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The differential cross section for direct radiative capture of protons by 0¹⁶ has been measured using ice targets of known thickness and 800 kev. protons. The differential cross section for the gamma ray transition to the first excited state in F¹⁷ was found to be (10.4 ± 1.3) x 10⁻⁻³² cm² per steradian at 90° to the incident proton beam direction. At the same energy, the ratio of the differential cross section at 90° for transitions to the ground state to that for transitions to the first excited state in F¹⁷ was found to be o.14 ± 0.03. The energy of the first excited state in F¹⁷ was determined by measuring the energy of the gamma ray from this level to the ground state. This method is difficult because of the presence of positron annihilation radiation of the same energy, within experimental errors, from the decay of F¹⁷. The first excited state energy was also measured by noting the difference between the capture gamma rays to this state and to the ground state. The energy of this level was found to be 0.50 ± 0.01 Mev. in agreement with the results of Marion and Bonner (55) and with earlier results obtained in this laboratory (Warren et. al., (54), An attempt to confirm that the source of the 873 kev. radiation from proton bombardment of natural oxide targets above 1.8 Mev. bombarding energy was the 0¹⁷(p, p',૪)0¹⁷ reaction, was made using separated 0¹⁶ and 0¹⁷ targets. The results were inconclusive due to the small percentage of oxygen that stuck to the targets and to the presence of several contaminants. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Eskom has embarked on the development of the Pebble Bed Modular Reactor to generate electricity at low cost compared to that of coal with no greenhouse gas emissions. The proposed PBMR requires about 600 tons of nuclear-grade graphite blocks in its construction. A considerable amount of pitch is required as impregnants for the manufacture of this graphite. Anthracene oil is a high-boiling coal-tar distillate fraction consisting of a complex mixture of mainly unsubstituted polyaromatic hydrocarbons, with some methyl substitution. Conversion to pitch may be effected by dehydrogenation, i.e. volatile oil molecules condensing together to yield material of high molecular mass. Two dehydrogenation reagents, elemental sulfur and a novel reagent, hexachlorocyclohexane (HCH), an industrial waste product, were used to give three series of pitches with differing softening points. These products were characterised by standard pitch analyses, elemental analyses, thermogravimetric analysis (TGA), proton nuclear magnetic resonance spectroscopy (1HNMR), gas chromatography (GC), gas chromatography–mass spectrometry (GC-MS), Maldi-TOF mass and mass probe spectrometry and then compared with commercial pitch grades from Mittal Coke and Chemicals. The results obtained showed that the reaction products reacted with 15 and 20 wt % of sulfur, with the softening points of 61.3 and 91.4 °C, would be suitable as commercial products. All pitches showed very little, if any, ash, making them suitable for use in the manufacture of high-purity graphite. Pitches prepared with 15-20 wt % of sulfur and HCH pitches may have more applications in impregnation because of their low QI content. Total sulfur content detected by elemental analysis is high. This is undesirable because sulfur may cause puffing which will result in poor quality of graphite. All pitches show very little, if any, ash, making them suitable for use in the manufacture of high-purity graphite. TGA results showed that pitches prepared with higher concentrations of sulfur can be suitably used as impregnants because of their high coking yields. The coking yields of HCH-derived pitches are lower than those of Mittal Coke and Chemicals commercial pitches. Results from the (1HNMR showed that cross-linking on reaction with sulfur is through aliphatic carbon atoms; the percentages of aliphatic protons decrease with the increase in the sulfur concentration, and aromatic percentages increase. There is a slight increase in the proportion of aromatic protons of the HCH pitches; therefore cross linking is through aromatic-aromatic carbon atoms. Model reactions were prepared by reacting pure aromatic hydrocarbons with HCH, demonstrating the incorporation of the dehydrogenation reagent. These reactions were used to determine the reactivity of the main components of anthracene oil. The reaction products were characterised by Maldi-TOF and GC-MS. The results obtained from HCH pitches showed the formation of chlorobenzenes. This is undesirable because chlorine may cause corrosion of the equipment during coking. / Dissertation (MSc (Chemistry))--University of Pretoria, 2008. / Chemistry / unrestricted

Standard computer techniques were developed for computation of ΔG° values from potentiometric titration data and ΔH° values from calorimetric titration data. The procedures developed can be used for any systems having the general equations HA = W^+ + A^- ; H_2A = 2H^+ + A^2- ; and H_3A = 3H^+ + A^3-, where the proton ionizations may be consecutive or simultaneous. The thermodynamic values ΔG°(K), ΔH° and ΔS° were determined for the ionization of protons from the protonated compounds methionine (-NH_3^+ group only), phosphoethanalomine and phosphoserine. The compounds studied were selected because they contain one, two and three ionizable protons respectively.

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