Measurement of the beta decay half-life of 17B.

The properties of nuclei far from stability provide a stringent test of the validity of shell model Hamiltonians derived from data for nuclei near the valley of stability. Failure of shell model calculations to extrapolate to nuclei far from stability may indicate a breakdown of symmetries assumed in the model. Such a breakdown may provide evidence for new regions of deformation as were found for neutron-rich sodium and magnesium isotopes. ' In the present experiment, the Michigan State University (MSU) reaction product mass separator (RPMS) (Ref. 2) was used to measure the beta decay half-lives of seven neutron-rich isotopes produced by the fragmentation of an E/A =35 MeV Ne beam incident on a tantalum stopping target. The MSU RPMS is designed to separate exotic nuclei produced in intermediate energy heavy ion reactions so that their decay properties can be studied. The RPMS is a "triply focusing" device— focusing in horizontal and vertical position as well as in velocity. It then disperses ions in the vertical direction according to their mass-to-charge ratio m /q. Jons of the mass to charge ratio of interest are focused onto a silicon detector telescope where they can be identified and their decay properties studied. The long target-to-focal-plane distance (14 m) assists in providing the relatively clean environment necessary for decay studies. The cyclotron beam was turned off following the detection of an ion of interest to allow detection of the beta decay of the ion without background from the reaction products. Using a 770 MeV Ne beam from the K500 cyclotron at the National Superconducting Cyclotron Laboratory, we made the first measurement of the half-life of ' B along with measurements of the half-lives of ' N, ' C, ' B, ' B, ' B, and Li. The neutron-rich isotope ' C was also observed in sufficient quantity for a lifetime measurement, but would have required a separate run for its measurement, since its half-life is significantly longer than the other isotopes of interest at the same mass to charge ratio. The isotope ' C was also observed, but not in sufficient quantities to make a half-life measurement. Beta decay half-lives were determined by event-by-event measurements of the absolute time of detection for an ion of interest and all subsequent events during the beam-off period. The technique, first used by Murphy et al. , is capable of measuring lifetimes as short as a few milliseconds. The beta decay time spectra measured in this experiment are shown in Fig. 1. The tantalum target thickness was chosen to stop the primary neon beam but allow light "projectilelike" fragments to escape. This allowed the RPMS to be operated at zero degrees. Reaction products were separated according to their mass-to-charge ratio, m/q, and detected in a focal plane detector consisting of a position sensitive single wire gas counter followed by a silicon detector telescope. The single wire gas proportional counter provided the horizontal position by resistive wire charge division and the vertical position by electron drift time. The silicon telescope was made up of three element, b,E, E, and veto, with thicknesses of 100 pm, 1 mm, and 100 pm, respectively. Aluminum foil degraders were placed in front of the telescope and adjusted in thickness such that isotopes of interest were stopped in the Si E detector. The third element of the telescope was used as a veto to reject He and Li isotopes that punched through the E detector. These isotopes had count rates which were much higher than those of the isotopes of interest. When an ion with Z )2 was detected that did not fire the veto, the beam was turned off in approximately 40 ps by switching the rf phase on one of the dees of the K500 cyclotron. The beam remained off for a preset time of either 125 ms or 1 s chosen to be several half-lives of the longest-lived isotope of interest present at the focal plane. At this time the silicon detector preamplifier gains were increased by a factor of 10 (in about 2 ms) in order to facilitate the detection of beta decay. The data acquisition electronics were inhibited during the gain-switching period. During the beam-off period, all counts in the E detector above a threshold of approximately 0.3 MeV were recorded. A scalar was used to count pulses from a prescaled 262. 144 kHz quartz-crystal oscillator, which gave the absolute time for the detection of the incident ion and for all events in the subsequent beam-off period. The data associated with the particle identification and beam-off events were then written to magnetic tape for off-line analysis. The analysis of the data for the cases of the short-lived isotopes ' B, ' B, ' B, and ' B was complicated by the presence of a short-lived background attributed to reactions induced by thermal neutrons in the vault. In addition, the background also contained a constant component due to long-lived isotopes implanted in the silicon