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Orientadores: Julio Cesar Hadler Neto, Peter Christian Hackspacher / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-11T05:57:37Z (GMT). No. of bitstreams: 1 Palissari_Rosane_D.pdf: 15939989 bytes, checksum: f455fc1254ec2dd67bb39d1df46cce88 (MD5) Previous issue date: 2007 / Resumo: O Método dos Traços de Fissão (MTF)é uma das técnicas mais poderosas usadas para descrever as histórias térmicas de rochas presentes na camada da superfície terrestre,sob condições de baixa temperatura.Se a rocha hospedeira teve a temperatura elevada em um dado período,os traços de fissão que se formaram desde então,são encurtados irreversivelmente ou apagados (os traços sofrem annealing ),como conseqüência de um processo de difusão termicamente ativada. Os minerais mais comumente utilizados no MTF são a apatita e o zircão,sendo este último o objetivo deste trabalho.O zircão é sensível ao annealing térmico sob temperaturas mais altas que a temperatura de fechamento da apatita,que é aproximadamente 120 o C . As características do annealing dos traços de fissão em zircão têm sido obtidas utilizando-se dados de experimentos de laboratório,que são executados em tempos menores que 1 ano,e comumente em tempos ainda menores.Estes dados são usados para calibrar modelos de annealing empíricos,que descrevem a dependência do encurtamento dos traços com o tempo e a temperatura.No entanto,um problema de ordem prática é a incerteza introduzida na extrapolação para tempos geológicos (10 6 ¿ 10 8 anos),através dos modelos calibrados pelos dados de laboratório (<1 ano). Um dos objetivos deste estudo é fornecer condições de contorno adicionais para a faixa de tempo-temperatura da zona de annealing do zircão,através de dados de zircões de poços, cujas amostras foram expostas a temperaturas estáveis por ~1 milhão de anos.Desta forma, o problema da extrapolação é abordado explicitamente ajustando-se os modelos a dados geológicos. Vários modelos de annealing foram propostos na literatura de traços de fissão.A formulação básica destes modelos baseia-se na equação de Arrhenius e foi apresentada por Laslett et al.(1987).Existem outros modelos de annealing que são baseados no mesmo princípio:Crowley et al.(1991);Laslett e Galbraith (1996);Rahn et al.(2004)e Yamada et al. (2007).Estes modelos têm sido usados preferencialmente,uma vez que inúmeros fenômenos, desde a formação dos traços até o ataque químico,não são bem entendidos ainda.orém,dois outros modelos calibrados no presente trabalho (Carlson,1990;Guedes et al.,2005a), procuram estabelecer uma correlação direta entre seus parâmetros e os fenômenos envolvidos.Os ajustes dos vários modelos foram comparados entre si,levando-se em conta considerações estatísticas. Além das calibrações efetuadas ajustando-se os modelos a dados geológicos, foram abordadas comparações de previsões dos modelos (relacionadas com as fronteiras da Zona de Annealing Parcial)com outros dados geológicos baseados em densidades de traços de fissão / Abstract: Fission-track (FT)analysis has developed into one of the most useful technique used to constrain the low-temperature thermal history of rocks in the upper part of the Earth's crust.If a host rock is subjected to elevated temperatures,fission tracks that have formed up to that point in time are shortened irreversibly or annealed,as a consequence of a thermally activated diffusion process. The most commonly analysed minerals are apatite and zircon and the latter is the focus of this work,since this mineral is sensitive to thermal annealing at higher temperatures,while the close temperature for apatite is around 120 o C. Short-term annealing characteristics of FT in zircon are generally based on laboratory experiments which are performed on timescales of less than 1 year,and typically much less. These data have been used to calibrate annealing empirical models,that describe the temperature and time dependence of fission track shortening.However,a practical problem is the uncertainty introduced through the extrapolation of laboratory calibrated models (<1 year)to geological timescales (10 6 ¿ 10 8 years). One of the purpose of this study is to give further constraints on the temperature range of the zircon annealing zone over a geological time scale using zircons data from boreholes,which samples have been exposed to a stable temperature for ~1 Ma.In this way,the extrapolation problem is explicity adressed by fit the zircon annealing models with geological timescale data. Several empirical models formulations have been proposed to perform these calibrations and have been compared in this work.The basic formulation is an Arrhenius-type model and is given by Laslett et al.