Measurement of lifetimes in ${}^{62,64}\mathrm{Fe},{}^{61,63}\mathrm{Co}$, and ${}^{59}\mathrm{Mn}$

Lifetimes of the $4_1^{+}$ states in ${}^{62,64}\mathrm{Fe}$ and the $11/2_1^{-}$ states in ${}^{61,63}\mathrm{Co}$ and ${}^{59}\mathrm{Mn}$ were measured at the Grand Accélérateur National d'Ions Lourds (GANIL) facility by using the Advanced Gamma Tracking Array (AGATA) and the large-acceptance variable mode spectrometer (VAMOS++). The states were populated through multinucleon transfer reactions with a ${}^{238}\mathrm{U}$ beam impinging on a ${}^{64}\mathrm{Ni}$ target, and lifetimes in the picosecond range were measured by using the recoil distance Doppler shift method. The data show an increase of collectivity in the iron isotopes approaching $N=40$. The reduction of the subshell gap between the $\nu 2p_{1/2}$ and $\nu 1g_{9/2}$ orbitals leads to an increased population of the quasi-SU(3) pair $\left(\nu 1g_{9/2},\nu 2d_{5/2}\right)$, which causes an increase in quadrupole collectivity. This is not observed for the cobalt isotopes with $N<40$ for which the neutron subshell gap is larger due to the repulsive monopole component of the tensor nucleon-nucleon interaction. The extracted experimental $B\left(E2\right)$ values are compared with large-scale shell-model calculations and with beyond-mean-field calculations with the Gogny D1S interaction. A good agreement between calculations and experimental values is found, and the results demonstrate in particular the spectroscopic quality of the Lenzi, Nowacki, Poves, and Sieja (LNPS) shell-model interaction.

Figure 1

Energy loss of the target-like reaction products in VAMOS++ as a function of total energy detected, with cuts on manganese (dotted), iron (dash-dotted), cobalt (dashed), and nickel (solid) isotopes.

Figure 2

Mass distributions for the manganese, iron, and cobalt isotopic chains.

Figure 3

$\gamma$-ray spectra in coincidence with ions identified as ${}^{59}\mathrm{Mn},{}^{62,64}\mathrm{Fe}$, and ${}^{61,63}\mathrm{Co}$, summed over all six distances.

Figure 4

Schematic level scheme illustrating the feeding and decay of the level of interest. Unobserved feeding is accounted for by introducing a fictive level (marked with v).

Figure 5

The spectra used to extract the lifetime of the $11/2_1^{-}$ state in ${}^{59}\mathrm{Mn}$ and ${}^{61,63}\mathrm{Co}$ and the $4_1^{+}$ in ${}^{62,64}\mathrm{Fe}$ for the different distances $d$ (in $\mu \mathrm{m})$. Shown as color coded areas are the intensities used for determining the decay curves. The area to the left is the shifted component while the right area is the unshifted component.

Figure 6

The decay curves used to extract the lifetime of the $11/2_1^{-}$ states in ${}^{59}\mathrm{Mn}$ and ${}^{61,63}\mathrm{Co}$ and the $4_1^{+}$ state in ${}^{62,64}\mathrm{Fe}$. Note that, for ${}^{62}\mathrm{Fe}$, two observed feeder states have been included in the fit. For ${}^{64}\mathrm{Fe}$ the fit used to extract the upper limit on the lifetime is shown.

Figure 7

$B(E2\downarrow )$ values for even iron isotopes for transitions from the first $2^{+}$ and the first $4^{+}$ state. The solid green lines with circles are shell-model calculations and the blue dashed lines with diamonds are D1S calculations, both a part of the present work. Previous experimental results with error bars are from Refs. [8, 25, 26] and Nuclear Data Sheets (NDS) [54, 55, 56], while the red stars are the new experimental results presented here.

Figure 8

$B(E2\downarrow )$ values for odd cobalt isotopes for transitions from the first $11/2^{-}$ to the ground state. The green solid line with circles represents shell-model calculations performed within this work. Previous experimental results with error bars are from Refs. [18, 36] and the compilations [47, 62] while the red stars are the new experimental results presented here. For comparison, the $B(E2\downarrow )$ values for the first $2^{+}$ state in the even nickel isotopes are shown also.