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S. E. Massen

Publications and source records attributed to S. E. Massen.

28 records · Page 2Linked to original sources

One-body density matrix and momentum distribution in s-p and s-d shell nuclei

Analytical expressions of the one- and two- body terms in the cluster expansion of the one-body density matrix and momentum distribution of the s-p and s-d shell nuclei with N=Z are derived. They depend on the harmonic oscillator parameter b and the parameter $β$ which originates from the Jastrow correlation function. These parameters have been determined by least squares fit to the experimental charge form factors. The inclusion of short-range correlations increases the high momentum component of the momentum distribution, $n({\bf k})$ for all nuclei we have considered while there is an A dependence of $n({\bf k})$ both at small values of k and the high momentum component. The A dependence of the high momentum component of $n({\bf k})$ becomes quite small when the nuclei $^{24}$Mg, $^{28}$Si and $^{32}$S are treated as 1d-2s shell nuclei having the occupation probability of the 2s-state as an extra free parameter in the fit to the form factors.

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Resonances and Off-Shell Characteristics of Effective Interactions

The importance of including experimental resonances in constructing effective inter-cluster interactions has been investigated. For this, we first address the question of how to obtain the analytical properties of the Jost function in regions of physical interest on the complex k-plane when the potential is given in a tabular form. We then employ the Marchenko inverse scattering method to construct, numerically, phase equivalent local potentials supporting the same bound state(s) but having different resonance spectra which affect the off-shell characteristics of the corresponding scattering amplitudes. This implies that the inclusion of the experimental resonances in constructing a potential would change its shape, strength, and range which in turn would modify the bound and scattering wave functions in the interior region. This is expected to have important consequences in calculations of transition amplitudes in nuclear reaction theories, which strongly depend on the behaviour of the wave functions at short distances. The influence of Supersymmetric Transformations on the position and movement of resonances has also been investigated.

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Systematic study of the effect of short range correlations on the form factors and densities of s-p and s-d shell nuclei

Analytical expressions of the one- and two-body terms in the cluster expansion of the charge form factors and densities of the s-p and s-d shell nuclei with N=Z are derived. They depend on the harmonic oscillator parameter b and the parameter $β$ which originates from the Jastrow correlation function. These expressions are used for the systematic study of the effect of short range correlations on the form factors and densities and of the mass dependence of the parameters b and $β$. These parameters have been determined by fit to the experimental charge form factors. The inclusion of the correlations reproduces the experimental charge form factors at the high momentum transfers ($q\geq 2 1/fm$). It is found that while the parameter $β$ is almost constant for the closed shell nuclei, $^4$He, $^{16}$O and $^{40}$Ca, its values are larger (less correlated systems) for the open shell nuclei, indicating a shell effect in the closed shell nuclei.

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Jost Function for Singular Potentials

An exact method for direct calculation of the Jost function and Jost solutions for a repulsive singular potential is presented. Within this method the Schrodinger equation is replaced by an equivalent system of linear first-order differential equations, which after complex rotation, can easily be solved numerically. The Jost function can be obtained to any desired accuracy for all complex momenta of physical interest, including the spectral points corresponding to bound and resonant states. The method can also be used in the complex angular-momentum plane to calculate the Regge trajectories. The effectiveness of the method is demonstrated using the Lennard-Jones (12,6) potential. The spectral properties of the realistic inter-atomic He4-He4 potentials HFDHE2 and HFD-B of Aziz and collaborators are also investigated.

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Universal property of the information entropy in atoms, nuclei and atomic clusters

The position- and momentum-space information entropies of the electron distributions of atomic clusters are calculated using a Woods-Saxon single particle potential. The same entropies are also calculated for nuclear distributions according to the Skyrme parametrization of the nuclear mean field. It turns out that a similar functional form S=a+b ln N for the entropy as function of the number of particles N holds approximately for atoms, nuclei and atomic clusters. It is conjectured that this is a universal property of a many-fermion system in a mean field. It is also seen that there is an analogy of our expression for S to Boltzmann's thermodynamic entropy S=k ln W.

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Information entropy as a measure of the quality of a nuclear density distribution

The information entropy of a nuclear density distribution is calculated for a number of nuclei. Various phenomenological models for the density distribution using different geometry are employed. Nuclear densities calculated within various microscopic mean field approaches are also employed. It turns out that the entropy increases on going from crude phenomenological models to more sophisticated (microscopic) ones. It is concluded that the larger the information entropy, the better the quality of the nuclear density distribution. An alternative approach is also examined: the net information content i.e. the sum of information entropies in position and momentum space $S_{r}+S_{k}$. It is indicated that $S_{r}+S_{k}$ is a maximum, when the best fit to experimental data of the density and momentum distributions is attained.

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Effects of Short Range Correlations on Ca Isotopes

The effect of Short Range Correlations (SRC) on Ca isotopes is studied using a simple phenomenological model. Theoretical expressions for the charge (proton) form factors, densities and moments of Ca nuclei are derived. The role of SRC in reproducing the empirical data for the charge density differences is examined. Their influence on the depletion of the nuclear Fermi surface is studied and the fractional occupation probabilities of the shell model orbits of Ca nuclei are calculated. The variation of SRC as function of the mass number is also discussed.

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Consistent Construction of Realistic One-Body Density Matrix in Nuclei

A phenomenological method based on the natural orbital representation is applied to construct the ground state one-body density matrix which describes correctly both density and momentum distributions in $^{4}He$, $^{16}O$ and $^{40}Ca$ nuclei. The parameters of the matrix are fixed by a best fit to the experimental density distribution and to the correlated nucleon momentum distribution. The method allows the natural orbitals, the occupation probabilities and the depletion of the Fermi sea to be obtained. Ground-state characteristics of $^{4}He$, $^{16}O$ and $^{40}Ca$ nuclei, such as rms radii and mean kinetic energies are calculated, as well.

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The dependence of the nuclear charge form factor on short range correlations and surface fluctuation effects

We investigate the effects of fluctuations of the nuclear surface on the harmonic oscillator elastic charge form factor of light nuclei, while simultaneously approximating the short-range correlations through a Jastrow correlation ~factor. Inclusion of surface-fluctuation effects within this description, by truncating the cluster expansion at the two-body part, is found to improve somewhat the fit to the elastic charge form-factor of $^{16}O$ and $^{40}Ca$. However, the convergence of the cluster expansion is expected to deteriorate. An additional finding is that the surface-fluctuation correlations produce a drastic change in the asymptotic behavior of the point-proton form factor, which now falls off quite slowly (i.e. as $const. \cdot q^{-4}$) at large values of the momentum transfer $q$.

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Proton momentum distribution in nuclei beyond Helium-4

Proton momentum distributions of the $^{12}C$, $^{16}O$, $^{40}Ca$, $^{56}Fe$ and $^{208}$Pb nuclei are calculated by a model using the natural orbital representation and the experimental data for the momentum distribution of the $^{4}He$ nucleus. The model allows realistic momentum distributions to be obtained using only hole-state natural orbitals (or mean-field single-particle wave functions as a good approximation to them). To demonstrate the model two different sets of wave functions were employed and the predictions were compared with the available empirical data and other theoretical results.

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