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

Background: The nuclei in the region around ${}^{68}$Ni 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 ${}_{27}$Co and ${}_{26}$Fe isotopes. Little spectroscopic information beyond the excited 2${}^{+}$ and 4${}^{+}$ is available in the even-even  ${}_{26}^{66,68}\mathrm{Fe}$ nuclei to compare with shell model calculations.

Purpose: Our goal is to determine the low-energy level schemes of ${}^{66,68}$Fe 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 ${}^{66,68}$Fe were populated through the beta decay of ${}^{66,68}$Mn 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 ${}^{66,68}$Fe 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 ${}^{68}$Ni to ${}^{66}$Fe 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 ${}^{66}$Fe.

Figure 1

(a) The beta-delayed gamma-ray energy spectrum observed within 500 ms following the implantation of a ${}^{66}$Mn ion. Gamma rays attributed to the decay of ${}^{66}$Mn are labeled with inverted triangles and are listed in Table . Gamma rays attributed to daughter and granddaughter activities are marked by circles. Gamma rays associated with the decay of nuclei populated through beta-delayed neutron emission are indicated by squares. The unlabeled “peak” at approximately 2800 keV is due to the overflow signal on one of the individual SeGA detectors. (b) The gamma-gamma coincidence spectrum gated by the 840-keV transition. Coincidences with the 573- and 1461-keV transitions are indicated.

Figure 2

The beta-decay curve for ${}^{66}$Mn from 0 to 1 second. The overall fit was composed of contributions from the beta decay of ${}^{66}$Mn, ${}^{66}$Fe, ${}^{66}$Co, and a constant background. Inset: The beta-decay curve for ${}^{66}$Mn from 0 to 600 ms detected in coincidence with the observation of a 573-keV gamma ray.

Figure 3

(Left) Low-energy level scheme of ${}^{66}$Fe inferred from the beta decay of ${}^{66}$Mn. Tentative spin and parity assignments, apparent beta-decay branching ratios, and ${\log }_{10}ft$ values are given on the left hand side of each state. The beta-decay $Q$ value was taken from a recent mass measurement [35]. (Right) Shell model calculations for ${}^{66}$Fe; see text for details.

Figure 4

(a) The beta-delayed gamma-ray energy spectrum detected within 300 ms following a ${}^{68}$Mn implanted ion. Gamma rays attributed to the decay of ${}^{68}$Mn are labeled with inverted triangles. Gamma rays attributed to daughter and granddaughter activity are marked by circles. Gamma rays associated with the decay of nuclei populated following beta-delayed neutron emission are indicated by squares. (b) The beta-decay curve for ${}^{68}$Mn from 0 to 1 second. The overall fit (black) was composed of contributions from the beta-decay of ${}^{68}$Mn, ${}^{68}$Fe, ${}^{68}$Co, and a constant background.

Figure 5

(Left) Low-energy level scheme of ${}^{68}$Fe inferred from the beta decay of ${}^{68}$Mn. Beta-decay branching ratios and apparent ${\log }_{10}ft$ values are given on the left-hand side of each state. The beta-decay $Q$ value of the decay was taken as 14 530 keV from Ref. [34]. (Right) Shell model calculations for ${}^{68}$Fe; see text for details. All calculated states below 3.5 MeV are shown. Above 3.5 MeV only the lowest excited states of a given spin and parity are shown.

Figure 6

Comparisons between the experimental level schemes below 2.5 MeV with the calculated energies for the low-energy 0${}^{+}$ and 2${}^{+}$ states in ${}_{28}^{68}$Ni${}_{40}$, ${}_{26}^{66}$Fe${}_{40}$, and ${}_{26}^{68}$Fe${}_{42}$. The model space used in the calculations is shown at the top of the figure. For the calculated states, the number of protons excited out of the $f_{7/2}$ single-particle state and the the number of neutrons excited into the $g_{9/2}$ single-particle state are shown schematically to the right of the theoretical state. See text for details.