5,437 research outputs found
Compression modes in nuclei: microscopic models with Skyrme interactions
The isoscalar giant monopole resonances (ISGMR) and giant dipole resonances
(ISGDR) in medium-heavy nuclei are investigated in the framework of HF+RPA and
HF-BCS+QRPA with Skyrme effective interactions. It is found that pairing has
little effect on these modes. It is also found that the coupling of the RPA
states to 2p-2h configurations results in about (or less than) 1 MeV shifts of
the resonance energies and at the same time gives the correct total widths. For
the ISGMR, comparison with recent data leads to a value of nuclear matter
compression modulus close to 215 MeV. However, a discrepancy between calculated
and measured energies of the ISGDR in Pb is found and remains an open
problem.Comment: To appear in: ``RIKEN Symposium and Workshop on Selected Topics in
Nuclear Collective Excitations'', proceedings of the meeting, RIKEN, Wako
city (Japan), March 20--24, 199
Neutrino Capture Cross Sections for Ar-40 and beta-decay of Ti-40
Shell-model calculations of solar neutrino absorption cross sections for
Ar, the proposed component of the ICARUS detector, are presented. It is
found that low-lying Gamow-Teller transitions lead to a significant enhancement
of the absorption rate over that expected from the Fermi transition between the
isobaric analog states, leading to an overall absorption rate of 6.7 SNU. We
also note that the pertinent Gamow-Teller transitions in ^{\sss 40}Ar are
experimentally accessible from the -decay of the mirror nucleus ^{\sss
40}Ti. Predictions for the branching ratios to states in ^{\sss 40}Sc are
presented, and the theoretical halflife of 53~ms is found to be in good
agreement with the experimental value of ~ms.Comment: 12 pages including references and table. NTGMI-94-
Elastic response of the atomic nucleus in gauge space:Giant Pairing Vibrations
Due to quantal fluctuations, the ground state of a closed shell system can become virtually excited in a state made out of the ground state of the neighbour nucleus () and of two uncorrelated holes (particles) below (above) the Fermi surface. These pairing vibrational states have been extensively studied with two-nucleon transfer reactions. Away from closed shells, these modes eventually condense, leading to nuclear superfluidity and thus to pairing rotational bands with excitation energies much smaller than , the energy separation between major shells. Pairing vibrations are the plastic response of the nucleus in gauge space, in a similar way in which low-lying quadrupole vibrations, i.e. surface vibrations with energies much smaller than whose eventual condensation leads to quadrupole deformed nuclei, provide an example of the plastic nuclear response in 3D space. While much is known, in particular concerning its damping, regarding the counterpart of quadrupole plastic modes, i.e. regarding the giant quadrupole resonances (GQR), elastic response of the nucleus with energies of the order of , little is known regarding this subject concerning pairing modes (giant pairing vibrations, GPV). Consequently, the recently reported observation of L = 0 resonances, populated in the reactions 12C(18O,16O)14C and 13C(18O,16O)15C and lying at an excitation energy of the order of , likely constitutes the starting point of a new field of research, that of the study of the elastic response of nuclei in gauge space. Not only that, but also the fact that the GPV have likely been serendipitously observed in these light nuclei when it has failed to show up in more propitious nuclei like Pb, provides unexpected and fundamental insight into the relation existing between basic mechanisms --Landau, doorway, compound damping-- through which giant resonances acquire a finite lifetime, let alone the radical difference regarding these phenomena displayed by correlated (ph) and (pp) modes
Challenges in the description of the atomic nucleus: Unification and interdisciplinarity
Nuclear physics, in general, and theoretical nuclear physics, in particular, have provided the physics community at large, among other things, with the paradigm of spontaneous symmetry breaking phenomena in finite many-body systems. The study of the associated mechanisms of symmetry restoration has shed light on the microscopic structure of the corresponding condensates, in particular on the superfluid phase, allowing to study Cooper pair tunnelling into superfluid nuclei (related to the Josephson effect), in terms of individual quantum states and reaching, in doing so, a new milestone: that of unifying structure and reactions, these last processes being found at the basis of the formulation of quantum mechanics (probability interpretation, Born). In the process, nuclear physicists have extended the validity of BCS theory of superconductivity to the single Cooper pair situation, let alone discovering unexpected mechanism to break gauge invariance. The insight obtained from pair transfer research is likely to have important consequences in the study of double charge exchange processes, and thus in the determination of the nuclear matrix element associated with neutrinoless double beta decay, eventually providing an important test of the Standard Model. Time, thus, seems ripe for nuclear theorists to take centre stage, backed by a wealth of experimental information and by their interdisciplinary capacity to connect basic physical concepts across the borders. With the help of these elements they can aim at fully revealing the many facets of their femtometer many-body system, from vacuum zero point fluctuations to new exotic modes of nuclear excitations and of their interweaving, resulting in powerful effective field theories. Unless. Unless they are not able to free themselves from words like ab initio or fundamental, and to adapt a relax attitude concerning Skyrme, tensor, etc., forces, as well as regarding the quest for âtheâ Hamiltonian
Pairing Matrix Elements and Pairing Gaps with Bare, Effective and Induced Interactions
The dependence on the single-particle states of the pairing matrix elements
of the Gogny force and of the bare low-momentum nucleon-nucleon potential
is studied in the semiclassical approximation for the case of a
typical finite, superfluid nucleus (Sn). It is found that the matrix
elements of follow closely those of on a wide range of
energy values around the Fermi energy , those associated with
being less attractive. This result explains the fact that around the
pairing gap associated with the Gogny interaction (and with a
density of single-particle levels corresponding to an effective -mass
) is a factor of about 2 larger than ,being
in agreement with = 1.4 MeV. The exchange of low-lying collective
surface vibrations among pairs of nucleons moving in time-reversal states gives
rise to an induced pairing interaction peaked at . The
interaction arising from the renormalization
of the bare nucleon-nucleon potential and of the single-particle motion
(mass and quasiparticle strength ) due to the
particle-vibration coupling leads to a value of the pairing gap at the Fermi
energy which accounts for the experimental value
Many-body effects in nuclear structure
We calculate, for the first time, the state-dependent pairing gap of a finite
nucleus (120Sn) diagonalizing the bare nucleon-nucleon potential (Argonne v14)
in a Hartree-Fock basis (with effective k-mass m_k eqult to 0.7 m), within the
framework of the BCS approximation including scattering states up to 800 MeV
above the Fermi energy to achieve convergence. The resulting gap accounts for
about half of the experimental gap. We find that a consistent description of
the low-energy nuclear spectrum requires, aside from the bare nucleon-nucleon
interaction, not only the dressing of single-particle motion through the
coupling to the nuclear surface, to give the right density of levels close to
the Fermi energy (and thus an effective mass m* approximately equal to m), but
also the renormalization of collective vibrational modes through vertex and
self-energy processes, processes which are also found to play an essential role
in the pairing channel, leading to a long range, state dependent component of
the pairing interaction. The combined effect of the bare nucleon-nucleon
potential and of the induced pairing interaction arising from the exchange of
low-lying surface vibrations between nucleons moving in time reversal states
close to the Fermi energy accounts for the experimental gap.Comment: 5 pages, 4 figures; author list correcte
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