384 research outputs found
Proton Decay from Excited States in Spherical Nuclei
Based on a single particle model which describes the time evolution of the
wave function during tunneling across a one dimensional potential barrier we
study the proton decay of Pb from excited states with non-vanishing
angular momentum . Several quantities of interest in this process like
the decay rate , the period of oscillation , the transient
time , the tunneling time and the average value of the proton
packet position are computed and compared with the WKB results.Comment: 12 pages, 4 figure
Emission of Scission Neutrons in the Sudden Approximation
At a certain finite neck radius during the descent of a fissioning nucleus
from the saddle to the scission point, the attractive nuclear forces can no
more withstand the repulsive Coulomb forces producing the neck rupture and the
sudden absorption of the neck stubs by the fragments. At that moment, the
neutrons, although still characterized by their pre-scission wave functions,
find themselves in the newly created potential of their interaction with the
separated fragments. Their wave functions become wave packets with components
in the continuum. The probability to populate such states gives evidently the
emission probability of neutrons at scission. In this way, we have studied
scission neutrons for the fissioning nucleus U, using two-dimensional
realistic nuclear shapes. Both the emission probability and the distribution of
the emission points relative to the fission fragments strongly depend on the
quantum numbers of the pre-scission state from which the neutron is emitted. In
particular it was found that states with = 1/2+ dominate the
emission. Depending on the assumed pre- and post-scission configurations and on
the emission-barrier height, 30 to 50% of the total scission neutrons are
emitted from 1/2+ states. Their emission points are concentrated in the region
between the newly separated fragments. The upper limit for the total number of
neutrons per scission event is predicted to lie between 0.16 and 1.73
(depending on the computational assumptions).Comment: 31 pages, 16 figures, 2 table
Gamow-Teller sum rule in relativistic nuclear models
Relativistic corrections are investigated to the Gamow-Teller(GT) sum rule
with respect to the difference between the and transition
strengths in nuclei. Since the sum rule requires the complete set of the
nuclear states, the relativistic corrections come from the anti-nucleon degrees
of freedom. In the relativistic mean field approximation, the total GT
strengths carried by the nucleon sector is quenched by about 12% in nuclear
matter, while by about 8% in finite nuclei, compared to the sum rule value. The
coupling between the particle-hole states with the nucleon-antinucleon states
is also discussed with the relativistic random phase approximation, where the
divergence of the response function is renormalized with use of the counter
terms in the Lagrangian. It is shown that the approximation to neglect the
divergence, like the no-sea approximation extensively used so far, is
unphysical, from the sum-rule point of view.Comment: 12 pages, Brief review for Mod. Phys. Lett. A, using ws-mpla.cl
Model independent study of the Dirac structure of the nucleon-nucleon interaction
Relativistic and non-relativistic modern nucleon-nucleon potentials are
mapped on a relativistic operator basis using projection techniques. This
allows to compare the various potentials at the level of covariant amplitudes
were a remarkable agreement is found. In nuclear matter large scalar and vector
mean fields of several hundred MeV magnitude are generated at tree level. This
is found to be a model independent feature of the nucleon-nucleon interaction.Comment: 5 pages, 2 figures, results for V_lowk added, to appear in PR
Associated hyperon-kaon production via neutrino-nucleus scattering
We present the investigation of the neutrino-induced strangeness associated
production on nuclei in the relativistic plane wave impulse approximation
(RPWIA) framework at the intermediate neutrino energies. In this study, the
elementary hadronic weak amplitudes are embedded inside the nuclear medium for
the description of the exclusive channels of neutrino-nucleus interactions.