(1987).There are other annealing models which are based on the same general formulation:Crowley et al.(1991);Laslett and Galbraith (1996);Rahn et al.(2004)and Yamada et al.(2007).These models equations have been preferred due to the great number of fenomena from track formation to chemical etching that are not well understood.However,two other calibrated models in this work (Carlson,1990;Guedes et al.,2005a)try to stablish a direct correlation between their parameters and the related fenomena.Several model fits are compared by themselves,considering statistical data. Besides calibrating annealing models with geological timescale data,model predictions (related with Parcial Annealing Zone boundaries)were compared with another geological data / Doutorado / Geofísica Nuclear / Doutor em Ciências

La mitose est une étape clé du cycle cellulaire, très préservée chez toutes les cellules eucaryotes, durant laquelle le matériel génétique de la cellule (les chromosomes) réparti de manière égale dans les deux cellules filles. Cette équipartition du matériel génétique est cruciale pour le maintien de la stabilité génétique. Durant ce processus, les chromosomes, composés des chromatides soeurs, établissent une plaque métaphasique au centre du fuseau mitotique. Chaque chromatide est attachée à un pôle du fuseau mitotique respectif (on parle d'attachement bipolaire) vers lequel elle se dirigera durant l'anaphase. Les chromatides sont l'unité indivisible du matériel génétique durant la mitose, à l'image des atomes dans une molécule. Initialement, une fois la chromatine condensée en chromosomes, chacun de ces " objets " est détaché et réparti suivant une position précise appellée territoires chromosomiques. Toute la complexité de la mitose est de capturer chacune des chromatides et de les positionner sur la plaque métaphasique avant leur séparation et migration vers leur pôle respectif durant l'anaphase. Cette étape de la division cellulaire requiert donc non seulement un réseau complexe d'interaction et de signalisation biochimique comme dans beaucoup d'autres processus biologiques mais aussi un fin contrôle spatio-temporel du mouvement et du positionnement de ces objets de grande taille à l'échelle de la cellule. Il semblerait que l'origine du mouvement des chromosomes provienne pour une grande part de la dynamique des microtubules. Ce qui est moins certain est la part relative accordée aux différents processus régulant cette dynamique; que ce soit la dynamique intrinsèque (appelée instabilité dynamique des microtubules) ou l'effet de différentes protéines sur les microtubules comme les MAPs (Microtubule Associated Proteins) et les kinésines (protéines motrices). On notera par ailleurs que le mécanisme de transfert d'énergie entre la dynamique des microtubules et le mouvement des chromosomes est encore très largement hypothétique. La dynamique des chromosomes durant la mitose est aussi largement contrôlée par un grand nombre d'acteurs autres que les microtubules. Certains d'entre eux étant responsables de l'attachement MTs-kinétochore comme les complexes NDC80 et DAM1, tandis que d'autres sont impliqués dans la régulation de la dynamique des microtubules comme la kinésine-8 et la kinésine-13. Durant mon travail de thèse, j'ai étudié la dynamique des chromosomes en mitose chez la levure à fission, modèle celulaire dont les mécanismes primordiaux qui contrôlent la mitose sont conservés avec les eucaryotes supérieurs. En effet, j'ai caractérisé deux de ces mécanismes conservés au cours de l'évolution: l'alignement des chromosomes durant la métaphase ainsi qu'un mouvement de va et vient plus ou moins régulier le long du fuseau aussi appelé oscillation des chromosomes. J'ai montré, en analysant les trajectoires des chromosomes que ces deux processus sont pour une large part indépendants [@Mary2015]. De plus, le processus d'alignement des chromosomes, encore mal compris, est en partie contrôlé par la kinésine-8 via une activité dépendante de la longueur des microtubules. Il semblerait donc que cette kinésine soit capable de fournir une information spatiale le long du fuseau mitotique afin de positionner correctement les chromosomes. Enfin, j'ai utilisé un modèle mathématique de la ségrégation des chromosomes précédemment développé dans l'équipe afin de tester de manière quantitative les hypothèses de mécanisme du centrage des chromosomes par la kinésine-8. L'ensemble de mon travail porte donc sur le contrôle du mouvement, de l'attachement et du positionnement des chromosomes durant la mitose afin de mieux comprendre les processus biophysiques associés à la mitose. / Mitosis is a highly preserved process