Low-lying GT${}^{+}$ strength in ${}^{64}\mathrm{Co}$ studied via the ${}^{64}\mathrm{Ni}$($d,{}^2\mathrm{He}$)${}^{64}\mathrm{Co}$ reaction

The ${}^{64}\mathrm{Ni}$($d$,${}^2\mathrm{He}$)${}^{64}\mathrm{Co}$ reaction was studied at the AGOR cyclotron of KVI, Groningen, with the Big-Bite Spectrometer and the EuroSuperNova detector using a 171-MeV deuteron beam. An energy resolution of about 110 keV was achieved. In addition to the $J{}^{\pi }$ = $1^{+}$ ground state, several other $1^{+}$ states could be identified in ${}^{64}\mathrm{Co}$ and the strengths of the corresponding Gamow-Teller transitions were determined. The obtained strength distribution was compared with theoretical predictions and former $(n,p)$ experimental results and displayed a good agreement. Due to the good energy resolution, detailed spectroscopic information was obtained, which supplements the data base needed for network calculations for supernova scenarios.

Figure 1

The ${}^{64}\mathrm{Co}$ spectrum taken at 0${}^{°}$ when the gate on prompt coincidences is imposed. For a better view of the spectrum, the ground-state peak was scaled down by a factor of 3. The random-coincidence spectrum is overlapped. Corrections for the limited spectrometer and detector acceptances for the two-proton system (see text for details) are not included. The peak around $E_x=$ −6.5 MeV is generated by a small contamination of the target with hydrogen. From about $E_x=$ −5.1 MeV there are very small contributions from the ${}^{58}\mathrm{Ni}$ component in the target.

Figure 2

Double-differential cross sections of the ${}^{64}\mathrm{Ni}$($d$,${}^2\mathrm{He}$)${}^{64}$ Co reaction for different scattering-angle gates. The BBS angular settings as well as the scattering-angle gates are indicated in the figure. Because of the considerable difference in intensities between the ground-state peak and the rest of the spectra, the ground-state peak was scaled by a factor of $1/2$.

Figure 3

Fit of the $E_x⩽$ 4 MeV energy region of the ${\theta }_{c.m.}⩽$ $1^{°}$ spectrum. The contribution of QFS and giant resonances to the spectrum has been estimated by a curve connecting the minima in the spectrum (as shown in the upper panel of Fig. 4) and subtracted (see text for details). The error bars are statistical only. Centroids of peaks corresponding to several states below 4 MeV are indicated.

Figure 4

Fit of the 4.5${}^{°}<{\theta }_{\mathrm{c.m.}}<$ 6.5${}^{°}$ spectrum. The upper panel shows our estimation of the contributions from giant resonances and QFS (see text for details) that have been subtracted from the initial spectrum. The error bars are statistical only. Centroids of peaks corresponding to several states are indicated.

Figure 5

Fit of the experimental angular distributions of the differential cross section for the peaks below 1.5 MeV. The shapes for different $\Delta L$ are obtained in a DWBA calculation (see text). The indicated errors on the experimental points account for statistical errors, uncertainties induced by the correction for the limited acceptance of the detection system for the proton pair and uncertainties in the fitting procedure.

Figure 6

Fit of the experimental angular distributions of the differential cross section for the selected groups of levels. The shapes for different $\Delta L$ are obtained in a DWBA calculation (see text). The indicated errors on the experimental points account for statistical errors, uncertainties induced by the correction for the limited acceptance of the detection system for the proton pair and uncertainties in the fitting procedure.

Figure 7

The experimental $B({\mathrm{GT}}^{+}$) distribution for $E_x⩽$ 4 MeV together with the $(n,p)$ results as shown in Fig. 12 of Ref. [18] and the LSSM calculations of the GT${}^{+}$ strength from ${}^{64}\mathrm{Ni}$ to individual levels [31]. A quenching factor $q=074$ was utilized for scaling the shell-model amplitudes. The block widths represent the integrated energy region and the block heights are proportional to the uncertainty on the $B$(GT) value. The error bars on the $(n,p)$ data are statistical only. For a better observation, the strength corresponding to the ground-state transition was scaled by 1/2 for both theoretical and experimental results.

Figure 8

Cumulative sum of $B$(GT${}^{+}$) values from the ($d$,${}^2\mathrm{He}$) experiment, the $(n,p)$ experiment as deduced from Fig. 12 of Ref. [18] and the LSSM calculations [31]. The block widths represent the integrated energy region and the block heights are proportional to the uncertainty on the $\sum$$B$(GT${}^{+}$). The error-bars on the $(n,p)$ data are statistical only.

Figure 9

Comparison of the ${}^{64}\mathrm{Co}$ level scheme obtained in this study with the ${}^{64}\mathrm{Ni}$($t$,${}^3\mathrm{He}$) results and γ-decay results shifted by 33 keV (see text). Suggested $J^{\pi }$ values are indicated.