The Structure of 33Si,35S and the magicity of the N = 20 gap at Z = 14; 16

Jongile, Sandile (2021-12)

Thesis (PhD)--Stellenbosch University, 2021.

Thesis

ENGLISH ABSTRACT: Nuclei along N = 20 provide an excellent region to investigate nuclear structure and interactions, with their evolution from the doubly magic nucleus 40Ca through the Z = 16 and Z = 14 nuclei 36S and 34Si, respectively, to 32Mg with a deformed 2p 􀀀 2h intruder ground state. This study is motivated by and focuses on: i) The robustness of the N = 20 shell gap from 40Ca (studied previously by Matoba et al., [1]) to 36S and 34Si, after removing 4 and 6 protons, respectively. A strong sd-shell closure would lead to a fully occupied neutron d3=2 orbital and no, or little occupancy, neutrons in the p3=2 and f7=2 orbitals located above N = 20. With the deformed 32Mg having only 2 protons removed from 34Si it is an interesting question if the magicity is somewhat gradually or abruptly eroded below Z = 14. ii) A significant reduction of the neutron 1d5=2 and 1d3=2 spin-orbit splitting between 40Ca and 36S, as protons are removed from the 1d3=2 orbital, would be indicative of the effect of proton-neutron tensor force. By comparing the neutron 1d5=2 hole strength between these nuclei, the strength of the tensor force can be probed in an unprecedented manner. Two separate studies were carried out to address the aforementioned motivations. Firstly, an inverse kinematics experiment with the 9Be(34Si,33Si+ )X and 9Be(36S,35S+ )X reactions which was performed at the National Superconducting Cyclotron Laboratory (NSCL) with 98.5 MeV/u 34Si and 88 MeV/u 36S secondary beams produced in the fragmentation of a 48Ca primary beam has been reanalysed. Reaction products were detected with the Gamma-Ray Energy Tracking In-beam Nuclear Array (GRETINA) coupled to the S800 magnetic spectrometer. This measurement focused on probing the Fermi surface in 34Si and 36S, and locating the strength of the neutron d5=2 orbital. From the spectroscopic factor values, which are derived from observed -ray decays, the neutron 1d3=2 appears to be fully occupied, while some fraction of the 1d5=2 orbital is observed only as the states lie above the neutron emission energy threshold. Secondly, the 36S(p; d)35S reaction is a useful tool to probe the change in neutron spinorbit splitting between 34Si and 36S. The 36S(p; d)35S reaction allows for an investigation into the 36S Fermi surface stiffness and the neutron d3=2 􀀀 d5=2 spin-orbit reduction. It also serves to probe the magicity of 36S through its Fermi surface, complementary to the (d; p) reaction previously performed, reported in Ref. [2]. Of course all this depends on the availability of a reliable 36S target. This was achieved by specifically developing a new target system at iThemba LABS which allows for a cost effective 36S target without heavy contaminants to be used. This novel target, which is in motion and encapsulates sulfur between two Mylar foils, has been shown to be an effective way to produce targets with a significant amount of material (0.5-1 mg/cm2) [3]. Using the developed moving 36S target system with 66 MeV incident protons, states in 35S were populated and studied with the K600 magnetic spectrometer at iThemba LABS. States up to 9 MeV are observed, identifying the neutron single-particle strength below and above the Fermi surface using the detection of the deuterons at the focal plane of the K600 spectrometer with an energy resolution of approximately FWHM = 30 keV in the center of mass.

