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

Solving the large discrepancy between inclusive and exclusive measurements of the 8Li+4He→11B+n{}^8{\rm Li}+{}^4{\rm He}\to{}^{11}{\rm B}+n reaction cross section at astrophysical energies

A solution of the large discrepancy existing between inclusive and exclusive measurements of the ${}^8{\rm Li}+{}^4{\rm He}\to{}^{11}{\rm B}+n$ reaction cross section at $E_{cm} <3$ 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 ${}^8{\rm Li}+{}^4{\rm He}\to{}^{11}{\rm B}+n$ 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 ${}^{11}{\rm B}$ excited levels, are presented and their possible origin, in terms of the cluster structure of the involved excited states of ${}^{11}{\rm B}$ and ${}^{12}{\rm B}$ nuclei, is discussed.Comment: 5 pages, 4 figures, 1 tabl

Radiation correction to astrophysical fusion reactions and the electron screening problem

We discuss the effect of electromagnetic environment on laboratory measurements of the nuclear fusion reactions of astrophysical interest. The radiation field is eliminated using the path integral formalism in order to obtain the influence functional, which we evaluate in the semi-classical approximation. We show that enhancement of the tunneling probability due to the radiation correction is extremely small and does not resolve the longstanding problem that the observed electron screening effect is significantly larger than theoretical predictions.Comment: 9 pages, 1 eps figure

Fusion and direct reactions for the system 6He + 64Zn at and below the Coulomb barrier

Fusion and transfer + breakup channels have been studied in the collision induced by the two-neutron-halo 6He on a 64Zn target at energies from below to above the Coulomb barrier. For comparison, the reaction induced by the stable isotope 4He on the same target has been studied. The fusion cross section has been measured by using an activation technique, detecting off-line the delayed x-ray activity following the electron capture decay of the evaporation residues. New measurements of the 4He + 64Zn fusion cross section at sub-barrier energies have been performed in order to cover the same energy range of the 6He + 64Zn fusion cross section and to compare the excitation functions for the two systems down to the lowest energy data point measured for 6He. From the new comparison a sub-barrier fusion enhancement has been observed in the 6He case with respect to the 4He one whereas no effect on the 6He fusion cross section has been seen at energies above the barrier. It has been concluded that such enhancement seems to be due to the diffuse halo structure properties of the 6He nucleus. Moreover, the reactions induced by 6He have shown a strong yield of α particles coming from direct processes

Halo effects on fusion cross section in 4,6He+64Zn collision around and below the Coulomb barrier

The structure of the halo nuclei is expected to influence the fusion mechanism at energies around and below the Coulomb barrier. Here new data of 4He+64Zn at sub-barrier energies are presented which cover the same energy region of previous measurements of 6He+64Zn. The fusion cross section was measured by using an activation technique where the radioactive evaporation residues produced in the reaction were identified by the X-ray emission which follows their electron capture decay. By comparing the two system, we observe an enhancement on the fusion cross section in the reaction induced by 6He, at energy below the Coulomb barrier. It is shown that this enhancement seems to be due to static properties of halo 2n 6He nucleus

Structure of low-lying states of 10,11^{10,11}C from proton elastic and inelastic scattering

NESTER PTH, expérience GANIL, équipement SISSITo probe the ground state and transition densities, elastic and inelastic scattering on a proton target were measured in inverse kinematics for the unstable $^{10}$C and $^{11}$C nuclei at 45.3 and 40.6 MeV/nucleon, respectively. The detection of the recoil proton was performed by the MUST telescope array, in coincidence with a wall of scintillators for the quasiprojectile. The differential cross sections for elastic and inelastic scattering to the first excited states are compared to the optical model calculations performed within the framework of the microscopic nucleon-nucleus Jeukenne-Lejeune-Mahaux potential. Elastic scattering is sensitive to the matter-root-mean square radius found to be 2.42$\pm$0.1 and 2.33$\pm$0.1 fm, for $^{10,11}$C, respectively. The transition densities from cluster and mean-field models are tested, and the cluster model predicts the correct order of magnitude of cross sections for the transitions of both isotopes. Using the Bohr-Mottelson prescription, a profile for the $^{10}$C transition density from the $0^+$ ground to the $2_1^+$ state is deduced from the data. The corresponding neutron transition matrix element is extracted: Mn=5.51$\pm$1.09 fm$^2$

Measurement of the Ge 70 (n,γ) cross section up to 300 keV at the CERN n-TOF facility

©2019 American Physical Society.Neutron capture data on intermediate mass nuclei are of key importance to nucleosynthesis in the weak component of the slow neutron capture processes, which occurs in massive stars. The (n,γ) cross section on Ge70, which is mainly produced in the s process, was measured at the neutron time-of-flight facility n-TOF at CERN. Resonance capture kernels were determined up to 40 keV neutron energy and average cross sections up to 300 keV. Stellar cross sections were calculated from kT=5 keV to kT=100 keV and are in very good agreement with a previous measurement by Walter and Beer (1985) and recent evaluations. Average cross sections are in agreement with Walter and Beer (1985) over most of the neutron energy range covered, while they are systematically smaller for neutron energies above 150 keV. We have calculated isotopic abundances produced in s-process environments in a 25 solar mass star for two initial metallicities (below solar and close to solar). While the low metallicity model reproduces best the solar system germanium isotopic abundances, the close to solar model shows a good global match to solar system abundances in the range of mass numbers A=60-80.Peer reviewedFinal Published versio

Towards the high-accuracy determination of the 238U fission cross section at the threshold region at CERN - N-TOF

The 238U fission cross section is an international standard beyond 2 MeV where the fission plateau starts. However, due to its importance in fission reactors, this cross-section should be very accurately known also in the threshold region below 2 MeV. The 238U fission cross section has been measured relative to the 235U fission cross section at CERN - n-TOF with different detection systems. These datasets have been collected and suitably combined to increase the counting statistics in the threshold region from about 300 keV up to 3 MeV. The results are compared with other experimental data, evaluated libraries, and the IAEA standards