In explosive astrophysical environments such as novae, X-ray bursters and supernovae, conditions of extreme temperature and density are achieved. Under such conditions, both the rate of energy release and path of nucleosynthesis are governed by reactions on unstable nuclei. In this light, direct reaction studies using radioactive ion beams play a vital role in determining nuclear reaction rates. However, in the vast majority of cases, direct measurements are not possible and as such, indirect measurements are equally important for the understanding of the main reaction processes driving astrophysical events. In this thesis work, indirect studies of the astrophysically important 19Ne(p,γ)20Na and 29P(p,γ)30S reactions have been performed. For the first reaction, a β-delayed proton decay study of 20Mg was performed to gain information about the spin-parity assignment of the first key resonance above the proton emission threshold in the compound nucleus 20Na. This resonance is expected to dominate the 19Ne(p,γ)20Na reaction rate in explosive astrophysical environments and its identity has been under discussion for a long time, with J π = 1+ and 3+ assignments suggested. In the present study an upper limit on the β-decay branch to this state of 0.02% with a con dence level of 90% is reported. This is signi cantly more stringent than previous studies and makes a 1+ assignment highly unlikely, favouring instead a 3+ assignment. A 3+ assignment is predicted to have a signi cantly higher resonance strength and produce a proportionately higher 19Ne(p,γ)20Na reaction rate in X-ray burst conditions. The second study performed was a detailed gamma-ray spectroscopy study of the nucleus 30S. Excitation energies have been determined with improved precision over previous studies and the first,firm spin-parity assignments of key 29P + p resonant states, expected to dominate the 29P(p,γ)30S reaction in stellar scenarios, have been made. An evaluation of the 29P(pγ)30S reaction over the temperature range T = 0.06-2.5 GK shows that the 3+ and 2+ resonant states located at Er = 292.0(9) and 413.1(10) keV, respectively, dominate the 29P(p,γ)30S reaction rate in ONe novae, while the 413 keV resonance is expected to govern the rate in X-ray burster environments. These new, precise resonance energy measurements and firm spin-parity assignments have signi cantly reduced uncertainties in the 29P(p,γ)30S reaction in ONe novae and X-ray bursters. In particular, the reaction rate is now speci ed precisely enough for calculations of isotopic abundances in ONe novae ejecta.

No description available.

>Magister Scientiae - MSc / This project aimed to investigate proton unbound states in 32Cl using the 32S(3He; t) charge-exchange reaction. This research is relevant both in the context of nuclear structure and astrophysics. Excited states in 32Cl up to Ex 6 MeV were produced using a 50 MeV 3He++ beam from the K200 separated sector cyclotron at iThemba LABS. The triton ejectiles were mass analysed and detected at the focal plane of the K600 magnetic spectrometer. An additional segmented silicon detector array called CAKE was used to detect the unbound protons from states in 32Cl in conjunction with the tritons. In this work we looked for potential sources of isospin admixture that could explain the apparent violation of the Isobaric Multiplet Mass Equation (IMME) for the A = 32, T = 2 quintet. We also investigated the possibility of determining the 31S(p; ) reaction rate indirectly, via measurements of the partial proton widths of unbound states in 32Cl.

No description available.

<p> Even with a good fit to nucleon-nucleon scattering data, there is considerable freedom in the off-energy-shell behaviour of the nuclear reaction matrix because of (1) the unavailability of the elastic scattering data in the high energy region and (2) the assumptions about the non-locality of the interaction. We have investigated off-energy shell behaviour by developing 'super' soft core potentials and several pairs of phase shift equivalent separable, local and momentum dependent potentials. Nuclear matter calculations were done using these potentials in order to study the sensitivity of the binding energy to the differences in the off-energy-shell behaviour. The effective range formula has been extended to the off-energy-shell case.</p> / Thesis / Doctor of Philosophy (PhD)

