Mechanisms in d + 6Li three-body reactions at low bombarding energies

The three-body reactions induced by low energy deuterons on ⁶Li have been studied. The contributions from the sequential processes through the excited states of the intermediate ⁴He, ⁵Li, ⁶Li and ⁷Li nuclei dominate the coincidence spectra. The quasi-free d + d → d + d, d + α → d + α, and d + d → p + t reactions have been observed and compared with plane wave impulse approximation (PWIA) calculations.     

Antikaon production and medium effects in proton-nucleus reactions at subthreshold beam energies

The inclusive K ^--meson production in proton-nucleus collisions in the subthreshold energy regime is analyzed in the framework of an appropriate folding model for incoherent primary proton-nucleon and secondary pion-nucleon production processes, which takes properly into account the struck target nucleon momentum and removal energy distribution (nucleon spectral function), novel elementary cross-sections for proton-nucleon reaction channel close to threshold as well as nuclear mean-field potential effects on the one-step and two-step antikaon creation processes. A detailed comparison of the model calculations of the K ^- differential cross-sections for the reactions p + ⁹Be and p + ⁶³Cu at subthreshold energies with the first experimental data obtained at the ITEP proton synchrotron is given, that displays both the relative role of the primary and secondary production channels at considered incident energies and the contributions to the K ^- production coming from the use of the single-particle part as well as high-momentum-energy part of the nucleon spectral function. It is found that the pion-nucleon production channel does not dominate in the subthreshold “hard” antikaon production in p ⁹Be-, p ⁶³Cu-collisions and the main contributions to the antikaon yields here come from the direct K ^- production mechanism. The influence of the nucleon, kaon and antikaon mean-field potentials on the K ^- yield is explored. It is shown that the effect of the nucleon mean-field is of importance in explaining the considered experimental data on “hard” antikaon production, whereas the K ⁺ and K ^- optical potentials play a minor role. The sensitivity of the subthreshold “soft” antikaon production in p ⁹Be-, p ¹²C-reactions to the nucleon, kaon and antikaon mean fields is studied. It is demonstrated that, contrary to the case of “hard” antikaon production, the K ^- potential has a very strong effect on the K ^- yield, which is greater than that from nucleon effective potential. Received: 20 July 2000 / Accepted: 4 December 2000     

Improved eikonal approach for charge exchange reactions at intermediate energies

In order to describe charge exchange reactions at intermediate energies,we implemented as a first step the formulation of the normal eikonal approach.The calculated differential cross-sections based on this approach deviated significantly from the conventional DWBA calculations for CE reactions at 140 MeV/nucleon.Thereafter,improvements were made in the application of the eikonal approximation so as to keep a strict three-dimensional form factor.The results obtained with the improved eikonal approach are in good agreement with the DWBA calculations and with the experimental data.Since the improved eikonal approach can be formulated in a microscopic way,it is easy to apply to CE reactions at higher energies,where the phenomenological DWBA is a priori difficult to use due to the lack,in most cases,of the required phenomenological potentials.     

Alpha-nucleus elastic scattering at intermediate energies

Elastic scattering of 288,340,480 and 699 MeV Alpha-particles was measured on ²⁰⁸Pb, ¹¹⁶Sn and ⁵⁸Ni. The data were analysed in terms of a phenomenological optical model. The optical potentials obtained were found to vary consistently with the target nucleus and the incident energy. The radial zone where the potentials are well determined was studied in detail. The data for ²⁰⁸Pb were also analysed with a folding model. The energy dependence of the strong-absorption radius and of the reaction cross section shows that the nuclear surface becomes slightly transparent for incident energies above 150 MeV per nucleon.     

