Investigating the $\gamma$ decay of ${}^{65}\mathrm{Ni}$ from particle-$\gamma$ coincidence data

The $\gamma$ decay of ${}^{65}\mathrm{Ni}$ has been studied from particle-$\gamma$ coincidence data on the ${}^{64}\mathrm{Ni}(d,p\gamma ){}^{65}\mathrm{Ni}$ reaction. $\gamma$-ray spectra at excitation energies below $E_x\approx 2\mathrm{MeV}$ have been studied and compared with previous measurements. Coincidences corresponding to $E_x\approx 4.4–6.1\mathrm{MeV}$ have been used to constrain the shape of the nuclear level density and $\gamma$-strength function of ${}^{65}\mathrm{Ni}$ by means of the Oslo method. The experimental $\gamma$-strength function presents an enhancement at $\gamma$ energies below $E_{\gamma }\approx 3\mathrm{MeV}$. In addition, a resonance-like structure centered at $E_{\gamma }\approx 4.6\mathrm{MeV}$ is seen together with accumulated strength at $E_{\gamma }\approx 2.6–3.6\mathrm{MeV}$. The obtained results contribute to the systematic study of $\gamma$ decay in the Ni isotopes, which is of great interest for the understanding of both single-particle and collective nuclear structure phenomena.

Figure 1

Original (a), unfolded (b), and first-generation (c) coincidence matrices for ${}^{65}\mathrm{Ni}$ from the ${}^{64}\mathrm{Ni}(d,p\gamma ){}^{65}\mathrm{Ni}$ reaction. The $x$ axis represents the $\gamma$ energy while the $y$ axis gives the excitation energy of ${}^{65}\mathrm{Ni}$. The number of counts is represented by the color scale.

Figure 2

Comparison between raw and unfolded $\gamma$-ray spectra from $E_x=1.017\mathrm{MeV}$.

Figure 3

Section of the unfolded coincidence matrix for ${}^{65}\mathrm{Ni}$ (a) compared to the first-generation matrix (b). The peaks marked with a $★$ correspond to removed secondary transitions: for instance, part (a) shows the decay cascade $E_x=1609\to 310\to 0$ keV, while in part (b) the secondary transition $310\to 0$ keV has been removed and only the peak for the $E_x=1609\to 310$ keV transition is seen.

Figure 4

Comparison between the first-generation matrix for ${}^{65}\mathrm{Ni}$ projected for a given $E_x$, and the corresponding product $\rho (E_x-E_{\gamma })\mathcal{T}\left(E_{\gamma }\right)$.

Figure 5

Normalized NLD for ${}^{65}\mathrm{Ni}$ using the CT model and the BSFG model for interpolation between the experimental data obtained in this work and the estimated value of $\rho \left(S_n\right)$. The bin width is 124 keV.

Figure 6

The $\gamma \mathrm{SF}$ of ${}^{65}\mathrm{Ni}$ together with its upper and lower limits. The error bars include statistical uncertainties plus systematic errors from the unfolding and extraction of the primary $\gamma$ rays.

Figure 7

Partial level scheme with some of the primary $\gamma$-ray transitions observed in this work. The $E_x=1920\to 693$ keV primary transition, of tentative $E1$ character according to the spin-parity assignments from Ref. [52], is marked in red. Note that the width of the arrows is not related to the decay intensities.

Figure 8

$\gamma \mathrm{SF}$ as obtained from different regions in the first-generation coincidence matrix. Part (a) shows the considered regions in the coincidence matrix while part (b) shows the resulting $\gamma \mathrm{SF}$. The total region corresponds to the standard $\gamma \mathrm{SF}$ as detailed in Sec. 3b.

Figure 9

Experimental excitation energies of ${}^{65}\mathrm{Ni}$ obtained from single-particle spectra (emitted protons). The $★$ indicates the excitation energy where the $E_x=1594$ keV and $E_x=1772$ keV levels should appear according to Ref. [52]. No evidence for these levels was found in the present work.

Figure 10

The NLD of ${}^{65}\mathrm{Ni}$ compared to the results for ${}^{64}\mathrm{Ni}$ [37] and the NLD of ${}^{59–63}\mathrm{Ni}$ given by Ref. [74].

Figure 11

The $\gamma \mathrm{SF}$ of ${}^{65}\mathrm{Ni}$ obtained in this work compared to the results for ${}^{64}\mathrm{Ni}$ [37] and ${}^{68}\mathrm{Ni}$ [29].