These amplitudes are extracted using a model-dependent evaluation of the
hadronic vertex using the Born term approximation in which the application of
the Cabibbo V-A theory and SU(3) symmetry are assumed to be valid. The nuclear
effects are included via the bound state wavefunctions of the nucleon obtained
from the relativistic mean field (RMF) models. Two kinematics settings are used
to examine various distributions of the differential cross section in the rest
frame of the target nuclei. The numerical results are obtained for the
neutrino-induced charged-current (CC) \,-production on bound
neutrons in and orbitals of C. The angular
distributions are forward peaked under both kinematic settings, whereas under
the quasifree setting the cross sections tend mimic the missing momentum
distribution of the bound nucleon inside the nucleus.Comment: This article is submitted to International Journal of Modern Physics
E (nuclear physics) and accepted on 31 October 20l
Characteristics of light charged particle emission in the ternary fission of 250Cf and 252Cf at different excitation energies
The emission probabilities and the energy distributions of tritons, α and ^6He particles emitted in the spontaneous ternary fission (zero excitation energy) of ^250Cf and ^252Cf and in the cold neutron induced fission (excitation energy â 6.5 MeV) of ^249Cf and 251Cf are determined. The particle identification was done with suited ÎE-E telescope detectors, at the IRMM (Geel, Belgium) for the spontaneous fission and at the ILL (Grenoble, France) for the neutron induced fission measurements. Hence particle emission characteristics of the fissioning systems ^250Cf and ^252Cf are obtained at zero and at about 6.5 MeV excitation energies. While the triton emission probability is hardly influenced by the excitation energy, the ^4He and ^6He emission probability in spontaneous fission is higher than for neutron induced fission. This can be explained by the strong influence of the cluster preformation probability on the ternary particle emission probability
The relativistic self-energy in nuclear dynamics
It is a well known fact that Dirac phenomenology of nuclear forces predicts
the existence of large scalar and vector mean fields in matter. To analyse the
relativistic self-energy in a model independent way, modern high precision
nucleon-nucleon () potentials are mapped on a relativistic operator basis
using projection techniques. This allows to compare the various potentials at
the level of covariant amplitudes were a remarkable agreement is found. It
allows further to calculate the relativistic self-energy in nuclear matter in
Hartree-Fock approximation. Independent of the choice of the nucleon-nucleon
interaction large scalar and vector mean fields of several hundred MeV
magnitude are generated at tree level. In the framework of chiral EFT these
fields are dominantly generated by contact terms which occur at next-to-leading
order in the chiral expansion. Consistent with Dirac phenomenology the
corresponding low energy constants which generate the large fields are closely
connected to the spin-orbit interaction in scattering. The connection to
QCD sum rules is discussed as well.Comment: 49 pages, 13 figure
Point-Coupling Models from Mesonic Hypermassive Limit and Mean-Field Approaches
In this work we show how nonlinear point-coupling models, described by a
Lagrangian density that presents only terms up to fourth order in the fermion
condensate , are derived from a modified meson-exchange
nonlinear Walecka model. The derivation can be done through two distinct
methods, namely, the hypermassive meson limit within a functional integral
approach, and the mean-field approximation in which equations of state at zero
temperature of the nonlinear point-coupling models are directly obtained.Comment: 18 pages. Accepted for publication in Braz. J. Phy
Energy distributions and yields of 3H, 4He and 6He-particles emitted in the 245Cm(n_th,f) reaction
The energy distributions and yields of light charged particles emitted during thermal neutron induced fission of 245Cm have been measured at the high flux reactor of the Institute Laue Langevin in Grenoble (France). The detection of the ternary particles was done using a Delta-E/E telescope, permitting a good separation of the ternary particles. In this way, the characteristics of the energy distribution (average energy and full width at half maximum) for 4He, 3H and 6He particles as well as their emission probabilities could be determined. For the emission probabilities per fission, the following values were obtained: LRA/B=(2.15+-0.05)E-3, 3H/B=(1.85+-0.10)E-4 and 6He/B=(4.95+-1.25)E-5
Bound states of bosons and fermions in a mixed vector-scalar coupling with unequal shapes for the potentials
The Klein-Gordon and the Dirac equations with vector and scalar potentials
are investigated under a more general condition, . These intrinsically relativistic and isospectral problems
are solved in a case of squared hyperbolic potential functions and bound states
for either particles or antiparticles are found. The eigenvalues and
eigenfuntions are discussed in some detail and the effective Compton wavelength
is revealed to be an important physical quantity. It is revealed that a boson
is better localized than a fermion when they have the same mass and are
subjected to the same potentials.Comment: 3 figure
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