Type I X-ray bursts (XRB) are highly energetic and explosive astrophysical events, observed as very sudden and intense emissions of X-rays. X-ray bursts are believed to be powered by a thermonuclear runaway on the surface of a neutron star in a binary system. XRB models are dependent on the accurate information of the nuclear reactions involved. The 18Ne(α,p)21Na reaction is considered to be of great importance as a possible breakout route from the Hot-CNO cycle preceding the thermonuclear runaway. In this thesis work, the 18Ne(α,p)21Na reaction cross-section was indirectly measured at Ecm(α,p) = 2568, 1970, 1758, 1683, 1379 and 1194 keV, using the time-reverse 21Na(p,α)18Ne reaction. Since the time-reverse approach only connects the ground states of 21Na and 18Ne, the cross sections measured here represent lower limits of the 18Ne(α,p)21Na cross-section. An experiment was performed using the the ISAC-II facility at TRIUMF, Vancouver, Canada. A beam of 21Na ions was delivered to a polyethylene (CH2)n target placed within the TUDA scattering chamber. The reaction 18Ne and 4He ions were detected using silicon strip detectors, with time-of-flight and ΔE/E particle identification techniques used to distinguish the ions from background. The measurement at Ecm = 1194 keV is the lowest energy measurement to date of the 18Ne(α,p)21Na cross section. The measured cross sections presented in this thesis were compared to the NON-SMOKER Hauser-Feshbach statistical calculations of the cross section and to the unpublished results of another time-reverse investigation performed by a collaboration at the Argonne National Laboratory. A 18Ne(α,p)21Na reaction rate calculation based on the measured cross sections was performed. In comparison with previous reaction rate estimates, our results indicate a rate that is about a factor 2-3 lower than Hauser-Feshbach calculations, suggesting that a statistical approach may not be appropriate for cross section calculations for nuclei in this mass region. The astrophysical consequences of our new results appear to remain nevertheless negligible. These are also presented in this thesis.

Os principais mecanismos da reação supracitada foram examinados: breakup, transferência, fusão completa e incompleta e excitação inelástica. / The mains mechanisms of the mencioned reaction were study, namely: breakup, fusion (complete and incomplete), transfer and inelastic excitation.

Os principais mecanismos da reação supracitada foram examinados: breakup, transferência, fusão completa e incompleta e excitação inelástica. / The mains mechanisms of the mencioned reaction were study, namely: breakup, fusion (complete and incomplete), transfer and inelastic excitation.

Thesis (PhD (Physics))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: The (p, pα) reaction on 12C was investigated experimentally using polarized incident protons of 100 MeV. Coincident data, which were obtained at ten quasifree angle pairs for proton angles ranging from 25◦ to 110◦, were analyzed in terms of the distorted-wave impulse approximation (DWIA). Calculated energy-sharing cross section and analyzing power distributions reproduce the data reasonably well. The observed agreement allows the extraction of distorted momentum distributions from experimental data. These distributions are very consistent over a wide range of angle pairs at which cross section energy-sharing distributions vary considerably. Since measurements of analyzing powers were made, spin-orbit distortions were included in the DWIA calculations. The effects of spin-orbit distortions were found to be very small near zero recoil momentum and did not destroy the validity of the factorization approximation where the two-body p-α cross section enters as a multiplicative factor in the three-body (p, pα) cross section expression. Spectroscopic factors derived from the data are fairly consistent with the trend of the theoretical predictions. Analyzing power data also follow the trend of free p-4He scattering data, and comparisons with DWIA predictions are in reasonable agreement. The theory reproduces also very well analyzing power angular distributions of the projectile-cluster two-body scattering at large angular momentum of the residual nucleus. This indicates that a quasifree knockout mechanism dominates the reaction. The two-body interaction response between the projectile and the α cluster was found to resemble the scattering of protons from a free α particle to a remarkable degree, the present results strongly imply the existence of preformed α clusters in 12C. / AFRIKAANSE OPSOMMING: Die (p, pα) reaksie op 12C is eksperimenteel ondersoek deur middel van gepolariseerde protone met n invalsenergie van 100 MeV. Ko¨ınsidensie data, wat verkry is by tien kwasievrye hoekpare, met proton hoeke tussen 25◦ en 110◦, is geanaliseer in terme van die vervormde-golf-impuls-benadering (DWIA). Die berekende energie-verdeelde kansvlak en analiseervermo¨e verspreidings reproduseer die data redelik goed. Die waargenome ooreenstemming maak dit moontlik om vervormde momentumverdelings uit die eksperimentele data te verkry. Aangesien analiseervermo¨e metings gedoen is, is spin-baan wisselwerking by die DWIA berekenige ingesluit. Die bydra as gevolg van spin-baan wisselwerking blyk baie klein te wees naby nul terugslag momentum en het nie die geldigheid van die faktoriseringsbenadering, waartydens die twee-deeltjie, p-α kansvlak as ’n vermenigvuldigingsfaktor in die uitdrukking vir die drie-deeltjie (p, pα) kansvlak verskyn, vernietig nie. Spektroskopiese faktore wat uit die data herlei is, is redelik konsistent met die verloop van die teoretiese voorspellings. Analiseervermo¨e data volg ook die verloop van die vrye p-4He verstrooiings-data en vergelyk redelik goed met DWIA voorspellings. Die teorie reproduseer ook die hoekverdelings in die analiseervermo¨e van die twee-deeltjie projektiel-bondel verstrooiing by groot hoekmomentum waardes vir die oorblywende kern baie goed. Dit dui daarop dat ’n kwasie-vrye uitslaanmeganisme die reaksie domineer. Die twee-deeltjie wisselwerkingsgedrag tussen die projektiel en die α-bondel toon sterk ooreenkomste met die verstrooiing van protone vanaf ’n vrye α-deeltjie. Die huidige resultate lewer sterk bewyse vir die bestaan van voorafgevormde α-bondels in 12C.

