| In summary, lifetimes of low-lying states in 70,72,74Zn
have been measured using the RDDS technique and
the AGATA demonstrator coupled to the PRISMA
magnetic spectrometer. Regarding the first 2+1 state,
a maximum of collectivity is found at N = 42, as in
the case of Ge and Se isotopic chains. The measured
lifetimes of the first 2+1 states in 70,72,74Zn from this
experiment are in very good agreement with previous
measurements. In the case of the first 4+1 states, the
10
values obtained in the present work contradict previous
measurements from which smaller lifetimes were systematically
deduced. A strong drop in B(E2;4+1 ! 2+1 )
systematics is observed at N = 44, but not reproduced
so far neither by shell model nor mean field approaches.
The present experimental study suggests that the nature
of the low-lying excitation in neutron-rich zinc isotopes
is similar to those of stable zinc isotopes with a ratio
B(E2;4+1 ! 2+)/B(E2;2+1 ! 0+1 ) lower than one. Neither
shell model predictions with the JUN45 or LNPS
interaction nor collective hamiltonian calculations with
the Gogny force reproduce this feature. In addition, the
B(E2;6+1 ! 4+1 ) has been measured to be 134+57
-31 e2fm4
in 72Zn much lower than the B(E2) values corresponding
to the decay of the 4+1 and 2+1 states. This tendency is
qualitatively reproduced by shell model predictions.
The effect of side feeding has been carefully taken
into account in the present work. The present study
illustrates that lifetime measurements with the RDDS
technique using deep inelastic scattering that populates
high-excitation-energy states requires sufficient statistics
to quantify weak side feeding from off-band states that
have a significant impact on the lifetime extraction.
Saying so, we conclude that lifetimes in neutron-rich
zinc isotopes deserve additional investigations on both
experimental and theoretical sides. In particular, the
confirmation of the unexpectedly long lifetime of the
4+1 state of 74Zn and the extension of the systematics
to richer neutron-rich even-even zinc isotopes are called
for. |
