Statistical properties of deformed Samarium isotopes and constraining the nucleosynthesis of 180Ta

Malatji, Kgashane Leroy (2019-12)

Thesis (PhD)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: This thesis focuses on two areas of research, i) statistical properties of well-deformed samarium isotopes and ii) constraining the nucleosynthesis of 180Ta. The rare-earth isotopic chain of samarium provides an excellent opportunity to systematically investigate the evolution of nuclear structure effects from the near spherical 154Sm isotope to the well-deformed 154Sm. As the nuclear shape changes, statistical properties such as the nuclear level density (NLD) and g-strength function (gSF) are expected to be affected. In particular resonance modes in the rare-earth region may reveal interesting features when their evolution is investigated across several nuclei in an isotopic chain. An experiment was performed at the Oslo Cyclotron Laboratory (OCL) to measure particle-g coincidences from which the NLDs and gSFs were extracted using the Oslo Method. A deuteron beam was used to populate excited states in 154,155Sm. Results from this work indicate a pronounced M1 scissors resonance at » 3 MeV in 154Sm. Surprisingly, the gSF of 154Sm appears to be more featureless. The measured integrated strength in both 154,155Sm shows nonetheless a reasonable agreement with previous measurements of other deformed samarium isotopes. Overall, the results provide complementary information to the (3He,ag)148Sm, (3He,3He’g)149Sm, (d,d’g)152Sm, (d,pg)153Sm, (p,dg)147,149,151,153Sm, (p,p’)154Sm and (a,a’g)154Sm data previously measured. A small number of naturally occurring neutron-deficient nuclides with Z > 33, referred to as p-nuclei, cannot be produced by (n,g) processes in the cosmos. Instead, these stable nuclides are thought to be produced by the photodisintegration (p-process) of both slow-neutron capture (s-process) and rapid-neutron capture (r-process) seed nuclei. However, for 180Ta, the p-process is not sufficient to explain the observed solar abundances and therefore additional processes are invoked. The 180Ta production mechanism causes controversy since calculations show that several processes, sometimes exclusively, can reproduce its observed abundance, making it a particularly interesting case to study. Since the astrophysical sites for the nucleosynthesis of 180Ta remain unknown, a combination of several processes is undeniably possible. However, the significance of individual processes cannot be clearly determined, as a result of the uncertainties on the reaction rates for 180Ta due to the unavailability of experimental data. The 180,181,182Ta NLDs and gSFs were recently measured at the OCL. From these results, the (n,g) cross sections are calculated using the nuclear reaction code TALYS and compared to previous results. Furthermore, the reaction rates are calculated and used in s-process calculations in low-mass AGB stars and p-process simulations in Type-II supernovae to investigate the nucleosynthesis of 180Ta. From this work the newly constrained reaction rates of the s-process show that in low-mass AGB stars the contribution to the production of 180Ta is negligible and only partially contributes to the production of 181Ta. The p-process nucleosynthesis of nature’s rarest stable isotope 180Ta in Type-II supernovae using newly constrained data is significant and therefore suggests the p-process to be the fundamental production mechanism of 180 Ta in the universe.

TSWANA BOIKGOPOLELO: Kakanyo-thero ye e lebišitše go karolo tše pedi tša nyakišišo, i) mekgwa ya dipalopalo ya di-isotope tša samarium tšeo di golofetšego, le ii) go tšweletšo ya 180Ta. Selokaloke sa isotope ya samarium se beela tšhono ye botse yago nyakišiša ka tlhagelelo ya ditlamorago tša tlhamego ya nuclear go tšwa go 144Sm isotope ya nkgokolo go ya go 154Sm yeo e golofetšego. Ge sebopego sa nuclear se fetoga, mekgwa ya dipalopalo bjalo ka tekanyo ya boima bja nuclear (NLD) le morero wa maatla (gSF), di lebeletšwe go ka amega. Kudukudu mekgwa ya go galagala e ka tšwetša kgakala dibopego ge tlhagelelo ya tšona e nyakišišwa mo go nuclei tše mmalwa mo di-isotopong. Go dirilwe boitekelo Oslo Cyclotron Laboratory (OCL) go lekola diNLD le di gSF, go šomišwa mokgwa wa Oslo. Go dirišitšwe deuteron beam go hlola seemo sa tlhalalo ka gare ga 154,155Sm. Dipoelo go tšwa mošomong wo, di laetša kwagatšo ya maatla a M1 gola » 3 MeV kago 155Sm. Eupja, 154Sm gSF e lebelelega e hloka sebopego. Le ge golebjalo, kelo ya maatla a kopantšwego a 154,155Sm, e laetša tumelelano yago kwagala le boitekelo bjo bo fetilego, ya di-isotope tše dingwe tša samarium tšago golofala. Ka kakaretšo, dipoelo di fa tshedimošo yago tlaleletšana go mešomo yago feta ya (3He,ag)148Sm, (3He,3He’g)149Sm, (d,d’g)152Sm, (d,pg)153Sm, (p,dg)147,149,151,153Sm, (p,p’)154Sm and (a,a’g)154Sm. Palo ye nnyane ya di-nuclide tšago tšwelela ka tlhago tša go bale tlhaelo ya dineutron tša Z > 33, e bitšwago p-nuclei, ga di kgone go ka tšweletšwa ka ditshepidišo tša (n,g) tlhagong. Bakeng seo, di-nuclides tšeo go šišinywa gore di tšweletšwa ke peu ya nuclei ka photodisintegration (p-process) ya slow- le rapid-neutron capture process (s- le r-process). Efela, ge ele 180Ta, p-process ga se ya lekanela go hlaloša boati bja yona mo tlhagong, ka fao go hlokega ditshepidišo tša go tlaleletša. Mekgwa ya go tšweletša 180Ta e hlola ngangišano ka ge dipalo di laetša gore ditshepidišo tše mmalwa, ka nnoši nako ye nngwe, di ka tšweletša gape temošo ya boati. Ka ge mafelo a tlhago a tlholego ya 180Ta a sa tsebje go fihla ga bjale, go na le kgonagalo mo go kase ganetšwego ya tlhakantšho ya ditshepedišo tše mmalwa. Fela ge, bohlokwa bja tshepedišo ya go ikema ga bo hlatselege, ka lebaka la dipelaelo lebelong la diphetogo tša 180Ta bakeng sa tlhokego ya dipalopalo tša boitekelo. Malobanyana 180,181,182Ta NLD le gSF di ile tša elwa OCL. Go tšwa dipoelong tšeo, karolo tšeo di farologanyego tša (n,g) di balwa go šomišwa sebalwa sa diphetogo tša nuclear sa TALYS gomme tša bapetšwa le dipoelo tša morago. Go feta fao, lebelo la diphetogo le šomišwa ka dipalopalo tša s-process ka gare ga dinaledi tša low-mass AGB le tutuetšo ya p-process go naledi ya go thuthupa ya Type-II go nyakišiša tšweletšo ya 180Ta. Go tšwa mošomong wo, mabelo a diphetogo ao a goretšwego a maswa a s-process, a laetša gore go dinaledi tša low-mass AGB, seabe sa tšweletšo ya 180Ta ga se bohlokwa gomme se kgatha tema ka boripanyana go tšweletšeng ga 181Ta. Tšweletšo ya p-process ya 180Ta ka go naledi ya go thuthupa ya Type-II go šomišwa dipalopalo tše diswa e bohlokwa gomme e šišinya gore p-process ke motheo wa tshepedišo ya tšweletšo ya 180Ta tlhagong.