O presente trabalho apresenta uma metodologia nova de introduzir o núcleo de prova 111In em amostras para medidas de Correlação Angular Gama-Gama Perturbada (CAP). Para esta finalidade utilizou-se reação nuclear do tipo 108Pd(6Li,3n)111In. Para testar o método de implantação iônica foram utilizadas amostras de compostos intermetalicas tipo liga de Heusler Pd2MnZ onde Z=Sn,Sb,Ge,In. O trabalho demonstra que, para as amostras nas quais o paládio é um dos componentes do alvo da reaçãoo, o processo de implantaçãoo de 111In usando esta reação nuclear é muito eficiente comparada com os métodos convencionais usados para introduzir este núcleo de prova. Após a implantação do núcleo de prova 111In foram realizadas medidas de Correlação Angular gama-gama Perturbada Diferencial em Tempo (CAPDT), utilizando um espectrometro gama com quatro detectores de BaF2 que gera 12 espectros de coincidências gama-gama atrasada. Utilizou-se nestas medidas a bem conhecida cascata gama de 172-245 keV do 111Cd populado no decaimento do 111In pela captura eletrônica. O nível intermediário do 111Cd a 245 keV com spin I=5/2 e T1/2 = 85 ns foi usado para medir o campo hiper¯no magnético Bhf do 111Cd nas ligas de Heusler. As medidas foram realizados na faixa de temperatura entre 10 a 295 K. A presente metodologia de implantação do 111In é comparada com outros métodos convencionais de introduzir o núcleo de prova nas amostras para medidas de CAP. Os resultados de Bhf são comparados e discutidos com resultados de medidas anteriores. / The present work describes a new and e±cient method to introduce the 111In nuclear probe in the samples for Perturbed Gama-Gama Angular Correlation (PAC) measurements. For this purpose a heavy ion nuclear reaction 108Pd(6Li,3n)111In was used. To test the ion implantation process we have used the samples of intermetallic compounds Pd2MnZ (Z=In, Sn, Ge, Sb) known as Heusler Alloys. It is shown that for samples, which contain Pd as one of the components of the reaction target the process of implantation of 111In using this reaction is much more eficient as compared to the other conventional methods of introducing the this probe nuclei. After the 111In nuclear probe implantation in the Heusler alloys these were measured by the Time Differential Perturbed Gamma-Gamma Angular Correlation (TDPAC) method using a gamma spectrometer consisting of four BaF2 detectors, which generates 12 simultaneous delayed gamma-gamma coincidence spectra. The well-known gamma cascade of 172-245 keV in 111Cd populated through electron capture decay of 111In has been used for the TDPAC measurements. The intermediate level of 111Cd at 245 keV with spin, I = 5/2 and half-life, T1=2 = 85ns was used to measure the hyperfine magnetic field (Bhf) in the Heusler alloys. The measurements were carried out in the temperature range of 10-295 K. The present ion implantation methodology is compared with other conventional methods of introducing the probe nuclei in samples for PAC measurements. The present results of magnetic hyperfine felds are discussed and compared with the results of earlier measurements.