378 research outputs found
Weak reactions on 12C within the Continuum Random Phase Approximation with partial occupancies
We extend our previous studies of the neutrino-induced reactions on 12C and
muon capture to include partial occupation of nuclear subshells in the
framework of the continuum random phase approximation. We find, in contrast to
the work by Auerbach et al., that a partial occupation of the p1/2 subshell
reduces the inclusive cross sections only slightly. The extended model
describes the muon capture rate and the 12C(nu_e,e-)12N cross section very
well. The recently updated flux and the improved model bring the calculated
12C(nu_mu,mu^-)12N cross section (~ 17.5 10^{-40} cm^2) and the data (12.4 +/-
0.3(stat.) +/- 1.8(syst.) 10^{-40} cm^2) closer together, but does not remove
the discrepancy fully.Comment: 12 pages, 2 figure
Estimates of weak and electromagnetic nuclear decay signatures for neutrino reactions in Super-Kamiokande
We estimate possible delayed β decay signatures of the neutrino induced reactions on 16O in a two-step model: the primary neutrino (ν,l) process, where l is the lepton in the final state, is described within the random phase approximation, while the subsequent decay of the excited nuclear state in the final channel is treated within the statistical model. We calculate partial reaction cross sections leading to β unstable nuclei. We consider neutrino energies up to 500 MeV, relevant for atmospheric neutrino detection in Super-Kamiokande, and supernova neutrino spectra
Quasielastic neutrino scattering from oxygen and the atmospheric neutrino problem
We examine several phenomena beyond the scope of Fermi-gas models that affect
the quasielastic scattering (from oxygen) of neutrinos in the 0.1 -- 3.0 GeV
range. These include Coulomb interactions of outgoing protons and leptons, a
realistic finite-volume mean field, and the residual nucleon-nucleon
interaction. None of these effects are accurately represented in the Monte
Carlo simulations used to predict event rates due to and neutrinos
from cosmic-ray collisions in the atmosphere. We nevertheless conclude that the
neglected physics cannot account for the anomalous to ratio observed
at Kamiokande and IMB, and is unlikely to change absolute event rates by more
than 10--15\%. We briefly mention other phenomena, still to be investigated in
detail, that may produce larger changes.Comment: In Revtex version 2. 14 pages, 3 figures (available on request from
J. Engel, tel. 302-831-4354, [email protected]
Shell Model Monte Carlo Methods
We review quantum Monte Carlo methods for dealing with large shell model
problems. These methods reduce the imaginary-time many-body evolution operator
to a coherent superposition of one-body evolutions in fluctuating one-body
fields; the resultant path integral is evaluated stochastically. We first
discuss the motivation, formalism, and implementation of such Shell Model Monte
Carlo (SMMC) methods. There then follows a sampler of results and insights
obtained from a number of applications. These include the ground state and
thermal properties of {\it pf}-shell nuclei, the thermal and rotational
behavior of rare-earth and -soft nuclei, and the calculation of double
beta-decay matrix elements. Finally, prospects for further progress in such
calculations are discussed
Neutrino–nucleus reactions and nuclear structure
The methods used in the evaluation of the neutrino–nucleus cross section are reviewed. Results are shown for a variety of targets of practical importance. Many of the described reactions are accessible in future experiments with neutrino sources from the pion and muon decays at rest, which might be available at the neutron spallation facilities. Detailed comparison between the experimental and theoretical results would establish benchmarks needed for verification and/or parameter adjustment of the nuclear models. Having a reliable tool for such calculation is of great importance in a variety of applications, e.g. the neutrino oscillation studies, detection of supernova neutrinos, description of the neutrino transport in supernovae and description of the r-process nucleosynthesis
^7Be(p,γ)^8B cross section and the properties of ^7Be
We study the nonresonant part of the ^7Be(p,γ)^8B reaction using a three-cluster resonating group model that is variationally converged and virtually complete in ^4He+^3He+p model space. The importance of using adequate nucleon-nucleon interaction is demonstrated. We find that the low-energy astrophysical S factor is linearly correlated with the quadrupole moment of ^7Be. A range of parameters is found where the most important ^8B, ^7Be, and ^7Li properties are reproduced simultaneously; the corresponding S factor at E_(c.m.)=20 keV is 24.6–26.1 eV b
Shell model calculations of stellar weak interaction rates: I. Gamow-Teller distributions and spectra of nuclei in the mass range A=45-65
Electron capture and beta-decay rates on nuclei in the mass range A=45-65
play an important role in many astrophysical environments. The determination of
these rates by large-scale shell model calculations is desirable, but it
requires to reproduce the Gamow-Teller strength distributions and spectra of
the pf shell nuclei. We show in this paper that large-scale shell model
calculations, employing a slightly monopole-corrected version of the wellknown
KB3 interaction, fulfill these necessary requirements. In particular, our
calculations reproduce the experimentally available GT+ and GT- strength
distributions and the nuclear halflives, and describe the nuclear spectra
appropriately.Comment: 11 pages, 5 figure
Analysis of the total 12C(α,γ)16O cross section based on available angular distributions and other primary data
Because a knowledge of the 12C/16O ratio is crucial to the understanding of the later evolution of massive stars, new R- and K-matrix fits have been completed using the available angular distribution data from radiative α capture and elastic α scattering on 12C. Estimates of the total 12C(α,γ)16O rate at stellar energies are reported. In contrast with previous work, the analyses generally involve R- and K-matrix fits directly to the primary data, i.e., the energy- and angle-dependent differential yields, with all relevant partial waves fitted simultaneously (referred to here as surface fits). It is shown that, while the E1 part of the reaction is well constrained by a recent experiment on the β-delayed α-particle decay of 16N, only upper limits can be placed on the E2 ground state cross section factor which we take conservatively as SE2(300)<140 keV b. Simulations were then carried out to explore what kind of new data could lead to better restrictions on SE2(300). We find that improved elastic scattering data may be the best short-term candidate for such restrictions while significantly improving S(300) with new radiative capture data may require a longer-term effort. Theoretical models and estimates from α-transfer reactions for the E2 part of 12C(α,γ)16O are then discussed for comparison with the R- and K-matrix fits of the present work
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