New $\beta$-decaying state in ${}^{214}\mathrm{Bi}$

A new $\beta$-decaying state in ${}^{214}\mathrm{Bi}$ has been identified at the ISOLDE Decay Station at the CERN-ISOLDE facility. A preferred $I^{\pi }=\left(8^{-}\right)$ assignment was suggested for this state based on the $\beta$-decay feeding pattern to levels in ${}^{214}\mathrm{Po}$ and shell-model calculations. The half-life of the $I^{\pi }=\left(8^{-}\right)$ state was deduced to be $T_{1/2}=9.39\left(10\right)$ min. The deexcitation of the levels populated in ${}^{214}\mathrm{Po}$ by the $\beta$ decay of this state was investigated via $\gamma \text{−}\gamma$ coincidences and a number of new levels and transitions was identified. Shell-model calculations for excited states in ${}^{214}\mathrm{Bi}$ and ${}^{214}\mathrm{Po}$ were performed using two different effective interactions: the H208 and the modified Kuo-Herling particle interaction. Both calculations agree on the interpretation of the new $\beta$-decaying state as an $I^{\pi }=8^{-}$ isomer and allow for tentative assignment of shell-model states to several high-spin states in ${}^{214}\mathrm{Po}$.

Figure 1

Singles $\gamma$-ray spectrum. Peaks without a special symbol in the label belong to transitions following the ${\beta }^{-}$ decay of ${}^{214}\mathrm{Bi}^m$, with the exception of the 406- and 609-keV peaks, which follow the ${\beta }^{-}$ decay of both ${}^{214}\mathrm{Bi}^m$ and ${}^{214}\mathrm{Bi}^g$. The remaining peaks are attributed to the following decays: ($+$) ${\beta }^{-}$ decay of ${}^{214}\mathrm{Bi}^g$, (*) $\mathrm{EC}/{\beta }^{+}$ decay of ${}^{210}\mathrm{Rn}$, ($♣$) $\mathrm{EC}/{\beta }^{+}$ decay of ${}^{205}\mathrm{Po}$, ($⧫$) $\mathrm{EC}/{\beta }^{+}$ decay of ${}^{210}\mathrm{At}$, and ($\circ$) $\mathrm{EC}/{\beta }^{+}$ decay of ${}^{205}\mathrm{At}$.

Figure 2

The decay scheme with levels in ${}^{214}\mathrm{Po}$ populated by the ${\beta }^{-}$ decay of ${}^{214}\mathrm{Bi}^m$. Transitions, levels, and spin and parity assignments written in blue italics are from the present study, while those in black normal font were reported in Refs. [8, 12]. For display purposes, the spacing of the g.s. band levels is compressed and not scaled according to their energies. Level energies are deduced from $\gamma$-ray energies listed in Table 1. In the case of parallel decay paths from the level, the path with the smaller number of $\gamma$-ray transitions was used. As a result of the systematic shift in energy calibration, discussed in Sec. 3, there may be a difference of up to 2 keV in the level energies compared with Ref. [12]. Apparent $\beta$-decay feeding intensities and resulting log($f_{0}t$), log($f_{1}t$) values are included, see details in Sec. 3e.

Figure 3

Background-subtracted spectra of $\gamma$ rays in coincidence with the following g.s. band transitions: (a) 245 keV ($8_1^{+}\to 6_1^{+}$), (b) 325 keV ($6_1^{+}\to 4_1^{+}$), (c) 406 keV ($4_1^{+}\to 2_1^{+}$), and (d) 609 keV ($2_1^{+}\to 0_1^{+}$). Peaks labeled only by energies belong to transitions following the ${\beta }^{-}$ decay of ${}^{214}\mathrm{Bi}^m$, transitions with labels in blue italic are new. Peaks marked by bullets are caused by summing of transitions in cascades (the list of these peaks and the relevant cascades are given in Table 2), peaks marked by plus signs belong to transitions following the ${\beta }^{-}$ decay of ${}^{214}\mathrm{Bi}^g$ [8]. The broader peak-like structures with depressions in the background around them are artificial peaks caused by Compton scattering.

Figure 4

Background-subtracted spectra of $\gamma$ rays in coincidence with (a) 426 keV, (b) 49 keV, (c) 475 keV, and (d) 240 keV. Peaks labeled only by energies belong to transitions following the ${\beta }^{-}$ decay of ${}^{214}\mathrm{Bi}^m$, transitions with labels in blue italics are new. Peaks marked by bullets are caused by summing of transitions in cascades (a list of these peaks and relevant cascades is given in Table 2), peaks marked by asterisks belong to transitions following the $\mathrm{EC}/{\beta }^{+}$ decay of ${}^{210}\mathrm{Rn}$ [1]. The broader peak-like structures with depressions in the background around them are artificial peaks caused by Compton scattering.

Figure 5

Background-subtracted time distributions for (a) 325-keV $\gamma$ ray and (b) 1764-keV $\gamma$ ray, obtained when the beam gate was closed. Each distribution was fit (solid red line) by an exponential function (dashed blue line) plus constant (dotted black line). The corresponding normalized residuals of the fits are plotted below each time distribution, dashed lines in these plots mark values of $-1\sigma$, $0\sigma$, and $1\sigma$.

Figure 6

The calculated energies of yrast and yrare states below 700 keV in ${}^{214}\mathrm{Bi}$ using KHPE (blue symbols) and H208 (red symbols) interactions are shown together with the experimental energies (black circles) from Refs. [10, 11]. Yrast SM states stemming from the $\pi 0h_{9/2}\otimes \nu 1g_{9/2}$ multiplet are connected by solid lines. Labels indicate dominant configurations of the odd proton and odd neutron.

Figure 7

Comparison of levels in ${}^{214}\mathrm{Po}$ from experiment and from SM calculations based on KHPE and H208 interactions. Experimental levels drawn by solid lines are from the present work, with two additional levels with spin 7 and 9 taken from Ref. [12] and drawn as dashed lines. From SM calculations, only levels with spins and energies relevant for comparison with the current experiment are displayed (see Sec. 4b). All of these SM levels have positive parities. Spins are given on the left sides of the levels, energies in keV are given on the right sides. Levels with the same spins are highlighted in the same colors.