Direct Mass Measurements on the Superallowed Emitter ${}^{74}\mathrm{R}\mathrm{b}$ and Its Daughter ${}^{74}\mathrm{K}\mathrm{r}$: Isospin-Symmetry-Breaking Correction for Standard-Model Tests

The decay energy of the superallowed $\beta$ decay ${}^{74}\mathrm{R}\mathrm{b}({\beta }^{+}){}^{74}\mathrm{K}\mathrm{r}$ was determined by direct Penning trap mass measurements on both the mother and the daughter nuclide using the time-of-flight resonance technique and was found to be $Q=10416.8(4.5)\mathrm{k}\mathrm{e}\mathrm{V}$. The exotic nuclide ${}^{74}\mathrm{R}\mathrm{b}$, with a half-life of only 65 ms, is the shortest-lived nuclide on which a high-precision mass measurement in a Penning trap has been carried out. Together with existing data for the partial half-life as well as theoretical corrections, the decay energy yields a comparative half-life of $Ft=3084(15)\mathrm{s}$ for this decay, in agreement with the mean value for the series of the lighter nuclides from ${}^{10}\mathrm{C}$ to ${}^{54}\mathrm{C}\mathrm{o}$. Assuming conserved vector current, this result allows for an experimental determination of the isospin-symmetry-breaking correction ${\delta }_{\mathrm{C}}$.

Figure 1

Sketch of the ISOLTRAP mass spectrometer used for the ${}^{74}\mathrm{K}\mathrm{r}$ and ${}^{74}\mathrm{R}\mathrm{b}$ mass measurements, consisting of a linear Paul trap located on a high-voltage (HV) platform, a cooling Penning trap, a precision Penning trap, an alkali reference ion source, and a carbon cluster ion source. Micro-channel-plate (MCP) detectors monitor ion transfer and record the time-of-flight (TOF) cyclotron resonances (MCP 5). The insets show typical TOF cyclotron resonances of ${}^{74}\mathrm{K}\mathrm{r}^{+}$ and ${}^{74}\mathrm{R}\mathrm{b}^{+}$. The solid lines are fits of the theoretically expected line shape [15] to the data.

Figure 2

Mass deviation between the measurements reported here and the literature values (data points labeled “AME1995”) [24] for ${}^{74}\mathrm{K}\mathrm{r}$ (left) and ${}^{74}\mathrm{R}\mathrm{b}$ (right). The zero line is defined by the weighted means of the two ISOLTRAP measurements for each of the nuclides (data points labeled “mean”). In the case of ${}^{74}\mathrm{R}\mathrm{b}$, the 720-keV error bar of the literature value is too large to be fully displayed. The insets show our data and their weighted means at a finer scale.

Figure 3

(a) $Ft$ value for the decay ${}^{74}\mathrm{R}\mathrm{b}({\beta }^{+}){}^{74}\mathrm{K}\mathrm{r}$, compared to the data from Ref. 3. The dashed lines represent the confidence interval of the weighted mean $\overline{Ft}=3072.2(9)\mathrm{s}$ for the nine lighter nuclides. (b) Relative contributions to the variance of the $Ft$ values by the four input parameters $Q$ (decay energy), $t$ (partial half-life), ${\delta }_{\mathrm{R}}$ (radiative correction), and ${\delta }_{\mathrm{C}}$ (isospin-symmetry-breaking correction).

Figure 4

Isospin-symmetry-breaking parameter ${\delta }_{\mathrm{C}}$ calculated by use of the $Q$ value measured by ISOLTRAP under the assumption of vector current conservation, compared with calculated values [2, 8, 21]. See the discussion in text.