Thesis (PhD)--Stellenbosch University, 2014. / 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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/96001 |
| Date | 12 1900 |
| Creators | Kheswa, Bonginkosi Vincent |
| Contributors | Wiedeking, Mathis, Papka, Paul, Giacoppo, Francesca, Stellenbosch University. Faculty of Science. Dept. of Physics. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | xviii, 100 p. : ill. (some col.) |
| Rights | Stellenbosch University |
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