Coulomb excitation of ${}_{28}^{68}{\mathrm{Ni}}_{40}$ at “safe” energies

The $B(E2;0^{+}\to 2^{+})$ value in ${}^{68}\mathrm{Ni}$ has been measured using Coulomb excitation at safe energies. The ${}^{68}\mathrm{Ni}$ radioactive beam was postaccelerated at the CERN on-line isotope mass separator (ISOLDE) facility to 2.9 MeV/u and directed to a ${}^{108}\mathrm{Pd}$ target. The emitted $\gamma$ rays were detected by the MINIBALL detector array. Not only directly registered but also indirectly deduced information on the nucleus emitting the $\gamma$ ray was used to perform the Doppler correction, leading to a larger center-of-mass angular range to infer the excitation cross section. The obtained value of $2.8_{-1.0}^{+1.2}\times {10}^2{e}^2\hspace{0.5em}{\mathrm{fm}}^4$ is in good agreement with the value measured at intermediate energy Coulomb excitation, confirming the low $0^{+}\to 2^{+}$ transition probability.

Figure 1

Release curves for ${}^{68}\mathrm{Ni}$ and the ${}^{68}\mathrm{Ga}$ isobaric contaminant as a function of the time difference with the start of the supercycle. 800 ms after the fifth proton pulse the beam gate is opened, and the produced ions are sent to the experimental setup.

Figure 2

γ-ray spectrum with particle coincidence Doppler corrected for ${}^{108}\mathrm{Pd}$ target excitation (a) and part of the $\gamma$-ray spectrum with particle coincidence Doppler corrected for ${}^{68}\mathrm{Ni}$ projectile excitation (b) focused around 2033 keV.

Figure 3

Scattering angles of the ${}^{68}\mathrm{Ni}$ projectile and the ${}^{108}\mathrm{Pd}$ target particles in the laboratory frame versus the scattering angle in the center-of-mass frame (a). From the angular range of the DSSSD detector it can be seen where the information on the $\gamma$-emitting nucleus was registered directly and where it was deduced indirectly. Also the differential cross sections for both nuclei are shown (b and c). All detected events occur in the angular range of safe Coulomb excitation, where the distance between the nuclear surfaces never exceeds 5 fm.

Figure 4

Particle-coincident $\gamma$-ray spectrum Doppler corrected for ${}^{68}\mathrm{Ni}$. Only the events occurring within the last 5 s of the proton pulse supercycle are included.