High-spin states in ${}^{91,92,93}\mathrm{Rb}$ and ${}^{155,156}\mathrm{Pm}$

The excited states of the neutron-rich nuclei ${}^{155,156}\mathrm{Pm}$ and ${}^{91,92,93}\mathrm{Rb}$ were studied from the spontaneous fission of ${}^{252}\mathrm{Cf}$. The $\gamma \text{−}\gamma \text{−}\gamma$ and $\mathrm{x}(\mathrm{Pm})\text{−}\gamma \text{−}\gamma$ triple coincidence relations were applied to identify the $\gamma$ transitions. Fourteen, six, three, twelve, and twelve new $\gamma$ transitions from high-spin states were observed in ${}^{155,156}\mathrm{Pm}$, ${}^{91,92}\mathrm{Rb}$, and ${}^{93}\mathrm{Rb}$ (first levels), respectively. The $\pi 5/2[532]$ rotational band in ${}^{155}\mathrm{Pm}$ was extended up to $23/2^{-}$. The $\pi g_{9/2}$ particle states and $\pi f_{5/2}$ particle states in ${}^{91,93}\mathrm{Rb}$ weakly coupled to ${}^{90,92}\mathrm{Kr}$, respectively, are reported.

Figure 1

(a) Relative Rb and Pm yields obtained from Wahl’s tables [4]. (b) Relative Rb yields extracted from the coincidence spectra gated on the $\gamma$ transitions in the partner ${}^{155,156}\mathrm{Pm}$ isotopes.

Figure 2

Coincidence spectrum double-gated on the new 912.7 keV transition in ${}^{93}\mathrm{Rb}$ and new 105.5 keV transition in ${}^{155}\mathrm{Pm}$. ${}^{93}\mathrm{Rb}$ and ${}^{155}\mathrm{Pm}$ are in the 4$n$ channel. Note that the previously known 67.4 and 87.2 keV transitions in ${}^{155}\mathrm{Pm}$ [9] and Pm $K$ x rays are clearly seen. See the text for more discussions of the new $\gamma$ transitions in ${}^{93}\mathrm{Rb}$ and ${}^{155}\mathrm{Pm}$.

Figure 3

(a) Coincidence spectrum double-gated on the 105.5 keV transition in ${}^{155}\mathrm{Pm}$ and 38.7 keV Pm $K$ x ray and (b) coincidence spectrum double-gated on the 103.6 keV transition in ${}^{156}\mathrm{Pm}$ and 38.7 keV Pm $K$ x ray. Note that there is a large background peak region around 600 keV. The $\mathit{xn}$ means the $\gamma$ transition of the partner Rb nucleus corresponding to the $\mathit{xn}$ channel.

Figure 4

$\pi 5/2[532]$ rotational band of ${}^{155}\mathrm{Pm}$ and partial level scheme of ${}^{156}\mathrm{Pm}$. Except for the 67.4, 87.2, and 154.7 keV transitions in ${}^{155}\mathrm{Pm}$ [9] and the 85.6 keV transition in ${}^{156}\mathrm{Pm}$ [8], all other transitions are new. The relative $\gamma$ transition intensities are not shown because of the large background under the low-energy transitions at 67.4 and 87.2 keV in ${}^{155}\mathrm{Pm}$ and 85.6 and 103.6 keV in ${}^{156}\mathrm{Pm}$ and uncertainties in the detector efficiencies at these low energies.

Figure 5

Partial level schemes of ${}^{92,93}\mathrm{Rb}$. All transitions except the transitions marked with $\ast$ are new. Relative intensities are shown in the parentheses. It is assumed that the intensity ratio of the 142.4 keV transition to the 142.5 keV transition in ${}^{92}\mathrm{Rb}$ is 0.7.

Figure 6

Coincidence spectrum double-gated on the new 286.9 and 719.7 keV transitions in ${}^{92}\mathrm{Rb}$. Two background peaks at 211.7 (${}^{113}\mathrm{Rh}$) and 1133.6 (${}^{135}\mathrm{I}$) keV appear because of the strong 288.0 keV transition in ${}^{135}\mathrm{I}$ [14].

Figure 7

Comparison of the levels in ${}^{89,91,93}\mathrm{Rb}$. Data for ${}^{89}\mathrm{Rb}$ are taken from Ref. [7].

Figure 8

Comparison between the experimental excited energies in ${}^{91,93}\mathrm{Rb}$ and ${}^{90,92}\mathrm{Kr}$ [16].