Characterization of the low-lying $0{}^{+}$ and $2{}^{+}$ states in ${}^{68}\mathrm{Ni}$ via $\beta$ decay of the low-spin ${}^{68}\mathrm{Co}$ isomer

The low-energy structure of the neutron-rich nucleus ${}^{68}\mathrm{Ni}$ has been investigated by measuring the $\beta$ decay of the low-spin isomer in ${}^{68}\mathrm{Co}$ selectively produced in the decay chain of ${}^{68}\mathrm{Mn}$. A revised level scheme has been built based on the clear identification of $\beta -\gamma -E0$ delayed coincidences. Transitions between the three lowest-lying $0^{+}$ and $2^{+}$ states are discussed on the basis of measured intensities or their upper limits for unobserved branches and state-of-the-art shell model calculations.

Figure 1

Time spectrum between two consecutive signals in the plastic detectors in the time region [$-1000,-125$] ns and [$+50,+1000$] ns, outside the prompt time distribution to exclude $\beta$ scattering from one detector to another.

Figure 2

Gamma spectrum in coincidence with the decay radiation from the $0_2^{+}$ to $0_1^{+}$ in ${}^{68}\mathrm{Ni}.$ The time condition required is that the $\gamma$ ray is detected within 600 ns after one of the two plastic signals (${}^{68}\mathrm{Co}\beta$ rays or delayed $E0$ signal, see text).

Figure 3

$\beta$-gated $\gamma$-ray spectrum in the time window [350, 2200] ms after the proton pulse impinged on the target when (blue) lasers are tuned to resonantly ionize Mn and (red) lasers are off. The laser-off spectrum was scaled to the laser-on one using the 1313-keV peak from ${}^{136}\mathrm{I}$ for comparison. Symbols indicate lines associated with: ${}^{68}\mathrm{Co}$ decay (stars), ${}^{68}\mathrm{Fe}$ decay (squares), ${}^{67}\mathrm{Fe}$ and ${}^{67}\mathrm{Co}$ decay after $\beta$-delayed neutron emission (triangles), laser off ${}^{68}\mathrm{Ga}$ and ${}^{136}\mathrm{I}$ contaminants decay (circles).

Figure 4

Decay scheme of ${}^{68}\mathrm{Co}.$ All the $\beta$-feeding percentages should be considered as upper limits (see text) and the missed feeding (possibly $\beta ,E0$, or $\gamma$ feeding) was determined using the method of Ref. [27]. Tentative spin-parity assignments in brackets for higher spin states $(4{}^{-},3{}^{-},4{}^{+})$ comes from Refs. [12, 23]. When indicated with a 1u exponent, $\log ft$ values were taken assuming a first-forbidden unique transition. Asterisk $\left({}^{*}\right)$ from Ref. [11], upsidedown triangle $\left({}^{\nabla }\right)$ from Ref. [32].

Figure 5

Level scheme of ${}^{68}\mathrm{Ni}$ focused on the low-lying $0^{+}$ and $2^{+}$ states with their experimental $B\left(E2\right)$ ratios [$R\left(E_{\gamma }\right)$] normalized to ground state transitions for each level. For the $0_3^{+}$ state at 2511 keV, limits on branching ratios are quoted instead of $B\left(E2\right)$ ratios (indicated with an asterisk).

Figure 6

Potential energy surfaces (PESs) obtained from MCSM calculations [6] for the $0^{+}$ and $2^{+}$ states of ${}^{68}\mathrm{Ni},$ coordinated by the usual quadrupole moments $Q_0$ and $Q_2$ (or $\gamma )$. Circles on the PES represent shapes of MCSM basis vectors (see Ref. [6] for more details).