Coulomb and nuclear excitations of ${}^{70}$Zn and ${}^{68}$Ni at intermediate energy

Abstract

The reduced transition probabilities $B(E2;0_{g.s.}^{+}\to 2_1^{+},2_2^{+})$ in ${}^{70}$Zn and the full $B(E2;0_{g.s.}^{+}\to 2^{+})$ strength up to S${}_n$=7.79 MeV in ${}^{68}$Ni have been determined at the LISE/GANIL facility using the Coulomb-excitation technique at intermediate beam energy on a ${}^{208}$Pb target. The ${\hat{I}}^3$ rays emitted in-flight were detected with an array of 46 BaF${}_2$ crystals. The angles of the deflected nuclei were determined in order to disentangle and extract the Coulomb and nuclear contributions to the excitation of the 2${}^{+}$ states. The measured $B(E2;0_{g.s.}^{+}\to 2_1^{+})$ of 1432(124) e${}^2$fm${}^4$ for ${}^{70}$Zn falls in the lower part of the published values which clustered either around 1600 or above 2000 e${}^2$fm${}^4$, while the $B(E2;0_{g.s.}^{+}\to 2_2^{+})$ of 53(7) e${}^2$fm${}^4$ agrees very well with the two published values. The relatively low $B(E2;0_{g.s.}^{+}\to 2_1^{+})$ of 301(38) e${}^2$fm${}^4$ for ${}^{68}$Ni agrees with previous studies and confirms a local magicity at $Z=28,N=40$. Combining the results of the low-energy spectra of ${}^{68}$Ni and ${}^{70}$Zn and their shell-model interpretations, it is interesting to notice that four different shapes (spherical, oblate, prolate and triaxial) are present. Finally, a summed $E2$ strength of only about 150 e${}^2$fm${}^4$ has been found experimentally at high excitation energy, likely due to proton excitations across the $Z=28$ gap. The experimental distribution of this high-energy $E2$ excitation agrees with SM calculations, but its strength is about two times weaker.