35 research outputs found
The nuclear electric polarizability of 6He
We present an estimate of the nuclear electric polarizability of the 6He halo
nucleus based on six-body microscopic calculations. Wave functions are obtained
from semi-realistic two-body interactions using the hyperspherical harmonics
expansion method. The polarizability is calculated as a sum rule of the dipole
response function using the Lanczos algorithm and also by integrating the
photo-absorption cross section calculated via the Lorentz integral transform
method. We obtain alpha_E=1.00(14) fm^3, which is much smaller than the
published value 1.99(40) fm^3 extracted from experimental data. This points
towards a potential disagreement between microscopic theories and experimental
observations.Comment: 8 pages, 8 figures, minor changes, added error analysi
Nuclear polarizability of helium isotopes in atomic transitions
We estimate the nuclear polarizability correction to atomic transition
frequencies in various helium isotopes. This effect is non-negligible for high
precision tests of quantum electrodynamics or accurate determination of the
nuclear charge radius from spectroscopic measurements in helium atoms and ions.
In particular, it amounts to kHz for 1S-2S transition in 4He+.Comment: 11 pages, 4 figures, to be published in Phys. Rev. A. Revised
version: misprints corrected, new references adde
Photodisintegration of light nuclei for testing a correlated realistic interaction in the continuum
An exact calculation of the photodisintegration cross section of 3H, 3He and
4He is performed using as interaction the correlated Argonne V18 potential,
constructed within the Unitary Correlation Operator Method (VUCOM).
Calculations are carried out using the
Lorentz Integral Transform method in conjunction with an hyperspherical
harmonics basis expansion. A comparison with other realistic potentials and
with available experimental data is discussed. The VUCOM potential leads to a
very similar description of the cross section as the Argonne V18 interaction
with the inclusion of the Urbana IX three-body force for photon energies 45< w
< 120 MeV, while larger differences are found close to threshold.Comment: 9 pages, 6 figure
Self-consistent continuum random phase approximation calculations of He electromagnetic responses
We study the electromagnetic responses of He within the framework of the
self-consistent continuum random phase approximation theory. In this approach
the ground state properties are described by a Hartree-Fock calculation. The
single particle basis constructed in this manner is used in the calculations of
the continuum responses of the system. Finite-range interactions are considered
in the calculations. We compare our results with photon absorption cross
sections and electron scattering quasi-elastic data. From this comparison, and
also from the comparison with the results of microscopic calculations, we
deduce that our approach describes well the continuum excitation.Comment: 6 pages, 1 table, 4 figures, accepted for publication in Physical
Review
Two-body Photodisintegration of He with Full Final State Interaction
The cross sections of the processes He()H and
He()He are calculated taking into account the full final
state interaction via the Lorentz integral transform (LIT) method. This is the
first consistent microscopic calculation beyond the three--body breakup
threshold. The results are obtained with a semirealistic central NN potential
including also the Coulomb force. The cross sections show a pronounced dipole
peak at 27 MeV which lies within the rather broad experimental band. At higher
energies, where experimental uncertainties are considerably smaller, one finds
a good agreement between theory and experiment. The calculated sum of three--
and four--body photodisintegration cross sections is also listed and is in fair
agreement with the data.Comment: 18 pages, 6 figure
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
New Constraints on Radiative Decay of Long-Lived Particles in Big Bang Nucleosynthesis with New He Photodisintegration Data
A recent measurement of He photodisintegration reactions,
He(,)H and He(,)He with laser-Compton
photons shows smaller cross sections than those estimated by other previous
experiments at MeV. We study big-bang nucleosynthesis
with the radiative particle decay using the new photodisintegration cross
sections of He as well as previous data. The sensitivity of the yields of
all light elements D, T, He, He, Li, Li and Be to the cross
sections is investigated. The change of the cross sections has an influence on
the non-thermal yields of D, He and He. On the other hand, the
non-thermal Li production is not sensitive to the change of the cross
sections at this low energy, since the non-thermal secondary synthesis of
Li needs energetic photons of MeV. The non-thermal
nucleosynthesis triggered by the radiative particle decay is one of candidates
of the production mechanism of Li observed in metal-poor halo stars
(MPHSs). In the parameter region of the radiative particle lifetime and the
emitted photon energy which satisfies the Li production above the abundance
level observed in MPHSs, the change of the photodisintegration cross sections
at MeV as measured in the recent experiment leads to
% reduction of resulting He abundance, whereas the Li
abundance does not change for this change of the cross sections of
He(,)H and He(,)He. The Li abundance,
however, could show a sizable change and therefore the future precise
measurement of the cross sections at high energy 50 MeV is
highly required.Comment: 10 pages, 7 figures, conclusion not changed, to be published in PR
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
Test of J-matrix inverse scattering potentials on electromagnetic reactions of few-nucleon systems
The J-matrix inverse scattering nucleon-nucleon potentials (JISP), describing
both two-nucleon data and bound and resonant states of light nuclei to high
accuracy, are tested on the total photoabsorption cross sections of Deuteron,
Triton, 3He and 4He. The calculations in the three- and four-body systems are
carried out via the Lorentz integral transform method and the hyperspherical
harmonics (HH) technique. To this end the HH formalism has been adapted to
accommodate non-local potentials. The cross sections calculated with the JISP
are compared to those obtained with more traditional realistic interactions,
which include two- and three-nucleon forces. While the results of the two kinds
of potential models do not differ significantly at lower energies, beyond the
resonance peak they show fairly large discrepancies, which increase with the
nuclear mass. We argue that these discrepancies may be due to a probably
incorrect long range behavior of the JISP, since the one pion exchange is not
manifestly implemented there.Comment: 18 pages, 4 figures, 1 tabl