An exactly solvable model of a superconducting to rotational phase transition

We consider a many-fermion model which exhibits a transition from a superconducting to a rotational phase with variation of a parameter in its Hamiltonian. The model has analytical solutions in its two limits due to the presence of dynamical symmetries. However, the symmetries are basically incompatible with one another; no simple solution exists in intermediate situations. Exact (numerical) solutions are possible and enable one to study the behavior of competing but incompatible symmetries and the phase transitions that result in a semirealistic situation. The results are remarkably simple and shed light on the nature of phase transitions.Comment: 11 pages including 1 figur

Establishing action levels for EPID-based QA for IMRT

Although portal dosimetry is used to provide quality assurance (QA) for intensity-modulated radiation therapy (IMRT) treatment plans, trends in agreement between the portal dose prediction (PDP) and the measured dose have not been clarified. In this work, we evaluated three scalar parameters of agreement for 152 treatment plans (1152 treatment fields): maximum gamma (γmax), average gamma (γavg), and percentage of the field area with a gamma value greater than 1.0 (γ %> 1). These data were then used to set clinical action levels based on the institutional mean and standard deviations. We found that agreement between measured dose and PDP was improved by recalculating the fields at lower dose rates. We conclude that action levels are a useful tool for standardizing the evaluation of EPID-based IMRT QA

Shape coexistence and electric monopole transitions in Pt184

Levels in Pt184 were studied from the decay of mass-separated Au184 by gamma-ray and conversion-electron spectroscopy and by gamma-ray angular distributions from low-temperature nuclear orientation. In addition to coexisting K=0+ bands, coexisting K=2+ bands are observed at low energy. String interband electric monopole transitions are observed between states of the two K=0+ bands and also between states of the two K=2+ bands. The coexistence of K=2+ bands at low energy represents a new type of collective structure. © 1992 The American Physical Society

On-line nuclear orientation study of\u3csup\u3e184\u3c/sup\u3eAu

The184Au→184Pt decay, studied on-line with the UNISOR facility at HHIRF, is discussed. Gamma-ray and conversion-electron spectroscopy of184Pt as well as on-line nuclear orientation measurements of184Au were done. A new low-lying level scheme of184Pt is proposed. Two coexisting bands with different deformations and their respective γ-vibrational bands are established. Internal conversion coefficients for interband transitions between states with the same spin are extracted from the spectroscopy measurements. The relative E0 contents of the transitions are determined by combining internal conversion coefficients with E2/M1 mixing ratios deduced from gamma-ray anisotropies measured from oriented nuclei. © 1992 J.C. Baltzer A.G., Scientific Publishing Company

Search for low-spin superdeformed states in nuclei

Since the first discovery in 1986 of a rotational band at high angular momentum with deformation parameters near β=0.6, a great deal of research has been devoted to the study of these superdeformed (SD) bands in nuclei leading to the identification of similar bands in numerous nuclei in the mass 130, 150, and 190 regions. While superdeformed shape isomers have been observed at low spin in the actinides via spontaneous fission and in a few cases via γ decay, below mass 200 the low-spin members of the SD bands have not been observed. Theoretical calculations are able to account for the relative stability of the superdeformed bands in the nuclei where they have been observed. These calculations suggest that in the HgPb region the SD shape should be stable at a rotational frequency of 0. Several experiments have been performed to search for the population and decay of the lower spin members of the SD bands in the mass 190 region and some are still in progress. To this date, none has provided conclusive observation of the 0+ bandhead of a superdeformed band. The experimental techniques utilized in several of these are described. © 1993