Narrow Results

In this contribution, we give a brief overview over the historical development of shell-model-related experimental and theoretical nuclear physics activities at KTH Royal Institute of Technology that benefited from the classic Kuo–Brown interaction and the fundamental insight of Kuo on the core polarization effect.

In a previous work it was shown that the radius of a nucleus R is determined by the α-cluster structure and can be estimated on the number of α-clusters disregarding the number of excess neutrons. A hypothesis was also made that the radius R_m of a β-stable isotope, which is actually measured at electron scattering experiments, is determined by the volume occupied by the matter of the core plus the volume occupied by the charge of the peripheral α-clusters. In this paper it is shown that the condition R_m = R restricts the number of excess neutrons filling the core to provide the β-stability. The number of peripheral clusters can vary from 1 to 5 and the value of R for heavy nuclei almost does not change, whereas the number of the excess neutrons should change with the number of peripheral clusters to provide the condition of R_m = R. It can explain the path of the β-stability and its width. The radii R_m of the stable isotopes with 12 ≤ Z ≤ 83 and the alpha-decay isotopes with 84 ≤ Z ≤ 116 that are stable to β-decay have been calculated.

Structures of excited states in ¹¹C and ¹¹B were studied based on variational calculations after spin-parity projection in a framework of antisymmetrized molecular dynamics (AMD). The calculations systematically reproduce experimental data of the transition strengths such as Gamov-Teller(GT) and M1 transitions. A well-developed cluster structure of 2α+³He(t) was found in the state of ¹¹C(¹¹B). We assigned this state to the observed state at 8 MeV. The calculations reproduce the small which has been measured in the recent experiments. It was found that the state has a dilute cluster state, and its features are similar to those of the which is considered to be a gas state of three α clusters.

The α-cluster model is based on two assumptions that the proton–neutron pair interactions are responsible for adherence between α-clusters and that the NN-interaction in the α-clusters is isospin independent. It allows one to estimate the Coulomb energy and the short range inter-cluster bond energy in dependence on the number of clusters. The charge radii are calculated from the number of α-clusters too. Unlike the Weizsäcker formula in this model the binding energies of alpha-clusters and excess neutrons are estimated separately. The calculated values are in a good agreement with the experimental data.

We study the role of the negative-energy nucleon states of relativistic nuclear models in nuclear ground states and excitations. The self-consistent calculation including the negative-energy states are performed by using the derivative expansion in terms of the meson field, which is known to provide a good description of the vacuum fluctuations in nuclear ground states quantitatively. A remarkable effect of the inclusion of the negative-energy contribution into the model is the increase of the nucleon effective mass. In particular, this produces the better description for the isoscalar quadrupole resonances, and essentially different results for the Gamow-Teller resonances.

To describe excited nuclear states, we developed the Brueckner-AMD combining the Brueckner theory with Antisymmetrized Molecular Dynamics (AMD), which has been recently proposed as a new framework to study nuclear structure of light nuclei based on the realistic nuclear force. In the present framework, we formulate a multi-configuration calculation with Generator Coordinate Method (GCM) in the Brueckner-AMD. An application is shown for the excited states with positive and negative parities in ⁴He, and the successful results and reliability of the present framework are discussed.

Effects of tensor interactions in nucleus are discussed from the experimental view points. Two recent studies, one is the mixing of s- and p- waves in ¹¹Li halo and the other is the high momentum component in the nucleon wave function from a transfer reaction. Preliminary results of ¹⁶O(p,d)¹⁵O reaction show an abnormal relative enhancement of the transition to the positive parity state of ¹⁵O, which is not a hole state, consistent with contents of high momentum nucleon expected from the tensor interactions. The importance of the inclusion of tensor force, that can not be described in the mean field model, is discussed.

The β-decay spectroscopy on ²⁴Si was carried out in order to investigate characteristic properties of proton-rich nuclei from a perspective of mirror asymmetry. We observed two β transitions to low-lying bound states in ²⁴Al for the first time. The B(GT) of ²⁴Si were deduced to be 0.13(2) and 0.14(2), which were smaller than those of the mirror transition by 22(11)% and 10(8)%, respectively. Through the comparison with theoretical calculations taking into account the Coulomb force and the Thomas-Ehrman shift, the mirror asymmetry of B(GT) is attributed to the lowering of the proton 1s_1/2 orbital.

