Precise Half-Life Measurement of ³⁰S

As part of our program to test the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) matrix via 0⁺→0⁺ superallowed β decays, we measured the half-life of ³⁰S. The radioactive beam was produced by the ¹H(³¹P, 2n)³⁰S reaction, in which a 30A MeV primary ³¹P beam bombarded a hydrogen target held at liquid nitrogen temperature and a pressure of 2.0 atm. With the MARS spectrograph we selected a high-purity ³⁰S beam from the recoils. The beam was then extracted in air, passed through a 0.3-mm-thick BC-404 plastic scintillator and a set of Al degraders, adjusted to stop the ³⁰S nuclei in the center of the 76-μm-thick aluminized-mylar tape of our fast tape-transport system. We collected ³⁰S nuclei for 2.4 s; then the beam was switched off and the activity was moved in less than 0.2 s to the center of a 4π proportional counter. The observed decays were then multi-scaled over 24 s. Such collect-move-detect cycles were repeated until a total of 1.4×10⁸ decays were recorded. We split the experiment into 33 runs, each differing from the others in its discriminator threshold, detector bias or dominant dead-time. No systematic effects were identified in these parameters. We found the half-life of ³⁰S to be 1.17992(34) s, a result that is 5 times more precise than any previous measurement.