Precise Half Life Measurement for the Superallowed $\beta ^{+}$ Emitter$^{34}$Ar

The half-life of the superallowed $\beta ^{+}$ emitter $^{34}$Ar was measured as part of our program to test the Standard Model via the unitarity of the CKM matrix, which requires 0.1{\%} precision or better. A 25$\cdot A$ MeV $^{35}$Cl beam from the Texas A{\&}M cyclotron initiated the $^{1}$H($^{35}$Cl, pn)$^{ 34}$Ar reaction, with recoils then passing through the MARS recoil separator. After being degraded, $^{34}$Ar ions were implanted as a 99.7{\%} pure source in the tape of a fast transport system. After a short collect time (0.7s or 1.0s), the beam was turned off and the implanted source moved in 175 ms to the center of a shielded 4$\pi $ proportional gas counter. Decay positrons were then multi-scaled for 12 s. The cycle was repeated until more than half a billion combined $^{34}$Ar and $^{34}$Cl decay events had been recorded. Critical detection parameters were changed periodically to expose any possible systematic effects; none were found. The extraction of a precise half-life for $^{34}$Ar presents a special challenge as its daughter $^{34}$Cl is itself $\beta ^{+}$ unstable with a half-life only 1.8-times longer. This results in an observed spectrum that is almost indistinguishable from pure $^{34}$Cl decay. A special method was developed to extract a precise half-life for the shorter-lived component in such a decay curve. Our preliminary result for the half-life of $^{34}$Ar is 843.8(7) ms.