A. F. Lisetskiy

Proton–neutron structure of the N=52 nucleus 92Zr

Abstract Following the successful identification of mixed-symmetric one- and two-phonon states in the N=52 nuclei 94Mo and 96Ru, we have performed a photon scattering experiment on the N=52 isotone 92Zr. Experimental data and shell model calculations show that both, single particle and collective degrees of freedom are present in the low-lying levels of 92Zr. The second excited quadrupole state shows the signatures of the one-phonon mixed-symmetric 2+ state, while calculations and data indicate an almost pure neutron configuration for the 2+1 state, in contradiction with the F-spin symmetric limit. Furthermore, two strong dipole excitations, which are candidates for the two-phonon quadrupole–octupole coupled E1 excitation and for the mixed-symmetric 1+ two-phonon state, were observed.

Half-lives and branchings for {beta}-delayed neutron emission for neutron-rich Co-Cu isotopes in the r-process

The {beta} decays of very neutron-rich nuclides in the Co-Zn region were studied experimentally at the National Superconducting Cyclotron Laboratory using the NSCL {beta}-counting station in conjunction with the neutron detector NERO. We measured the branchings for {beta}-delayed neutron emission (P{sub n} values) for {sup 74}Co (18{+-}15%) and {sup 75-77}Ni (10{+-}2.8%, 14{+-}3.6%, and 30{+-}24%, respectively) for the first time, and remeasured the P{sub n} values of {sup 77-79}Cu, {sup 79,81}Zn, and {sup 82}Ga. For {sup 77-79}Cu and for {sup 81}Zn we obtain significantly larger P{sub n} values compared to previous work. While the new half-lives for the Ni isotopes from this experiment had been reported before, we present here in addition the first half-life measurements of {sup 75}Co (30{+-}11 ms) and {sup 80}Cu (170{sub -50}{sup +110} ms). Our results are compared with theoretical predictions, and their impact on various types of models for the astrophysical rapid neutron-capture process (r-process) is explored. We find that with our new data, the classical r-process model is better able to reproduce the A=78-80 abundance pattern inferred from the solar abundances. The new data also influence r-process models based on the neutrino-driven high-entropy winds in core collapse supernovae.

Shell model description of “mixed-symmetry” states in 94Mo

Abstract Shell model calculations have been performed for the nucleus 94 Mo. The calculated excitation energies and electromagnetic properties of low-lying states are in good agreement with the data, which include states with mixed-symmetry (MS) assignments in previous interacting boson model studies. In the shell model large isoscalar E2 matrix elements are found between states with MS assignments indicating that they form a class of states with similar proton-neutron symmetry.

First observation of a mixed-symmetry two-Q-phonon 22,ms+ state in 94Mo

Abstract A two-Q-phonon mixed-symmetry Jπ=22,ms+ state was identified in the nucleus 94Mo. This identification is based on lifetime information for the first nine 2+ states. The proton–neutron version of the interacting boson model (IBM-2) suggests the interpretation of the 26+ state of 94Mo as a two-Q-phonon 22,ms+ state. Recent microscopic calculations support this interpretation.

Ab initio shell model with a core

We construct effective 2- and 3-body Hamiltonians for the p-shell by performing 12{h_bar}{Omega} ab initio no-core shell model (NCSM) calculations for A=6 and 7 nuclei and explicitly projecting the many-body Hamiltonians onto the 0{h_bar}{Omega} space. We then separate these effective Hamiltonians into 0-, 1- and 2-body contributions (also 3-body for A=7) and analyze the systematic behavior of these different parts as a function of the mass number A and size of the NCSM basis space. The role of effective 3- and higher-body interactions for A > 6 is investigated and discussed.

Quasideuteron configurations in odd-oddN=Znuclei

The isovector M1 transitions between low-lying T=1 and T=0 states in odd-odd N=Z nuclei are analyzed. Simple analytical expressions for M1 transition strengths are derived within a single-j-shell approximation for both j=l+1/2 and j=l-1/2 cases. The large B(M1) values for the j=l+1/2 case are attributed to quasi-deuteron configurations. The B(M1) values for the j=l-1/2 case are found to be small due to partly cancellation of spin and orbital parts of the M1 matrix element.

Quasideuteron states with deformed core

Abstract The M1 transitions between low-lying T =1 and T =0 states in deformed odd–odd N = Z nuclei are analyzed in the frames of the rotor-plus-particle model. Using the representation of an explicit coupling of angular momenta we show that strong coupling of the quasideuteron configurations to the axially deformed core results in a distribution of the total 0 + →1 + strength among a few low-lying 1 + states. Simple analytical formulae for B(M1) values are derived. The realization of the M1 sum rule for the low-lying 1 + , T =0 states is indicated. The calculated B(M1) values are found to be in good agreement with experimental data and reveal specific features of collectivity in odd–odd N = Z nuclei.

Approaching rotational collectivity in odd-odd NZ nuclei in pf-shell

Abstract The analysis of shell model wave functions and new experimental data for odd-odd NZ nuclei 46V and 50Mn is presented in terms of collective rotor-plus-quasideuteron model. The low-lying low-spin states exhibiting collective properties appropriate for the K=0, K=3, and K=5 bands are identified. The strong isovector M1 transitions of quasideuteron origin in 46V and 50Mn are shown to be important indicators of collective features and to give more complete information on the structure of the intrinsic states than E2 matrix elements.

Quasideuteron configurations in 46V and 58Cu

The data on low spin states in the odd-odd nuclei 46V and 58Cu investigated with the 46Ti(p,ngamma)46V, 32S(16O,pn)46V and 58Ni(p,ngamma)58Cu reactions at the FN-TANDEM accelerator in Cologne are reported. The states containing large quasideuteron components are identified from the strong isovector M1 transitions, from shell model calculations and from experimental data for low-lying states.

Low spin structure of odd-odd N=Z nuclei

Isovector M1 transitions between low-lying T=1 and T=0 states in odd-odd N=Z nuclei are discussed. Simple analytical expressions for M1 transition strengths are given in single-j-shell approximation. The large B(M1) values for the j = l + 12 case are attributed to quasi-deuteron configurations. The B(M1) values for the j = l − 12 case are found to be small due to a cancellation of spin and orbital parts of the M1 matrix element. The experimental data on low spin states in the odd-odd nuclei 54Co and 46V investigated with the 54Fe(p, nγ)54Co and 46Ti(p, nγ)46V fusion reactions at the EN-tandem accelerator in Cologne are reported. The Jπ = 01+, 21+, 41+ T=1 and Jπ = 11+, 31+, T=0 states in 54Co are identified as predominantly πf72−1 × νf72−1quasi-deuteron states.