Precise half-life measurements for the superallowed β⁺ emitters ³⁴Ar and ³⁴Cl

To contribute meaningfully to any test of the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, the measured ft value of a superallowed 0⁺→0⁺ β⁺ transition must be obtained to a precision of 0.1% or better. We have determined the half-life of the superallowed emitter ³⁴Ar to be 843.8(4)ms; the quoted precision, 0.05%, is a factor of five improvement on the best previous measurement and meets this demanding requirement. Our measurement employed a high-efficiency gas counter, which was sensitive to positrons from both ³⁴Ar and its daughter ³⁴Cl. We achieved the required precision on ³⁴Ar by analyzing the parent-daughter composite decay with a new fitting technique. We also obtained an improved half-life for ³⁴Cl of 1.5268(5) s, which has 0.03% precision and is a factor of two improvement on previous results. As a by-product of these measurements, we determined the half-life of ³⁵Ar to be 1.7754(11) s.