Precise branching ratios from $\beta -\gamma $ coincidences: the case of $^{^{34}}Ar$.

The experiment reported here aims to extract a precise \textit{ft}-value for the superallowed $0^+\to 0^+ \quad \beta ^+$-decay of $^{^{34}}Ar$. Such measurements are essential in testing the Standard Model via the unitarity of the Cabibbo-Kobayashi-Maskawa matrix. One ingredient in the $ft$-value is the branching ratio and, to be useful, it must be determined with a precision of $\sim $0.1{\%} or better. Since the $\beta ^+$-decay of $^{^{34}}Ar$ populates the ground-state as well as excited states of the $^{3^{34}}Cl$ daughter, to determine the branching ratio, we require $\beta -\gamma $ coincidences as well as $\beta $ singles, both with well determined intensities. Gammas were measured with our precisely calibrated HPGe detector whose absolute efficiency is known to 0.2{\%} for energies between 50 and 1400 keV and to 0.4{\%} from 1400 to 3500 keV. For the $\beta $ detector we also require accurate relative efficiencies. To this end, we have compared the recorded beta spectra -- in singles and in coincidence with individual $\gamma $-ray peaks -- with Monte Carlo calculations performed with the DOSRZNRC program (from the EGS package) [1]. Good agreement was obtained over a wide energy range, allowing us to extract precise branching-ratio results. [1] NRCC Report PIRS-701 and http://www.irs.inms.nrc.ca/inms/irs/EGSnrc/EGSnrc.html