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Title: Electromagnetic properties of light proton-rich nuclei
Author: Caraca, Joao Manuel Gaspar
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 1972
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An experimental study of electromagnetic transition rates of some proton-rich nuclei of the sd shell was made. In order to perform the neutron-gamma coincidence measurements necessary to study these nuclei an associated γ-ray neutron time of flight spectrometer was constructed. The neutron detector has an annular shape and is divided in four quadrants, which are filled with NE213 liquid scintillator. The detector is viewed by eight XP1040 photomultiplier tubes (two per quadrant) and the neutron events in the scintillator are identified by means of pulse shape discrimination. The fast timing pulses are derived from constant fraction pulse height discriminators, thus enabling the use of wide dynamic ranges in both the neutron and the γ-ray detectors. The time resolution of the system, when the γ-detector is a large volume Ge(Li) crystal (82 cm3), is approximately 6 ns (FWHM). The decay modes and branching ratios of states in 37Ar up to 3.60 MeV excitation energy have been determined using the 37Cl(p,n)37Ar reaction. A new level at Ex = 3184±3 keV excitation energy has been found. The decay of the 7andfrasl;2, 3535 keV state has also been observed. Using the Doppler shift attenuation method (DSAM) P-values for some 37 35 levels in 37Ar and 35Cl were measured by means of the reactions 37Cl(p,n)37Ar and 35Cl(p,pandprime;)35Cl*. The lifetimes deduced were (in fs): τ(1409) = 370 +190/-110, τ(2489) = 88±24 and τ(2795)<8 in 37Ar; and τ(2646) = 83±21 and τ(2695) = 11±7 in 35Cl. The data provide further evidence that the commonly used stopping power theory is in error under certain circumstances and a discussion of the reliability of the DSAM measurements is presented. Further investigations have been made on the nucleus 18Ne, which was populated by means of the 16O(3He,n)18Ne reaction. The first excited 0+ state was definitely located at Ex = 3576 keV excitation energy and the lifetimes of both this level and the 4+ state at Ex = 3376 keV were measured with the Doppler shift recoil-distance method. The values obtained were (in ps) : τ(3376) = 4.4±0.6 and τ(3576)<6. Angular correlation measurements of the γ-decay of the 2+ level at 3616 keV were also made. As a result, the strength of the cross over branch of this state to the ground was observed to be (12.5±2.5)% and the E2/M1 mixing ratio of the 3616 → 1887 transition was also determined, yielding the value δ = +0.03±0.09. These results are compared with current theoretical calculations. Decay modes and lifetimes of low-lying levels in 30S and 34Ar were investigated by the Doppler shift attenuation method using the 28Si(3He,n)30S and the 32S(3He,n)34Ar reactions, respectively. The lifetimes measured were (in fs): τ(2210) = 310±75, τ(3402) = 160±45 and τ(3664)>1400 in 30S; and τ(2090) = 150±50, τ(3286) = 120±65 and τ(4513) = 300±80 in 34Ar. These results are compared with theoretical calculations. A comparison of the E2 decay strengths in masses 30 and 34 is also made to test the isospin dependence of electromagnetic transitions.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available