The upcoming FAIR facility in Darmstadt, Germany, will produce intense high energy beams of exotic nuclei, which will be used to explore the properties of new regions of the chart of nuclides of key importance for both nuclear structure and nuclear astrophysics. In the EXL project, heavy ion beams are first cooled in the New Experimental Storage Ring (NESR) and then used to induce reactions on windowless thin Hydrogen, Deuterium and Helium gas targets in the ring. High luminosities can be achieved because of the recirculation of the beam with a frequency of about 1 MHz. The EXL system will be ideal for high resolution reaction studies at low momentum transfer. The design of the detector system considered is universal in the sense that it should allow the use of a large variety of nuclear reactions, addressing numerous physics questions. The detector system provides the capability of fully exclusive kinematical measurements, with target recoil detectors, fast ejectile forward detectors and an in-ring heavy-ion spectrometer. The physics case and detector design considerations for EXL along with tests experiments performed at KVI and GSI, paving the way to the full EXL detection system, are presented in this contribution.

Low-lying excitation properties of exotic nuclei in the vicinity of the canonical magic number 8 have been investigated through the level lifetime measurement on the neutron-rich isotope ¹³B with N = 8 and the missing-mass spectroscopy on the proton unbound nucleus ¹²O with Z = 8. The observed trends of the transition probabilities and level schemes have been discussed in comparison with the advanced shell model calculations, which points to the universality as well as the charge independence of the shell quenching phenomena far from stability.

The Glauber model is modified with the Fermi-motion effect in the calculation of elastic differential cross-sections and momentum distributions of a fragment from mother nucleus. Different reaction systems at low energies are calculated with the modified Glauber model. It is found that calculations including the Fermi-motion provide a better prescription relating the model to a proper nuclear density distribution by comparing with the experimental data. On the basis of the studies, the influence of the correction on the extracted nuclear radius is quantified. The results further confirm the importance of the Fermi-motion in the nucleus–nucleus collision reactions at low energies.

In this paper, we intend to extend the nuclear data of ${}^{244}\mathrm{\text{Cm}}$, ${}^{241}\mathrm{\text{Am}}$, ${}^{238}\mathrm{\text{Pu}}$, ${}^{210}\mathrm{\text{Po}}$, ${}^{147}\mathrm{\text{Pm}}$, ${}^{137}\mathrm{\text{Cs}}$, ${}^{90}\mathrm{\text{Sr}}$ and ${}^{63}\mathrm{\text{Ni}}$ nuclei used in nuclear battery technology, because, these nuclei are quite important for space investigations in radioisotope thermoelectric generator (RTG) and for microelectronic technologies in betavoltaic batteries. Therefore, the nuclear structure properties of nuclei such as separation energies, neutron skin thicknesses, proton, charge and neutron density distributions as a function of radius, the root mean square (rms) proton, charge and neutron radii, binding energies per particle, have been investigated by Hartree–Fock with eight different Skyrme forces. The obtained results have been compared with the experimental data in literature and relativistic mean field theory (RMFT) results.

The $\gamma$-bands are analyzed through the variation of the energy of the $2_{\gamma }^{+}$ excitation and the energies of excited level sequence of $\gamma$-bands with respect to various parameters. The shape phase transition observed at N = 88–90 is reviewed through its influence on the energies of $\gamma$-band. The correlation of the $2_{\gamma }^{+}$ excitation energies with the collective shape signature observable $R_{4/2}$ indicates a connection with the nuclear equilibrium structure. The study of excited level sequence in the $\gamma$-band with respect to the ground band signifies that the two bands differ in deformation.