in all eukaryotic cells during which the genetic material (chromosomes) is divided in two parts which spread in both daughter cells. This equipartition is crucial for maintaining genetic stability. During this process, chromosomes form a metaphasic plate at the center of the mitotic spindle. Each chromatid is attached to its respective spindle pole (called bipolar attachment) toward which it will move during anaphase. Chromatids are the indivisible units of genetic material during mitosis just like atoms in a molecule. Originally each of these "\ objects\ " are detached and organized in chromosomes territories. All the complexity of mitosis resides in the capture of each chromatid by the spindle pole to exert forces to position them on the metaphase plate before their separation and migration towards their respective poles in anaphase. This step of cell division not only requires complex interaction networks and metabolic signaling pathways just like many other biological processes but also a fine spatio-temporal control of movement and positioning of these big objects relative to cell size. It is usually accepted that the origin of chromosome movement arises from microtubule dynamics. However, what is less clear is the relative importance of each of these processes regulating chromosome movement: the intrinsic dynamic instability of microtubules or the effect of their associated proteins such as MAPs and kinesins. It is also important to note that the mechanism controlling the transfer of energy between microtubule dynamics and chromosome movement is still largely hypothetical. Moreover, chromosome dynamics during mitosis is regulated by a large number of actors apart from microtubules. Some of them being responsible for MT-kinetochore attachment such as NDC80 and DAM1 complex. While others are involved in the regulation of MT dynamics such as Kinesin-8 and Kinesin-13. During my PhD, I studied fission yest chromosome dynamic during mitosis. This cellular model has the advantage of sharing many fundamental mechanisms of symmetrically dividing higher eukaryotic cells. I characterized two of these conserved mechanisms: chromosome alignment during metaphase and back and forth movement along the spindle, called chromosome oscillation. By analyzing chromosome trajectories, I showed that both processes are performed through independent mechanisms [@Mary2015]. Moreover, chromosome alignment process, which is still poorly understood, is regulated by Kinesin-8 via a length dependent activity on microtubules. This suggests that Kinesin-8 is able to provide spatial information along the mitotic spindle to properly position chromosomes. Finally, I used a mathematical model of chromosome segregation in order to test quantitatively different hypotheses of chromosome centering process. This work is thus deciphering the control of movement, attachment and positioning of chromosomes during mitosis and seeks to better understand the biophysical processes controlling mitosis.

The performance of a growing sulphate reducing bacteria consortium for Sr2+, Co2+ and Cs+ removal from solution in a batch sulphidogenic bioreactor was investigated. Metal removal by the growing bacterial consortium, and microbial culture growth and metabolic activities (biological sulphate removal) were continuously monitored in the bioreactors over the duration of the treatment period. On the other hand, diversity changes within the bacterial consortium before and after bioreactor operation (28 days) were performed using the partial 16S rRNA fingerprinting method. In the original bacterial consortium, Enterococcus and Staphylococcus sp. were the dominant bacterial species. However, the presence of Sr2+, Co2+ and Cs+ in the growth media, resulted in the emergence of new bacterial species belonging to the Citrobacter, Paenibacillus, and Enterococcus and Stenotrophomonas genera, respectively. The Citrobacter and Paenibacillus sp. demonstrated high tolerance towards the presence of the divalent cations, Sr2+ and Co2+, respectively, while the Enterococcus and Stenotrophomonas sp., demonstrated Cs+ high tolerance. The bacterial growth and sulphate removal rate were significantly decreased at initial metal ion concentrations ≥100 mg/L. The toxicity and inhibitory effects of the metals on the present SRB consortium was observed in the order Sr>Co>Cs. The metal uptake capacity (qτ) of the bacterial consortium decreased with increasing initial metal concentration, and complete Sr2+, Co2+ and Cs+ removal was observed at initial metal concentrations ≤75 mg/L. Overall, the present SRB consortium demonstrated a superior Sr2+ removal capacity (qmax= 405 mg/g), and the least