$\beta$ decay of ${}^{75}\mathrm{Ni}$ and the systematics of the low-lying level structure of neutron-rich odd-$A$ Cu isotopes

Background: Detailed spectroscopy of neutron-rich odd-$A$ Cu isotopes is of great importance for studying the shell evolution in the region of ${}^{78}\mathrm{Ni}$. While there is experimental information on excited states in ${}^{69-73,77,79}\mathrm{Cu}$ isotopes, the information concerning ${}^{75}\mathrm{Cu}$ is very limited.

Purpose: Experimentally observed single-particle, core-coupling, and proton-hole intruder states in ${}^{75}\mathrm{Cu}$, will complete the systematics of these states in the chain of isotopes.

Method: Excited states in ${}^{75}\mathrm{Cu}$ were populated in the $\beta$ decay of ${}^{75}\mathrm{Ni}$ isotopes. The Ni nuclei were produced by the in-flight fission of ${}^{238}\mathrm{U}$ projectiles, and were separated, identified, and implanted in a highly segmented Si detector array for the detection of the $\beta$-decay electrons. The $\beta$-delayed $\gamma$ rays were detected in a HPGe cluster array. Monte Carlo shell model calculations were performed using the A3DA interaction built on the $pf{g}_{9/2}{d}_{5/2}$ model space for both neutrons and protons.

Results: A level scheme of ${}^{75}\mathrm{Cu}$ was built up to $\approx 4$ MeV by performing a $\gamma \text{−}\gamma$ coincidence analysis. The excited states below 2 MeV were interpreted based on the systematics of neutron-rich odd-$A$ Cu isotopes and the results of the shell model calculations.

Conclusions: The evolution of the single-particle, core-coupling, and proton-hole intruder states in the chain of neutron-rich odd-$A$ Cu isotopes is discussed in the present work, in connection with the newly observed level structure of ${}^{75}\mathrm{Cu}$.

Figure 1

Singles spectrum of $\gamma$ rays measured in coincidence with the first position-correlated electrons detected within 2.5 s after the implantation of ${}^{75}\mathrm{Ni}$ ions. Transitions labeled by their energy only were assigned to ${}^{75}\mathrm{Cu}$ and have been placed in the level scheme of Fig. 4. Transitions labeled by their energy in parentheses were tentatively assigned to ${}^{75}\mathrm{Cu}$, but could not be placed in the level scheme because of insufficient coincidence relations. Transitions identified to originate from isotopes other than ${}^{75}\mathrm{Cu}$ are labeled with the respective symbol of the nuclide. Transitions that could not be assigned to any specific nuclide are labeled with their energy followed by an asterisk ($★$).

Figure 2

Time difference between implantation of ${}^{75}\mathrm{Ni}$ ions and detection of electrons inside the area of spatial correlation. The various curves show the fit of contributions from individual decays based on known half-lives and probability for $\beta$-delayed neutron emission (see text for more details).

Figure 3

Background-subtracted $\gamma$-ray spectra gated on the four strongest transitions of ${}^{75}\mathrm{Cu}$ with energies 992, 884, 1484, and 506 keV. All transitions which are labeled by their energy have been placed in the level scheme of Fig. 4, except those at 908 and 1557 keV (labeled by $★$).

Figure 4

Level scheme of ${}^{75}\mathrm{Cu}$. The energies of the states and the transitions are given in keV, and the uncertainties are within 1 keV. The relative intensities of the transitions (in brackets) are normalized to the 992-keV transition and corrected for internal conversion. Lower and upper limits are given for the relative intensity of the 66.2-keV transition. For the discussion of the $\beta$-decay branching ratios ($I_{\beta }$) to the 61.8- and 66.2-keV states, and to the ground state, see the text (Sec. 3).

Figure 5

Background-subtracted $\gamma$-ray spectra gated on the 884-, 950-, 1417-, and 1484-keV transitions in the energy range of the two isomeric transitions of 61.8 and 66.2 keV.

Figure 6

(Left) Experimental energy states of ${}^{75}\mathrm{Cu}$ with assigned spins and parities. (Right) MCSM calculations.

Figure 7

The circles drawn on the potential energy surface of the nucleus indicate the shapes of the MCSM basis vectors of calculated excited states of ${}^{75}\mathrm{Cu}$. See Ref. [38] for details.

Figure 8

Systematics of the energies of the first $1/2^{-}$ states (red circles), and $B(E2;1/2^{-}\to g.s.)$ values (blue squares) in odd-$A {}^{69-79}\mathrm{Cu}$ isotopes [10, 18, 20]. Results from the MCSM calculations (open symbols) are shown together with experimental values (filled symbols). For ${}^{79}\mathrm{Cu}$, the assignment of the $1/2^{-}$ state was based on the results of the MCSM calculations (see Ref. [18]).

Figure 9

Systematics of the energies of particle-core coupling states in odd-$A {}^{69-77}\mathrm{Cu}$ isotopes [8, 9, 15]. The levels corresponding to the Ni cores [48, 49, 50, 51, 52, 53, 54, 55] are shown in dashed lines. The $3/2^{-}, 7/2^{-}$, and $11/2^{-}$ states (in blue), can be associated with the $|\pi 2p_{3/2}\otimes 0_1^{+},2_1^{+},4_1^{+}\rangle$ configurations, respectively, while the $5/2^{-}, 9/2^{-}$, and $13/2^{-}$ states (in red) can be associated with the $|\pi 1f_{5/2}\otimes 0_1^{+},2_1^{+},4_1^{+}\rangle$ configurations, respectively. In ${}^{73}\mathrm{Cu}$, the spin assignment of the state at 1287 keV is not clear, but the systematics suggest an important $|\pi 1f_{5/2}\otimes 2_1^{+}\rangle$ component in its wave function.

Figure 10

Systematics of the decay sequences of particle-core coupling states in odd-$A {}^{69-77}\mathrm{Cu}$ isotopes [8, 9, 15]. The widths of the transitions correspond with the relative intensities, normalized to the strongest transition shown in each isotope. In ${}^{71}\mathrm{Cu}$, the $11/2^{-}\to 9/2^{-}$ transition was only observed in Ref. [15] and its intensity was normalized according to the observed branching ratio.

Figure 11

(a) Systematics of the intruder states in odd-$A {}^{69-77}\mathrm{Cu}$ isotopes. The widths of the $11/2^{-}\to 9/2^{-}$ transitions are normalized to those of the $9/2^{-}\to 7/2^{-}$ transitions in each isotope. The relative intensities were taken from the $\beta$-decay experiment of Ref. [9], except for ${}^{69}\mathrm{Cu}$, where the $11/2^{-}$ state was only seen in Ref. [14]. (b) Intruder $0^{+}$ states in the corresponding Ni cores. The experimental $0_{\text{Exp}}^{+}$ states in ${}^{68,70}\mathrm{Ni}$ are those in Refs. [48, 50]. The MCSM values of the $0_{\text{Th}}^{+}$ energies have been previously presented in Ref. [38].