High-precision $B\left(E2\right)$ measurements of semi-magic ${}^{58,60,62,64}\mathrm{Ni}$ by Coulomb excitation

High-precision reduced electric-quadrupole transition probabilities $B(E2;0_1^{+}\to 2_1^{+})$ have been measured from single-step Coulomb excitation of semi-magic ${}^{58,60,62,64}\mathrm{Ni}$ ($Z=28$) beams at 1.8 MeV per nucleon on a natural carbon target. The energy loss of the nickel beams through the carbon target were directly measured with a zero-degree Bragg detector and the absolute $B\left(E2\right)$ values were normalized by Rutherford scattering. The $B\left(E2\right)$ values disagree with recent lifetime studies that employed the Doppler-shift attenuation method. The present high-precision $B\left(E2\right)$ values reveal an asymmetry about ${}^{62}\mathrm{Ni}$, midshell between $N=28$ and 40, with larger values towards ${}^{56}\mathrm{Ni}$ ($Z=N=28$). The experimental $B\left(E2\right)$ values are compared with shell-model calculations in the full $pf$ model space and the results indicate a soft ${}^{56}\mathrm{Ni}$ core.

Figure 1

Single-particle states with shell closures at 20, 28, and 50, and a subshell closure at 40.

Figure 2

The carbon gated and Doppler corrected $2_1^{+}\to 0_1^{+}$ $\gamma$-ray transitions of ${}^{58,60,62,64}\mathrm{Ni}$ from Coulomb excitation and decay.

Figure 3

The $2_1^{+}\to 0_1^{+}$ $\gamma$-ray angular distributions for ${}^{64}\mathrm{Ni}$, gated on the carbon target recoils in (a) BareBall ring 2 and (b) BareBall ring 3.

Figure 4

$B(E2;0_1^{+}\to 2_1^{+})$ systematics of the stable Ni isotopes from the present study compared to Raman2001 [24] and the recent DSAM studies Kenn2000 [21], Orce2008 [22], and Chak2011 [23].

Figure 5

Comparison of experimental and theoretical $B(E2;0_1^{+}\to 2_1^{+})$ values for the $N=28$ isotones and the nickel isotopes (see text). The experimental data are from the present study and Pritychenko2012 evaluation [37].