$\mathbf{\beta }$ decay of ${}^{71,73}$Co: Probing single-particle states approaching doubly magic ${}^{78}$Ni

Low-energy excited states in ${}^{71,73}$Ni populated via the $\beta$ decay of ${}^{71,73}$Co were investigated in an experiment performed at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University (MSU). Detailed analysis led to the construction of level schemes of ${}^{71,73}$Ni, which are interpreted using systematics and analyzed using shell-model calculations. The 5/2${}^{-}$ states attributed to the the $f_{5/2}$ orbital and positive-parity 5/2${}^{+}$ and 7/2${}^{+}$ states from the $g_{9/2}$ orbital have been identified in both ${}^{71,73}$Ni. In ${}^{71}$Ni the location of a 1/2${}^{-}$ $\beta$-decaying isomer is proposed, and limits are suggested as to the location of the isomer in ${}^{73}$Ni. The location of positive-parity cluster states are also identified in ${}^{71,73}$Ni. Beta-delayed neutron branching ratios obtained from this data are given for both ${}^{71,73}$Co.

Figure 1

The identification plot with time of flight (arbitrary units) on the horizontal axis and energy loss (arbitrary units) in a 0.5 mm PIN detector ($\Delta E$) on the vertical axis. The cobalt isotopes (${}^{71,73}$Co) reported in this work and ${}^{76m}$Ni, and ${}^{78m}$Zn used for identification have been pointed out with arrows. Rows of isotopes with the same number of protons have been labeled with their respective element symbol to give a bearing on the range of nuclei studied here.

Figure 2

Upper panel: The background-subtracted ion-$\beta$-gated $\gamma$-ray spectrum of ${}^{71}$Co is shown. The ion-$\beta$ correlation time used was 400 ms. The 1260 keV line is from $\beta$-delayed neutron emission (${\beta }_n$) of ${}^{71}$Co to ${}^{70}$Ni. All other labeled peaks correspond to the $\beta$ decay of ${}^{71}$Co to ${}^{71}$Ni. The inset illustrates the presence of the 534 keV peak in the raw spectrum but it is not observed in the background-subtracted spectrum. The blue line in the inset shows the raw spectrum and the red line is the background spectrum taken for a correlation time of 401 to 801 ms. Middle panel: A $\gamma$-$\gamma$ coincidence spectrum gated on the 813 keV peak is shown. Lower panel: A background coincidence spectrum obtained with a gate made before the 813 keV peak. The width of the background gate was the same as that of the 813 keV peak.

Figure 3

The experimental (Exp) level schemes for ${}^{69}$Ni, ${}^{71}$Ni, and ${}^{73}$Ni. The decay energy ($Q_{\beta }$) values shown were obtained from Ref. [32]. (a) The experimental level scheme for ${}^{69}$Ni as obtained from Ref. [6]. (b) The level scheme for ${}^{71}$Ni as proposed in this work. Feeding from GT allowed transitions populates the two 5/2${}^{-}$ states at 1066 and 1273 keV. This newly proposed level scheme differs from an earlier, preliminary presentation in Ref. [33]. A $\beta$-decaying isomeric state is proposed at 499 keV. (c) The new level scheme for ${}^{73}$Ni. Again the GT-allowed $\beta$ decay feeds the 5/2${}^{-}$ state at 1298 keV. The 1/2${}^{-}$ $\beta$-decaying isomeric state observed in ${}^{69}$Ni and ${}^{71}$Ni is not observed in this nucleus. All energy values assigned to the states and transitions are in keV.

Figure 4

Upper panel: The transitions populated in the granddaughter decay into ${}^{71}$Cu are shown with an ion-$\beta$ time correlation of 400 ms to 6 seconds. The 534 and 454 keV transitions are clearly observed, although there appears to be a 449 keV background peak beside the 454 keV transition. Middle panel: A background subtraction is attempted, taking 0.4–6 s and subtracting off $3\times ($6–8) s. The resulting granddaughter spectrum has a peak slightly lower than 454 keV. Lower panel: A background-subtracted ${}^{71}$Ni spectrum identical to that in Fig. 2 is shown to compare the states populated in the daughter and the granddaughter decays.

Figure 5

Upper panel: Gamma-ray spectrum from $\beta$-decay of ${}^{73}$Co. The 1095 keV line is from ${\beta }_n$ of ${}^{73}$Co to ${}^{72}$Ni, all other labeled peaks are from the $\beta$-decay of ${}^{73}$Co to ${}^{73}$Ni. Middle panel: $\gamma$-$\gamma$ coincidence gated on the 775 keV peak and background subtracted. Lower panel: Same as the middle but gated on the 285 keV $\gamma$-peak.

Figure 6

Partial level schemes for ${}^{69–77}$Ni obtained from shell-model calculations using the NR78 residual interaction [14]. The 1/2${}^{-}$, 5/2${}^{-}$, 7/2${}^{+}$, and 9/2${}^{+}$ states are all dominated by their respective single-particle wave functions due to $\nu p_{1/2}$, $\nu f_{5/2}$, and $\nu g_{9/2}$ orbitals, respectively. The dominance of the single-hole configurations ($\nu p_{1/2}$)${}^{-1}$ and ($\nu f_{5/2}$)${}^{-1}$ for the 1/2${}^{-}$ and 5/2${}^{-}$ levels, respectively, is shown as a percentage of total wave-function contribution for each state. All energies assigned to the respective states are in keV.

Figure 7

The upper limits of $\beta {}_n$-emission branching ratios for the indicated nuclei are plotted against the difference between the $\beta$-decay $Q$ value and neutron separation energy. The values for ${}^{73}$Co and ${}^{75}$Co (triangles) were obtained from Ref. [47] and the value for ${}^{69}$Co (circle) was obtained from Ref. [32].