3,112 research outputs found
On the measurement of B(E2, 0+ -> 2+) using intermediate-energy Coulomb excitation
Coulomb excitation is a standard method used to extract quadrupole excitation
strengths of even-even nuclei. In typical analyses the reaction is assumed to
be one-step, Coulomb only, and is treated within a semi-classical model. In
this work, fully-quantal coupled-channel calculations are performed for three
test cases in order to determine the importance of multi-step effects, nuclear
contributions, feeding from other states and corrections to the semi-classical
approximation. We study the excitation of 30S, 58Ni and 78Kr on 197Au at ~ 50
AMeV. We find that nuclear effects may contribute more than 10% and that
feeding contributions can be larger than 15%. These corrections do not alter
significantly the published B(E2) values, however an additional theoretical
error of up to 13% should be added to the experimental uncertainty if the
semi-classical model is used. This theoretical error is reduced to less than 7%
when performing a quantal coupled-channel analysis.Comment: 9 pages, accepted for publication in J. Phys. G: Nucl. Phy
Phase transitions in the Interacting Boson Fermion Model: the gamma-unstable case
The phase transition around the critical point in the evolution from
spherical to deformed gamma-unstable shapes is investigated in odd nuclei
within the Interacting Boson Fermion Model. We consider the particular case of
an odd j=3/2 particle coupled to an even-even boson core that undergoes a
transition from spherical U(5) to gamma-unstable O(6) situation. The particular
choice of the j=3/2 orbital preserves in the odd case the condition of
gamma-instability of the system. As a consequence, energy spectrum and
electromagnetic transitions, in correspondence of the critical point, display
behaviours qualitatively similar to those of the even core. The results are
also in qualitative agreement with the recently proposed E(5/4) model, although
few differences are present, due to the different nature of the two schemes.Comment: In press in PRC as rapid communication. 7 pages, 4 figure
Phase structure of the two-fluid proton-neutron system
The phase structure of a two-fluid bosonic system is investigated. The
proton-neutron interacting boson model (IBM-2) posesses a rich phase structure
involving three control parameters and multiple order parameters. The surfaces
of quantum phase transition between spherical, axially-symmetric deformed, and
SU*(3) triaxial phases are determined.Comment: RevTeX 4, 4 pages, as published in Phys. Rev. Let
Distribution of occupation numbers in finite Fermi-systems and role of interaction in chaos and thermalization
New method is developed for calculation of single-particle occupation numbers
in finite Fermi systems of interacting particles. It is more accurate than the
canonical distribution method and gives the Fermi-Dirac distribution in the
limit of large number of particles. It is shown that statistical effects of the
interaction are absorbed by an increase of the effective temperature. Criteria
for quantum chaos and statistical equilibrium are considered. All results are
confirmed by numerical experiments in the two-body random interaction model.Comment: 4 pages, Latex, 4 figures in the form of PS-file
On the relation between models and the interacting boson model
The connections between the models (the original E(5) using an
infinite square well, , and ), based
on particular solutions of the geometrical Bohr Hamiltonian with
-unstable potentials, and the interacting boson model (IBM) are
explored. For that purpose, the general IBM Hamiltonian for the
transition line is used and a numerical fit to the different models
energies is performed, later on the obtained wavefunctions are used to
calculate B(E2) transition rates. It is shown that within the IBM one can
reproduce very well all these models. The agreement is the best for
and reduces when passing through ,
and E(5), where the worst agreement is obtained (although still very good for a
restricted set of lowest lying states). The fitted IBM Hamiltonians correspond
to energy surfaces close to those expected for the critical point. A phenomenon
similar to the quasidynamical symmetry is observed
Relationship between X(5)-models and the interacting boson model
The connections between the X(5)-models (the original X(5) using an infinite
square well, X(5)-, X(5)-, X(5)-, and
X(5)-), based on particular solutions of the geometrical Bohr
Hamiltonian with harmonic potential in the degree of freedom, and the
interacting boson model (IBM) are explored. This work is the natural extension
of the work presented in [1] for the E(5)-models. For that purpose, a quite
general one- and two-body IBM Hamiltonian is used and a numerical fit to the
different X(5)-models energies is performed, later on the obtained wave
functions are used to calculate B(E2) transition rates. It is shown that within
the IBM one can reproduce well the results for energies and B(E2) transition
rates obtained with all these X(5)-models, although the agreement is not so
impressive as for the E(5)-models. From the fitted IBM parameters the
corresponding energy surface can be extracted and it is obtained that,
surprisingly, only the X(5) case corresponds in the moderate large N limit to
an energy surface very close to the one expected for a critical point, while
