Isomer-tagged differential-plunger measurements in proton-unbound 144Ho

The lifetime of an excited state above a weakly populated isomer in the proton-unbound odd–odd nucleus 144Ho has been measured using the recoil distance Doppler shift method. This measurement represents the first differential-plunger lifetime measurement to utilize recoil-isomer tagging. The first excited Iπ=(10+)Iπ=(10+) state above the two-quasiparticle πh11/2⊗νh11/2(8+)πh11/2⊗νh11/2(8+) isomer was determined to have a lifetime of τ=6(1) psτ=6(1) ps. Potential energy surface calculations, based on the configuration-constrained blocking method, predict the isomeric state to have γ  -soft triaxial-nuclear shape with |γ|≈24°|γ|≈24°. The lifetime of the (10+)(10+) state can be understood from these calculations if there is a degree of rotational alignment in this band, with the K value being lower than the bandhead spin. However, the validity of the K quantum number with large predicted triaxiality and gamma softness requires further theoretical study.