C. Riedel

Two-Neutron Transfer Reactions and the Pairing Model

The description of many-body systems at low energy in terms of “elementary modes of excitation” (see, e.g., Noz 65) is very useful in the case of nuclei (see BM 69). “Elementary modes of excitation” as used here comprise collective (rotations and vibrations) as well as quasiparticle excitations. In order to study the various modes, experiments are required which selectively excite one such degree of freedom while all the others remain inactive spectators.

Nuclear spectroscopy on deformed nuclei with two-neutron transfer reactions

Abstract The formalism for calculating (t, p) and (p, t) reactions on quadrupole deformed nuclei is presented. It is applied to the calculation of the cross sections associated with the excitations of members of the ground state rotational band of nuclei in the rare-earth region. Comparison with the available experimental material is carried out. Special attention is paid to the relation between multipole pairing moments and reaction cross sections. The possibility of determining deformation parameters is pointed out.

Two-nucleon transfer and pairing phase transition

Abstract A system of n particles moving in two non-degenerate j-shells and interacting via the pairing force is amenable to an exact treatment. This model exhibits a phase transition, namely the one between normal and superconductive phases. The behaviour of the cross section for the two-particle transfer reaction in the seniority zero channel is studied. Special attention is paid to the transition region.

First-forbidden β-decays in the lead region

Abstract Fourteen first-forbidden β-decays in the region around 203Pb are analysed in terms of only six unknown β-matrix elements between single-particle or single-hole states. Each of the matrix elements is of multipolarity λ = 0 or λ = 1 and is a combination of the usual first-forbidden matrix elements. The analysis gives experimental values for the six β-matrix elements and tests the consistency of the nuclear wave functions. The decay of 206Tl is too fast by a factor of two as compared to the rest of the decays. The experimental values of the β-matrix elements are compared to values calculated with the β-interaction.

The pairing vibrational model and the analysis of the 116,118Sn(t, p) reactions

Abstract The reactions 116,118 Sn(t, p) are analysed by using the eigenfunctions of a Rosenfeld type force diagonalized in a two-quasiparticle basis. Special attention is paid to the π = 0 + channel which is also studied in the pairing vibrational model. It is concluded that the pairing vibration carries a (t, p) strength of the same order as that carried by two-quasiparticle intrinsic excitations in superconductive nuclei. The presence of high-lying (core excited) 0 + states with large (t, p) intensities is predicted.

Sum rules and two-particle units in the analysis of two-neutron transfer reactions

Abstract The problem of two-particle units and sum rules in two-neutron transfer reactions is discussed. Using (t, p) data on 16 O, 40 Ca, 48 Ca, 118 Sn, 204 Pb, 206 Pb, 207 Pb and 208 Pb targets and available model wave functions, a universal value of the absolute normalization factor D 0 2 is obtained. Utilizing this value the adequacy of the shell-model subspace used in generating the wave functions is studied. With the aid of conveniently defined two-particle units in terms of D 0 2 , enhancement factors are calculated for all transitions analyzed, and a quantitative definition of enhanced cross sections is obtained. The relation between the two-particle units introduced here and the Weisskopf units used in connection with electromagnetic reduced transition probabilities is pointed out.

The structure of 42Ca and 42Sc investigated by two-nucleon transfer reactions

Abstract The low-lying states of 42Ca and 42Sc have been calculated in the co-existence model using effective matrix elements obtained from a Hamada-Johnston potential. The resulting wave functions are used to analyse the reactions 40Ca(t, p)42Ca and 40Ca(3He, p) 42Sc. A detailed comparison between theory and experiment is presented. The influence of a spin-orbit coupling in the optical potential on angular distributions and proton polarization is discussed briefly.

COHERENCE PROPERTIES OF TWO-NEUTRON TRANSFER REACTIONS AND THEIR RELATION TO INELASTIC SCATTERING.

Abstract The correlation aspects of two-neutron transfer and inelastic scattering reactions are discussed from a unified point of view. The experimentally observed correlations between the two types of collective states and corresponding cross sections are qualitatively explained.

Two-nucleon transfer reaction on oxygen and the nuclear coexistence model

Abstract The experimental data of the reactions 16 O(t, p) 18 O and 18 O(t, p) 20 O are analysed using the nuclear coexistence model for the description of the initial and final states. It is concluded that the admixture of deformed configurations in the nuclear states involved is essential to explain the observed relative cross sections. Two-particle transitions connecting deformed components do not carry, in general, any appreciable (t, p) strength. The main effect of the deformed state admixture is to provide destructive coherence in weakly excited final states. The detailed analysis of the relative cross sections for different levels provides information about the amplitude of the deformed component in the target ground state and the excited final states. Theoretical predictions are given for the reaction 18 O(p, t) 16 O.

Qualitative description of nuclear collectivity

Abstract The characteristic feature of many types of collective states in the shell model is a spatial correlation between particles and holes. Several examples are discussed, which clarify the relation between different types of collectivity.