the rest of models seat a little farther.Comment: Accepted in Physical Review
Electromagnetic transition strengths in soft deformed nuclei
Spectroscopic observables such as electromagnetic transitions strengths can
be related to the properties of the intrinsic mean-field wave function when the
latter are strongly deformed, but the standard rotational formulas break down
when the deformation decreases. Nevertheless there is a well-defined, non-zero,
spherical limit that can be evaluated in terms of overlaps of mean-field
intrinsic deformed wave functions. We examine the transition between the
spherical limit and strongly deformed one for a range of nuclei comparing the
two limiting formulas with exact projection results. We find a simple criterion
for the validity of the rotational formula depending on ,
the mean square fluctuation in the angular momentum of the intrinsic state. We
also propose an interpolation formula which describes the transition strengths
over the entire range of deformations, reducing to the two simple expressions
in the appropriate limits.Comment: 16 pages, 5 figures, supplemental material include
On-shell Techniques and Universal Results in Quantum Gravity
We compute the leading post-Newtonian and quantum corrections to the Coulomb
and Newtonian potentials using the full modern arsenal of on-shell techniques;
we employ spinor-helicity variables everywhere, use the Kawai-Lewellen-Tye
(KLT) relations to derive gravity amplitudes from gauge theory and use
unitarity methods to extract the terms needed at one-loop order. We stress that
our results are universal and thus will hold in any quantum theory of gravity
with the same low-energy degrees of freedom as we are considering. Previous
results for the corrections to the same potentials, derived historically using
Feynman graphs, are verified explicitly, but our approach presents a huge
simplification, since starting points for the computations are compact and
tedious index contractions and various complicated integral reductions are
eliminated from the onset, streamlining the derivations. We also analyze the
spin dependence of the results using the KLT factorization, and show how the
spinless correction in the framework are easily seen to be independent of the
interacting matter considered.Comment: 34 pages, 7 figures, typos corrected, published versio
A simple and surprisingly accurate approach to the chemical bond obtained from dimensional scaling
We present a new dimensional scaling transformation of the Schrodinger
equation for the two electron bond. This yields, for the first time, a good
description of the two electron bond via D-scaling. There also emerges, in the
large-D limit, an intuitively appealing semiclassical picture, akin to a
molecular model proposed by Niels Bohr in 1913. In this limit, the electrons
are confined to specific orbits in the scaled space, yet the uncertainty
principle is maintained because the scaling leaves invariant the
position-momentum commutator. A first-order perturbation correction,
proportional to 1/D, substantially improves the agreement with the exact ground
state potential energy curve. The present treatment is very simple
mathematically, yet provides a strikingly accurate description of the potential
energy curves for the lowest singlet, triplet and excited states of H_2. We
find the modified D-scaling method also gives good results for other molecules.
It can be combined advantageously with Hartree-Fock and other conventional
methods.Comment: 4 pages, 5 figures, to appear in Phys. Rev. Letter
Structural evolution in Pt isotopes with the Interacting Boson Model Hamiltonian derived from the Gogny Energy Density Functional
Spectroscopic calculations are carried out, for the description of the
shape/phase transition in Pt nuclei in terms of the Interacting Boson Model
(IBM) Hamiltonian derived from (constrained) Hartree-Fock-Bogoliubov (HFB)
calculations with the finite range and density dependent Gogny-D1S Energy
Density Functional. Assuming that the many-nucleon driven dynamics of nuclear
surface deformation can be simulated by effective bosonic degrees of freedom,
the Gogny-D1S potential energy surface (PES) with quadrupole degrees of freedom
is mapped onto the corresponding PES of the IBM. Using this mapping procedure,
the parameters of the IBM Hamiltonian, relevant to the low-lying quadrupole
collective states, are derived as functions of the number of valence nucleons.
Merits of both Gogny-HFB and IBM approaches are utilized so that the spectra
and the wave functions in the laboratory system are calculated precisely. The
experimental low-lying spectra of both ground-state and side-band levels are
well reproduced. From the systematics of the calculated spectra and the reduced
E2 transition probabilities (E2), the prolate-to-oblate shape/phase
transition is shown to take place quite smoothly as a function of neutron
number in the considered Pt isotopic chain, for which the -softness
plays an essential role. All these spectroscopic observables behave
consistently with the relevant PESs and the derived parameters of the IBM
Hamiltonian as functions of . Spectroscopic predictions are also made for
those nuclei which do not have enough experimental E2 data.Comment: 11 pages, 5 figure
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