Enhanced low-energy $\gamma \text{-decay}$ strength of ${}^{70}\mathrm{Ni}$ and its robustness within the shell model

Neutron-capture reactions on very neutron-rich nuclei are essential for heavy-element nucleosynthesis through the rapid neutron-capture process, now shown to take place in neutron-star merger events. For these exotic nuclei, radiative neutron capture is extremely sensitive to their $\gamma$-emission probability at very low $\gamma$ energies. In this work, we present measurements of the $\gamma \text{-decay}$ strength of ${}^{70}\mathrm{Ni}$ over the wide range $1.3\le E_{\gamma }\le 8$ MeV. A significant enhancement is found in the $\gamma \text{-decay}$ strength for transitions with $E_{\gamma }<3$ MeV. At present, this is the most neutron-rich nucleus displaying this feature, proving that this phenomenon is not restricted to stable nuclei. We have performed $E1$-strength calculations within the quasiparticle time-blocking approximation, which describe our data above $E_{\gamma }\simeq 5$ MeV very well. Moreover, large-scale shell-model calculations indicate an $M1$ nature of the low-energy $\gamma$ strength. This turns out to be remarkably robust with respect to the choice of interaction, truncation, and model space, and we predict its presence in the whole isotopic chain, in particular the neutron-rich ${}^{72,74,76}\mathrm{Ni}$.

Figure 1

(a) ${}^{70}\mathrm{Ni}$ raw $\gamma$ spectra versus $E_x$; (b) unfolded $\gamma$ spectra corrected for the SuN response functions for both $E_{\gamma }$ and $E_x$; (c) distribution of primary $\gamma$ rays for each $E_x$ bin. The dashed lines show the region used for the analysis. The pixels are 200 keV wide.

Figure 2

Matrix of primary $\gamma$ rays as a function of excitation energy. The dashed lines show the direct decay to the low-energy levels marked with their spin/parity assignment (see text). The bin width is 100 keV both on the $\gamma$-ray and $E_x$ axis.

Figure 3

Projections of the unfolded $E_x\text{−}{E}_{\gamma }$ matrix onto (a) the $E_x$ axis, and (b) the $E_{\gamma }$ axis for a gate on $E_x=3.4–3.8$ MeV (dashed lines in the top panel). Transitions are labeled with their $\gamma$-ray energies in keV.

Figure 4

(a) Extracted NLD for ${}^{70}\mathrm{Ni}$ from the analysis in Ref. [33] (open squares) and the present $E_x$-unfolded SuN data (blue points) with upper/lower limits (blue shaded area) and the HFB+c calculations used for normalization (dashed line). (b) Extracted $\gamma \mathrm{SF}$ for ${}^{70}\mathrm{Ni}$. The data of Rossi et al. [52] on ${}^{68}\mathrm{Ni}$ (red points) are used for normalization.

Figure 5

Comparison of the experimental low-energy structure of ${}^{70}\mathrm{Ni}$ to calculations with ca48mh1g and jun45. The experimental data is from [49] except the $0_2^{+}$ state, which is revised according to [50].

Figure 6

Calculated shell-model level densities compared to the ${}^{70}\mathrm{Ni}$ data. The gray band indicates the total experimental uncertainty, systematic, and statistical.

Figure 7

Calculated $\gamma \mathrm{SFs}$ within the shell model (for $4.0\le E_x\le 6.5$ MeV) and the QTBA (for all $E_x$) compared with data. The blue shaded band indicates the total experimental uncertainty. In panel (a) all SM $M1$ transitions in the extraction region are included, while in (b) only transitions originating from levels with $J^{\pi }=5^{-},6^{-}$, or $7^{-}$ are shown (see text).

Figure 8

Sketch of a summed $\gamma \mathrm{SF}$, $f_{\mathrm{tot}}\left(E_{\gamma }\right)=f_{E1}^{\mathrm{QTBA}\left(\mathrm{mod}\right)}\left(E_{\gamma }\right)+f_{M1}^{\mathrm{SM}}\left(E_{\gamma }\right)$ for the different shell-model calculations. The QTBA calculation has been replaced by a flat $E1$ strength for $E_{\gamma }<3.5$ MeV, as suggested by recent shell-model $E1$ calculations [28].

Figure 9

Calculated $M1\gamma$ strengths for a wide range of Ni isotopes with the shell model, using the ca48mh1g interaction.