Speaker
Description
Neutron rich Ge isotopes represent a puzzle so far as the few existing data
yield a contradicting picture. A recently determined large $B(E2;2^+ \to 0^+)$
value with a large uncertainty in $^{84}$Ge hints for a completely unexpected
shape transition from soft triaxiality in neutron rich Se isotopes with
Z=34 to prolate deformation for neighboring Ge with Z=32 [1]. Such an
"island of inversion" is not expected. The shell model only predicts
such for much lighter systems (Z<28) [1]. On the other hand, recent
investigations of the level schemes of $^{84,86,88}$Ge hint for a new region of
rigid triaxiality similar to $^{76}$Ge [2]. However, these interpretations
suffer heavily from the lack of knowledge on transition strengths between
the lowest states. Therefore, we aim to determine transition strengths
between the lowest states from level lifetimes in the picosecond range
in extremely neutron-rich $^{84,86,88}$Ge. These observables will be determined
at the RIBF facility at RIKEN with the recoil distance Doppler-shift method
after proton knockout using fast (v/c~60%) secondary beams of $^{85,87,89}$As,
a dedicated Cologne plunger device and a segmented highly efficient gamma-ray
spectrometer coupled to the zero degree spectrometer for recoil
identification. It should be stressed that $^{88}$Ge is the most neutron
rich Ge isotope where excited states are known so far at all.
[1] C. Delafosse et al., Phys. Rev. Lett. 121, 192502 (2018)
[2] M. Lettmann et al., Phys. Rev. C 96, 011301(R) (2017)
