Precise branching ratio measurements in ${}^{19}\mathrm{Ne}\beta$ decay and fundamental tests of the weak interaction

We used the $8\pi \gamma$-ray spectrometer at the TRIUMF-ISAC radiocative ion beam facility to obtain high-precision branching ratios for ${}^{19}\mathrm{Ne}{\beta }^{+}$ decay to excited states in ${}^{19}\mathrm{F}$. Together with other previous work, our measurements determine the superallowed $1/2^{+}\to 1/2^{+}\beta$ branch to the ground state in ${}^{19}\mathrm{F}$ to be 99.9878(7)%, which is three times more precise than known previously. The implications of these measurements for testing a variety of weak interaction symmetries are discussed briefly.

Figure 1

Nominal decay scheme and $\gamma$ transitions for ${}^{19}\mathrm{Ne}{\beta }^{+}$ decay. The decay proceeds predominantly via the superallowed $1/2^{+}\to 1/2^{+}$ transition to the ground state in ${}^{19}\mathrm{F}$. The solid red lines represent the most intense $\gamma$ transitions from excited states in ${}^{19}\mathrm{F}$. Level energies are in keV.

Figure 2

Schematic picture of one hemisphere of the $8\pi$ array, shown together with the tape system. This model is drawn to scale and was used for the simulations described in the text.

Figure 3

Upper panel: $\gamma$-Ray spectrum for ${}^{19}\mathrm{Ne}$ decay in coincidence with observed positrons. No beam contaminants are apparent from this spectrum, which expectedly is dominated by counts from 511-keV annihilation photons. The broad peak at approximately 680 keV arises from the backscatter of two simultaneous 511-keV $\gamma$ rays. Lower panel: Overlay of $\gamma$-ray singles data with the coincidence spectrum. The data show that room background peaks were significantly reduced by gating on the prompt time differences between successive $\beta$ and $\gamma$ triggers registered with the ULM.

Figure 4

Fits to the 110- and 1357-keV peaks in the coincident $\gamma$-ray spectrum. These peaks arise from the $110\to 0$- and $1554\to 197$-keV transitions shown in Fig. 1.

Figure 5

Experimentally determined $\gamma$-ray efficiencies for the $8\pi$ array in the range $80\le E_{\gamma }\le 1408$ keV. The dashed curve outlines normalized simulated efficiencies for individual $\gamma$ rays determined using the PENELOPE code. The gray band represents our conservative estimate of the total uncertainty in the simulations. The red squares show the determined efficiencies at 110 and 1357 keV, respectively, that were eventually used to determine the branching ratios of interest.

Figure 6

Best fit to the dead-time-affected $\beta$ singles activity curve. The total ${\chi }^2$ for the fit is $30627$ for 193 degrees of freedom, which is rather poor. However, this has an inconsequential effect on the eventual determination of $\beta$-decay branches. The data described in this analysis were restricted to those runs that were collected at initial $\beta$ rates of $\lesssim 1$ kHz per paddle [14]. In comparison, the measured $\gamma$-ray rates averaged approximately 150 Hz over the whole experiment.

Figure 7

The axial-vector form factor for ${}^{19}\mathrm{Ne}{\beta }^{+}$ decay obtained from independent $\beta$ asymmetry measurements. The standard model prediction $c_{\mathrm{SM}}$ obtained from this work is shown for comparison.

Figure 8

A comparison of 90% CL constraints on $\delta$ and $\zeta$ set by two independent $A_{\beta }\left(0\right)$ measurements for ${}^{19}\mathrm{Ne}$ decay (black solid line [1] and red dashed line [50]), together with the $\mathcal{F}t$ value obtained in this work. The allowed regions were generated assuming the manifest LR model.