Excited states in ${}^{87}\mathrm{Br}$ populated in $\beta$ decay of ${}^{87}\mathrm{Se}$

Excited levels in ${}^{87}\mathrm{Br}$, populated in $\beta$ decay of ${}^{87}\mathrm{Se}$, have been studied by means of $\gamma$-ray spectroscopy using an array of broad energy Ge detectors. ${}^{87}\mathrm{Se}$ nuclei were produced by irradiating a natural Th target with 25-MeV protons. Fission products were extracted from the target chamber using the IGISOL technique, then separated on a dipole magnet and Penning trap (JYFLTRAP) setup. The scheme of excited levels of ${}^{87}\mathrm{Br}$ has been significantly extended. 114 new transitions and 51 new levels were established. $\beta$ feedings and $log\left(ft\right)$ values of levels were determined. The upper limit for $\beta$ feeding to the ground state of ${}^{87}\mathrm{Br}$ was determined to be 23(5)%. Ground state spin and parity $5/2^{-}$ was confirmed, as suggested by previous studies. We also confirm the low-energy excited state at 6.02 keV. The ground state and two lowest excited states in ${}^{87}\mathrm{Br}$ were interpreted as the ${\left(\pi f_{5/2}\right)}_{j,j-1,j-2}^3$ triplet produced by the so-called anomalous coupling. The 333.61- and 699.26-keV levels were interpreted as $\pi p_{3/2}$ and $\pi p_{1/2}$ single-particle excitations. The $9/2^{+}$ level reported previously as corresponding to the $\pi g_{9/2}$ single-particle excitation is proposed to be an isomer with half-life 20 ns. Large-scale shell-model calculations performed in this work are in good agreement with experimental results.

Figure 1

Spectrum of singly charged ions with mass number $A=87$ obtained using a mass selective buffer-gas cooling technique [19] and detected on a MCP detector located behind the Penning traps. Peaks corresponding to various isotopes are marked with their element symbol.

Figure 2

Experimental setup used in this experiment. ${}^{87}\mathrm{Se}^{+}$ ions, released from JYFLTRAP, are implanted in the middle of a Ge array. $\gamma$ rays emitted in $\beta$ decay are registered by six BEGe detectors (blue) and one large-volume Ge detector (red). See text for more details.

Figure 3

Part I. Partial scheme of excited levels in ${}^{87}\mathrm{Br}$, populated in $\beta$ decay of ${}^{87}\mathrm{Se}$, as observed in the present work. Thickness of arrows representing transitions is proportional to their relative $\gamma$-ray intensities.

Figure 4

Examples of gated $\gamma$-ray spectra obtained in this work. (a) $\gamma$-ray spectrum gated on the 242.55-keV transition. (b) $\gamma$-ray spectrum gated on the 333.62-keV transition. (c) $\gamma$-ray spectrum gated on the 662.60-keV transition. Energies are in keV.

Figure 5

Partial excitation scheme of ${}^{87}\mathrm{Br}$, drawn from Fig. 3 to assist the discussion.

Figure 6

Systematics of excited levels relative to $5/2^{-}$ levels in $N=52$ [(a) and (b)] isotones and $Z=35$ isotopes (c). Data are taken from [35]. Lines are drawn to guide the eye.

Figure 7

Comparison of excited states populated in $\beta$ decay of ${}^{87}\mathrm{Se}$ with shell-model calculations. Data are normalized to the $5/2^{-}$ ground state. Experimental points presented above 2 MeV are taken from Ref. [12].