for Cs2+, where qmax = 192 mg/g. The present SRB culture exhibited a superior Sr+ and Cs+ binding capacity, compared to other studies in literature. Results from Sr2+, Co2+ and Cs+ biosorption kinetics indicate that initial concentration and solution pH played a vital role in determining the rate of metal removal kinetics. The experimental data was successfully analysed by the pseudo-second-order rate model, demonstrating that chemisorption is the main rate limiting step for the removal of Sr2+, Co2+ and Cs+ from solution. In this study, the adsorption behaviour of protons and of Sr2+, Co2+ and Cs+ onto the bacterial consortium cell surfaces was evaluated under anaerobic conditions as a function of pH (4-10), ionic strength (0.01, 0.05, 0.1M) and temperature (25, 50 and 75°C). Acid-base titrations of the bacterial suspension indicated that the titration data could be adequately described by a four site non-electrostatic model, with pKa values of 4.41, 6.69, 8.10 and 10. The Sr2+, Co2+ and Cs+ adsorption data could be fitted with a two site non-electrostatic model, involving the type 1 and 2 sites (carboxylic and phosphoryl sites). Increasing the ionic strength had a negative effect on the adsorption of metal ions from solution. There was no observed temperature dependence on the adsorption of Co2+ and Cs+ from solution. In summary, results obtained in this study have shown that the processes involved in microbial Sr2+, Co2+ and Cs+ removal from contaminated sources is a direct function of the microbial characteristics and efficiency, mass transfer and surface complexation effects under varying environmental conditions. One important goal to be achieved in future studies will be the determination of the intrinsic stability constants and the structure of the formed metal complexes species. These constants can be used directly in risk assessment programs. / Thesis (PhD(Eng))--University of Pretoria, 2011. / Chemical Engineering / unrestricted

La volonté d'améliorer l'efficacité énergétique des réacteurs nucléaires a motivé de nouvelles solutions dans leur conception. L'une d'elles est l’utilisation d’un réflecteur lourd dans les réacteurs de génération III+ et les futurs réacteurs de génération IV. Lorsque la matière est traversée par des rayons γ, les excitations induites entraînent une élévation de sa température. Ce processus, appelé échauffement γ, est responsable de plus de 90% de la production de chaleur dans la région hors combustible d'un réacteur nucléaire. C’est également le cas dans le réflecteur. Pour simuler l'effet de l’échauffement γ en fonction de la composition du combustible, il faut disposer de données précises sur les γ prompts émis par les différents fragments produits dans le processus de fission. En 2012, une campagne d’expériences inédite, EXILL, a été menée au réacteur de recherche de l'ILL. Un grand nombre de détecteurs HPGe a été placé autour d’une cible fissile et a mesuré les rayons γ émis par la cible alors qu’elle était irradiée par un faisceau intense de neutrons froids. Dans ce travail, nous avons analysé les données obtenues avec des cibles ²³⁵U. Elles nous ont permis d’étudier la désexcitation de plusieurs fragments de fission et plus globalement le processus de fission induite par des neutrons. Dans un premier temps, nous avons utilisé la méthode standard d'analyse par coïncidence γ-γ-γ. Nous avons pu filtrer les données expérimentales, identifier les transitions γ dans des fragments bien produits et calculer leur intensité relative. Les problèmes que nous avons rencontrés concernent le bruit de fond. Les résultats obtenus dépendent de ce choix et présentent donc des problèmes de reproductibilité. Nous avons développé et testé une nouvelle méthodologie d'analyse. Son principe est un balayage des portes de coïncidence selon trois directions, ce qui permet de trouver le bruit de fond le mieux adapté. L'idée principale était finalement de passer d'une méthode "spectroscopique", dont le but est de trouver de nouvelles transitions et des états excités dans un noyau, à une méthode "spectrométrique", qui nous permet d'obtenir plus précisément l’intensité de transitions γ connues, avec une meilleure estimation de leur incertitude. Cela nous a amené à développer un logiciel d'analyse semi-automatique d'ajustement des pics. Divers schémas de calcul de l'intensité des transitions γ ont été également élaborés pour tenir compte des contaminations possibles, selon leur emplacement dans la matrice de coïncidence et leur intensité. La méthode standard et la nouvelle méthode d'analyse ont été comparées pour l'analyse du ¹⁴²Ba. Dans ce travail, nous avons également comparé nos résultats sur quelques