Collective excitations and shape changes in ${}^{80}\mathrm{Y}$

Lifetimes of 30 high-spin levels in ${}^{80}\mathrm{Y}$ were measured using the Doppler-shift attenuation method. The high-spin states were populated using the ${}^{54}\mathrm{Fe}({}^{28}\mathrm{Si},pn)$ reaction at $90\mathrm{MeV}$, with a thick $14\mathrm{mg}∕{\mathrm{cm}}^2$ ${}^{54}\mathrm{Fe}$ target used to stop all recoils. Prompt $\gamma –\gamma$ coincidences were detected using a Compton-suppressed Ge array consisting of three Clover detectors and seven single-crystal detectors. Lifetimes were determined from experimental line shapes measured at both $35°$ and $145°$ whenever possible. Transition quadrupole moments $Q_t$ inferred from the lifetimes in the lowest positive- and negative-parity bands are large $(>3e\mathrm{b})$ at low spin, but show a rather abrupt decrease at high spin. Results of calculations using the projected shell and cranked Woods-Saxon models in conjunction with the cranked-shell model suggest that these drops in $Q_t$ are due to quasiparticle alignment, which introduce shape changes in each band from nearly prolate below the alignment to triaxial above. Magnetic dipole transition strengths $B\left(M1\right)$ in the yrast positive-parity band show a strong alternating pattern with spin, similar to that observed in other neighboring odd-odd nuclei.

Figure 1

Partial level scheme of ${}^{80}\mathrm{Y}$ [15] showing the bands in which lifetimes were measured in the present work. The vertical energy scale is compressed by a factor of three above an excitation energy of $2000\mathrm{keV}$. The numbers above the decay sequences are intended only to facilitate the discussion and follow the same convention as in Ref. 15.

Figure 2

Fits to the $35°$ and $145°$ line shapes of the $888\mathrm{keV}$ transition in band 1 with (solid line) and without (dashed line) feeding corrections. Each spectrum was obtained from a sum of gates on the 257 and $570\mathrm{keV}$ transitions in band 1.

Figure 3

Simultaneous fit to the line shapes of the 1013 and $1031\mathrm{keV}$ transitions in band 3 measured at $145°$. The uncertainty limits of the overall best fit (solid curve) are indicated by the broken curves. The spectra were obtained from a sum of gates on the 257 and $802\mathrm{keV}$ transitions.

Figure 4

Fits to the $145°$ line shapes of the 788, 992, and $1180\mathrm{keV}$ transitions in band 1. The uncertainty limits of the best fit (solid curve) are indicated by the broken curves.

Figure 5

Sample total Routhian surface plots for the lowest positive- and negative-parity configurations in ${}^{80}\mathrm{Y}$ at three rotational frequencies (and their corresponding spin $I$) as indicated in each plot. The $aA$ configuration corresponds to $(\pi ,\alpha )=(+,1)$ (odd spins in band 3), and the $aF$ configuration corresponds to $(\pi ,\alpha )=(-,1)$ (odd spins in band 1). The spacing between contour lines is $200\mathrm{keV}$.

Figure 6

Transition quadrupole moments $Q_t$ as a function of the initial-state spin $I_i$ for the yrast positive-parity bands of ${}^{78}\mathrm{Rb}$ [13] and ${}^{82}\mathrm{Y}$ [4] (top), and yrast positive-parity band 3 in ${}^{80}\mathrm{Y}$ (bottom). Filled (open) symbols represent values obtained from signature $\alpha =0$ $(\alpha =1)$ transitions. Symbols with arrows indicate lower limits established from effective lifetimes. The curves represent theoretical predictions from projected shell-model (PSM) and cranked Woods-Saxon (CWS) calculations, as indicated in the figure.

Figure 7

Normalized energy differences between adjacent states in band 3 (top) and $B\left(M1\right)$ strengths in bands 1 and 3 (bottom) as a function of the initial-state spin $I_i$ in ${}^{80}\mathrm{Y}$. Filled (open) symbols represent values with even (odd) $I_i$. The solid (dashed) curves represent the theoretical predictions from projected shell-model (PSM) calculations using quadrupole deformation ${ϵ}_2=0.311$ $({ϵ}_2=0.251)$. Not all available points at high spin are shown in the top portion of the figure.

Figure 8

Transition quadrupole moments $Q_t$ as a function of initial-state spin $I_i$ for the yrast negative-parity bands of ${}^{78}\mathrm{Rb}$ [13] and ${}^{82}\mathrm{Y}$ [4] (top), and yrast negative-parity band 1 in ${}^{80}\mathrm{Y}$ (bottom). Filled (open) symbols represent values obtained from signature $\alpha =0$ $(\alpha =1)$ transitions. Symbols with arrows indicate lower limits established from effective lifetimes. The broken curves represent the theoretical predictions for the $\alpha =0$ and $\alpha =1$ signatures inferred from cranked Woods-Saxon (CWS) calculations, as indicated in the figure.