Diffusion-influenced reactions

We summarize three of our recent results on diffusion-influenced reactions in solutions. All deal with the concentration dependence of the reaction rate when the reactants must first diffuse together before reaction can occur. When one species (the sink species) is not dilute, the rate cannot be obtained by solution of a pair diffusion equation; the correlations among the sinks for the diffusing species must be accounted for. First, we consider fluorescence quenching when the quenchers are not dilute. For charged quenchers and fluorophores we discuss how the solution dielectric constant and ionic strength can strongly influence the deviations from the linear Stern-Volmer behavior (the dilute sink result) which arise due to the sink correlations. Second, we consider heterogeneous catalysis where a reactive species is adsorbed onto a surface and must surface diffuse to reactive sites (the sinks). We find that surface diffusion can be an important factor contributing to the rate of reaction; especially when surface diffusion is rapid relative to the adsorption/desorption rate. Third, we discuss diffusion influenced reactions with sinks which are long ellipsoids. Dilute long ellipsoids provide a large rate enhancement relative to a spherical sink; we show that this rate enhancement survives when nondilute ellipsoids are considered.     

Scattering of α-particles at medium energies

Elastic scattering of α-particles is treated within the context of high energy multiple diffraction theory. Terms up to fourth order in the expansion of the optical phase-shift function are retained (when necessary). The theoretical predictions are compared with the recent 5.05 and 4.30 GeV/c α-α measurements. The effects of short-range dynamical correlations are discussed.     

Computing activation energies of non-thermal reactions

The chemistry in bulk gases involves reactions of nascent radicals that are almost invariably non-thermal. The energy requirements of reactions involving radicals depend on the reactions that produce them and the intra- and inter-molecular energy transfer they may undergo. Here, we extend the generalised Tolman activation energy (GTEa) method to non-thermal reactions in molecular dynamics (MD) simulations. We compute the energy requirements, which we refer to as chemical-activation energies (CE _a), of reactions of radicals formed by the decomposition of hydrogen peroxide. The equipartition theorem is adapted to compute average energies of small isolated systems with internal degrees of freedom in MD simulations with periodic boundary conditions, which is necessary for application of the GTEa method to non-thermal reactions. To illustrate the applicability of the GTEa method to non-thermal reactions, we present CE _a results for H₂O₂?+?OH → H₂O?+?HO₂, a key reaction in hydrogen combustion, as described by the ReaxFF force field. The OH radicals are the products of the self-dissociation of H₂O₂ and subsequent reactions. We define the chemical-activation energy for a back reaction (BCE _a) as the difference between the energy of the products and the average energy of the system. We show that the BCE _a and CEa are linearly correlated.     

Proton capture by 15N at stellar energies

Excitation functions of the ¹⁵N(p, γ)¹⁶O proton capture reaction have been obtained at θ_γ = 45° and E_p = 150–2500 keV. Below E_p = 400 keV, the reaction is dominated by capture into the ground state of ¹⁶O. The observed excitation function for the latter process can be explained if, in addition to the two well-known J^π = 1^? resonances at E_p = 338 and 1028 keV, a direct radiative capture process (DC → 0) is included in the analysis. The direct capture component in the capture reaction is enhanced through interference effects on the tails of the two resonances. From the observed direct capture cross section, a single-particle spectroscopic factor of C²S(1p) = 1.8 ± 0.4 has been deduced for the ground state in ¹⁶O. The extrapolated astrophysical S-factor of S(0) = 64 ± 6 keV · b for the ¹⁵N(p, γ₀)¹⁶O reaction is a factor of 2.5 larger than previously reported. This result amplifies the role of the oxygen side cycle in the CNO hydrogen burning process.The observed excitation function of the ¹⁵N(p, α₁γ₁)¹²C reaction at E_p = 150 – 2500 keV shows that this reaction makes a negligible contribution to hydrogen burning at stellar energies [S(0) ≈ 0.1 keV · b] compared to ${}^{15}\text{N}\text{(}\text{p}\text{, γ}{}_0\text{)}{}^{16}\text{O and}{}^{15}\text{N}\text{(}\text{p}\text{, α}{}_{\mathrm{<mtext>o</mtext>}}\text{)}{}^{12}\text{C}$.     

