, which are limited by the signal-to-noise ratio of the MRI methodology.

Nucleon and Nuclear Structure with Electronmagnetic Interactions

Scientists at MIT's Bates Linear Accelerator Center have made recent measurements of coincidence reactions with proton, deuteron and carbon targets, using a newly constructed proton focal plane polarimeter. The initial measurements were of the spin transfer coefficient in elastic scattering of polarized electrons from hydrogen, and in quasielastic scattering from the deuteron. According to MIT's S.P. van Verst, preliminary analysis indicates that in quasifree kinematics, the spin transfer coefficients on deuterium are consistent with those on hydrogen to within a few percent, and all three components of the proton polarization appear consistent with many-body theory.

Using the same technique, Van Verst and his colleagues also completed recoil polarization measurements of the deuteron away from quasifree kinematics. In addition, with an unpolarized beam, they measured the induced proton polarization with a carbon target, and the normal component to the proton polarization with a proton target. Future measurements of the beam helicity dependent polarization components are also planned.

New experiments with gaseous internal targets at the NIKHEF Internal Target Facility in the Netherlands have produced measurements of target asymmetries with unprecedented accuracy for the elastic and the break-up channel of tensor-polarized deuterium. Planned upgrades to further improve luminosity will allow researchers to extend these measurements even further. According to NIKHEF's C.W. de Jager, in future experiments, the availability of a longitudinally polarized beam with a high degree of polarization will be used in combination with polarized targets of 3He and D to measure the neutron electric form factor and various aspects of the internal structure of those elements.

New Aspects of Nuclear Structure

Neutron rich nuclei are of particular interest to scientists since they might reveal new aspects of nuclear structure associated with an excess of neutrons, such as a new region of deformation, shell effects, and modes of excitation. According to I.Y. Lee of Los Alamos National Laboratory, deep-inelastic reactions have been shown to produce neutron-rich nuclei with a high multiplicity of gamma-ray emission.

However, the lack of sensitivity of available gamma-ray detector arrays have made it difficult to study these reactions. Lee and his colleagues carried out gamma-spectroscopy studies of neutron rich nuclei using a silicon-strip detector to detect the projectile-like fragments, and coincident gamma rays were detected in the gammasphere. The group also studies pairing strength as a function of spin, and the variation of the interaction strength of the first backbending.

Scientists at Argonne National Laboratory have been investigating the positron and positron-electron line phenomena in heavy ion collisions using a new device called APEX, which was designed specifically for this purpose. Earlier work reporting the observation of line structure in the spectra of positrons produced in low-energy collisions of very heavy nuclei has persistently puzzled researchers. But according to LANL's Alan Wuosmaa, the new data obtained with APEX, measured under similar conditions, do not show evidence for the reported lines. In fact, in the case of the isolated two-body decay of a neutral object, the cross section limits obtained were far below those implied by the previous results.

Superheavy Elements

New results of research on the synthesis and investigation of properties of heavy nuclei at Russia's Flerov Laboratory of Nuclear Reactions have led to the observation of a new region of nuclear stability near the closed deformed shells of 108Z and 162N, predicted by the macro-microscopic theory. The experiments were conducted with beams from the facility's heavy ion accelerator using a gas-filled separator of recoils. The discovery of this new region allows scientists to make much more accurate assessments regarding the properties of heavy nuclides.

According to K.E. Rehm of Argonne National Laboratory, the study of heavy-ion induced fusion reactions at sub-Coulombic barrier energies reveals a rich and interesting interplay between reaction dynamics and nuclear structure. Considerable progress has been made in recent years advancing present understanding of the sub-barrier fusion enhancement by including additional degrees of freedom, such as static deformation, vibrational motion, and nucleon transfer reactions. New measurements at ANL involving transitional nuclei exhibit especially strong enhancement effects, Rehm reported, and these processes are expected to significantly influence the fusion of very heavy nuclei using stable and radioactive beams.

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Editor: Barrett H. Ripin