noyaux, tel que le ¹⁰⁰Zr, avec des simulations réalisées avec le code FIFRELIN. Ce dernier est un code Monte-Carlo qui simule le processus de fission et la désexcitation des fragments de fission. FIFRELIN utilise plusieurs modèles différents pour décrire ces processus. Nous avons testé le comportement des différents modèles, trouvé les valeurs optimales des paramètres de simulation et testé comment ces configurations reproduisaient les résultats expérimentaux. FIFRELIN n'a pas été en mesure de reproduire simultanément les intensités des transitions γ émises par les fragments de ¹⁰⁰Zr et la multiplicité de neutrons prompts moyennée sur tous les fragments de fission. Cependant, avec des paramètres modifiés, FIFRELIN a fourni localement une multiplicité de neutrons prompts correcte pour les fragments de masse atomique A=100 et des intensités de transition γ bien reproduites pour le noyau de ¹⁰⁰Zr. Nous avons également comparé nos résultats expérimentaux sur les fragments de ¹⁰⁰Zr provenant du processus ²³⁵U(n,f) avec les autres données expérimentales disponibles provenant des expériences sur ²⁴⁸Cm(sf) et ²⁵²Cf(sf), et une autre expérience sur ²³⁵U(n,f). / The desire to improve the fuel efficiency of nuclear reactors has motivated new solutions in their design. One of them is the heavy reflector used in the generation III+ and in the future generation IV reactors. γ-rays passing through matter cause its excitation and temperature rise. It is a process called γ-heating, and it is responsible for more than 90% of the heat production in the non-fuel region of the nuclear reactor. This is also the case of the heavy reflector. To simulate the γ-heating effect in every state of the nuclear reactor it is necessary to have precise data on the prompt γ-rays emitted by different fission fragments produced in the course of the nuclear chain reaction. In 2012, at the research reactor of the ILL, an innovative experiment, called EXILL, was conducted. It produced a large amount of useful data on the de-excitation of the fission fragments. A large number of HPGe detectors were used to study the neutron induced fission process by measuring the emitted γ-rays. Fissile targets were irradiated by an intense cold neutron beam. In this work we analyzed the ²³⁵U targets. We studied several fission fragments and more generally the fission process by using high-resolution γ-ray spectroscopy. At the beginning, we used the standard γ-γ-γ coincidence analysis method. We were able to filter experimental data, identify the well produced γ-rays, and calculate their relative intensities. The problems we have encountered are related to the background. The results obtained with this method were background dependent and thus presented some problems with reproducibility. We therefore developed and tested a new analysis methodology. Its crucial feature is a coincidence gates scanning in three directions which helps to find the most suitable background. The idea was to move from a “spectroscopic” method, which main purpose is finding new transitions and excited states in a nucleus, to a “spectrometric” method, which allows us to obtain more precise γ-ray intensities. We developed a semi-automatic analysis software which facilitates fitting of the chosen γ-ray peak, the contamination and the background. Various γ-ray intensity calculation schemes were derived to take into account different contamination strengths and placements. The results of the analysis with the new technique are reproducible and more reliable. The standard and the new analysis method were compared in the ¹⁴²Ba analysis. In this work, we also compared our experimental results on some nuclei, such as ¹⁰⁰Zr, with the simulation results performed with the FIFRELIN code. It is a Monte-Carlo code which simulates the fission process and the de-excitation of the fission fragments. It uses various models to describe these processes. We were able to test the behavior of different models implemented in FIFRELIN to find the optimal simulation parameter values and to test how well these setups reproduce the experimental results. FIFRELIN was unable to simultaneously reproduce the γ-ray intensities of ¹⁰⁰Zr and the prompt-neutron multiplicity averaged over all fission fragments. However, with modified simulation parameters, FIFRELIN locally provided correct prompt-neutron multiplicity for the fission fragment with the atomic mass A=100 and well reproduced γ-ray intensities of ¹⁰⁰Zr. We also compared our experimental results on ¹⁰⁰Zr coming from the ²³⁵U(n,f) process with the other available experimental data coming from the experiments on ²⁴⁸Cm(sf) and ²⁵²Cf(sf), and another experiment on ²³⁵U(n,f).

