Collectivity along the N=Z line: Lifetime studies in self-conjugate even-even nuclei in the 0f7/2 shell

Nuclear level lifetimes and corresponding reduced transition strengths are signatures for collective properties of atomic nuclei. They allow for stringent tests of modern shell-model interactions. Self-conjugate nuclei (with equal numbers of protons and neutrons) provide special access to nuclear-structure phenomena such as the isospin symmetry. Along the N=Z line, these features can be studied for spherical nuclei in the vicinity of shell-closures and for well-deformed nuclei with various shapes. Within this work lifetimes and reduced transition strengths are investigated in 44Ti, 48,50Cr, 52Fe, and 56Ni. These nuclei are located in the 0f7/2 orbital of the lower pf shell between the doubly-magic shell closures 40Ca and 56Ni. Lifetime measurements were performed at the FN tandem accelerator at the Institute for Nuclear Physics (University of Cologne, Germany) combining a setup of twelve high-purity germanium detectors and the Cologne coincidence-plunger device. Excited states in the nuclei of interest were populated via favorable fusion-evaporation reactions. Lifetimes in the range of several hundreds femtoseconds up to one hundred picoseconds were determined utilizing the Doppler-shift attenuation method and the recoil-distance Doppler-shift technique. The deduced reduced transition probabilities expressed with B(σλ) values are compared within the systematics along their isotopic and isotonic chains. Within this comparison, the experimental findings show an enhanced collective behavior for the 2+ states in 44Ti, 48Cr, and 52Fe as well as for the 4+ and 6+ states in the doubly-magic nucleus 56Ni. Further studies on 44Ti address the impact of multi-particle multi-hole cross-shell configurations on the experimental spectrum. Lifetimes of the 3- and 4- states are determined and are confronted with previous, in part preliminary, values. Large deviations are found for the 4- state which lifetime appeared to be 28 times larger than originally measured. The finite E1 strength of the 3- -> 2+ transition gives indication of isospin-symmetry breaking in this nucleus. All experimental values are confronted with results from shell-model calculations utilizing the established GXPF1A, FPD6, and KB3G effective interactions complemented by the novel PFSDG-U interaction. Moreover, valence ab-initio calculations are performed using modern Hamiltonians derived from chiral perturbation theory with and without contributions of three-body forces. For sake of completeness, the ZBM2M interaction within the pn-symmetric 1s1/2 0d3/2 0f7/2 1p3/2 model space is used to calculate negative-parity states and to investigate cross-shell excitations in 44Ti. The newly determined B(E2) values in doubly-magic 56Ni are used to compare with theoretical predictions in the N=Z doubly-magic partner nucleus 100Sn. For this purpose, a large-scale shell-model calculation using the gds valence space as well as results from a Hartree-Fock random-phase approximation are utilized. The combined results could indicate higher B(E2; 6+ -> 4+) values for 100Sn which would contradict the assumption of a 6+ isomer in this nucleus