Lifetimes of N

The best known element of the Kobayashi-Maskawa quark mixing matrix is U„d. It is determined' by comparing the vector coupling constant Gz of nuclear beta decay to the Fermi coupling constant Gz determined from muon decay, after correcting Gz for "inner" radiative effects' that depend upon the assumed substructure of the nucleon. An accurate determination of U„d is necessary to test the unitarity of the Kobayashi-Maskawa matrix, a measure of the number of generations. The ft values of the 0+~0+ superallowed Fermi beta decays provide the most sensitive measurement of Gz. Eight cases, from ' 0 to Co, are known to &0.2%. Before Gv may be extracted from these measured ft values, they must be corrected for "outer" radiative and Coulomb effects. The outer radiative corrections depend on the nuclear charge and size and on the total energy released in the decay. They are believed to be known ' to g 0.1%. The Coulomb corrections, ' which depend upon the detailed structures of the parent and daughter nuclei, are considerably less certain. In general, they tend to increase approximately as Z, so it is of interest to measure heavier cases to test the accuracy of these calculations. The highest Z 0+~0+ Fermi transition whose ft value has been measured to date is Ga, which is currently known to 1.5%. This uncertainty is dominated by our knowledge of the Ga endpoint energy, as the half-life is known to 0.2%. By contrast, the half-lives of As and Br, the next two nuclei with 0+~0+ Fermi beta decays, have only been determined to 0.4% and 1.0%, respectively, and their endpoint energies have not yet been measured. As a first step in an effort to extend our knowledge of superallowed Fermi decays to higher Z systems, we have remeasured the half lives of As and Br, using a detection system that is subject to different systematic efFects from those associated with the previous experiment. While our As half-life agrees with the previous measurement, our Br half-life determination differs somewhat from the previous result. In Sec. II we describe the detector system developed for this measurement. In Sec. III we discuss the data analysis and results. In Sec. IV we provide some conclusions. As was produced with the reaction Ni(' B,2n ) As, using 29.5 and 30.5 MeV ' B + beams from the Texas A&M University 224-cm Cyclotron. Br was produced with the reaction Ni(' N, 2n) Br, using a 42.5 MeV ' N + beam. Figure 1 shows a schematic layout of the beamline and target chamber, including the P-ray detector and shielding. The beams passed through movable X-Y collimator slits located 5.3 m before the target. These slits defined the beam size at the target and ensured that no beam passed around a piston-driven beam shutter, the forward Faraday cup (FCf ), when it was closed. FCf was used to chop the beam on target mechanically. We observed &2 pA on target with FCf shut when the slits were opened to 2)&2 cm . FCf also served as the primary aid used in keeping the cyclotron beams tuned, since the beam remained on FCf over 90% of the time. Typical beam currents on target were 600 nA, with 80% transmission between FCI and the target. To reduce the buildup of long-lived activities, a rapidtransport target system was employed. Twenty equally spaced targets, each consisting of 5.0 mg/cm of 99.89% enriched Ni, were mounted on a 15.2-cm diameter Al wheel. A Ta collimator placed at the entrance to the target chamber prevented the beam from hitting the target wheel during the irradiation periods. A Slo-Syn stepping motor (M series), coupled to the target wheel via a Ferrofluidic feedthrough, rotated it on demand. A preset