Observation of a large $\beta$-delayed neutron emission component in ${}^{102}\mathrm{Rb}$ decay and identification of excited states in ${}^{102}\mathrm{Sr}$

The $\beta$ decay and $\beta$-delayed neutron emission of ${}^{102}\mathrm{Rb}$ have been studied using the $8\pi$ spectrometer at TRIUMF-ISAC. The level scheme of ${}^{102}\mathrm{Sr}$ has been expanded from results of a $\gamma -\gamma$ coincidence analysis, including the identification of the $(4^{+})$ member of the ground-state band. The $\beta$-delayed neutron branching ratio of ${}^{102}\mathrm{Rb}$ was found to be significantly larger than previously reported. This larger value has the potential to modify the results of $r$-process calculations and motivates further measurements as well as refinements of theoretical models.

Figure 1

(a) Portion of the $\gamma$-ray energy spectrum in coincidence with the 126.1-keV transition in ${}^{102}\mathrm{Sr}$. (b) The $\gamma$-ray energy spectrum gated on the 285.2-keV transition in ${}^{102}\mathrm{Sr}$. Both spectra require a coincidence with a $\beta$ particle and are random-background subtracted. Peaks from other decay species are indicated by the following symbols: $▾$ for ${}^{101}\mathrm{Sr}–{}^{101}\mathrm{Y}$ and $•$ for ${}^{102}\mathrm{Nb}–{}^{102}\mathrm{Mo}$.

Figure 2

Proposed ${}^{102}\mathrm{Sr}$ level scheme from the present ${}^{102}\mathrm{Rb}\beta$-decay experiment. The width of the arrows represents their relative intensity. The placements in the level scheme are based on mutual $\gamma -\gamma$ energy coincidences. The systematic comparison suggests a tentative ($4^{+}$) state at an energy of 411.3 keV.

Figure 3

Coincidence-energy-gated and random-background-subtracted $\gamma$-ray spectra in coincidence with a $\beta$ particle, gated on the (a) 271.0-, (b) 134.8-, (c) 160.8-, (d) 111.8-, (e) 145.5-, and (f) 232.3-keV transitions in ${}^{101}\mathrm{Sr}$. Peaks from other decay species are indicated by the following symbols: $▴$ for ${}^{102}\mathrm{Zr}–{}^{102}\mathrm{Nb}, \blacksquare$ for ${}^{102}\mathrm{Sr}–{}^{102}\mathrm{Y}$, and $*$ for ${}^{101}\mathrm{Zr}–{}^{101}\mathrm{Nb}$.

Figure 4

Proposed ${}^{101}\mathrm{Sr}$ level scheme following ${}^{102}\mathrm{Rb}\beta$-delayed neutron emission. The width of the transitions represent their relative intensities. The transition assignments are based on mutual coincidence in a $\gamma -\gamma$ analysis. The tentative $J^{\pi }$ assignments are from Ref. [17].

Figure 5

The ratio of $P_n/T_{1/2}$ plotted as a function of the $Q_{\beta n}$ value for Rb isotopes. Experimental data are from Refs. [1, 23]. New experimental data for ${}^{100}\mathrm{Rb}$ are taken from Mathieu et al. [26] and the ${}^{102}\mathrm{Rb}$ value is taken from the present work. The calculations use the QRPA [20] and KHF [27] models with values taken from Ref. [28], in addition to the prediction from Ref. [24], which is a fit to the ENSDF data [23].