15 research outputs found
Ab initio study of the photoabsorption of He
There are some discrepancies in the low energy data on the photoabsorption
cross section of He. We calculate the cross section with realistic nuclear
forces and explicitly correlated Gaussian functions. Final state interactions
and two- and three-body decay channels are taken into account. The cross
section is evaluated in two methods: With the complex scaling method the total
absorption cross section is obtained up to the rest energy of a pion, and with
the microscopic -matrix method both cross sections He()H
and He()He are calculated below 40\,MeV. Both methods give
virtually the same result. The cross section rises sharply from the H+
threshold, reaching a giant resonance peak at 26--27\,MeV. Our calculation
reproduces almost all the data above 30\,MeV. We stress the importance of
H+ and He+ cluster configurations on the cross section as well as
the effect of the one-pion exchange potential on the photonuclear sum rule.Comment: 15 pages, 12 figure
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
The Lorentz Integral Transform (LIT) method and its applications to perturbation induced reactions
The LIT method has allowed ab initio calculations of electroweak cross
sections in light nuclear systems. This review presents a description of the
method from both a general and a more technical point of view, as well as a
summary of the results obtained by its application. The remarkable features of
the LIT approach, which make it particularly efficient in dealing with a
general reaction involving continuum states, are underlined. Emphasis is given
on the results obtained for electroweak cross sections of few--nucleon systems.
Their implications for the present understanding of microscopic nuclear
dynamics are discussed.Comment: 83 pages, 31 figures. Topical review. Corrected typo
Big-Bang Nucleosynthesis and Hadronic Decay of Long-Lived Massive Particles
We study the big-bang nucleosynthesis (BBN) with the long-lived exotic
particle, called X. If the lifetime of X is longer than \sim 0.1 sec, its decay
may cause non-thermal nuclear reactions during or after the BBN, altering the
predictions of the standard BBN scenario. We pay particular attention to its
hadronic decay modes and calculate the primordial abundances of the light
elements. Using the result, we derive constraints on the primordial abundance
of X. Compared to the previous studies, we have improved the following points
in our analysis: The JETSET 7.4 Monte Carlo event generator is used to
calculate the spectrum of hadrons produced by the decay of X; The evolution of
the hadronic shower is studied taking account of the details of the energy-loss
processes of the nuclei in the thermal bath; We have used the most recent
observational constraints on the primordial abundances of the light elements;
In order to estimate the uncertainties, we have performed the Monte Carlo
simulation which includes the experimental errors of the cross sections and
transfered energies. We will see that the non-thermal productions of D, He3,
He4 and Li6 provide stringent upper bounds on the primordial abundance of
late-decaying particle, in particular when the hadronic branching ratio of X is
sizable. We apply our results to the gravitino problem, and obtain upper bound
on the reheating temperature after inflation.Comment: 94 pages, 49 figures, to appear in Phys. Rev. D. This is a full
length paper of the preprint astro-ph/040249
Radiative decay of a massive particle and the non-thermal process in primordial nucleosynthesis
We consider the effects on big bang nucleosynthesis (BBN) of the radiative
decay of a long-lived massive particle. If high-energy photons are emitted
after the BBN epoch ( sec), they may change the abundances of
the light elements through photodissociation processes, which may result in a
significant discrepancy between standard BBN and observation. Taking into
account recent observational and theoretical developments in this field, we
revise our previous study constraining the abundance of the
radiatively-decaying particles. In particular, on the theoretical side, it was
recently claimed that the non-thermal production of Li, which is caused by
the photodissociation of \hefour, most severely constrains the abundance of
the radiatively-decaying particle. We will see, however, it is premature to
emphasize the importance of the non-thermal production of Li because (i)
the theoretical computation of the Li abundance has large uncertainty due
to the lack of the precise understanding of the Li production cross
section, and (ii) the observational data of Li abundance has large errors.Comment: 15 pages, using REVTeX and 3 postscript figure
Electromagnetic Cascades and Cascade Nucleosynthesis in the Early Universe
We describe a calculation of electromagnetic cascading in radiation and
matter in the early universe initiated by the decay of massive particles or by
some other process. We have used a combination of Monte Carlo and numerical
techniques which enables us to use exact cross sections, where known, for all
the relevant processes. In cascades initiated after the epoch of big bang
nucleosynthesis -rays in the cascades will photodisintegrate He,
producing He and deuterium. Using the observed He and deuterium
abundances we are able to place constraints on the cascade energy deposition as
a function of cosmic time. In the case of the decay of massive primordial
particles, we place limits on the density of massive primordial particles as a
function of their mean decay time, and on the expected intensity of decay
neutrinos.Comment: compressed and uuencoded postscript. We now include a comparison with
previous work of the photon spectrum in the cascade and the limits we
calculate for the density of massive particles. The method of calculation of
photon spectra at low energies has been improved. Most figures are revised.
Our conclusions are substantially unchange
Femtometer Toroidal Structures in Nuclei
The two-nucleon density distributions in states with isospin , spin
=1 and projection =0 and 1 are studied in H, He,
Li and O. The equidensity surfaces for =0 distributions are
found to be toroidal in shape, while those of =1 have dumbbell shapes
at large density. The dumbbell shapes are generated by rotating tori. The
toroidal shapes indicate that the tensor correlations have near maximal
strength at fm in all these nuclei. They provide new insights and simple
explanations of the structure and electromagnetic form factors of the deuteron,
the quasi-deuteron model, and the , and =2 (-wave)
components in He, He and Li. The toroidal distribution has a
maximum-density diameter of 1 fm and a half-maximum density thickness of
0.9 fm. Many realistic models of nuclear forces predict these values,
which are supported by the observed electromagnetic form factors of the
deuteron, and also predicted by classical Skyrme effective Lagrangians, related
to QCD in the limit of infinite colors. Due to the rather small size of this
structure, it could have a revealing relation to certain aspects of QCD.Comment: 35 pages in REVTeX, 25 PostScript figure