Spectroscopy of weakly deformed bands in Zr87 : First observation of the shears mechanism in a Zr isotope

Abstract

Excited states of $^{87}\mathrm{Zr}$, populated in the reaction $^{60}\mathrm{Ni}(^{31}\mathrm{P}$, $3pn$) at a beam energy of 112.5 MeV, have been studied. Experimental information on two negative-parity bands has been significantly upgraded with the addition of new $\ensuremath{\gamma}$ rays and levels. Small values of the reduced transition probability $B(E2$) and a general absence of a measurable Doppler shift in the transitions suggest that the states are weakly deformed. Several positive-parity levels have been grouped into two bands based on their observed properties. Spin parities have been proposed for a majority of the states belonging to the different bands. Lifetimes have been measured for the eight states belonging to the two positive-parity bands from Doppler shift attenuation data, including an upper limit for the highest energy state. The behavior of the deduced $B(M1$) and $B(E2$) values as a function of level spin supports the interpretation of one of these bands within the framework of the shears mechanism. A study of these reduced transition probabilities in the light of the semiclassical model of the shears mechanism, proposed by Macchiavelli and co-workers, confirms this interpretation. The effective gyromagnetic ratio and the interaction strength between the two blades of the shears have been estimated from this study.