Low-energy level schemes of 66^,68Fe and inferred proton and neutron excitations across Z=28 and N=40

Background: The nuclei in the region around 68Ni display an apparent rapid development of collectivity as protons are removed from the f_7/2 single-particle state along the N=40 isotonic chain. Proton and neutron excitations across the Z=28 and N=40 gaps are observed in odd-A ²⁷Co and ²⁶Fe isotopes. Little spectroscopic information beyond the excited 2⁺ and 4⁺ is available in the even-even 2666,68Fe nuclei to compare with shell model calculations. Purpose: Our goal is to determine the low-energy level schemes of 66^,68Fe and compare the observed excitations with shell model calculations to identify states wherein a contribution from excitations across Z=28 and N=40 are present. Method: The low-energy states of 66^,68Fe were populated through the beta decay of 66^,68Mn produced at the National Superconducting Cyclotron Laboratory. Beta-delayed gamma-ray transitions were detected and correlated to the respective parent isotope to construct a low-energy level scheme. Results: The low-energy level schemes of 66^,68Fe were constructed from observed gamma-ray coincidences and absolute gamma-ray intensities. Tentative spin and parity assignments were assigned based on comparisons with shell model calculations and systematics. The two lowest 0⁺ and 2⁺ states were characterized in terms of the number of protons and neutrons excited across the respective shell gaps. Conclusion: The removal of two protons from 68Ni to 66Fe results in an inversion of the normal configuration and the one characterized by significant excitation across the Z=28 and N=40 gaps. Approximately, one proton and two neutrons are excited across their respective single-particle gaps in the ground state of 66Fe.