220 research outputs found
Correlation studies of fission fragment neutron multiplicities
We calculate neutron multiplicities from fission fragments with specified
mass numbers for events having a specified total fragment kinetic energy. The
shape evolution from the initial compound nucleus to the scission
configurations is obtained with the Metropolis walk method on the
five-dimensional potential-energy landscape, calculated with the
macroscopic-microscopic method for the three-quadratic-surface shape family.
Shape-dependent microscopic level densities are used to guide the random walk,
to partition the intrinsic excitation energy between the two proto-fragments at
scission, and to determine the spectrum of the neutrons evaporated from the
fragments. The contributions to the total excitation energy of the resulting
fragments from statistical excitation and shape distortion at scission is
studied. Good agreement is obtained with available experimental data on neutron
multiplicities in correlation with fission fragments from U(n,f). At higher neutron energies a superlong fission mode appears which
affects the dependence of the observables on the total fragment kinetic energy.Comment: 12 pages, 10 figure
Microscopic Structure of Rotational Damping
The damping of collective rotational motion is studied microscopically, making use of shell model calculations based on the cranked Nilsson deformed mean-field and on residual two-body interactions, and focusing on the shape of the gamma-gamma correlation spectra and on its systematic behavior. It is shown that the spectral shape is directly related to the damping width of collective rotation, \Gammarot, and to the spreading width of many-particle many-hole configurations, \Gammamu. The rotational damping width is affected by the shell structure, and is very sensitive to the position of the Fermi surface, besides mass number, spin and deformation. This produces a rich variety of features in the rotational damping phenomena
Linear response of light deformed nuclei investigated by self-consistent quasiparticle random-phase-approximation
We present a calculation of the properties of vibrational states in deformed,
axially--symmetric even--even nuclei, within the framework of a fully
self--consistent Quasparticle Random Phase Approximation (QRPA). The same
Skyrme energy density and density-dependent pairing functionals are used to
calculate the mean field and the residual interaction in the particle-hole and
particle-particle channels. We have tested our software in the case of
spherical nuclei against fully self consistent calculations published in the
literature, finding excellent agreement. We investigate the consequences of
neglecting the spin-orbit and Coulomb residual interactions in QRPA.
Furthermore we discuss the improvement obtained in the QRPA result associated
with the removal of spurious modes. Isoscalar and isovector responses in the
deformed Mg, Mg isotopes are presented and compared to
experimental findings
Poisson and Porter-Thomas Fluctuations in off-Yrast Rotational Transitions
Fluctuations associated with stretched E2 transitions from high spin levels
in nuclei around Yb are investigated by a cranked shell model extended
to include residual two-body interactions. It is found that the gamma-ray
energies behave like random variables and the energy spectra show the Poisson
fluctuation, in the cranked mean field model without the residual interaction.
With two-body residual interaction included, discrete transition pattern with
unmixed rotational bands is still valid up to around 600 keV above yrast, in
good agreement with experiments. At higher excitation energy, a gradual onset
of rotational damping emerges. At 1.8 MeV above yrast, complete damping is
observed with GOE type fluctuations for both energy levels and transition
strengths(Porter-Thomas fluctuations).Comment: 21 pages, phyzzx, YITP/K-99
Shell Model for Warm Rotating Nuclei
In order to provide a microscopic description of levels and E2 transitions in
rapidly rotating nuclei with internal excitation energy up to a few MeV, use is
made of a shell model which combines the cranked Nilsson mean-field and the
residual surface delta two-body force. The damping of collective rotational
motion is investigated in the case of a typical rare-earth nucleus, namely \Yb.
It is found that rotational damping sets in at around 0.8 MeV above the yrast
line, and the levels which form rotational band structures are thus limited. We
predict at a given rotational frequency existence of about 30 rotational bands
of various lengths, in overall agreement with the experimental findings. The
onset of the rotational damping proceeds quite gradually as a function of the
internal excitation energy. The transition region extends up to around 2 MeV
above yrast and it is characterized by the presence of scars of discrete
rotational bands which extend over few spin values and stand out among the
damped transitions, and by a two-component profile in the
correlation. The important role played by the high-multipole components of the
two-body residual interaction is emphasized.Comment: 28 pages, LaTe
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