Total reaction and transfer cross sections for 11B + 9Be and 13C + 9Be reactions at low energies

The total reaction cross sections for ¹¹B + ⁹Be and ¹³C + ⁹Be have been measured by the total γ-ray yield method over the energy intervals E_c.m. = 1.4–4.4 MeV and E_c.m. = 2.0–5.2 MeV, respectively. The cross sections for the neutron transfer reactions ¹¹B(⁹Be, ⁸Be)¹²B, leading to the ¹²B 0.953 and 1.674 MeV states, and ¹³C(⁹Be, ⁸Be)¹⁴C, leading to the ¹⁴C 6.094, 6.728 and 6.902 MeV states, have been obtained from the yields of the characteristic γ-rays. The α-transfer reaction ¹¹B(⁹Be, ⁵He)¹⁵N, leading to many unresolved ¹⁵N states, has been observed with large cross section. There is, however, no evidence for the ¹³C(⁹Be, ⁵He)¹⁷O transfer process in the ¹⁷O + nα channels. This different behaviour of the ¹¹B + ⁹Be and ¹³C + ⁹Be systems seems to indicate that the α-transfer reaction at sub-barrier energies is not a direct transfer process, and that it probably occurs via molecular state formation.     

Hydrogen-antihydrogen interactions at low energies

The main cause of loss of trapped AH is due to collisions with H₂ and He. As a first step towards treating these reactions we are studying the interaction of AH with H. We have carried out variational calculations to determine an upper bound to the smallest internuclear distance at which the light particles are still bound to the nuclei. We are currently in the process of taking into account the motion of the nuclei. This will enable us to calculate cross-sections for low energy H-AH scattering. This revised version was published online in August 2006 with corrections to the Cover Date.     

Cross sections for 6Li production in the reactions 10B + 16O and 12C + 14N at low energies

Absolute cross sections have been measured for the reactions ¹⁰B(¹⁶O, ⁶Li)²⁰Ne and ¹²C(¹⁴N, ⁶Li)²⁰Ne at several energies in the range E_c.m. = 7.5–16.2 MeV, and 13.8–16.6 MeV, respectively. The predictions of Hauser-Feshbach calculations show generally good agreement with the experimental data without parameter variation. The consequences of an angular momentum cutoff in the entrance channel and in the compound nucleus are discussed.     

Multistep direct reactions at low energies

The cross sections for neutron inelastic scattering by niobium at 20 MeV and for the charge-exchange (p,n) reaction on copper at 27 MeV have been calculated using the multistep direct reaction theory of Feshbach, Kerman and Koonin [1]. The theory was modified to include the non-DWBA matrix elements and the collective vibrations. The results show enhanced contributions from two-, three- and four-step direct reactions in agreement with experiment.     

Cluster transfer reactions at low energies

Acta physica Academiae Scientiarum Hungaricae - The cluster transfer processes below the Coulomb barrier are investigated for arbitrary charges and angular momentum transfer, taking into account...     

Elastic and inelastic scattering of carbon ions at intermediate energies

Elastic and inelastic scattering have been measured at E_lab = 360 MeV and 1016 MeV for the ¹²C + ¹²C system, as well as elastic scattering for ¹³C + ²⁰⁸Pb at 390 MeV. An optical-model analysis is reported and nuclear surface transparency effects are discussed, together with the energy dependence of the nuclear potential. A DWBA analysis of data on the 2⁺, 4.4 MeV state of ¹²C is reported and trends in the energy dependence of mean-field excitations are deduced.     

Application of EFT at thermal energies

We have evaluated some observables of n-d systems by using pionless effective field theory (EFT( ${\not}\pi$ )) and insertion of the three-body force up to next-to-next to leading order (N²LO). The evaluated data has been compared with experiment and a three-nucleon calculation of the total cross section with modern realistic two- and three-nucleon force AV18/UrbIX potential models.     

