P*

As part of our program to test for physics beyond the standard model via β decays, we measured the half-life of ²⁹P that undergoes a T=1/2 nuclear mirror transition. The radioactive beam was produced in the ¹H(³⁰Si,2n)²⁹P reaction, with a 24 A MeV ³⁰Si beam impinging on a hydrogen target kept at 2.0 atm and liquid nitrogen temperature. Using the MARS spectrograph, a high purity ²⁹P beam at 22 A MeV was extracted from the reaction products. Then, the radioactive beam was extracted in air through a 51 μm thick Kapton window, and further passed through a 0.3 mm thick BC-404 plastic scintillator and an Al degrader. The thickness of the degrader was set to stop the desired activity in the 76-μm-thick Mylar tape of our fast tape transport system. The radio-purity of the collected activity exceeded 99.9%. After collecting the sample, the beam was turned off, and the activity was moved in the center of our 4π proportional counter, where it was multiscaled for 84 s (~40 half-lives). Repeating such collect-move-detect cycles, we collected more than 10⁸ events. The experiment was split in sub-runs differing in detector bias, signal discrimination threshold, and dominant dead-time. No systematic bias could be identified. Our preliminary half-life value is 4.1140(8) s, which is the most precise measurement to date.