243 research outputs found
Quasi Free 238U (e,e'f)-Cross Section in Macroscopic-Microscopic Approach
We present the result of a theoretical study of inclusive quasi free
electrofission of U. The off-shell cross sections for the quasi free
reaction stage have been calculated within the Plane Wave Impulse Approximation
(PWIA), using a Macroscopic -Microscopic description of the proton and neutron
single particle momentum distributions. Electron wave function distortion
corrections were included using the effective momentum approximation, and the
Final State Interaction (FSI) effects were calculated using an optical
potential. The fissility for the proton single hole excited states of the
residual nucleus Pa was calculated both without and with contributions
of the pre-equilibrium emission of the particles. The fissility for
residual nuclei was calculated within the compound nucleus model.
The cross sections thus obtained were compared with available
experimental data.Comment: 26 pages, 7 figure
Enhanced Fusion-Evaporation Cross Sections in Neutron-Rich Sn on Ni
Evaporation residue cross sections have been measured with neutron-rich
radioactive Sn beams on Ni in the vicinity of the Coulomb
barrier. The average beam intensity was particles per second
and the smallest cross section measured was less than 5 mb. Large subbarrier
fusion enhancement was observed. Coupled-channels calculations taking into
account inelastic excitation and neutron transfer underpredict the measured
cross sections below the barrier.Comment: 4 pages including 1 table and 3 figure
Spectroscopy of Po
Prompt, in-beam rays following the reaction Yb + 142 MeV
Si were measured at the ATLAS facility using 10 Compton-suppressed Ge
detectors and the Fragment Mass Analyzer. Transitions in Po were
identified and placed using -ray singles and coincidence data gated on
the mass of the evaporation residues. A level spectrum up to
J10 was established. The structure of Po is more
collective than that observed in the heavier polonium isotopes and indicates
that the structure has started to evolve towards the more collective nature
expected for deformed nuclei.Comment: 8 pages, revtex 3.0, 4 figs. available upon reques
Angular anisotropy of the fusion-fission and quasifission fragments
The anisotropy in the angular distribution of the fusion-fission and
quasifission fragments for the O+U, F+Pb and
S+Pb reactions is studied by analyzing the angular momentum
distributions of the dinuclear system and compound nucleus which are formed
after capture and complete fusion, respectively. The orientation angles of
axial symmetry axes of colliding nuclei to the beam direction are taken into
account for the calculation of the variance of the projection of the total spin
onto the fission axis. It is shown that the deviation of the experimental
angular anisotropy from the statistical model picture is connected with the
contribution of the quasifission fragments which is dominant in the
S+Pb reaction. Enhancement of anisotropy at low energies in the
O+U reaction is connected with quasifission of the dinuclear
system having low temperature and effective moment of inertia.Comment: 17 pages 8 figures. Submitted to Euro. Phys. Jour.
Very high rotational frequencies and band termination in 73Br
Rotational bands in 73Br have been investigated up to spins of 65/2 using the
EUROBALL III spectrometer. One of the negative-parity bands displays the
highest rotational frequency 1.85 MeV reported to date in nuclei with mass
number greater than 25. At high frequencies, the experimental dynamic moment of
inertia for all bands decrease to very low values, indicating a loss of
collectivity. The bands are described in the configuration-dependent cranked
Nilsson-Strutinsky model. The calculations indicate that one of the
negative-parity bands is observed up to its terminating single-particle state
at spin 63/2. This result establishes the first band termination case in the A
= 70 mass region.Comment: 6 pages, 6 figures, submitted to Phys. Rev. C as a Rapid
Communicatio
Neutrino-induced neutron spallation and supernova r-process nucleosynthesis
In order to explore the consequences of the neutrino irradiation for the
supernova r-process nucleosynthesis, we calculate the rates of charged-current
and neutral-current neutrino reactions on neutron-rich heavy nuclei, and
estimate the average number of neutrons emitted in the resulting spallation.
Our results suggest that charged-current captures can be important in
breaking through the waiting-point nuclei at N=50 and 82, while still allowing
the formation of abundance peaks. Furthermore, after the r-process freezes out,
there appear to be distinctive neutral-current and charged-current
postprocessing effects. A subtraction of the neutrino postprocessing effects
from the observed solar r-process abundance distribution shows that two mass
regions, A=124-126 and 183-187, are inordinately sensitive to neutrino
postprocessing effects. This imposes very stringent bounds on the freeze-out
radii and dynamic timescales governing the r-process. Moreover, we find that
the abundance patterns within these mass windows are entirely consistent with
synthesis by neutrino interactions. This provides a strong argument that the
r-process must occur in the intense neutrino flux provided by a core-collapse
supernova.Comment: 34 pages, 4 PostScript figures, RevTe
Prompt dipole radiation in fusion reactions
The prompt gamma ray emission was investigated in the 16A MeV energy region
by means of the 36,40Ar+96,92Zr fusion reactions leading to a compound nucleus
in the vicinity of 132Ce. We show that the prompt radiation, which appears to
be still effective at such a high beam energy, has an angular distribution
pattern consistent with a dipole oscillation along the symmetry axis of the
dinuclear system. The data are compared with calculations based on a collective
bremsstrahlung analysis of the reaction dynamics
Extended Hauser-Feshbach Method for Statistical Binary-Decay of Light-Mass Systems
An Extended Hauser-Feshbach Method (EHFM) is developed for light heavy-ion
fusion reactions in order to provide a detailed analysis of all the possible
decay channels by including explicitly the fusion-fission phase-space in the
description of the cascade chain. The mass-asymmetric fission component is
considered as a complex-fragment binary-decay which can be treated in the same
way as the light-particle evaporation from the compound nucleus in
statistical-model calculations. The method of the phase-space integrations for
the binary-decay is an extension of the usual Hauser-Feshbach formalism to be
applied to the mass-symmetric fission part. The EHFM calculations include
ground-state binding energies and discrete levels in the low excitation-energy
regions which are essential for an accurate evaluation of the phase-space
integrations of the complex-fragment emission (fission). In the present
calculations, EHFM is applied to the first-chance binary-decay by assuming that
the second-chance fission decay is negligible. In a similar manner to the
description of the fusion-evaporation process, the usual cascade calculation of
light-particle emission from the highly excited complex fragments is applied.
This complete calculation is then defined as EHFM+CASCADE. Calculated
quantities such as charge-, mass- and kinetic-energy distributions are compared
with inclusive and/or exclusive data for the S+Mg and
Cl+C reactions which have been selected as typical examples.
Finally, the missing charge distributions extracted from exclusive measurements
are also successfully compared with the EHFM+CASCADE predictions.Comment: 34 pages, 6 Figures available upon request, Phys. Rev. C (to be
published
- âŠ