Type of Document	Dissertation
Author	Wiedeking, Mathis
Author's Email Address	wiedekin@nucmar.physics.fsu.edu
URN	etd-12142004-163314
Title	Nuclear Structure of 86Zr and 20O and Beam Pulsing Techniques
Degree	PhD
Department	Physics, Department of
Advisory Committee	Advisor Name Title Ingo L. Wiedenhoever Committee Member Jorge Piekarewicz Committee Member Samuel L. Tabor Committee Member Timothy M. Logan Committee Member Vasken Hagopian Committee Member
Keywords	tetrahedral symmetry isomer shell gap p-sd shell model 20O 86Zr band termination
Date of Defense	2004-12-02
Availability	unrestricted
Abstract High angular momentum states in $^{86}$Zr were populated through the $^{58}$Ni($^{32}$S,4p) reaction at 135 MeV using the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory. Recoiling $^{86}$Zr nuclei were stopped in a thick Ta backing. Prompt multi-$gamma$ coincidences with evaporated charged particles were detected using the full array of {sc Gammasphere} and the {sc Microball}. Mean lifetimes of 36 levels in $^{86}$Zr were measured using the Doppler-shift attenuation method. Transition quadrupole moments $Q_t$ were found in the range of about 0.3 to 1.5 $e$b in the positive-parity bands. The negative-parity bands show $Q_t$ values from about 0.25 to 1.2 $e$b. In the yrast positive-parity band, a sharp drop in collectivity approaching the 30$^+$ state supports the interpretation of band termination in this configuration. Decreasing $Q_t$ values approaching the 24$^+$ and 27$^-$ states also provide an indication of terminating structures. Two beam pulsing devices were implemented at the Florida State University Superconducting Linear Accelerator Laboratory. The $mu$s pulsing device consists of parallel plates which deflect the charged ion beam by applying a high voltage. Pulsing electronics controlling the beam deflection and acquisition system with variable time intervals were designed and built. This setup is suitable for lifetime measurements from about a hundred ns up to several ms. The ns pulsing device uses a 48 MHz buncher and two choppers operating at 24 and 6 MHz. This setup is suitable for lifetime measurements in the range of $approx$ 1 to 100 ns. An extensive test experiment was performed to verify the accurate operation of these devices by populating known isomeric states with a wide range of lifetimes. Excited states in $^{20}$O were populated in the reaction $^{10}$Be($^{14}$C,$alpha$) at Florida State University. Charged particles were detected with a particle telescope consisting of 4 annularly segmented Si surface barrier detectors. The FSU $gamma$ detector array was used for $gamma$ radiation detection. Five new states were observed below 6 MeV from the $alpha$-$gamma$ and $alpha$-$gamma$-$gamma$ coincidence data. Shell model calculations suggest that most of the newly observed states are core-excited 1p-1h excitations across the $N = Z = 8$ shell gap. Comparisons between experimental data and calculations for the neutron-rich O and F isotopes imply a steady reduction of the $p$-$sd$ shell gap as neutrons are added.
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