52 research outputs found
The resonance triplet at E_alpha = 4.5 MeV in the 40Ca(alpha,gamma)44Ti reaction
The 40Ca(alpha,gamma)44Ti reaction is believed to be the main production
channel for the radioactive nuclide 44Ti in core-collapse supernovae. Radiation
from decaying 44Ti has been observed so far for two supernova remnants, and a
precise knowledge of the 44Ti production rate may help improve supernova
models. The 40Ca(alpha,gamma)44Ti astrophysical reaction rate is determined by
a number of narrow resonances. Here, the resonance triplet at E_alpha = 4497,
4510, and 4523 keV is studied both by activation, using an underground
laboratory for the gamma counting, and by in-beam gamma spectrometry. The
target properties are determined by elastic recoil detection analysis and by
nuclear reactions. The strengths of the three resonances are determined to
omega gamma = (0.92+-0.20), (6.2+-0.5), and (1.32+-0.24) eV, respectively, a
factor of two more precise than before. The strengths of this resonance triplet
may be used in future works as a point of reference. In addition, the present
new data directly affect the astrophysical reaction rate at relatively high
temperatures, above 3.5 GK.Comment: 12 pages, 11 figures; submitted to Phys. Rev.
Activation measurement of the 3He(alpha,gamma)7Be cross section at low energy
The nuclear physics input from the 3He(alpha,gamma)7Be cross section is a
major uncertainty in the fluxes of 7Be and 8B neutrinos from the Sun predicted
by solar models and in the 7Li abundance obtained in big-bang nucleosynthesis
calculations. The present work reports on a new precision experiment using the
activation technique at energies directly relevant to big-bang nucleosynthesis.
Previously such low energies had been reached experimentally only by the
prompt-gamma technique and with inferior precision. Using a windowless gas
target, high beam intensity and low background gamma-counting facilities, the
3He(alpha,gamma)7Be cross section has been determined at 127, 148 and 169 keV
center-of-mass energy with a total uncertainty of 4%. The sources of systematic
uncertainty are discussed in detail. The present data can be used in big-bang
nucleosynthesis calculations and to constrain the extrapolation of the
3He(alpha,gamma)7Be astrophysical S-factor to solar energies
First Direct Measurement of the ^{17}O(p,\gamma)^{18}F Reaction Cross-Section at Gamow Energies for Classical Novae
Classical novae are important contributors to the abundances of key isotopes,
such as the radioactive ^{18}F, whose observation by satellite missions could
provide constraints on nucleosynthesis models in novae. The
^{17}O(p,\gamma)^{18}F reaction plays a critical role in the synthesis of both
oxygen and fluorine isotopes but its reaction rate is not well determined
because of the lack of experimental data at energies relevant to novae
explosions. In this study, the reaction cross section has been measured
directly for the first time in a wide energy range Ecm = 200 - 370 keV
appropriate to hydrogen burning in classical novae. In addition, the E=183 keV
resonance strength, \omega \gamma=1.67\pm0.12 \mueV, has been measured with the
highest precision to date. The uncertainty on the ^{17}O(p,\gamma)^{18}F
reaction rate has been reduced by a factor of 4, thus leading to firmer
constraints on accurate models of novae nucleosynthesis.Comment: accepted by Phys. Rev. Let
The 3He(alpha,gamma)7Be S-factor at solar energies: the prompt gamma experiment at LUNA
The 3He(alpha,gamma)7Be process is a key reaction in both Big-Bang
nucleosynthesis and p-p chain of Hydrogen Burning in Stars. A new measurement
of the 3He(alpha,gamma)7Be cross section has been performed at the INFN Gran
Sasso underground laboratory by both the activation and the prompt gamma
detection methods. The present work reports full details of the prompt gamma
detection experiment, focusing on the determination of the systematic
uncertainty. The final data, including activation measurements at LUNA, are
compared with the results of the last generation experiments and two different
theoretical models are used to obtain the S-factor at solar energies.Comment: Accepted for publication in Nucl. Phys.
