Type of Document Dissertation Author Reynolds, Robert R. URN etd-07132010-112454 Title A Study of the Single Neutron Knockout Reaction from Si-26 and S-30 Degree Doctor of Philosophy Department Physics, Department of Advisory Committee
Advisor Name Title Paul Cottle Committee Chair Hon-Kie Ng Committee Member Ingo Wiedenhoever Committee Member Jorge Piekarewicz Committee Member Linda DeBrunner University Representative Keywords
- Nuclear Structure
- Nuclear Spectroscopy
- Knockout Reaction
- Shell Model
Date of Defense 2010-06-30 Availability unrestricted AbstractThe use of single nucleon and two-like nucleon knockout reactions of medium to heavy mass exotic beams on light targets has proven an invaluable tool in exploring nuclear properties away from the valley of beta-stability up to the proton and neutron drip-lines. The nuclear shell model has had a great amount of success in describing structural properties of nucleons populating states from the 1s1p-shells up to the 2p1f -shells, of particular interest is the success of the USD shell model used in the truncated 2s1d-shell space. The USD Hamiltonian was updated in 2005 to include the effects of exotic nuclei. Using relativistic beam velocities greater then 30% of the speed of light allows for the direct exploration of underlying single particle valence state structure. Comparisons between current shell models and experimental results are showing discrepancies between measured and theoretical cross sections.
The focus of the present work is on the single neutron knockout reactions 9Be(26Si,25Si+gamma) and 9Be(30S,29S+gamma). Relativistic beams containing 26Si and 30S were created at the National Superconducting Cyclotron Laboratory's Coupled-Cyclotron Facility using the A1900 fragment separator. The secondary 376 mg/cm2 thick 9Be target was located at the pivot point of the S800, a large-acceptance, high-resolution spectrometer with a specialized detector system that allowed for accurate event-by-event particle identification of the incident and residual particles based on their mass and charge, as well as providing accurate longitudinal momentum distribution measurements of the post-target beam. The secondary target was also surrounded by SeGA, a gamma-ray detector array specifically designed for accurate Doppler reconstruction of observable gamma-rays into the emitting particles rest frame. Measurements were made of the direct inclusive and individual state population cross sections in the residual states, as well as the first measurements of electromagnetic transitions between energy levels in both 25Si and 29S.
Two new gamma-rays were observed for 25Si at 821(15) keV and 1088(22) keV. The 821(15) keV gamma-ray is a direct decay from the first 1/2+ state to the 5/2+ ground state, while the 1088(22) keV gamma-ray is actually a result of a 3/2+ state at 1909(27) keV feeding into the 1/2+ state at 821(15) keV. The measured cross sections for the 26Si to 25Si reaction were sigma(inclusive) = 26.1(35) mb, sigma(1909keV ) = 1.06(21) mb, and sigma(821keV ) = 4.06(61) mb. There were three new gamma-ray transitions seen for 29S with energies of 1160(16) keV, 1222(20) keV, and 1727(37) keV, which correspond to the first experimental observation of excited states in 29S. The 1222(20) keV decay is the direct decay from the 1/2+ first excited
state to the 5/2+ ground state, while the 1727(37) keV gamma-ray is the decay of the 7/2+ state to the ground state, however the 1727(37) keV state is only populated by direct feeding from the 5/2+ state at 2887(40) keV by the observed 1160(16) keV gamma-ray. The measured cross sections for the 30S to 29S reaction were sigma(inclusive) = 27.5(26) mb, sigma(1222keV ) = 3.09(33) mb, and sigma(2887keV ) = 1.86(15) mb.
The reduction factor to the shell model spectroscopic strength for the inclusive cross section for the 26Si to 25Si reaction is 0.55(7) and the reduction factor for the inclusive cross section for the 30S to 29S reaction is 0.46(4). With valence binding separation energy differences of dS = 13.5 MeV for 26Si and dS = 14.6 MeV for 30S, these reduction factors are in good agreement with the systematic behavior of reduction factors from other single nucleon knockout reaction studies. The calculated electromagnetic multipole transition strengths in both 25Si and 29S, when taken into consideration with the observed decay branching ratios, allows some speculation that these nuclei are well-deformed and that the known region of deformation in the sd-shell extends out to both. Based on the evidence in the current work, intermediate-energy Coulomb excitation studies would help in completing the level schemes and also verify the branching strength of the E2 transitions characteristic of deformed nuclei.
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