AFRIKAANSE OPSOMMING: Kerne in die N = 20 isotoon bied vrugbare grond vir kernstruktuur en kernreaksie studies aangesien dit toegang verleen tot ’n verskeidenheid van verskynsels, vanaf die 40Ca kern met tower getalle vir beide protone en neutrone, die Z = 16 36S en die Z = 14 34Si kerne tot by 32Mg met sy vervormde 2p 􀀀 2h indringer grondtoestand. In hierdie studie word daar op die volgende aspekte gefokus: i) Die karakter van die N = 20 skilmodel gaping met verandering van proton getal vanaf 40Ca (vantevore bestudeer deur Matoba et al.,) tot by 36S en 34Si, soos wat 4 en 6 protone respektiewelik verwyder word uit die kern. In die geval van ’n robuuste sd-skil afsluiting moet die neutron d3=2 orbitaal heeltemal gevul wees, en behoort die p3=2 en f7=2 orbitale bokant N = 20 feitlike leeg te wees. Aangesien die vervormde 32Mg kern verskil van 34Si slegs deur die verwydering van twee protone, sal dit interessant wees om vas te stel of die verandering in voorkeurgetal eienskappe van die kern geleidelik of skielik verander benewens Z = 14. ii) ’n Beduidende verlaging van die neutron 1d5=2 en 1d3=2 spinbaan splyting in 36S in vergelyking met die 40Ca kern, soos wat protone verwyder word uit die 1d3=2 orbitaal, kan dui op die impak van die proton-neutron tensor wisselwerking. Inligting aangaande die sterkte van die tensor wisselwerking kan verkry word deur vergelykings te maak tuseen die neutron 1d5=2 holte sterkte van hierdie kerne. Twee onafhanklike experimente is onderneem ten einde bogenoemde aspekte te ondersoek. Eerstens is die 34Si(-1n)33Si en 36S(-1n)35S reaksies eksperimenteel ondersoek by die National Superconducting Cyclotron Laboratory (NSCL). In hierdie inverse kinematiese metings was die 98.5 MeV/u 34Si en 88 MeV/u 36S sekondêre bundels verkry deur die fragmentasie van ’n primêre 48Ca bundel, waarna reaksieprodukte waargeneem is met behulp van die Gamma-Ray Energy Tracking In-beam Nuclear Array (GRETINA) tesame die S800 magnetiese spektrometer. Hierdie meting het ten doel gehad om die Fermi oppervlakte van die 34Si en 36S kerne te ondersoek, en om vas te stel wat die posisie van die neutron d5=2 orbitaal is. Met behulp van die geassosieerde spektroskopies faktore, soos bepaal deur middel van metings van verval met GRETINA, is dit bepaal dat die neutron 1d3=2 orbitaal volledig gevul is. Verder is ook vasgestel dat slegs ’n gedeelte van die 1d5=2 orbitaal waargeneem, aangesien die toestand geleë is bokant die neutron emissie drempel. Tweedens is 36S(p,d)35S reaksie benut om die verandering in die spinbaan splyting tussen 34Si en 36S te ondersoek. Hierdie reaksie kan verder gebruik word om die 36S Fermi oppervlak styfheid te ondersoek, asook die voorkeurgetal karakter van 36S, komplementê tot die resultate van die (d,p) reaksie wat vantevore ondersoek is. Vanselfsprekend kan hierdie ondersoek slegs gedoen word indien ’n betroubare 36S teiken beskikbaar is. ’n Nuwe, unieke en koste effektiewe teiken meganisme is ontwerp by iThemba LABS spesifiek vir die doel van hierdie studie. Hierdie bewegende teiken bestaan uit twee Mylar foelies wat die swael omhul, en het die voordeel dat dit geen swaar kerne as kontaminante bevat nie. Dit is verder bewys dat hierdie proses suksesvol gebruik kan word om swael teikens te maak wat 0.5-1 mg/cm2 dik is. So ’n bewegende 36S teiken was gebruik tesame met ’n 66 MeV proton bundel om kerntoestande in 35S te ondersoek met behulp van die K600 magnetiese spektrometer by iThemba LABS. Tydens die eksperiment is deutrone waargeneem in die fokale vlak van die spektrometer met energie resolusie van die meting ongeveer 30 keV (FWHM). Opgewekte toestande tot en met 9 MeV is waargeneem, wat dit moontlik maak om neutron enkeldeeltjie sterkte aan beide kante van die Fermi oppervlakte te ondersoek.

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