arXiv subjects

A. Bracco

Publications and source records attributed to A. Bracco.

80 records · Page 5Linked to original sources

arXiv2003

Evidence for the Jacobi shape transition in hot 46Ti

The gamma-rays from the decay of the GDR in 46Ti compound nucleus formed in the 18O+28Si reaction at bombarding energy 105 MeV have been measured in an experiment using a setup consisting of the combined EUROBALL IV, HECTOR and EUCLIDES arrays. A comparison of the extracted GDR lineshape data with the predictions of the thermal shape fluctuation model shows evidence for the Jacobi shape transition in hot 46Ti. In addition to the previously found broad structure in the GDR lineshape region at 18-27 MeV caused by large deformations, the presence of a low energy component (around 10 MeV), due to the Coriolis splitting in prolate well deformed shape, has been identified for the first time.

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arXiv2003

Search for the Jacobi shape transition in light nuclei

The gamma-rays following the reaction 105 MeV 18O + 28Si have been measured using the EUROBALL IV, HECTOR and EUCLIDES arrays in order to investigate the predicted Jacobi shape transition. The high-energy gamma-ray spectrum from the GDR decay indicates a presence of large deformations in hot 46Ti nucleus, in agreement with new theoretical calculations based on the Rotating Liquid Drop model.

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arXiv2002

Rotational Damping and Compound Formation in Warm Rotating Nuclei

The rotational damping width Γ_{rot} and the compound damping width Γ_{comp} are two fundamental quantities that characterize rapidly rotating compound nuclei having finite thermal excitation energy. A two-component structure in the strength function of consecutive E2 transitions reflects the two widths, and it causes characteristic features in the double and triple gamma-ray spectra. We discuss a new method to extract experimentally values of Γ_{rot} and Γ_{comp}. The first preliminary result of this method is presented.

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arXiv2001

Search for the Jacobi Instability in Rapidly Rotating 46Ti* Nuclei

The possible existence of Jacobi shape transition in hot 46Ti at high angular momenta was investigated with the Giant Dipole Resonance exclusive experiments. The GDR spectra and the angular distributions are consistent with predictions of the thermal shape fluctuation model indicating elongated nuclear shapes.

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arXiv2001

Gamma-ray strength functions in thermally excited rotating nuclei

A general discussion and illustration is given of strength functions for rotational transitions in two-dimensional E(gamma_1) x E(gamma_2) spectra. Especially, a narrow component should be proportional to the compound damping width, related to the mixing of basis rotational bands into compound bands with fragmented transition strength. Three E(gamma_1) x E(gamma_2) spectra are made by setting gates on triple coincidences, selecting cascades which feed into specific low-lying bands in the nucleus 168Hf. In each of the gated spectra, we find a ridge, carrying about 100 decay paths. This ridge is ascribed to rotational transitions in the excitation energy range of 1.0 to 1.5 MeV above the yrast line. The FWHM of the ridges are around 40 keV, about a factor of two smaller than calculated on the basis of mixed cranked mean field bands.

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arXiv1999

Particle Aggregation in a turbulent Keplerian flow

In the problem of planetary formation one seeks a mechanism to gather small solid particles together into larger accumulations of solid matter. Here we describe a scenario in which turbulence mediates this process by aggregating particles into anticyclonic regions. If, as our simulations suggest, anticyclonic vortices form as long-lived coherent structures, the process becomes more powerful because such vortices trap particles effectively. Even if the turbulence is decaying, following the upheaval that formed the disk, there is enough time to make the dust distribution quite lumpy.

astro-ph↗

arXiv1998

Spotted Disks

Rotating, turbulent cosmic fluids are generally pervaded by coherent structures such as vortices and magnetic flux tubes. The formation of such structures is a robust property of rotating turbulence as has been confirmed in computer simulations and laboratory experiments. We defend here the notion that accretion disks share this feature of rotating cosmic bodies. In particular, we show that the intense shears of Keplerian flows do not inhibit the formation of vortices. Given suitable initial disturbances and high enough Reynolds numbers, long-lived vortices form in Keplerian shear flows and analogous magnetic structures form in magnetized disks. The formation of the structures reported here should have significant consequences for the transport properties of disks and for the observed properties of hot disks.

astro-ph↗

arXiv1996

Behavior of the giant-dipole resonance in $^{120}$Sn and $^{208}$Pb at high excitation energ

The properties of the giant-dipole resonance (GDR) are calculated as a function of excitation energy, angular momentum, and the compound nucleus particle decay width in the nuclei $^{120}$Sn and $^{208}$Pb, and are compared with recent experimental data. Differences observed in the behavior of the full-width-at-half-maximum of the GDR for $^{120}$Sn and $^{208}$Pb are attributed to the fact that shell corrections in $^{208}$Pb are stronger than in $^{120}$Sn, and favor the spherical shape at low temperatures. The effects shell corrections have on both the free energy and the moments of inertia are discussed in detail. At high temperature, the FWHM in $^{120}$Sn exhibits effects due to the evaporation width of the compound nucleus, while these effects are predicted for $^{208}$Pb.

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