arXiv subjects
Publications and source records attributed to W. Sandhas.
Elastic eta-d scattering is considered within the Alt-Grassberger-Sandhas (AGS) formalism for various eta-N input data. A three-body resonant state is found close to the eta-d threshold. This resonance is sustained for different choices of the two-body eta-N scattering length a_{ηN}. The position of the resonance moves towards the eta-d threshold when Re a_{ηN} is increased, and turns into a quasi-bound state at Re a_{ηN}=0.733 fm.
The quality of two different separable expansion methods ({\sl W} matrix and Ernst-Shakin-Thaler) is investigated. We compare the triton binding energies and components of the triton wave functions obtained in this way with the results of a direct two-dimensional treatment. The Paris, Bonn {\sl A} and Bonn {\sl B} potentials are employed as underlying two-body interactions, their total angular momenta being incorporated up to $j \leq 2$. It is found that the most accurate results based on the Ernst-Shakin-Thaler method agree within 1.5% or better with the two-dimensional calculations, whereas the results for the {\sl W}-matrix representation are less accurate.
The differential cross section for radiative capture of protons by deuterons is calculated using different realistic NN interactions. We compare our results with the available experimental data below $E_x = 20 MeV$. Excellent agreement is found when taking into account meson exchange currents, dipole and quadrupole contributions, and the full initial state interaction. There is only a small difference between the magnitudes of the cross sections for the different potentials considered. The angular distributions, however, are practically potential independent.
The process $γ+ t \to n + d$ is treated by means of three-body integral equations employing in their kernel the W-Matrix representation of the subsystem amplitudes. As compared to the plane wave (Born) approximation the full solution of the integral equations, which takes into account the final state interaction, shows at low energies a 24% enhancement. The calculations are based on the semirealistic Malfliet-Tjon and the realistic Paris and Bonn B potentials. For comparison with earlier calculations we also present results for the Yamaguchi potential. In the low-energy region a remarkable potential dependence is observed, which vanishes at higher energies.
Three- and four-nucleon photodisintegration processes are quite efficiently treated by means of effective two-body integral equations in momentum space. We recall some aspects of their derivation, present previous and most recent results obtained within this framework, and discuss general features, trends and effects observed in these investigations: At low energies final-state interaction plays an important role. Even more pronounced is the effect of meson exchange currents. A considerable potential dependence shows up in the low-energy peak region. The different peak heights are found to be closely correlated with the corresponding binding energies. Above the peak region only the difference between potentials with or without p-wave contributions remains relevant. In the differential cross sections the electric quadrupole contributions have to be taken into account. The remarkable agreement between theory and experiment in $p$-$d$ radiative capture is achieved only when incorporating this contribution, together with most of the above-mentioned effects. In the final part of this report we briefly review also methods developed, and results achieved in three- and four- nucleon electrodisintegration. We, in particular, compare them with a recent access to this problem, based on the construction of nucleon-nucleus potentials via Marchenko inversion theory.
The photodisintegration of $^{3}$H is treated by means of coupled integral equations using separable versions of the Paris and the Bonn potentials in their kernel. The differential cross section for the inverse reaction is obtained via detailed balance. For the latter process good agreement with the data is found when including final-state interaction, meson exchange currents, higher partial waves in the potential, and electric quadrupole contributions in the electromagnetic interaction.
Several nuclear reactions involving the Be7 nucleus, not included into the standard model of the pp-chain, are discussed. A qualitative analysis of their possible influence on the fate of the Be7 in solar plasma and of their role in the interpretation of the solar neutrino experiments is given. As an example, the reaction rate of the nonradiative production of B8 in the triple collision p + e^- + Be7 ---> B8 + e^- is estimated in the framework of the adiabatic approximation. For the solar interior conditions the triple collision reaction rate is approximately 10^{-4} of that for the binary process p + Be7 ---> B8 + gamma .
We calculated the energy and the size of the three-deuteron muonic molecule (3d 2e^- μ^-) = D_3 μ. It turns out that this system possesses two equilibrium positions, one at distances typical for muonic molecules and a second one at the usual molecular size. We show, moreover, that the fusion probability of the three deuterons is considerably enhanced due to the existence of a ^6Li^* threshold resonance. Our estimates indicate that this probability is considerably higher than the decay rate of the competing Auger transition.
We consider several nuclear fusion reactions that take place at the center of the sun, which are omitted in the standard pp-chain model. More specifically the reaction rates of the nonradiative production of ^3He, ^7Be, and ^8B nuclei in triple collisions involving electrons are estimated within the framework of the adiabatic approximation. These rates are compared with those of the corresponding binary fusion reactions.
It is widely accepted that in molecular systems the nuclear interaction plays a negligible role, because of the strong Coulomb repulsion of the nuclei at small distances. We are going to show that this is not always true. The existence of an extended nuclear resonance may lead to considerably enhanced nuclear reaction rates in appropriately prepared molecules. Especially we point out that p+p+ ^{16}O, i.e., the constituents of water, can form a ^{18}Ne(1^-) threshold resonance which decays under energy release into ^{17}F and a proton.
The probability of nuclear transitions $p+p+{}^{16}{O} \, \rightarrow \, {}^{18}{Ne}\, (4.522,1^-)$ in molecular water is estimated. Due to the practically exact agreement of the energy of the $Ne$ resonance and of the $p+p+{}^{16}{O}$ threshold, the transition probability is found to be considerably enhanced. This indicates the possibility of nuclear fusion in rotationally excited $H_2O$ molecules in the state $1^-$.
The position and movement of poles of the amplitude for elastic eta-meson scattering off the light nuclei d, t, ^3He, and ^4He are studied. It is found that, within the existing uncertainties for the elementary eta-N interaction, all these nuclei can support a quasi-bound state. The values of the eta-nucleus scattering lengths corresponding to the critical eta-N interaction which produces a quasi--bound state are given.
The resonance and bound--state poles of the amplitude describing elastic scattering of $η$-meson off the light nuclei $^2H$,\,$^3H$,\, $^3He$, and $^4He$ are calculated in the framework of a microscopic approach based on few--body equations. For each of the nuclei, the two--body parameters that enhance the $ηN$--attraction which generate quasi--bound states, are also determined.
A microscopic treatment of $η$--nucleus scattering is presented. When applying the underlying exact integral equations, the excitation of the target are neglected, and an input $ηN$ amplitude is chosen which reproduces the $S_{11}(1535)$ resonance. It is shown that the $η$--nucleus scattering lengths are quite sensitive to the $ηN$ parameters and the nuclear wave functions. For a special choice of the parameters an $η{^4\!H}e$ quasi--bound state occurs.
The scattering lengths of eta-meson collisions with light nuclei d,t,3He, and 4He are calculated on the basis of few-body equations in coherent approximation. It is found that the eta-nucleus scattering length depends strongly on the number of nucleons and the potential-range parameter. By taking into account the off-shell behavior of the eta-N amplitude, the eta-4He scattering length increases considerably.
In this report we will discuss properties of and models for nuclear transitions in muonic molecules formed via collisions of muonic atoms of hydrogen isotopes with light nuclei like $L\!i$ and $B\!e$. Their importance for nuclear astrophysics and nuclear physics at super low energies is emphasized.