Insulin-like growth factor 1 deficiency exacerbates hypertension-induced cerebral microhemorrhages in mice, mimicking the aging phenotype
LBL
Revision of the 15N(p,{\gamma})16O reaction rate and oxygen abundance in H-burning zones
The NO cycle takes place in the deepest layer of a H-burning core or shell,
when the temperature exceeds T {\simeq} 30 {\cdot} 106 K. The O depletion
observed in some globular cluster giant stars, always associated with a Na
enhancement, may be due to either a deep mixing during the RGB (red giant
branch) phase of the star or to the pollution of the primordial gas by an early
population of massive AGB (asymptotic giant branch) stars, whose chemical
composition was modified by the hot bottom burning. In both cases, the NO cycle
is responsible for the O depletion. The activation of this cycle depends on the
rate of the 15N(p,{\gamma})16O reaction. A precise evaluation of this reaction
rate at temperatures as low as experienced in H-burning zones in stellar
interiors is mandatory to understand the observed O abundances. We present a
new measurement of the 15N(p,{\gamma})16O reaction performed at LUNA covering
for the first time the center of mass energy range 70-370 keV, which
corresponds to stellar temperatures between 65 {\cdot} 106 K and 780 {\cdot}106
K. This range includes the 15N(p,{\gamma})16O Gamow-peak energy of explosive
H-burning taking place in the external layer of a nova and the one of the hot
bottom burning (HBB) nucleosynthesis occurring in massive AGB stars. With the
present data, we are also able to confirm the result of the previous R-matrix
extrapolation. In particular, in the temperature range of astrophysical
interest, the new rate is about a factor of 2 smaller than reported in the
widely adopted compilation of reaction rates (NACRE or CF88) and the
uncertainty is now reduced down to the 10% level.Comment: 6 pages, 5 figure
Preparation and characterisation of isotopically enriched TaO targets for nuclear astrophysics studies
The direct measurement of reaction cross sections at astrophysical energies
often requires the use of solid targets of known thickness, isotopic
composition, and stoichiometry that are able to withstand high beam currents
for extended periods of time. Here, we report on the production and
characterisation of isotopically enriched TaO targets for the study of
proton-induced reactions at the Laboratory for Underground Nuclear Astrophysics
facility of the Laboratori Nazionali del Gran Sasso. The targets were prepared
by anodisation of tantalum backings in enriched water (up to 66% in O
and up to 96% in O). Special care was devoted to minimising the presence
of any contaminants that could induce unwanted background reactions with the
beam in the energy region of astrophysical interest. Results from target
characterisation measurements are reported, and the conclusions for proton
capture measurements with these targets are drawn.Comment: accepted to EPJ
Resonance strengths in the 14N(p, \gamma)15O and 15N(p, \alpha \gamma)12C reactions
The 14N(p, \gamma)15O reaction is the slowest reaction of the
carbon-nitrogen-oxygen cycle of hydrogen burning in stars. As a consequence, it
determines the rate of the cycle. The 15N(p, \alpha \gamma)12C reaction is
frequently used in inverse kinematics for hydrogen depth profiling in
materials. The 14N(p, \gamma)15O and 15N(p, \alpha \gamma)12C reactions have
been studied simultaneously, using titanium nitride targets of natural isotopic
composition and a proton beam. The strengths of the resonances at Ep = 1058 keV
in 14N(p, \gamma)15O and at Ep = 897 and 430 keV in 15N(p, \alpha \gamma)12C
have been determined with improved precision, relative to the well-known
resonance at Ep = 278 keV in 14N(p, \gamma)15O. The new recommended values are
\omega \gamma = 0.3530.018, 36220, and 21.91.0 eV for their
respective strengths. In addition, the branching ratios for the decay of the Ep
= 1058 keV resonance in 14N(p, \gamma)15O have been redetermined. The data
reported here should facilitate future studies of off-resonant capture in the
14N(p, \gamma)15O reaction that are needed for an improved R-matrix
extrapolation of the cross section. In addition, the data on the 430 keV
resonance in 15N(p, \alpha \gamma)12C may be useful for hydrogen depth
profiling.Comment: 10 pages, 8 figures. Corrected typos in the abstract, table IV made
more comprehensible. As accepted in Phys.Rev.
Treatment with the mitochondrial-targeted antioxidant peptide SS-31 rescues neurovascular coupling responses and cerebrovascular endothelial function and improves cognition in aged mice
L
Relational mobility predicts social behaviors in 39 countries and is tied to historical farming and threat
International audienc
- âŠ