803 research outputs found
Structure of the vacuum states in the presence of isovector and isoscalar pairing correlations
The long standing problem of proton-neutron pairing and, in particular, the
limitations imposed on the solutions by the available symmetries, is revisited.
We look for solutions with non-vanishing expectation values of the proton, the
neutron and the isoscalar gaps. For an equal number of protons and neutrons we
find two solutions where the absolute values of proton and neutrons gaps are
equal but have the same or opposite sign. The behavior and structure of these
solutions differ for spin saturated (single l-shell) and spin unsaturared
systems (single j-shell). In the former case the BCS results are checked
against an exact calculation.Comment: 19 pages, 5 postscript figure
Spin distribution of nuclear levels using static path approximation with random-phase approximation
We present a thermal and quantum-mechanical treatment of nuclear rotation
using the formalism of static path approximation (SPA) plus random-phase
approximation (RPA). Naive perturbation theory fails because of the presence of
zero-frequency modes due to dynamical symmetry breaking. Such modes lead to
infrared divergences. We show that composite zero-frequency excitations are
properly treated within the collective coordinate method. The resulting
perturbation theory is free from infrared divergences. Without the assumption
of individual random spin vectors, we derive microscopically the spin
distribution of the level density. The moment of inertia is thereby related to
the spin-cutoff parameter in the usual way. Explicit calculations are performed
for 56^Fe; various thermal properties are discussed. In particular, we
demonstrate that the increase of the moment of inertia with increasing
temperature is correlated with the suppression of pairing correlations.Comment: 12 pages, 8 figures, accepted for publication in Physical Review
Expression of TLP-3 gene without signal peptide in tobacco plants using Agrobacterium mediated transformation
Plants are exploited as a source of food by a wide range of parasites, including viruses, bacteria, fungi, nematodes, insects and even other plants. So paying attention to their protection is very important. One of the most important groups of defensive gene is pathogenesis-related (PR) proteins. PR proteins are a group of plant proteins which accumulate as a result of different types of pathogens attack. Thaumatinlike proteins (TLPs) belonging to the PR proteins, plays an important role in plant defense against pathogen invasions. Agrobacterium-mediated transformation is routinely used for the transformation of many plants. In the present study, thaumatin-like TLP-3 gene without signal peptide was transferred into tobacco plants through Agrobacterium-mediated technology. The PCR and RT-PCR assays confirmed the presence of TLP-3 gene and TLP-3 mRNA in transgenic tobacco plants, respectively. The bioassay results showed the activity of TLP-3 against Alternaria alternata in microscopic experiment. This is the first report of the expression of TLP-3 gene without signal peptide in tobacco plants using Agrobacterium mediated transformation and it confirmed the ability of the gene action in another plant.Key words: Thaumatin like protein, Agrobacterium, signal peptide, Alternaria alternate
Triaxial quadrupole deformation dynamics in sd-shell nuclei around 26Mg
Large-amplitude dynamics of axial and triaxial quadrupole deformation in
24,26Mg, 24Ne, and 28Si is investigated on the basis of the quadrupole
collective Hamiltonian constructed with use of the constrained
Hartree-Fock-Bogoliubov plus the local quasiparticle random phase approximation
method. The calculation reproduces well properties of the ground rotational
bands, and beta and gamma vibrations in 24Mg and 28Si. The gamma-softness in
the collective states of 26Mg and 24Ne are discussed. Contributions of the
neutrons and protons to the transition properties are also analyzed in
connection with the large-amplitude quadrupole dynamics.Comment: 16 pages, 18 figures, submitted to Phys. Rev.
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
Study of Giant Pairing Vibrations with neutron-rich nuclei
We investigate the possible signature of the presence of giant pairing states
at excitation energy of about 10 MeV via two-particle transfer reactions
induced by neutron-rich weakly-bound projectiles. Performing particle-particle
RPA calculations on Pb and BCS+RPA calculations on Sn, we
obtain the pairing strength distribution for two particles addition and removal
modes. Estimates of two-particle transfer cross sections can be obtained in the
framework of the 'macroscopic model'. The weak-binding nature of the projectile
kinematically favours transitions to high-lying states. In the case of (~^6He,
\~^4He) reaction we predict a population of the Giant Pairing Vibration with
cross sections of the order of a millibarn, dominating over the mismatched
transition to the ground state.Comment: Talk presented in occasion of the VII School-Semina r on Heavy Ion
Physics hosted by the Flerov Laboratory (FLNR/JINR) Dubna, Russia from May 27
to June 2, 200
Microscopic description of large-amplitude shape-mixing dynamics with inertial functions derived in local quasiparticle random-phase approximation
On the basis of the adiabatic self-consistent collective coordinate method,
we develop an efficient microscopic method of deriving the five-dimensional
quadrupole collective Hamiltonian and illustrate its usefulness by applying it
to the oblate-prolate shape coexistence/mixing phenomena in proton-rich
68,70,72Se. In this method, the vibrational and rotational collective masses
(inertial functions) are determined by local normal modes built on constrained
Hartree-Fock-Bogoliubov states. Numerical calculations are carried out using
the pairing-plus-quadrupole Hamiltonian including the quadrupole-pairing
interaction. It is shown that the time-odd components of the moving mean-field
significantly increase the vibrational and rotational collective masses in
comparison with the Inglis-Belyaev cranking masses. Solving the collective
Schroedinger equation, we evaluate excitation spectra, quadrupole transitions
and moments. Results of the numerical calculation are in excellent agreement
with recent experimental data and indicate that the low-lying states of these
nuclei are characterized as an intermediate situation between the
oblate-prolate shape coexistence and the so-called gamma unstable situation
where large-amplitude triaxial-shape fluctuations play a dominant role.Comment: 17 pages, 16 figures, Submitted to Phys. Rev.
- …