Mapping of fragmented $\nu f_{5/2}\to \pi f_{7/2}$ transitions in the ${}^{73}\mathrm{Co}\to {}^{73}\mathrm{Ni}$ decay

Excited states in ${}^{73,75}\mathrm{Ni}$ were investigated through the $\beta$ decay of ${}^{73,75}\mathrm{Co}$ in an experiment performed at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University (MSU). The experimental results extended the level scheme of ${}^{73}\mathrm{Ni}$ to 3.2-MeV excitation energy and provided the experimental information on excited states in ${}^{75}\mathrm{Ni}$. The $\beta$-delayed neutron branching ratio for ${}^{73}\mathrm{Co}$ was obtained. The experimental results are discussed in comparison with shell-model calculations.

Figure 1

Schematic drawing for the Gamow-Teller (GT) transitions in Co isotopes. The transitions from the neutron to the proton proceed between their spin-orbit partners.

Figure 2

(Top) $\beta$-gated $\gamma$-ray energy spectrum for the decay of ${}^{73}\mathrm{Co}$ up to 3.6 MeV (correlation time 120 ms, background subtracted). The 454-, 843-, and 1095-keV peaks correspond to transitions in ${}^{72}\mathrm{Ni}$ populated in the $\beta n$ channel. The 1460-keV line labeled with BG corresponds to the known ${}^{40}\mathrm{K}$ natural background line. (Bottom) $\beta -\gamma -\gamma$ coincidence spectra for the decay of ${}^{73}\mathrm{Co}\to {}^{73}\mathrm{Ni}$. From top to bottom, the spectra are gated on the 239-, 284-, 524-, and 774-keV transitions.

Figure 3

The partial decay scheme of ${}^{73}\mathrm{Co}$ as obtained in this work. The decay energy ($Q_{\beta }$) value stems from Ref. [31]. See text for details.

Figure 4

(Top) $\beta$-gated $\gamma$-ray energy spectrum for the decay of ${}^{75}\mathrm{Co}$. The black and red lines show the spectra up to 100-ms correlation time and background (200–300 ms), respectively. (Bottom) Time distribution of the $\beta$-decay events of ${}^{75}\mathrm{Co}$ correlated with the 232-keV transition.

Figure 5

The partial decay scheme of ${}^{75}\mathrm{Co}$ obtained in this work. See text for details.

Figure 6

(Left) ${}^{73}\mathrm{Ni}$ levels from shell-model calculations using the $fpgpn$ interactions with ${}^{40}\mathrm{Ca}$ core [11] and corresponding $B$(GT) strength distribution from ${}^{73}\mathrm{Co}$. Positive- and negative-parity states are colored in black and red, respectively. The $B$(GT) values are plotted without any quenching factor. The experimental values correspond to $B$(GT) $<0.13$ and $B$(GT) $<0.06{\mathrm{MeV}}^{-1}$ for the first and second excited states. (Right) Three panels show occupancies for the neutron single-particle levels for three different spins.

Figure 7

(Left) Excitation energies and calculated $B$(GT) strengths for $5/2_{1,2,3}^{-}$ states as a function of energy between single-particle $f_{5/2}$ and $p_{3/2}$. Each color corresponds to the $5/2_{1,2,3}^{-}$ states. Black line represents sums of $B$(GT) for the second and third $5/2^{-}$ states. (Right) Occupations for $f_{5/2}$ and $p_{1/2}$ orbitals for the $5/2_{1,2,3}^{-}$ states. Black line represents average occupations for second and third $5/2^{-}$ states.

Figure 8

Calculated levels by the $fpgpn$ interaction [11] with ${}^{40}\mathrm{Ca}$ core and $jj44pna$ [34] interactions with ${}^{56}\mathrm{Ni}$ core. The positive- and negative-parity states are in black and red.

Figure 9

Comparison of experimental levels for odd-$A$ nickel isotopes. Levels for ${}^{69}\mathrm{Ni}$ stem from Ref. [10], while part of the levels for ${}^{71,73}\mathrm{Ni}$ are obtained from Ref. [23]. The levels with $(5/2_1^{-})$ and $(7/2^{+})$ are in red and blue, respectively.

Figure 10

Neutron emission probabilities ($P_n$) for the indicated nuclei as a function of the difference between the $\beta$-decay $Q$ value and neutron separation energy $S_n$. The $Q_{\beta }$ and $S_n$ are taken from Ref. [36]. The experimental upper limits were taken from Ref. [23] and Ref. [33]. The green squares correspond to predicted $P_n$ values from FRDM calculations [36].