Heavy-ion induced fission reactions at near-barrier energies — what have we learnt?

Systematic studies of heavy-ion induced fission reactions at near-barrier energies carried out in the last decade have brought out many interesting aspects of fission process in general. The recent experimental findings which show dependence of fission fragment angular distributions on entrance channel, shape, size and spin of the interacting nuclei and shell closure of the intermediate compound nucleus are summarised in the present paper.     

Fusion cross sections for 10,11B+12C at sub-Coulomb energies

Total fusion cross sections for the ¹⁰B + ¹²C and ¹¹B + ¹²C reactions have been determined over a 5 MeV (c.m.) energy range extending to ≈ 3 MeV below the Coulomb barrier. Absolute γ-ray yields for specific transitions in the de-excitation of the heavy products following compound nucleus decay were measured using a Ge(Li) detector. Statistical model calculations of the decay modes of the compound nucleus have been used to deduce, from the γ-ray data, cross sections for single proton, neutron and α-particle emission, and to determine total cross sections for compound nucleus formation. No evidence has been found for sub-Coulomb resonances in either reaction. The total reaction cross sections are compared with optical model calculations using different parameter sets and the observed trend in the very low energy cross sections is discussed relative to other reactions in the same mass region.     

Pion small-angle multiple scattering at energies spanning the (3,3) resonance

The small-angle multiple scattering of positive and negative pions has been measured for C, Al, Cu and Pb targets throughout an energy range spanning the (3, 3) resonance. The measurements were made using two-dimensional multiwire proportional counters placed in the pion beam. All previous multiple scattering data for electrons and protons as well as these new data for pions are used to recalculate the empirical terms in the Moliere theory of multiple scattering. A second-order Born approximation multiple scattering theory has been devised for spin zero particles. Modified Moliere theory gives a better fit to the experimental data than the second-order Born calculation. Because the updated Moliere theory contains much simpler equations and gives closer agreement with experiment, it is recommended in place of the more sophisticated theory for the interpretation of new experimental data.     

14N fusion with 13C and 16O at sub-barrier energies

Total fusion cross sections for the ¹⁴N+¹²C and ¹⁴N+¹⁶O reactions have been measured in the c.m. energy ranges 3.6–9.2 MeV and 5.6–12.6 MeV, respectively. Cross sections are also reported for important individual exit channels which were studied by observing discrete γ-ray transitions in the evaporation residues with a Ge(Li) detector. Excitation functions reveal no evidence of intermediate structure in these reactions. The fusion cross sections for ¹⁴N induced reactions on ¹²C, ¹⁴N and ¹⁶O are compared with an IWB calculation using recently published semi-empirical parameters for the real ion-ion interaction potential. Such a comparison supports the view that low-energy fusion studies may be sensitive probes of the nuclear potential in the interior of the interaction barrier.     

Antiproton-deuteron annihilation at low energies

Recent experimental data obtained by the OBELIX collaboration on ˉpD and ˉp⁴ He total annihilation cross sections are analyzed. The combined analysis of these data with existing antiprotonic atom data allows, for the first time, the imaginary parts of the S-wave scattering lengths for the two nuclei to be extracted. The obtained values are: Im a^sc ₀= [−0.62 ± 0.02 stat) ± 0.04 (sys)] fm for ˉpD and Im a^sc ₀= [−0.36 ± 0.03 (stat)^+0.19 _−0.11 (sys)] fm for ˉp ⁴He. This analysis indicates an unexpected behaviour of the imaginary part of the ˉp-nucleus S-wave scattering length as a function of the atomic weight A: |Im a^sc ₀| (macr;pp) > |Im a^sc ₀| (ˉpD) > |Im a^sc ₀ (ˉp⁴ He). Received: 3 November 1999 / Revised version: 15 December 1999