39 research outputs found
Photonuclear Reactions of Three-Nucleon Systems
We discuss the available data for the differential and the total cross
section for the photodisintegration of He and H and the corresponding
inverse reactions below MeV by comparing with our calculations
using realistic interactions. The theoretical results agree within the
errorbars with the data for the total cross sections. Excellent agreement is
achieved for the angular distribution in case of He, whereas for H a
discrepancy between theory and experiment is found.Comment: 11 pages (twocolumn), 12 postscript figures included, uses psfig,
RevTe
Influence of Gamma-Ray Emission on the Isotopic Composition of Clouds in the Interstellar Medium
We investigate one mechanism of the change in the isotopic composition of
cosmologically distant clouds of interstellar gas whose matter was subjected
only slightly to star formation processes. According to the standard
cosmological model, the isotopic composition of the gas in such clouds was
formed at the epoch of Big Bang nucleosynthesis and is determined only by the
baryon density in the Universe. The dispersion in the available cloud
composition observations exceeds the errors of individual measurements. This
may indicate that there are mechanisms of the change in the composition of
matter in the Universe after the completion of Big Bang nucleosynthesis. We
have calculated the destruction and production rates of light isotopes (D, 3He,
4He) under the influence of photonuclear reactions triggered by the gamma-ray
emission from active galactic nuclei (AGNs). We investigate the destruction and
production of light elements depending on the spectral characteristics of the
gamma-ray emission. We show that in comparison with previous works, taking into
account the influence of spectral hardness on the photonuclear reaction rates
can increase the characteristic radii of influence of the gamma-ray emission
from AGNs by a factor of 2-8. The high gamma-ray luminosities of AGNs observed
in recent years increase the previous estimates of the characteristic radii by
two orders of magnitude. This may suggest that the influence of the emission
from AGNs on the change in the composition of the medium in the immediate
neighborhood (the host galaxy) has been underestimated.Comment: 13 pages, 13 figures, 3 table
Photodisintegration of the triton with realistic potentials
The process is treated by means of three-body integral
equations employing in their kernel the W-Matrix representation of the
subsystem amplitudes. As compared to the plane wave (Born) approximation the
full solution of the integral equations, which takes into account the final
state interaction, shows at low energies a 24% enhancement. The calculations
are based on the semirealistic Malfliet-Tjon and the realistic Paris and Bonn B
potentials. For comparison with earlier calculations we also present results
for the Yamaguchi potential. In the low-energy region a remarkable potential
dependence is observed, which vanishes at higher energies.Comment: 16 pages REVTeX, 8 postscript figures included, uses epsfig.st
Triton photodisintegration in three-dimensional approach
Two- and three- particles photodisintegration of the triton is investigated
in a three-dimensional (3D) Faddeev approach. For this purpose the Jacobi
momentum vectors for three particles system and spin-isospin quantum numbers of
the individual nucleons are considered. Based on this picture the three-nucleon
Faddeev integral equations with the two-nucleon interaction are formulated
without employing the partial wave decomposition. The single nucleon current as
well as and like exchange currents are used in an appropriate
form to be employed in 3D approach. The exchange currents are derived from AV18
NN force. The two-body t-matrix, Deuteron and Triton wave functions are
calculated in the 3D approach by using AV18 potential. Benchmarks are presented
to compare the total cross section for the two- and three- particles
photodisintegration in the range of . The 3D Faddeev
approach shows promising results
Radiative capture of protons by deuterons
The differential cross section for radiative capture of protons by deuterons
is calculated using different realistic NN interactions. We compare our results
with the available experimental data below . Excellent agreement
is found when taking into account meson exchange currents, dipole and
quadrupole contributions, and the full initial state interaction. There is only
a small difference between the magnitudes of the cross sections for the
different potentials considered. The angular distributions, however, are
practically potential independent.Comment: 4 pages (twocolumn), 4 postscript figures included, submitted for
publication, revised versio
Exploring the value of a cyber threat intelligence function in an organization
Organizations can struggle to cope with the rapidly advancing threat landscape. A cyber threat intelligence (CTI) function broadly aims to understand how threats operate to better protect the organization from future attacks. This seems like a natural step to take in hardening security. However, CTI is understood and experienced differently across organizations. To explore the value of this function this study used a qualitative method, guided by the Socio-Technical Framework, to understand how the CTI function is interpreted by organizations in South Africa. Thematic analysis was used to provide an in-depth view of how each organization implemented its CTI function and what benefits and challenges they’ve experienced. Findings show that CTI tasks tend to be more manual and resource-intensive, but these challenges can be resolved through automation. It was noted that only larger organizations seem to have the budget and resources available to implement the CTI function, whereas smaller organizations put more reliance on tools. It was observed that skills for the CTI function can be learned on the job, but that formal education provides a good foundation. The findings illustrate the value the CTI function can provide an organization but also the challenges, thereby enabling other organizations to improve preparation before such a function is adopted
Determination of Omega_b From Big Bang Nucleosynthesis in the Presence of Regions of Antimatter
Production of regions of antimatter in the early universe is predicted in
many baryogenesis models. Small scale antimatter regions would annihilate
during or soon after nucleosynthesis, affecting the abundances of the light
elements. In this paper we study how the acceptable range in Omega_b changes in
the presence of antimatter regions, as compared to the standard big bang
nucleosynthesis. It turns out that it is possible to produce at the same time
both a low 4He value (Y_p < 0.240) and a low D/H value (D/H < 4e-5), but
overproduction of 7Li is unavoidable at large Omega_b.Comment: 9 pages, PRD version, ref. 6 correcte
Antimatter Regions in the Early Universe and Big Bang Nucleosynthesis
We have studied big bang nucleosynthesis in the presence of regions of
antimatter. Depending on the distance scale of the antimatter region, and thus
the epoch of their annihilation, the amount of antimatter in the early universe
is constrained by the observed abundances. Small regions, which annihilate
after weak freezeout but before nucleosynthesis, lead to a reduction in the 4He
yield, because of neutron annihilation. Large regions, which annihilate after
nucleosynthesis, lead to an increased 3He yield. Deuterium production is also
affected but not as much. The three most important production mechanisms of 3He
are 1) photodisintegration of 4He by the annihilation radiation, 2) pbar-4He
annihilation, and 3) nbar-4He annihilation by "secondary" antineutrons produced
in anti-4He annihilation. Although pbar-4He annihilation produces more 3He than
the secondary nbar-4He annihilation, the products of the latter survive later
annihilation much better, since they are distributed further away from the
annihilation zone.Comment: 15 pages, 9 figures. Minor changes to match the PRD versio