AFRIKAANSE OPSOMMING: Hierdie tesis het twee navorsings fokus areas naamlik, i) die statistiese eienskappe van goedvervormde samarium isotope, en ii) die inperking van nukleosintetiese reaksies van 180Ta. Die isotoop ketting van die seldsame aardelement samarium bied ’n uitstekende geleentheid om sistematies die verandering van kernstruktuur effekte vanaf die byna sferiese 144Sm isotoop tot die heel-vervormde 154Sm te bestudeer. Met die vervorming van die kern, word verwag dat statistiese eienskappe van die kern soos die vlak-digtheid (KVD) en die g-sterke funksie (gSF) sal verander. In besonder word verwag dat die opwekking van resonansies in hierdie massa gebied moontlik interessante kenmerke sal oopenbaar met ’n ondersoek oor verskeie isotope van dieselfde element. Uit data wat by die Oslo Siklotron Fasiliteit (OSF) opgeneem is, is KVDe en gSFs ge-ekstraeer met gebruik van die Oslo metode. ‘n Deuteron bundel is gebruik om opgewekte toestande in 154,155Sm te vul. Die data toon aan dat daar ’n opmerklikke M1 skêr-resonansie by ’n opwekking van » 3 MeV in 155Sm voorkom terwyl die gSF van 154Sm vormloos is. Nieteenstaande is die gemete geintegreerde sterkte van beide 154,155Sm goed vergelykbaar met vorige gemete waardes van ander vervormde samarium isotope. Hierdie resultate voorsien komplimentere informasie tot die reeds gepubliseerde gemete stelle (3He,ag)148Sm, (3He,3He’g)149Sm, (d,d’g)152Sm, (d,pg)153Sm, (p,dg)147,149,151,153Sm, (p,p’)154Sm and (a,a’g)154Sm data. ’n Kleiner getal neutron-arm nukliedes wat naturlik voorkom, met Z > 33, en as p-nukliedes bekend staan, kan nie met die (n,g) reaksie in die heelal geproduseer word nie. Daar word verwag dat hierdie nukliedes deur die fotodisintegrasie (p-proses) van kerne gevorm word deur beide stadige-neutron vangs (s-proses) en vinnige-neutron vangs (r-proses) van stabiele kerne. Vir 180Ta kan nie net die p-proses die waargenome natuurlike voorkoms van die isotoop verklaar nie. Teoreties is daar verskeie produksie reaksies wat die korrekte natuurlike voorkoms van 180Ta voorspel en waarvan sommiges sterk genoeg is om selfs alleenlik die natuurlike voorkoms te kan verklaar. Dit is wat 180Ta so interessant maak om te bestudeer. Waar 180Ta geproduseer word is nie bekend nie sodat dit heel waarskynlik is dat meer as een proses tot die produksie bydra. Weens ’n gebrek aan eksperimentele data was dit nie moontlik om uitsluitsel te gee oor die moontlike reaksies nie, nog minder hul sterktes, wat bydra tot die produksie van 180Ta. Vanuit die KVDe en gSFs van 180,181,182Ta wat ook by OSF gemeet is, is die (n,g) kansvlakke bereken met gebruik van die kode TALYS en vergelyk met vorige resultate. Daaruit is reaksie obrengse bereken wat as invoer data vir die s-proses produksie van 180Ta in lae-massa AGB sterre gedien het sowel as vir die p-proses produksie in Tipe-II supernovas. Die kleiner limiete van die reaksie sterktes bereken in hierdie tesis wys daarop dat in lae massa AGB sterre die bydrae van die s-proses, tot die produksie van 180Ta, weglaatbaar klein is en net gedeeltelik die produksie van 181Ta verklaar. Daarbenewens is uit die data nuwe limiete vasgestel wat daarop dui dat die p-proses in Tipe-II supernovas, die oorwegende produksie meganisme van 180Ta is, die skaarste stabiele isotoop in die heelal.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/107063
This item appears in the following collections: