Search for Shape Transitions in Neutron-rich W isotopes and Optimizing Hole Mobilities for Gamma-Ray Tracking.

摘要

Nuclear structure studies are intimately tied to advanced detector arrays, both current and emerging. The present work describes two independent projects that have advanced detector arrays in common. The first explores the physics of nuclear shapes and shape evolution with angular momentum in the A~180 region using Gammasphere, arguably the world's most advanced 4pi array for high-resolution gamma-ray spectroscopy at present, coupled to CHICO2, a state- of-the-art position-sensitive detector for heavy-ions in binary reactions near the Coulomb barrier. The second involves simulations and analysis for GRETINA, a next-generation segmented gamma-ray tracking array which can resolve the in- teraction position of a gamma-ray to a few mm, and is slated to evolve into the germanium shell GRETA, for use at the national Facility for Rare Isotope Beams.;The reasonably rare phenomenon of a transition from prolate collective to oblate collective rotation along the yrast line, long predicted and possibly observed recently in 180Hf, are expected to occur at even lower spins in 186W. Prompt collective excitations in 186 W were populated via inelastic excitation using a 136Xe beam from the ATLAS accelerator at Argonne incident on a thin 186W target. Emitted gamma-rays were detected using Gammasphere, coincident with the binary reaction fragments detected in the heavy-ion counter CHICO2. Level schemes were extended in 186W, although experimental constraints limited the observation of states above 14+. The observed gamma-ray yields were compared to Coulomb excitation predictions. A large staggering between the even- and odd-spin members of the gamma-vibrational band suggests increased triaxiality in this nucleus. Transfer reaction products were also analyzed, and the yrast band in 185W extended to higher spins. A sudden increase in signature splitting in the ground state band of 187W, compared to 185W suggests a boundary for the onset of gamma softness.;The GRETINA array consists of "quad" modules of segmented position- sensitive Ge crystals, each with a central contact along the cylindrical axis that collects the electrons and 36 contacts along the cylindrical surface which collect the "hole" current pulse generated by a gamma ray interaction inside the crystal. A signal decomposition algorithm is used to fit the observed waveform from each crystal contact with a linear combination of stored basis signals to localize the gamma-ray interaction within the detector volume. In this work, the sensitivity of the hole mobility parameter for position reconstruction in GRETINA is investigated. Calibration data on position resolution is analyzed, together with simulations that isolate the signal decomposition dependence from electronics cross-talk. The chi-square fits exhibit a shallow minimum for +/-15% variation in the hole mobility, effectively excluding it as a parameter in play for addressing remaining challenges in reconstructing gamma-ray interaction points in GRETINA.