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Daniel Sperber
Professor Emeritus
Education:
M.Sc.,The Hebrew University, Jerusalem, 1954.
M.A., Ph.D., Princeton University, 1957, 1960.
Career Highlights:
From 1960 to 1967 Sperber held joint appointments at IIT (Illinois Institute of Technology) and Illinois Institute of Technology research Institute, the former Armour Research Foundation (IITRI). At IIT he rose from the rank of assistant professor to the rank of associate professor. He joined IITRI as an associate physicist, being promoted successively to research physicist, senior physicist and finally becoming science adviser.
Sperber came to Rensselaer as an associate professor in the fall of 1967. He was promoted to the rank of full professor in 1972. His honors include a fellowship in the American Physical Society, a sabbatical spent at the Niels Bohr Institute as a NORDITA professor, a sabbatical spent as distinguished visiting professor at the Institute for Heavy Ion Research in Darmstadt Germany and a sabbatical spent as senior Fulbright fellow at the Saha Institute for Nuclear Physics in Calcutta India.
Research Interests:
Sperber’s research touches on various aspects of theoretical nuclear physics. The main theme of this work is the study of the nuclear many body problems; in particular the study of heavy nuclei. Nuclear fission exhibits some of the features of a many body system made of strongly interacting particles. Originally the theory of fission was limited to the study non-rotating nuclei. The understanding of fission of rotating nuclei offered a challenging topic of theoretical research, was investigated is now well understood. This work was followed by the study of fission isomers, and the double humped fission barrier suggesting a much more complicated potential map for the interaction between the emerging fission fragments.
Since the early seventies, heavy ion accelerators offered a new way of unraveling the structure of heavy nuclei and opened new vistas to investigate the nuclear many body nuclear problem. Hence since 1973 the research focused on the theoretical aspects of heavy ion collision or on the study of heavy ion scattering. In such a scattering two nuclei at close proximity behave like two atoms stripped of their electrons, thus totally ionized. The collision between such ions can lead to three different processes: a) elastic scattering, b) inelastic scattering and c) fusion. In the first case they ions come out with their original energy, in the second case the come out with a reduced kinetic energy, part of the energy being transferred into internal degrees of freedom, in the last case the ions fuse to form a very heavy nuclear system, which may be stable or alternatively may be unstable and decay. Developing models for the angular distribution of the emerging ions, the energy transfer from relative motion to internal degrees of freedom and the determination of the fusion cross section were the challenges of these projects. The energy transfer is an irreversible process, the slower emerging ions can be considered as heated up, thus the loss of kinetic energy is accounted by the of the random thermal energy of the scattered ions. This is an indication, that in addition to mechanical considerations, thermo dynamical and statistical mechanical consideration must be included, and hence were incorporated in the theoretical models. Finally, the emerging ions do not have the same mass as the original colliding ions. Thus the emerging ions come out with a spectrum of masses. The successful explanations of this spectrum, and the correlation between the loss of kinetic energy and mass spectrum are of great significance. The above experimental observation and corresponding theoretical models are applicable to heavy ion collisions at low energies, roughly to bombarding energies below 10 MeV/nucleon. At higher energies new and unexpected phenomena were observed. In a nutshell the experiments suggested a liquid-gas phase transition. These results were very exciting and called for further theoretical work, they also confirmed that thermo dynamical and statistical mechanical considerations play a key role in the study of heavy ion collisions. The overriding issues were the study of the equation of state for heavy, but finite size nuclei and the determination of the critical temperature. This leads to a comprehensive study of hot nuclei and hot nuclear matter. Most attempts to solve this problem are based on using mean field theory. It is essential to recognize that mean field theory fails for systems that are made of constituents which interact strongly only at very short distances. Thus the equation of state was derived using an effective two-body momentum dependent non-local nucleon-nucleon interaction, which reproduces nuclear ground states properties very well. Minimizing the Helmholtz free emery determines the equation of state and the relevant thermodynamic parameter to study the liquid-gas phase transition in nuclei, these models are consistent with experimental results. It is worthwhile to note that in the early days of nuclear physics; people talked loosely about the “ liquid drop model” and the “Fermi gas model of nuclei”. Now these intuitive models are on firm theoretical ground. We also studied heavy ion collisions as stochastic processes solving either the master equation or the Fokker Planck equation.
The liquid -gas phase transition is not the only phase transition observed in nuclei, there are indication that at bombarding energies of about 200 MeV/nucleon another phase transition occurs. This transition corresponds to dissolution of the building blocks or the constituents of the nucleus, namely the protons and neutrons, and formation of short-lived quark-gluon plasma. Thus this can be considered as the melting of the hadrons. The quark-gluon plasma freezes very quickly and hadronizes again. There are speculations that in the very early moments the universe was made of quark-gluon plasma, which hadronizes very fast. Thus the study of this plasma and its thermodynamics properties is also very significant for cosmological considerations, and we are trying to apply our model for phase transition is to these new phenomena. In summary this research program covers most of the topics of heavy ion physics and is closely connected to the experimental work in this field.
Contact:
(518) 276-8409
sperbd@rpi.edu
Home Page: http://www.rpi.edu/dept/phys/sperber.html
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