491 research outputs found
Solving the large discrepancy between inclusive and exclusive measurements of the reaction cross section at astrophysical energies
A solution of the large discrepancy existing between inclusive and exclusive
measurements of the reaction
cross section at MeV is evaluated. This problem has profound
astrophysical relevance for this reaction is of great interest in Big-Bang and
r-process nucleosynthesis. By means of a novel technique, a comprehensive study
of all existing cross section
data is carried out, setting up a consistent picture in which all the inclusive
measurements provide the reliable value of the cross section. New unambiguous
signatures of the strong branch pattern non-uniformities, near the threshold of
higher excited levels, are presented and their possible
origin, in terms of the cluster structure of the involved excited states of
and nuclei, is discussed.Comment: 5 pages, 4 figures, 1 tabl
The Bare Astrophysical S(E) Factor of the 7Li(p, α)α Reaction
The astrophysically important 7Li(p, α)α reaction has been studied via the Trojan horse method in the energy range E = 10-400 keV. A new theoretical description, based on the distorted-wave Born approximation approach, allows one to extract information on the bare astrophysical S-factor, Sb(E), with Sb(0) = 55 ± 3 keV barns. The results are compared with direct experimental data leading to a model-independent value of the electron screening potential energy, Ue = 330 ± 40 eV, much higher than the adiabatic limit Uad = 175 eV
Fusion rate enhancement due to energy spread of colliding nuclei
Experimental results for sub-barrier nuclear fusion reactions show cross
section enhancements with respect to bare nuclei which are generally larger
than those expected according to electron screening calculations. We point out
that energy spread of target or projectile nuclei is a mechanism which
generally provides fusion enhancement. We present a general formula for
calculating the enhancement factor and we provide quantitative estimate for
effects due to thermal motion, vibrations inside atomic, molecular or crystal
system, and due to finite beam energy width. All these effects are marginal at
the energies which are presently measurable, however they have to be considered
in future experiments at still lower energies. This study allows to exclude
several effects as possible explanation of the observed anomalous fusion
enhancements, which remain a mistery.Comment: 17 pages with 3 ps figure included. Revtex styl
Theory of the Trojan-Horse Method
The Trojan-Horse method is an indirect approach to determine the energy
dependence of S-factors of astrophysically relevant two-body reactions. This is
accomplished by studying closely related three-body reactions under quasi-free
scattering conditions. The basic theory of the Trojan-Horse method is developed
starting from a post-form distorted wave Born approximation of the T-matrix
element. In the surface approximation the cross section of the three-body
reaction can be related to the S-matrix elements of the two-body reaction. The
essential feature of the Trojan-Horse method is the effective suppression of
the Coulomb barrier at low energies for the astrophysical reaction leading to
finite cross sections at the threshold of the two-body reaction. In a modified
plane wave approximation the relation between the two-body and three-body cross
sections becomes very transparent. The appearing Trojan-Horse integrals are
studied in detail.Comment: 27 pages, REVTeX4, 4 figures, 1 tabl
Pulse shape discrimination of charged particles with a silicon strip detector
Abstract A simple and effective pulse shape discrimination technique is applied to a silicon strip detector array. Excellent charge identification from H up to the Ni projectile has been obtained and isotope separation up to N has also been observed. The method we systematically studied is essentially based on a suitable setting of the constant fraction discriminators, and its main advantage is that no additional electronic modules are needed compared to the ones used in the standard TOF technique
- âŠ