$\gamma$-strength functions in ${}^{60}\mathrm{Ni}$ from two-step cascades following proton capture

The two-step cascade method previously used in neutron-capture experiments is now applied to a proton-capture reaction. The spectrum of two-step cascades populating the first $2^{+}$ level of ${}^{60}\mathrm{Ni}$ has been measured with the ${}^{59}\mathrm{Co}$($p,2\gamma$)${}^{60}\mathrm{Ni}$ reaction. The simulation technique used for the spectrum analysis allows one to reveal the range of possible shapes of both $E1$ and $M1$ $\gamma$-strength functions. The low-energy enhancement previously observed in ${}^3\mathrm{He}$-induced reactions is seen to appear in $M1$ strength functions of ${}^{60}\mathrm{Ni}$.

Figure 1

The spectrum of the sum amplitude of two coincident pulses from the ${}^{59}\mathrm{Co}$($p,2\gamma$)${}^{60}\mathrm{Ni}$ reaction.

Figure 2

The spectrum of two-step $\gamma$ transitions populating the $2^{+}$ level of the ${}^{60}\mathrm{Ni}$ nucleus.

Figure 3

Ratio of the density of positive-parity levels to the total level density for ${}^{60}\mathrm{Ni}$. The histogram represents the calculations of Ref. [16]. The line shows the approximation used in our calculations.

Figure 4

Results of the simulations of the two-step cascade spectrum populating the $2^{+}$ level of ${}^{60}\mathrm{Ni}$. The black thick lines in panels (a), (b), and (c) are input strength functions, which result in the black TSC spectrum in panel (d). The thin red lines show the output functions and the corresponding TSC spectra resulting from the simulation procedure. In panel (d), $R$ is the ratio of the TSC intensity and the intensity of the $2^{+}\to 0^{+}$ ground-state transition in ${}^{60}\mathrm{Ni}$. Note that the absolute scale of the strength functions is approximate.

Figure 5

Same as in Fig. 4 but for the experimental TSC spectrum. The black points in panel (c) are data on ${}^{56}\mathrm{Fe}$ from the Oslo experiment [7].

Figure 6

Strength function calculated from individual $\gamma$-ray transitions between discrete levels of ${}^{60}\mathrm{Ni}$. The histogram shows the average value of the data points binned in 300-keV energy intervals.