Penning trap mass spectrometry Q-value determinations for highly-forbidden $\beta $-decay*

Over the last several decades, extremely sensitive, ultra-low background beta-particle and gamma-ray detection techniques have been developed. These techniques have enabled the observation of very rare processes, such as highly forbidden beta decays e.g. of $^{\mathrm{113}}$Cd , $^{\mathrm{50}}$V, and $^{\mathrm{138}}$La. Half-life measurements of highly forbidden beta decays provide a testing ground for theoretical nuclear models, and the comparison of calculated and measured energy spectra could enable a determination of the values of the weak coupling constants. Precision Q-value measurements also allow for systematic testing of the beta-particle detection techniques. We will present the results and current status of Q value determinations for highly forbidden beta decays of $^{\mathrm{113}}$Cd, $^{\mathrm{50}}$V, and $^{\mathrm{138}}$La. The Q values, corresponding to the mass difference between parent and daughter nuclides, are measured using the high precision Penning trap mass spectrometer LEBIT at the NSCL.