Impact of the 138,139La radiative strength functions and nuclear level densities on the galactic production of 138La

Kheswa, Bonginkosi Vincent (2014-12)

Thesis (PhD)--Stellenbosch University, 2014.

Thesis

ENGLISH ABSTRACT: 138La is a very long-lived and low abundant p-isotope. Most p-nuclei with Z > 54 are thought to be produced through photodisintegration of s- and r-process seed nuclei. However, this p-process cannot satisfactorily explain the observed abundance of 138La, and more exotic processes, such as ve + 138Ba → 138La + e− have to be considered. This v-process can reproduce the observed solar abundance of 138La, but the significance of the p-process cannot be ruled out due to very high uncertainties in its predicted reaction rates. These errors have been discussed to be mainly due to the unavailability of the experimental nuclear level densities and radiative strength functions of 138,139La, which are critical ingredients for astrophysical reaction rate calculations based on the Hauser-Feshbach approach. Thus, nuclear physics measurements are necessary to place the nuclear properties on a strong footing, in order to make statements regarding the importance of p- and v-processes. In this research project the experimental nuclear level densities and radiative strength functions of 138,139La were measured below the neutron thresholds. From this new experimental data, the Maxwellian averaged cross sections for the 137La(n, y) and 138La(n, y) reactions, at the p-process temperature of 2.5⇥109 K, were computed with the TALYS code. Using these reaction rates the nucleosynthesis calculations in the O/Ne-rich layers of Type II supernovae were performed. The results imply that the standard p-process still under-produces 138La, which puts the v-process on a very strong footing as the main production process for 138La.

AFRIKAANSE OPSOMMING: 138La is ’n p-isotoop met ’n baie lang halfleeftyd. Daar word tans vermoed dat p-nukiede met Z > 54 geproduseer word deur fotodisintegrasie van sen r-proses saadnukliede. Nogtans verklaar hierdie p-proses die waargenome natuurlike voorkoms van 138La nie behoorlik nie, en meer eksotiese prosesse soos byvoorbeeld ve+ 138Ba → 138La + e− moet in aanmerking geneem word. Hierdie v-proses kan die waargenome natuurlike voorkoms van 138La verklaar, maar die belangrikheid van die p-proses kan nie afgewys word nie weens die onsekerheid in die voorspelde reaksie snelheid. Sodanige onsekerhede word bespreek en word hoofsaaklik toegeskryf aan die gebrek aan eksperimentele vlakdigthede en stralings sterkefunksies van die kerne 138,139La, wat van kritiese belang is vir berekeninge van astrofisiese reaksie snelhede gebaseer op die Hauser-Feshbach benadering. Kernfisiese metings is derhalwe noodsaaklik om die eienskappe van kerne op ’n stewige grondslag te plaas sodat uitlatings gemaak kan word omtrent die belangrikheid van p- en v-prosesse. In hierdie esperimentele navorsingsprojek is die kern vlakdigthede en stralings sterkefunksies van 138,139La onder die neutron reaksiedrumpels gemeet. Die nuwe gemete data maak dit moontlik om die Maxwell-gemiddelde kansvlakke vir die 137La(n, y) en 138La(n, y) reaksies by ’n p-proses temperatuur van 2.5 x 109 K met die TALYS program te bereken. Hierdie reaksie snelhede is daarna gebruik om berekeninge van elementvorming in die O/Ne-ryke lae van Tipe-II supernovas te maak. Die resultate wys uit dat die stadaard p-proses nie genoegsame 138La produseer nie, wat derhalwe die v-proses op ’n baie stewige grondslag plaas as die hoof produksie proses vir 138La.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/96001
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