An extensive study of $\alpha$-decay half-lives for various decay chains of isotopes of $Z=120$ is performed within the axially deformed relativistic mean-field (RMF) formalism by employing the NL3, NL3${}^{\ast }$, and DD-ME2 parameter set. The structural properties of the nuclei appearing in the decay chains are explored. The binding energy, quadrupole deformation parameter, root-mean-square charge radius, and pairing energy are calculated for the even–even isotopes of $Z=100\text{–}120$, which are produced in five different $\alpha$-decay chains, namely, ${}^{296}120\to {}^{260}\mathrm{\text{No}}$, ${}^{298}120\to {}^{262}\mathrm{\text{No}}$, ${}^{300}120\to {}^{264}\mathrm{\text{No}}$, ${}^{302}120\to {}^{266}\mathrm{\text{No}}$, and ${}^{304}120\to {}^{268}\mathrm{\text{No}}$. A superdeformed prolate ground state is observed for the heavier nuclei, and gradually the deformation decreases towards the lighter nuclei in the considered decay chains. The RMF results are compared with various theoretical predictions and experimental data. The $\alpha$-decay energies are calculated for each decay chain. To determine the relative numerical dependency of the half-life for a specific $\alpha$-decay energy, the decay half-lives are calculated using four different formulas, namely, Viola–Seaborg, Alex–Brown, Parkhomenko–Sobiczewski and Royer for the above said five $\alpha$-decay chain. We notice a firm dependency of the half-life on the $\alpha$-decay formula in terms of $Q_{\alpha }$-values for all decay chains. Further, this study also strengthens the prediction for the island of stability in terms of magic number at the superheavy valley in the laboratories.

The nuclear structure properties of heavy and superheavy nuclei lying at the extremes of charge, especially $Z=102\text{–}126$, and their density profiles have been explored using the relativistic Hartree–Bogoliubov in axial symmetry and the Skyrme–Hartree–Fock–Bogoliubov framework in transformed harmonic oscillator basis. In this study, we found that the semi-bubble (reduced central density) structure is mainly driven by Coulomb repulsion which stabilizes the nuclei either by pushing the nuclear matter in the surface region or by shape deformation. The shell effect is found maximum in spherical symmetric nuclei as the vanishing fission barrier in Liquid Drop formula indicates that the shell correction energy is responsible for the stabilization of these nuclei.

The author's earlier research with Professor Bruce McKellar, some 30 years ago, led to the Tucson-Melbourne three-nucleon interaction, which is still relevant today. The new significance of three-nucleon forces in determining the structure of atomic nuclei will be discussed. This has led to increased efforts to learn more about the nature of these three-nucleon interactions, both experimentally and theoretically. The recently developed no-core shell model (NCSM) has the ability to test different theoretical models for three-nucleon forces by making direct comparisons of results produced by these forces with experimental data.

We present a brief history of the structure study of ¹⁶O as a typical example in the development of the cluster model theory over 50 years.

This presentation focuses on some of the recent developments in low-energy nuclear structure theory, with emphasis on applications of coupled-cluster theory. We report on results for ground and excited states in ⁴He and ¹⁶O, and about extensions of coupled-cluster theory to treat three-body forces.

The bases of the translation invariant shell model are used to construct the ground-state wave functions of ³T, ⁵He and ⁶Li. For ³T the bases used correspond to the number of quanta of excitation N up to ten. For ⁵He and ⁶Li the bases used correspond to the number of quanta of excitation N up to six. The model is applied to calculate the binding energy and the root mean square radius for ³T, ⁵He and ⁶Li nuclei. The residual interactions used consist of central, tensor, spin-orbit and quadratic spin-orbit terms with Gaussian radial dependence. The parameters of these interactions are chosen in such away that they represent the long range attraction and the short range repulsion of nucleon interactions. It was found that this potential is more suitable for calculating the characteristics of these nuclei, and better than other potentials, such as our previous potentials which were represented by the parameters of long range attraction forces only. For ³T we obtained good agreement between calculated and experimental values of both the ground state binding energy and the root mean square radius. For ⁵He and ⁶Li nuclei we obtained an acceptable improvement with these calculations over other potentials.

In the framework of the Alpha Cluster Model extended calculations of form factors of elastic and inelastic and (7.66 MeV) electron scattering on ¹²C were performed. Two possible alpha cluster configurations (linear and triangular) were considered for states assigned to different rotational bands and although the configuration mixture was found to be important for explanation of the inelastic monopole form factor, a fully consistent picture, including all available data, was not obtained.