Population and decay of a $K^{\pi }=8^{-}$ two-quasineutron isomer in ${}^{244}\mathrm{Pu}$

The decay of a $K^{\pi }=8^{-}$ isomer in ${}^{244}\mathrm{Pu}$ and the collective band structures populating the isomer were studied using deep inelastic excitations with ${}^{47}\mathrm{Ti}$ and ${}^{208}\mathrm{Pb}$ beams, respectively. Precise measurements of $M1/E2$ branching ratios in the band confirm a $9/2^{-}{\left[734\right]}_{\nu }\otimes 7/2^{+}{\left[624\right]}_{\nu }$ configuration assignment for the isomer, validating the systematics of $K^{\pi }=8^{-}$, two-quasineutron isomers observed in even-$Z, N=150$ isotones. These isomers around the deformed shell gap at $N=152$ provide critical benchmarks for theoretical predictions of single-particle energies in this gateway region to superheavy nuclei.

Figure 1

(a) Sum of double gates (163-217, 163-671, 163-681, 217-671, and 217-681 keV) from events collected in the beam-off periods, showing decay transitions from the ${}^{244}\mathrm{Pu}$ isomer; (b) the decay curve of the isomer.

Figure 2

Partial level scheme of ${}^{244}\mathrm{Pu}$, showing the ground-state band (a), the octupole vibrational band (b), and a rotational band [signature partners (c) and (d)] built on the suggested $K^{\pi }=8^{-}$ isomer. An unobserved 10-keV transition connects the isomer to the $7^{-}$ level of the octupole band.

Figure 3

(a) Spectrum in coincidence with 163- to 473-keV ground-state-band transitions in ${}^{244}\mathrm{Pu}$ and Pu x rays. (b) Spectrum in coincidence with Pu x rays and selected gates of $M1$ and $E2$ transitions in the band built on the isomer. The 231-keV peak, indicated with an asterisk, has contributions from both an $M1$ and an $E2$ transition in the band. Insets in both panels show the coincident 2615-keV transition from the first excited state in ${}^{208}\mathrm{Pb}$ (reaction partner).

Figure 4

Measured $|(g_K-g_R)/Q_0|$ for transitions in the band built on the isomer, clearly favoring the $9/2^{-}{\left[734\right]}_{\nu }\otimes 7/2^{+}{\left[624\right]}_{\nu }$ configuration (see text). Expected values for other low-lying $9/2^{+}{\left[624\right]}_{\pi }\otimes 7/2^{-}{\left[514\right]}_{\pi }$ and $9/2^{-}{\left[734\right]}_{\nu }\otimes 7/2^{+}{\left[613\right]}_{\nu }$ configurations are shown as well.

Figure 5

(a) Kinematic and (b) dynamic moments of inertia for the ground-state (solid symbols) and isomeric (open symbols) bands in $N=150$ nuclei as a function of rotational frequency $\hslash \omega$, calculated as $E_{\gamma }/[\sqrt{(I+1)(I+2)-K^2}-\sqrt{(I-1)I-K^2}]$, where the transition is between states with $I_i=I+1$ and $I_f=I-1$, and $K$ is the bandhead spin. $J^{\left(1\right)}=(2I+1)/E_{\gamma }$ and $J^{\left(2\right)}=4/\mathrm{\Delta }{E}_{\gamma }$.