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Gamma-Gamma Angular Correlation Measurements With GRIFFIN

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Title: Gamma-Gamma Angular Correlation Measurements With GRIFFIN
Author: MacLean, Andrew
Department: Department of Physics
Program: Physics
Advisor: Svensson, Carl
Abstract: When an excited nuclear state emits successive γ-rays in a γ − γ cascade, an anisotropy is generally found in the spatial distribution of the second γ-ray, γ2, with respect to the first, γ1. By defining the direction of γ1 to be the z-axis, the intermediate level, in general will have an uneven distribution of m-states. This causes an anisotropy in the angular correlation of the second γ-ray with respect to the first. Angular correlation measurements can be used for the assignment of spins and parities to the nuclear states, and thus provide a powerful means to elucidate the structure of nuclei away from stability through β − γ − γ coincidence measurements. In order to explore the sensitivity of the new 16 high-purity germanium (HPGe) clover-detector Gamma-Ray Infras- tructure For Fundamental Investigations of Nuclei (GRIFFIN) at TRIUMF-ISAC, a series of measurements were taken to establish a methodology for such γ − γ angular correlations. The first case studied to test the performance of GRIFFIN for these measurements was the well-known 4+ → 2+ → 0+ γ−γ cascade from 60Co β− decay. Geant4 simulated experiments and experimental source data were collected and analyzed as the primary test of GRIFFIN. The next case studied was a 66Ga beam implanted in the centre of the array. With 66Ga, three cascades were examined to analyze the γ − γ angular correlations. The first angular correlation being a 2+ →2+ →0+ cascade of 833-1039 keV γ-rays with a known mixing ratio of δ = -1.9(3). Next was a known 0+ →2+ →0+ cascade with the γ-ray energies in coincidence being 1333-1039 keV . The final correlation for this nucleus was a 1+ →2+ →0+ cascade of 2752-1039 keV γ-rays with a mixing ratio of -0.12(2). The next nucleus studied was the β+ decay of 62Ga, which is a superallowed Fermi β emitter. This nucleus was a first attempt to discern a recently challenged spin assignment to the 2.34 MeV excited state. The labelling of this state as a 2+ or a 0+ has effects on the isospin symmetry breaking correction factor used for calculating Ft values. Forthe60Comeasurementsthespinassignmentswerefullyconsistentwitha4+ →2+ →0+ cascade, but due to the similarity of the 2+ →2+ →0+ with a mixing ratio of δ = 0.18(1) which gives a nearly identical angular correlation and was indistinguishable. Also in 66Ga decay, the precision of the mixing ratio for the 2+ →2+ →0+ cascade involving the 833- 1039 keV γ-rays was improved giving a value of δ = −2.1(2). Finally, the mixing ratio for the 1+ →2+ →0+ cascade involving the 2752-1039 keV γ-rays was determined to be δ = −0.08(3). This measurement was completed without realizing the value was known and is in complete agreement with the previous measurement. In the 62Ga measurement the assignment of the spin of the 2.34 MeV excited state was unable to be discerned due to reduced statistics from a faulty extraction electrode. The measurement seemed to favour the assignment as a 0+ state but additional statistics are required for a definitive assignment.
Date: 2016-03
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