Shape coexistence scenario in ${}^{150}\mathrm{S}m$ from a $\gamma \text{−}\gamma$ fast-timing measurement

Lifetimes are measured for low lying states of ${}^{150}\mathrm{Sm}$, populated from ${\beta }^{-}$ decay of ${}^{150}\mathrm{Pm}$ produced through $(p,n)$ reactions with a ${}^{150}\mathrm{Nd}$ target. The VENTURE array comprising of eight fast ${\mathrm{CeBr}}_3$ detectors is used for lifetime measurement with $\gamma \text{−}\gamma$ fast timing technique. The lifetime of $0_3^{+}$ level of ${}^{150}\mathrm{Sm}$ is measured for the first time to be 36(10) ps. The $0_3^{+}$ level is found to have enhanced decay strengths to the $K^{\pi }=0_2^{+}$ structure compared with $K^{\pi }=0_1^{+}$. A high ${\rho }^2\left(E0\right)$ strength for the $0_3^{+}\to 0_2^{+}$ decay confirms shape coexistence and shape mixing in $N=88{}^{150}\mathrm{Sm}$.

Figure 1

The ${\mathrm{CeBr}}_3$ (blue, solid) and Clover (red, dash-dot-dot) total projections obtained from ${\mathrm{CeBr}}_3\text{−}{\mathrm{CeBr}}_3$ and ${\mathrm{CeBr}}_3$-Clover coincidence data, respectively. This was used to identify the deexciting $\gamma$ lines from excited level of ${}^{150}\mathrm{Sm}$, as detected in ${\mathrm{CeBr}}_3$ and Clover detectors of the setup.

Figure 2

The ${\mathrm{CeBr}}_3$ energy-gated projections of ${\mathrm{CeBr}}_3$ (blue, solid) and Clover (red, dash-dot) detectors are shown with gate on (a) feeder (1324 keV) and (b) decay (334 keV) $\gamma$ rays of 1324–334 keV cascade corresponding to $2_1^{+}$ level. The delayed (red, solid) and antidelayed (blue, dash-dot-dot) time difference spectra are shown in panel (c). The background analysis for this cascade are shown in panels (d) and (e) where $\mathrm{\Delta }{C}_{\text{BG}}$ values are shown with respect to $\text{PRD}=0$ at $E_{\text{ref}}$. The PRD curve is also shown with black dashed line by making $\text{PRD}=0$ at $E_{\text{ref}}$. The $\mathrm{\Delta }C=0$ line is out of the scale of the background plot and is not shown.

Figure 3

The ${\mathrm{CeBr}}_3$ energy gated projections of ${\mathrm{CeBr}}_3$ (blue, solid) and Clover (red, dash-dot) detectors are shown with gate on (a) feeder (1223 keV) and (b) decay (406 keV) $\gamma$ rays of 1223-406 keV cascade ($0_2^{+}$ level). The delayed (red, solid) and antidelayed (blue, dash-dot-dot) time difference spectra are shown in panel (c). The background analysis for this cascade are shown in panels (d) and (e) where $\mathrm{\Delta }{C}_{\text{BG}}$ values are shown with respect to $\text{PRD}=0$ at $E_{\text{ref}}$. The PRD curve is also shown with black dashed line by making $\text{PRD}=0$ at $E_{\text{ref}}$. The $\mathrm{\Delta }C=0$ line is out of the scale of the background plot and is not shown. The peaks neighboring to the gamma line of interest are shown which are arising out of coincidence with underlying Compton background or random (#). The range of $\gamma$ gates given in such cases for the generation of time distribution spectra are shown with solid vertical lines.

Figure 4

The ${\mathrm{CeBr}}_3$ energy gated projections of ${\mathrm{CeBr}}_3$ (blue, solid) and Clover (red, dash-dot) detectors are shown with gate on (a) feeder (1004 keV) and (b) decay (922 keV) $\gamma$ rays of 1004-922 keV cascade corresponding to the $0_3^{+}$ level. The delayed (red, solid) and antidelayed (blue, dash-dot-dot) time difference spectra are shown in (c). The background analysis for this cascade are shown in panels (d) and (e) where $\mathrm{\Delta }{C}_{\text{BG}}$ values are shown with respect to $\text{PRD}=0$ at $E_{\text{ref}}$. The PRD curve is also shown with black dashed line by making $\text{PRD}=0$ at $E_{\text{ref}}$. The red dash-dotted line is drawn to guide the eye for $\mathrm{\Delta }\mathrm{C}$ or $\text{PRD}=0$. The peaks neighboring to the gamma line of interest in panels (a) and (b) are shown, which are arising out of coincidence with photopeak (*), underlying Compton background or random (#). The range of $\gamma$ gates given in such cases for the generation of time distribution spectra to minimize the effect of contamination are shown with solid vertical lines.

Figure 5

The partial level schemes of ${}^{150}\mathrm{Sm}$ and ${}^{152}\mathrm{Sm}$ are compared for the decays from the excited $0_2^{+}$ and $0_3^{+}$ levels. The width of the transitions indicate the corresponding reduced transition strengths. The $E0$ decays are shown in red and the $E2$ decays in black. See text for details.

Figure 6

The $B\left(E2\right)$ and ${\rho }^2\left(E0\right)$ values for the decays of $0_2^{+}$ and $0_3^{+}$ levels in Sm and Gd nuclei are shown as a function of neutron number. The $B\left(E2\right)$ value for $N=88$ Sm has been taken from present work and that of $0_2^{+}$ levels of Gd isotopes are taken from Ref. [22]. The other values shown are deduced from the known lifetimes in literature [34]. The ${\rho }^2\left(E0\right)$ values have been calculated from the $B\left(E2\right)$ values.