Analogous intruder behavior near Ni, Sn, and Pb isotopes

Near shell closures, the presence of unexpected states at low energies provides a critical test of our understanding of the atomic nucleus. New measurements for the $N=42$ isotones ${}_{27}{}^{69}\mathrm{Co}$ and ${}_{29}{}^{71}\mathrm{Cu}$, along with recent data and calculations in the Ni isotopes, establish a full set of complementary, deformed, intruder states astride the closed-shell ${}_{28}\mathrm{Ni}$ isotopes. Nuclei with a one-proton hole or one-proton particle adjacent to $Z=28$ were populated in $\beta$-decay experiments and in multinucleon transfer reactions. A $\beta$-decaying isomer, with a 750(250)-ms half-life, has been identified in ${}_{27}{}^{69}\mathrm{Co}_{42}$. It likely has low spin and accompanies the previously established $7/2^{-}$ state. Complementary data for the levels of isotonic ${}_{29}{}^{71}\mathrm{Cu}_{42}$ support the presence of a deformed, $\mathrm{\Delta }J=1$ band built on the proton intruder $7/2^{-}$ level at 981 keV. These data, together with recent studies of lower-mass Co and Cu isotopes and extensive work near ${}^{68}\mathrm{Ni}$, support the view that intruder states based on particle-hole excitations accompany all closed proton shells with $Z\ge 28$.

Figure 1

Top: Single-particle level ordering for the In and Sb isotopes. Normal proton configurations in In (Sb) consist of a proton hole (particle) below (above) $Z=50$. Schematic particle-hole excitations in the In and Sb isotopes are also shown; see text for details. Bottom: The energies of the Sb $9/2^{+}$ and In $1/2^{+}$ intruder levels plotted with respect to their respective ground states. The experimentally observed spins and parities of the intruder states are obtained from the projection of the angular momentum on the symmetry axis.

Figure 2

(a) $\beta$-delayed $\gamma$ rays detected within one second of the arrival of a ${}^{69}\mathrm{Co}$ ion at the experimental station. The transitions in ${}^{69}\mathrm{Ni}$ are marked by their respective energies. Inset: $\gamma$-ray energy spectrum between 1040 and 1500 keV. The circle indicates a transition assigned in ${}^{69}\mathrm{Cu}$ following the decay of ${}^{69}\mathrm{Ni}$ [24]. (b) $\beta$-delayed $\gamma$-ray transitions identified within one second of the arrival of a ${}^{69}\mathrm{Fe}$ ion. Transitions attributed to the decay of ${}^{69}\mathrm{Fe}$ are marked by their respective energies. Arrows mark the locations of the ${}^{69}\mathrm{Co} \beta$-delayed $\gamma$ rays at 1128 and 1319 keV.

Figure 3

(a) Decay curve in coincidence with the 594-keV transition following the decay of directly produced ${}^{69}\mathrm{Co}$, fit with an exponential decay and a constant background. (b) Sum of the ${}^{69}\mathrm{Fe}$ decay curves generated from gates placed on the 250-, 291-, and 446-keV transitions fit with an exponential decay and constant background. (c) Decay curve in coincidence with the 594-keV transition following the decay of ${}^{69}\mathrm{Co}$ produced from ${}^{69}\mathrm{Fe}$. The curve is fit with the growth and decay of ${}^{69}\mathrm{Co}$ and two background components. (d) The total decay curve for ${}^{69}\mathrm{Fe}$ with components attributed to the decay of ${}^{69}\mathrm{Fe}$, two states in ${}^{69}\mathrm{Co},{}^{69}\mathrm{Ni}$, and a constant background. The grow-in and decay curve for ${}^{69}\mathrm{Ni}$ is explicitly separated into two components corresponding to the two ${}^{69}\mathrm{Co} \beta$-decaying states.

Figure 4

Location of the $\gamma$ rays identified in the present work in the level scheme of ${}^{71}\mathrm{Cu}$, establishing the band to a spin of $(17/2^{-})$. The lifetime of the 2756-keV $19/2^{-}$ state was taken from [29].

Figure 5

(top) Single-particle level ordering for the Co and Cu isotopes. “Normal” proton configurations in Co (Cu) consist of a proton hole (particle) below (above) $Z=28$. Schematic particle-hole excitations in the Co, Ni, and Cu isotopes are also shown; see text for details. The experimentally observed spins and parities of the deformed intruder states are obtained from the projection of the angular momentum on the symmetry axis. (bottom) The energies of the Cu $\left(7/2^{-}\right)$ (copper-colored) and Co $\left(1/2^{-}\right)$ (blue) intruder levels with respect to their ground states. The energy of the Co intruder states were taken from Refs. [30, 31]. The range of values for the energy difference between the two states in ${}^{69}\mathrm{Co}$ were determined from two different assumptions for the strength of the unobserved $M3 \gamma$-ray transition; see text for details. The energies of the experimentally observed intruding $2^{+}$ levels in the even-even Ni isotopes are shown divided by two (black solid line) along with theoretical calculations (black dashed line) [17, 47].