Depuis les années 60 les modèles de structure nucléaire prédisent l'existence d'un îlot de stabilité des noyaux super-lourds, situé autour du prochain noyau doublement magique, attendu selon les modèles à N=172 ou 184 et entre Z=114 et 126. Ces noyaux posséderaient de très hautes barrières de fission, de l'ordre de quelques MeV, générées par les effets microscopiques, et responsables de larges distributions de temps de fission s'étendant à des temps très longs. Ainsi nous avons entrepris des mesures de temps de fission du noyau superlourd Z=124 par fluorescence X, une méthode basée sur le remplissage des lacunes électroniques internes crées durant la collision conduisant à la formation du noyau composé. Cette expérience repose sur la détection des fragments de fission en coïncidence avec les raies XK caractéristiques du Z=124, formé lors de la réaction U+70,76Ge. La difficulté majeure a été d'identifier ces raies XK, du fait de la présence de raies gamma émises par des fragments de fission dans la gamme d'énergie des XK du 124 entachant les multiplicités de photons mesurées pour différentes sélections de fragments. Cette difficulté met clairement en évidence une limitation importante à la méthode de fluorescence X pour certains systèmes super-lourds. Des simulations de spectres d’émission XK, réalisées à partir d'un calcul MCDF (Multi-Configuration-Dirac-Fock) ont été comparées aux spectres expérimentaux. Une limite maximale de multiplicité d'XK compatible avec les données, de l'ordre de 6 à 7 % pour le 76Ge et de 12 à 14 % pour le 70Ge ont ainsi pu être extraites. Ces valeurs restent compatibles avec les signatures de temps longs obtenues sur le même système par la technique de blocage cristallin. / Since the 1960s nuclear structure model have predicted the existence of an island of stability of superheavy elements. It should be located around the next magic numbers expected at N=172 or 184 and between Z=114 and 126 depending on the model. Very high fission barrier of a few MeV are predicted to be generated by microscopic effects for those nuclei for which large fission times distributions extended to very high fission times are induced. Fission time measurements of the superheavy element Z=124 have been made by us using the X-ray fluorescence technique, a method based on the filling of inner-shell electronic vacancies created during the collison leading to the formation of the compound nucleus. The aim of this experiment was to detect in coincidence both fission fragments and characteristic X-rays from the Z=124, created by the reaction U+70,76Ge.The main difficulty was to identify those X-rays due to the fact that gamma-rays from fission fragments were emitted in the same energy range, which affected our photon multiplicities for any fragment selection. This new difficulty brings an mportant limitation to the study of some particular superheavy elements by the X-ray fluorescence method. K X-rays spectra have been simulated using MCDF (Multi-Configuration-Dirac-Fock) and then compared to the experimental ones in order to get a maximal K X-ray multplicity compatible with our data. The extracted results were about 6-7% for 76Ge and from 12 to 14% for 70Ge. Those values remain compatible with the experimental signature of long lifetime component observed for the same system but using a blocking tehcnique in single crystals.

Planaria (class Turbellaria) are the highest life forms which exhibit fission (Hyman, 1951). The laboratory planarian, Dugesia, lives in lakes, ponds, streams and springs, where it occupies bottom habitats. Fissioning is an asexual mode of reproduction. The fission plane is usually transverse, forming behind the pharynx, and separation appears to be dependent upon locomotion (Curtis, 1902). The posterior half of the worm adheres to the substratum while the anterior half continues to move forward until the worm snaps in two. Each half then regenerates the missing parts, forming two new smaller worms.

Isotopic abundances of the elements xenon, krypton, and cesium formed in the 14-Mev neutron fission of 238U have been measured using the mass-spectrometric method. The relative yields of some isotopes of krypton, strontium, zirconium, molybdenum, ruthenium, iodine, xenon, barium, cerium and neodymium were measured using a Ge(Li) detector. The ratios were normalized through isobaric nuclides, and absolute yields were obtained by normalizing the sum of the heavy-mass yields to 100%. A semi-empirical method has been developed for constructing neutron yield curves. Such a curve was used to obtain a primary-yield curve from the cumulative yields reported here. The results indicate that considerable structure might exist in the primary-yield curve at the higher excitation energy. / Thesis / Doctor of Philosophy (PhD)

<p> Part I describes experiments carried out to search for possible products of ternary fission of 235U. Inert gases extracted from neutron-irradiated 235u were analyzed mass-spectrometrically for the presence of stable and radioactive neon and argon isotopes. No evidence for fission product neon or argon was found. Upper limits obtained for the yields are orders of magnitude lower than those suggested by some other studies.</p> <p> Part II describes measurements of relative yields and energy distributions of 3H, 3He, and 4He produced in fission of 235U. A short-range (<8 Mev) component in 4He, not previously established, has been detected in this work. The upper limit obtained for direct formation of 3He is lower by two to four orders of magnitude than the frequency of formation of 3He found in studies of other fissile nuclides.</p> / Thesis / Doctor of Philosophy (PhD)

<p> Fission-product bromine was isolated from a uranium target by the hot-atom reaction of the fission recoils with methane to form organic bromides. The organically-bound bromine was shown to be formed preferentially by primary (independently-formed) bromine with little contribution from secondary bromine. </p> <p> The delayed-neutron activity of the short-lived bromine isotopes was analyzed to obtain the relative yields of delayed neutrons from Br8?, Br88 and Br89 produced as primary fission products. The relative delayed-neutron yields are summarized below. (see abstract in text) </p> / Thesis / Doctor of Philosophy (PhD)