101 research outputs found
Isotopic distribution of fission fragments in collisions between 238U beam and 9Be and 12C targets at 24 MeV/u
Inverse kinematics coupled to a high-resolution spectrometer is used to
investigate the isotopic yields of fission fragments produced in reactions
between a 238U beam at 24 MeV/u and 9Be and 12C targets. Mass, atomic number
and isotopic distributions are reported for the two reactions. These
informations give access to the neutron excess and the isotopic distribution
widths, which together with the atomic-number and mass distributions are used
to investigate the fusion-fission dynamics.Comment: Submitted to PR
Resonances in 19Ne with relevance to the astrophysically important 18F(p,{\alpha})15O reaction
The most intense gamma-ray line observable from novae is likely to be from
positron annihilation associated with the decay of 18F. The uncertainty in the
destruction rate of this nucleus through the 18F(p,{\alpha})15O reaction
presents a limit to interpretation of any future observed gamma-ray flux.
Direct measurements of the cross section of both this reaction and the
18F(p,p)18F reaction have been performed between center of mass energies of 0.5
and 1.9 MeV. Simultaneous fits to both data sets with the R-Matrix formalism
reveal several resonances, with the inferred parameters of populated states in
19Ne in general agreement with previous measurements. Of particular interest,
extra strength has been observed above ECM \sim1.3 MeV in the 18F(p,p)18F
reaction and between 1.3-1.7 MeV in the 18F(p,{\alpha})15O reaction. This is
well described by a broad 1/2+ state, consistent with both a recent theoretical
prediction and an inelastic scattering measurement. The astrophysical
implications of a broad sub-threshold partner to this state are discussed.Comment: 7 pages, 4 figures, 2 table
Production of neutron-rich fragments with neutron number N > Nprojectile in the reaction Ca (60 MeV/nucleon) + Ta
Expérience GANIL, Spectrometre LISEInternational audienceThe goal of the present paper is to attempt to clarify the nuclear reaction mechanism leading to the production of fragments at zero degree with neutron number larger than that in the 48Ca projectile, at about 60 MeV per nucleon. The production cross sections of the extremely neutron-rich Si and P isotopes were measured. Concerning the nuclear reaction mechanism leading to the production of these isotopes, one should probably refer to a particular type of transfer mechanism, which results in low excitation energy for the fragments, rather than to the ‘genuine' fragmentation mechanism. An upper limit of about 0.05 pb was estimated for the production cross section for the 47P isotope for which no count was observed
Spatio-temporal permanence and plasticity of foraging trails in young and mature leaf-cutting ant colonies (Atta spp.)
The distribution and formation of foraging trails have largely been neglected as factors explaining harvesting
patterns of leaf-cutting ants.We applied fractal analysis, circular, and conventional statistics to published and newly
recorded trailmaps of seven Atta colonies focusing on three aspects: permanence, spatio-temporal plasticity and colony
life stage. In the long term, trail patterns of young and mature Atta colonies revealed that foraging activities were
focused on distinct, static sectors that made up only parts of their potentially available foraging range. Within these
foraging sectors, trails were typically ephemeral and highly variable in space and time. These ephemeral trails were
concentrated around permanent trunk trails in mature and around nest entrances in young colonies. Besides these
similarities, the comparison of trail systems between the two life stages indicated that young colonies exploited fewer
leaf sources, used smaller and less-complex systems of foraging trails, preferred different life forms as host plants, and
switched hosts more often compared with mature colonies. Based on these analyses, we propose a general hypothesis
which describes the foraging pattern in Atta as a result of initial foraging experiences, spatio-temporal distribution of
suitable host plants, energetic constraints, and other factors such as seasonality and interspecific predatio
Neutron-proton pairing in the N=Z radioactive fp-shell nuclei 56Ni and 52Fe probed by pair transfer
The isovector and isoscalar components of neutron-proton pairing are
investigated in the N=Z unstable nuclei of the \textit{fp}-shell through the
two-nucleon transfer reaction (p,He) in inverse kinematics. The combination
of particle and gamma-ray detection with radioactive beams of Ni and
Fe, produced by fragmentation at the GANIL/LISE facility, made it
possible to carry out this study for the first time in a closed and an
open-shell nucleus in the \textit{fp}-shell. The transfer cross-sections for
ground-state to ground-state (J=0,T=1) and to the first (J=1,T=0) state
were extracted for both cases together with the transfer cross-section ratios
(0,T=1) /(1,T=0). They are compared with second-order
distorted-wave born approximation (DWBA) calculations. The enhancement of the
ground-state to ground-state pair transfer cross-section close to mid-shell, in
Fe, points towards a superfluid phase in the isovector channel. For the
"deuteron-like" transfer, very low cross-sections to the first (J=1,T=0)
state were observed both for \Ni\phe\, and \Fe\phe\, and are related to a
strong hindrance of this channel due to spin-orbit effect. No evidence for an
isoscalar deuteron-like condensate is observed.Comment: 7 pages, 4 figure
Cross-shell states in C: a test for p-sd interactions
The low-lying structure of C has been investigated via the
neutron-removal C reaction. Along with bound neutron sd-shell
hole states, unbound p-shell hole states have been firmly confirmed. The
excitation energies and the deduced spectroscopic factors of the cross-shell
states are an important measure of the neutron
configurations in C. Our results show a very good agreement with
shell-model calculations using the SFO-tls interaction for C. However, a
modification of the - and - monopole terms was applied in order
to reproduce the isotone O. In addition, the excitation energies
and spectroscopic factors have been compared to the first calculations of
C with the self-consistent Green's function method
employing the NNLO interaction. The results show the sensitivity to the
size of the shell gap and highlight the need of going beyond the current
truncation scheme in the theory
Neutron-rich nuclei produced at zero degrees in damped collisions induced by a beam of 18O on a 238U target
Cross sections and corresponding momentum distributions have been measured for the first time at zero degrees for the exotic nuclei obtained from a beam of 18O at 8.5 MeV/A impinging on a 1 mg/cm2 238U target. Sizable cross sections were found for the production of exotic species arising from the neutron transfer and proton removal from the projectile. Comparisons of experimental results with calculations based on deep-inelastic reaction models, taking into account the particle evaporation process, indicate that zero degree is a scattering angle at which the differential reaction cross section for production of exotic nuclei is at its maximum. This result is important in view of the new generation of zero degrees spectrometers under construction, such as the S3 separator at GANIL, for example
Low-lying single-particle structure of 17C and the N = 14 sub-shell closure
The first investigation of the single-particle structure of the bound states of 17C, via the C transfer reaction, has been undertaken. The measured angular distributions confirm the spin-parity assignments of and for the excited states located at 217 and 335 keV, respectively. The spectroscopic factors deduced for these states exhibit a marked single-particle character, in agreement with shell model and particle-core model calculations, and combined with their near degeneracy in energy provide clear evidence for the absence of the sub-shell closure. The very small spectroscopic factor found for the ground state is consistent with theoretical predictions and indicates that the strength is carried by unbound states. With a dominant valence neutron configuration and a very low separation energy, the excited state is a one-neutron halo candidate
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