Speaker
Description
Zr isotopes exhibit a sudden change of nuclear shape from spherical to deformed at $N=60$ as can be seen by the jump of the $B(E2)$ transition strength from the lowest $2^+$ state to the ground state. This shape transition results from the shape coexistence of spherical and deformed bands at low energy. $^{100}\text{Zr}$ $(N=60)$ is deformed more easily than $^{98} \text{Zr}$, which is a subshell closure of $N=58$, so a deformed $0^+$ state comes down below the spherical $0^+$ state at $N=60$.
The detailed nuclear structure has been described particularly by Monte Carlo shell model (MCSM) calculations. Comparing the results with recent gamma-ray spectroscopy, we can however find some discrepancies between the MCSM calculations and experiment for the energy spectra and $B(E2)$ transition strengths.
In this talk, we present a recent study by using the Quasi-particle vacua shell model (QVSM) calculations. In the QVSM, a nuclear wave function is expressed with a superposition of quasi-particle vacua. The U and V matrices, which characterize each basis vector, are optimized to describe some low-lying states. We developed a phenomenological effective interaction which can be applied to describe not only Zr $(Z=40)$ isotopes, but also Kr $(Z=36)$, Sr $(Z=38)$, Mo $(Z=42)$, and Ru $(Z=44)$ for $N=50-70$. We are successful in systematically reproducing energy spectra and $B(E2)$ strengths. We discuss the magnitude of deformation, triaxiality, and the mixing of different shapes based on the analysis of wave functions obtained by the QVSM calculations. In contrast to Zr isotopes, the Kr, Sr, Mo, and Ru isotopes exhibit more gradual shape transitions. Our calculations reproduce this trend and describe the detail of shape coexistence in those isotopes.
